Equipment Flashcards
Give an overview of Sources of Central Gas Supply, with relevance to anaesthetics.
Define the key components, functionality and safety features of cylinder manifolds
Recognize the important aspects of liquid oxygen stores (vacuum insulated evaporators) and their associated safety features
Recognize the key components, functionality and safety features of oxygen concentrators
Cylinder manifolds can be used to supply piped gases at a constant pressure. Large size cylinders are used in two banks, ‘duty’ and ‘standby’. There is automatic changeover between the two banks
Liquid oxygen is usually used to supply piped oxygen in hospitals. It is stored in vacuum insulated evaporators at a temperature of -150oC to -170oC (below its critical temperature of -118oC) and at a pressure of 5–10 atmospheres
Oxygen concentrators are used to extract oxygen from air using differential adsorption via a molecular zeolite. Oxygen concentrations of 93–95% can be achieved
Automatic changeover from the duty to the standby bank should occur at a cylinder pressure that will ensure the maximum usage of the contents of the duty bank. Each bank of cylinders has separate pressure regulator valves.
Although the pipelines are fed from pressure regulators and work at about 400 kPa, changes in demand can lead to small fluctuations in pipeline pressure. Pressure gauges are also used to indicate cylinder bank pressures and distribution pressure.
The number of cylinders in each bank is determined by the expected demand.
Question: How many days’ supply would you expect the total storage capacity of a manifold to be?
It should be one week’s supply. Each bank of cylinders should contain no less than two days’ supply, with a three days’ supply of spare cylinders kept in the manifold room.
The nitrous oxide manifold is often larger than the oxygen manifold. This is because nitrous oxide is present in cylinders only, whereas liquid oxygen is normally used to supply piped oxygen in hospitals, so the oxygen manifold is a back-up.
Can you identify the correct areas of the gas supply?
Liquid oxygen in vacuum insulated evaporators:
A. Should be stored at -150°C, below its critical temperature
B. Should be stored at pressures of 5–10 atmospheres
C. A safety valve opens at pressures above 1700 kPa
D. Can give more than 800 times its volume as gas at a temperature of 15°C and atmospheric pressure
A. Correct.
B. Correct.
C. Correct. A safety valve opens at 1700 kPa allowing the oxygen gas to escape, thus reducing pressure. This occurs if the pressure in the VIE starts to build up due to an under-demand of oxygen.
D. Correct.
Oxygen can also be stored as a liquid, in a vacuum insulated evaporator made of steel (the inner wall of stainless steel and the outer wall of carbon steel). This is the most economical way to store and supply oxygen, and allows for easy maintenance and access (Fig 1).
Liquid oxygen is stored at a temperature of -150 ° C to -170 ° C (below its critical temperature of -118 ° C) and at a pressure of 5–10 atmospheres. The temperature of the VIE is maintained by the high-vacuum shell (effectively a vacuum flask).
For the liquid oxygen to evaporate, it requires heat (the latent heat of vaporization). This heat is taken from the liquid oxygen, helping to maintain its low temperature. The outside surface of the VIE is painted white to reduce the absorption of ambient heat.
The VIE should be capable of delivering up to a maximum of 3000 L/min of oxygen. At a temperature of 15 ° C and atmospheric pressure, liquid oxygen can give 842 times its volume as gas. VIEs can be supplied in up to 50 different sizes depending on the oxygen use in the hospital.
Features of oxygen concentrators include:
A. Oxygen concentrators concentrate O2 that has been delivered from an oxygen cylinder manifold
B. Argon accumulation can occur when they are used with a circle system
C. Oxygen concentrators are made of columns of a zeolite molecular sieve
D. Oxygen concentrators can achieve O2 concentrations of up to 100%
E. Oxygen concentrators use air at atmospheric pressure
A. False. Oxygen concentrators extract oxygen from air using a zeolite molecular sieve. Many columns of zeolite are used.
B. True. The maximum oxygen concentration achieved by oxygen concentrators is 95%. The rest is mainly argon. Using low flows with the circle-breathing system can lead to the accumulation of argon. Higher fresh gas flows are required to avoid this.
C. True. The zeolite molecular sieve selectively retains nitrogen and other unwanted gases in air. These are released into the atmosphere. The changeover between zeolite towers is made by a time switch.
D. False. Oxygen concentrators can deliver a maximum oxygen concentration of 95%.
E. False. Air used is compressed to a pressure of 137 kPa.
Oxygen concentrators can be used to supply oxygen by extracting it from air by differential adsorption (Fig 1).
Ambient compressed air is filtered and pressurized to about 137 kPa and enters one of the two parallel, alternating adsorber towers located on either side of the central ‘mix tank’.
The air in the tower is then forced through a molecular sieve composed of columns of microporous crystals known as zeolites.
When under pressure, the zeolites strongly attract nitrogen molecules, while allowing oxygen molecules to pass through. By the time the air has made it to the top of the tower, all of the nitrogen and most other impurities have been removed.
All that remains is the oxygen and trace amounts of inert argon. This oxygen passes into the mix tank, completing the one cycle of the concentrator.
Adsorption
Occurs when the host material does not change its characteristics when the added substance adheres to it. The zeolite material is a crystalline structure that is quite rigid. Nitrogen molecules chemically attach to it but do not change its physical structure.
Absorption
Occurs when a substance combines with another substance to change the physical characteristics of the host material. (Paper towels, Kleenex, toilet tissue all depend on absorption).
When using cylinder manifolds:
A. Size E cylinders are customarily used
B. There is an automatic changeover between the banks of the manifold
C. Cylinders in each bank are connected through non-return valves to a common pipe
D. The manifold should be housed as near as possible to the main hospital building
E. Each bank of cylinders should contain no less than two days’ supply
A. False. Larger cylinders, e.g. size J, are usually used.
B. True.
C. True.
D. False. Due to fire hazard, manifolds should be housed away from the main hospital building.
E. True.
One source of central hospital gas supply is cylinder manifolds (Fig 1). These are used to supply nitrous oxide, Entonox and oxygen at a constant pressure (Fig 2).
This is normally achieved via a control panel from two equal banks of large cylinders (e.g. size J) known as:
Duty banks
Standby banks
The cylinders in each bank are connected through non-return valves to a common pipe. All cylinders in each bank are turned on and interconnected.
The duty and the standby banks alternate in supplying the pipelines. The changeover from one to the other is an automatic process.
Oxygen can also be supplied from vacuum insulated evaporators where it is stored as liquid oxygen, or from large oxygen concentrators.
Give an overview of gas cylinders, and the relevance to anaesthetics.
identify commonly used medical gas cylinders and their associated valves
understand the safety features of cylinders, valves and connections
be able to interpret pressure changes in cylinders in relation to their content
Traditional gas cylinders are made of thin-walled molybdenum steel to withstand high pressures.
Newer ‘CD’ oxygen cylinders are constructed of aluminium alloy wrapped in carbon fibre or kevlar. This makes them lighter and able to hold oxygen at higher pressure.
Oxygen cylinders contain gas whereas nitrous oxide cylinders contain a mixture of liquid and vapour. In the UK, they are 75% filled with liquid nitrous oxide (filling ratio).
At a constant temperature, the pressure in a gas cylinder decreases linearly and proportionally as the cylinder empties. This is not true in a cylinder containing liquid and vapour.
Gas cylinders are colour coded and display labelling and marking.
They undergo regular testing and checking.
A cylinder valve is mounted on the neck of the cylinder which acts as an on/off device for the discharge of the contents. The pin index system prevents cylinder identification errors.
Entonox is a gas mixture of 50% oxygen and 50% nitrous oxide by volume.
The colour of a medical gas cylinder indicates the contents within it. In the UK, ISO standards determine the colour which represents each gas. Different manufacturers may vary the colour pattern slightly, but in general the colour of the shoulders of the cylinder (the curved top) will indicate the contents within.
Can you match these gases to the correct cylinder?
Cylinders of different sizes are labelled from ‘A’ to ‘J’. Sizes ‘A’ and ‘H’ are not routinely used for medical gases, but other sizes can be seen in the healthcare setting.
Question: What size of oxygen cylinder would we routinely see on the back of our anaesthetic machine as in Figure 1 and what size is normally used in cylinder manifolds?
Those attached to the anaesthetic machine are usually size ‘E’, while size ‘J’ cylinders are commonly used for cylinder manifolds.
Question: How does a nitrous oxide cylinder change pressure as it empties?
As the cylinder contains liquid and vapour, initially the pressure remains constant as more vapour is produced to replace what has been used (Figure 2). Once all the liquid has evaporated, the pressure in the cylinder decreases.
In actual fact, the cylinder pressure may drop a little as the liquid is evaporating, as the latent heat of vapourisation cools the cylinder and its contents, leading to a slightly lower vapour pressure. We may also see ice crystals forming on the outside of the cylinder.
The key message is that the cylinder pressure does not accurately indicate the cylinder contents, staying roughly the same until no liquid is left.
Nitrous oxide is used in anaesthesia as an adjunct to other anaesthetic drugs and also as an analgesic. It is usually stored in steel cylinders which are French blue in colour (Figure 1).
Nitrous oxide cylinders contain liquid nitrous oxide in equilibrium with its vapour. When we open a cylinder, vapour is released and more liquid nitrous oxide evaporates to replace it.
The pressure in a nitrous oxide cylinder represents the saturated vapour pressure of nitrous oxide at the temperature of the cylinder: usually 5,200kPa at room temperature (21C).
Question: Why do nitrous oxide cylinders contain liquid, when oxygen cylinders do not?
The critical temperature of nitrous oxide is 36.5C, much higher than the value for oxygen (-118C). The critical temperature is the temperature above which a substance cannot exist in its liquid form.
At room temperature, nitrous oxide is below its critical temperature and as such it can be liquified by pressure.
Oxygen is above its critical temperature and therefore cannot be liquified by pressure alone.
Because nitrous oxide cylinders contain liquid, a larger mass is present in a full cylinder, compared to oxygen.
Question: Do you know the volume of gas and vapour present in a size E oxygen and nitrous oxide cylinders?
Size E oxygen cylinders contain 680 litres whereas size E nitrous oxide cylinders can release 1800 litres.
Question: Why do you think nitrous oxide cylinders are only partially filled with liquid?
If ambient temperature increases beyond the critical temperature, all liquid nitrous oxide would turn to gas, potentially causing a significant increase in pressure and consequent risk of explosion.
Having cylinders only partly full of liquid nitrous oxide reduces the risk of a dangerous increase in pressure at higher temperatures.
The filling ratio of a cylinder is defined as the mass of the contents, divided by the mass of water it could hold when full.
In the UK, the filling ratio for nitrous oxide and carbon dioxide is 0.75. In hotter climates, the filling ratio is reduced to 0.67.
Which of these statements about gas cylinders is accurate?
A. There is no need for cylinders to undergo regular checks
B. The only agent identification on the cylinder is its colour
C. When turning on a cylinder, the cylinder valve must be opened slowly
D. Cylinders are made of thick-walled steel to withstand the high internal pressure
E. Bodok refers to the type of metal from which cylinders are made
A. Incorrect. Cylinders should be checked regularly by the manufacturers. Internal endoscopic examination, pressure testing, flattening, bending and impact testing, ultrasonic testing and tensile testing should be carried out on a regular basis.
B. Incorrect. In addition to the colour coding of the cylinder, the name, chemical symbol, pharmaceutical form and specification of the agent are displayed on the cylinder to identify the agent.
C. Correct. When turning on a cylinder, the cylinder valve must be opened slowly to prevent a rapid rise in the pressure within the machine’s pipelines.
D. Incorrect. For ease of transport, cylinders are made of thin-walled seamless molybdenum steel. They are designed to withstand considerable internal pressures and tested up to pressures of about 22,000kPa.
E. Incorrect. Bodok refers to the Bodok seal washer that makes a seal between the cylinder and the yoke of the anaesthetic machine.
When the pressure of a molybdenum steel oxygen cylinder is 6,850kPa, the cylinder is:
A. Full
B. Three-quarters full
C. Half full
D. One-quarter full
A. Incorrect.
B. Incorrect.
C. Correct. Oxygen is stored in cylinders at a pressure of 13,700kPa, so this option is correct: it is half full. Oxygen is stored as a gas, so at a constant temperature, pressure changes are related to volume. The pressure of a gas accurately represents the cylinder contents.
D. Incorrect.
The critical temperature of oxygen (the temperature above which it cannot be liquified by pressure alone) is -118 degrees celsius. This means that storage of oxygen as a liquid in normal ambient conditions is not possible. As such, oxygen cylinders contain gaseous oxygen.
Traditional steel oxygen cylinders are filled to a pressure of 13,700kPa, a pressure chosen to maximise content without risking the structural integrity of the cylinder.
A standard size ‘E’ cylinder (Figure 1) contains 680L of oxygen when full.
Gay-Lussac’s law tells us that for a fixed volume of an ideal gas, pressure is proportional to absolute temperature (Figure 1).
The regulator assembly of a CD oxygen cylinder can effectively be considered to be a box of fixed volume.
If a CD cylinder valve is rapidly turned on, the pressure in this fixed volume box increases rapidly and, potentially, so does the temperature. This is called adiabatic compression.
It is thought that the temperature increase may exceed the ‘auto-ignition temperature’ of any contaminants in the valve assembly and thus cause ignition.
Adiabatic changes can occur with any gas cylinder. However, newer CD cylinders with their higher filling pressure and integral regulators may be particularly prone if they are not used in accordance with the manufacturer’s guidance.
Regarding Entonox:
A.
Entonox is a 50:50 mixture by weight of O2 and N2O
True False
B.
Pressure in a full entonox cylinder is 13,700kPa
True False
C.
Cylinders should be stored upright
True False
D.
At room temperature, cylinders contain only gas
True False
E.
Entonox cylinders have blue bodies and white and blue quarters on the shoulder
True
A. False. Entonox is a 50:50 mixture of O2 and N2O by volume and not by weight.
B. True. Pressure in a full Entonox cylinder is 13,700kPa.
C. False. This increases the risk of the liquefaction and separation of the components. To prevent this, Entonox cylinders should be stored horizontally for about 24 hours at temperatures of or above 5C. This position increases the area for diffusion. With repeated inversion, Entonox cylinders can be used sooner than 24 hours.
D. True. The liquefaction and separation of nitrous oxide and oxygen occurs at or below -5.5C.
E. True. Entonox cylinders have blue bodies and white and blue quarters on the cylinder shoulder.
Entonox (BOC Medical) is a mixture of gaseous oxygen and nitrous oxide in a 50:50 ratio by volume. It is used in hospital and prehospital settings to provide analgesia and sedation.
Entonox is manufactured by bubbling oxygen gas through liquid nitrous oxide. The resulting gaseous mixture is stored in cylinders, usually at 13,700kPa (Figure 1). Lightweight ED cylinders with an integral valve are sometimes seen, particularly in the pre-hospital setting. These cylinders have an integral valve and are thus very similar to CD oxygen cylinders. ED cylinders are filled to a pressure of 21,700kPa.
Entonox is usually self-administered. A 2-stage pressure demand regulator is attached to the cylinder to regulate delivery to the patient (Figure 2). As the patient inhales through the mask or mouthpiece, the valve opens and gas flow is permitted. Flow ceases at the end of an inspiratory effort.
If a cylinder of Entonox is cooled below -5.5C, the nitrous oxide component starts to liquefy and the gas mixture separates. This is called the Poynting effect. The result of the separation is:
a liquid mixture at the bottom of the cylinder, containing mostly nitrous oxide with approximately 20% oxygen dissolved in it
a gas mixture of high oxygen concentration at the top of the cylinder
If this cold cylinder is used, the oxygen-rich gas mixture at the top of the cylinder will be delivered first. The oxygen-poor liquid at the bottom of the cylinder then evaporates, leading to delivery of gas of decreasing oxygen concentration. This may lead to the supply of hypoxic mixtures with less than 20% oxygen as the cylinder empties.
This separation and liquefaction can be reversed by re-warming the cylinder and mixing its contents by inverting it repeatedly.
Give an overview of the piped gas supply, and relevance to anaesthetics.
Recognize the key features and functionality of the piped gas supply network and outlets
Demonstrate an awareness of the piped gas supply safety features including how the supply connects to the anaesthetic machine
Piped gas is supplied by a network of copper pipelines throughout the hospital from central supply points
The outlets are named, colour-coded and shape-coded to accept matching probes
The anaesthetic machine is connected to the outlets via flexible and colour-coded pipelines that are permanently fixed
Single-hose and tug tests are performed to test for cross-connection and misconnection respectively
There is a risk of fire from worn and damaged hoses
Gases are supplied under pressure of 400 kPa. In addition, air is also supplied under pressure of 700 kPa
A vacuum of -53 kPa (-400 mmHg) is generated
Identify the NIST colour-coded hoses.
Medical air is supplied either for clinical use, such as during anaesthesia via the anaesthetic machine, or to drive power tools, such as orthopaedic surgical tools or tourniquet equipment.
For clinical use, medical air is supplied at a pressure of 400 kPa, while for power tools it is supplied at 700 kPa.
As you can see in Fig 1, the terminal outlets for the two pressures are different, to prevent misconnection.
Question: Where do you think 400 kPa is most commonly used?
Anaesthetic machines and most intensive care ventilator blenders accept a 400 kPa supply.
Regarding PMGV systems:
A. Oxygen is supplied under two pressures
B. The NIST system is the British Standard
C. Systems are made of copper pipework because of its bacteriostatic properties
D. Outlets can be installed in various ways
E. Medical air is usually supplied from a compressor
A. False. Oxygen is supplied under one pressure of 400 kPa. Medical air is supplied under two pressures – 400 kPa and 700 kPa.
B. True. The British Standard states that the end of the hose connected to the anaesthetic machine should be permanently fixed using a nut and liner union and gas-specific and non-interchangeable screw threads (the NIST system).
C. True. In addition, copper also prevents the degradation of the gases.
D. True. Outlets can be installed as flush-fitting units, surface-fitting units, on booms or pendants, suspended on a hose and gang-mounted.
E. True. Air can be supplied from cylinder manifolds, or more economically, from a compressor.
PMGV system outlets are recognized by:
A. Gas colour coding
B. Gas colour coding and gas name
C. Gas colour coding, gas name and their shape
D. Gas colour coding, gas name, their shape and pressure
Submit
A. Incorrect.
B. Incorrect.
C. Incorrect.
D. Correct.
Which of the following statements about suction in PMGV systems is correct?
A. The pump should be capable of creating at least a negative pressure of 53 kPa (400 mmHg)
B. The pump should be capable of creating at least a negative pressure of -53 kPa (-400 mmHg)
C. A unit should take no longer than 100 seconds to generate a vacuum (500 mmHg) with a displacement of air of 25 L/min
D. The pump should be capable of creating a positive pressure of -53 kPa (-400 mmHg)
A. Incorrect.
B. Correct. The pump should be capable of creating a negative pressure of -53 kPa (-400 mmHg). A unit should take no longer than 10 seconds to generate a vacuum (500 mmHg) with a displacement of air of 25 L/min.
C. Incorrect.
D. Incorrect.
Give an overview of Pressure Regulator, Pressure Gauge, Flowmeters, and the relevance to anaesthetic machines.
Identify the key features and functionality of the pressure gauges, regulators and flow restrictor components of the anaesthetic machine
Recognize the components, basic designs and functionality of the of anaesthetic machine flowmeters and demonstrate an awareness of their safety features
Define the anti-hypoxic features in flowmeter design and the Quantiflex ® anaesthetic machine
Colour coded and gas specific pressure gauges use the Bourdon pressure gauge principle to measure pressures in cylinders and pipelines
Pressure regulators reduce pressure of gases from cylinders to about 400 kPa (similar to pipeline pressure). This allows fine control of the gas flow and protects the anaesthetic machine from high pressures
Flow restrictors are used on pipeline supply instead
Flowmeters are gas specific where both laminar and turbulent flows are encountered, making both the viscosity and density of the gas relevant. The bobbin should not stick to the tapered tube
Oxygen is the last gas to be added to the mixture. The flowmeter is very accurate with an error margin of +/- 2.5%. Oxygen and nitrous oxide flowmeters are interlinked to prevent the delivery of hypoxic mixtures
Select each of the labels outlined in red for details of how the regulator works.
The valve and the spring
As gas from the cylinder enters the high pressure chamber and passes into the low pressure chamber via the valve, it exerts a force that works to close the valve.
There is an opposing force from the diaphragm and spring that works to open the valve. A balance is reached between the two opposing forces. This maintains a gas flow under a constant pressure of about 400 kPa from the outlet of the regulator.
The diaphragm
If the diaphragm ruptures, the valve will fail. For safety purposes, relief valves (usually set at 700 kPa) are fitted downstream of the regulators and allow the escape of gas in the event of a regulator failure.
Should such a failure occur, the high flow rate of escaping gas will produce a hissing sound loud enough to alert the anaesthetist to the problem.
High pressure inlet and high pressure chamber
The high pressure inlet leads to a high pressure chamber fitted with a valve.
Low pressure chamber
If the supplying cylinder contains water vapour, this may condense and freeze as a result of the heat lost when gas expands on entry into the low pressure chamber. This can lead to the formation of ice inside the regulator which may block the flow of gas.
Cylinder gases should be water-vapour-free to prevent ice formation.
When the gas flowmeters in the anaesthetic machine are turned off, flow from the low-pressure chamber ceases. This causes the pressure in the low-pressure chamber to increase, so pushing the diaphragm upwards. This in turn closes the valve, halting the flow of gas from the cylinder into the regulator.
The low pressure outlet
Gas at the regulated pressure leaves via the low pressure outlet and is conveyed to the flowmeters of the anaesthetic machine before being delivered to the patient.
Pressure regulators are not the only means by which gas is supplied. When pipeline supply is used (at a pressure of 400 kPa), flow restrictors are used instead (Fig 1). Flow restrictors are simple constrictions between the pipeline supply and the rest of the anaesthetic machine.
Flow restrictors function in a similar way to pressure regulators. They significantly reduce the pressure while maintaining a constant flow rate, despite any changes in pipeline pressure that might occur. In this way, lower pressures of 100-200 kPa can be achieved.
Question: Why are flow restrictors used on the pipeline supply?
Flow restrictors are used on the pipeline supply instead of pressure regulators because pipeline supply pressure is not as high and variable as that delivered by cylinders.
Variable cylinder pressure
As gases are stored under high pressure in cylinders, regulators are necessary to reduce the variable cylinder pressure to a constant safe operating pressure of about 400 kPa.
Maintaining constant flow
As the gas in the cylinder is used up, the temperature and pressure of the cylinder contents decrease. Regulators allow for a constant flow to be maintained in spite of this.
Were they absent, constant adjustment of the gas flow would be required.
Pressure surging
Regulators are positioned between the cylinders and the rest of the anaesthetic machine.
This protects the low-pressure components of the anaesthetic machine against pressure surges.
Gas leaks
The use of pressure regulators allows low pressure piping and connectors to be used in the machine.
This means that the machine‘s pipe work is safe and easy to use, making the consequences of any gas leak much less serious.
The following statements relate to pressure regulators.
A. They are mainly used to protect the patient from the high pressure
B. They are only used to reduce the high pressure of cylinder gases
C. They maintain a gas flow at a constant pressure of about 400 kPa
D. Relief valves open at 700 kPa in case of failure
E. Flow restrictors can be used instead in cylinder gas supply
A. False. Pressure regulators offer no protection to the patient. Their main function is to protect the anaesthetic machine from the high pressure of the cylinder and to maintain a constant flow of gas.
B. False. Pressure regulators are used to reduce pressure of gases and also to maintain a constant flow. In the absence of pressure regulators, the flowmeters need to be adjusted regularly to maintain constant flows as the contents of the cylinders are used up. The temperature and pressure of the cylinder contents decrease with use.
C. True. Pressure regulators are designed to maintain a gas flow at a constant pressure of about 400 kPa, irrespective of the pressure and temperature of the contents of the cylinder.
D. True. In situations where the pressure regulator fails, a relief valve set to open at 700 kPa prevents the build up of excessive pressure.
E. False. Flow restrictors can be used in a pipeline supply only and not in cylinder gas supply. Flow restrictors are designed to protect the anaesthetic machine from pressure surges in the system. They consist of a constriction between the pipeline supply and the anaesthetic machine.
Describe each of the bobbins used in flowmeters.
In flowmeters, different designs of bobbin or ball are used (Fig 1). Most modern anaesthetic machines use a skirted bobbin with a dot to facilitate easy and accurate measurement of flows even when the flow rate is very low.
The tops of bobbins 3 and 4 are shown with slits (flutes) cut into the top. Gas flowing through these slits causes the bobbin to rotate inside the flowmeter tube.
The bobbin’s rotation and features such as the dot on bobbin 3 indicate to the operator that the bobbin is floating in the gas flow and not stuck inside the tube. Thus, an accurate reading of gas flow can be obtained.
The following statements relate to flowmeters in an anaesthetic machine.
A. Each flowmeter is gas calibrated
B. They are made of a tube that is wide at the bottom and tapers at the top
C. The flow reading is taken from the bottom
of the bobbin
D. Flowmeters have a linear scale
E. Laminar flow is the principal cause of the bobbin’s ‘floating’ in the tube
A. True. This means that it is not possible to use oxygen flowmeter to measure air or nitrous oxide flow and the opposite is correct.
B. False. The tube is tapered at the bottom and widens at the top.
C. False. The flow reading should be taken from the top of the bobbin.
D. False. The flowmeters do not have a linear scale. There are different scales for low and high flow rates.
E. False. Both laminar and turbulent flows are encountered. This depends on the position of the bobbin in the flowmeter. At low flows, the flowmeter acts as a tube, as the clearance between the bobbin and the wall of the tube is longer and narrower. This leads to laminar flow which is dependent on the viscosity of the gas (Poiseuille’s law). At high flows, the flowmeter acts as an orifice. The clearance is shorter and wider. This leads to turbulent flow which is dependent on gas density.
The flowmeters (Fig 1) in the anaesthetic machine measure the flow rate of a gas passing through them. Each flowmeter is individually calibrated for each gas. Calibration occurs at room temperature and atmospheric pressure (sea level). Flowmeters have an accuracy of about +/- 2.5%.
Units are calibrated as L/min for flows above one L/min. For flows below that, measurement is shown in increments of 100 ml/min. This allows for more accurate flow measurement when a low-flow breathing system is used.
The flow of the gas is controlled by the flow control (needle) valve, as shown on the previous page. The gas then flows into a tapered transparent plastic or glass tube that is wider at the top than the bottom (Fig 2).
A light-weight rotating bobbin or ball is used to measure the flow. The bobbin-stops at either end of the tube ensure that the bobbin is always visible to the operator at extremes of flow.
During an anaesthetic using the circle breathing system, Dr Smith reduces the flows of the gases. He notices that, despite adjusting the control knob of flowmeter, the bobbin is not responding accordingly. He also notices that the bobbin is not rotating.
A. It can only be due to a build up of static electricity in the flowmeter tube
B. The gas supply pressure is too low
C. An inaccurate gas mixture might be delivered
D. The bobbin is stuck inside the tube
E. There is a crack in an adjacent flowmeter
A. Incorrect. While this is a likely reason for the bobbin’s lack of response, the bobbin can stick inside the flowmeter for other reasons too:
a) There may be an accumulation of dirt in the tube, perhaps from a from a contaminated gas supply
b) The anaesthetic machine might be on an uneven surface
B. Incorrect. This would be unlikely.
C. Correct. Inaccurate gas mixtures can be delivered if the bobbin is not responding. Bobbin sticking due to the build up of static electricity can lead to in-accuracies of up to 35%.
D. Correct. This is the most likely reason, especially if the gas flow is low.
E. Incorrect. This would not cause the bobbin to stick.
The following statements relate to pressure gauges in an anaesthetic machine.
A. They are interchangeable between different gases
B. They are interchangeable between cylinder and pipeline supplies
C. They use the Bourdon pressure gauge principle
D. They are colour coded for a particular gas or vapour
E. The pressure accurately reflects the contents for both oxygen and nitrous oxide cylinders
A. False. Each gauge is calibrated to measure a specific gas.
B. False. Cylinders are kept under much higher pressures
(13 700 kPa for oxygen and 5400 kPa for nitrous oxide) than the pipeline gas supply (about 400 kPa). Using the same pressure gauges for both cylinders and pipeline gas supply can lead to inaccuracies and/or damage to pressure gauges.
C. True. A pressure gauge consists of a coiled tube that is subjected to pressure from the inside. The high pressure gas causes the tube to uncoil. The movement of the tube causes a needle pointer to move on a calibrated dial indicating the pressure.
D. True. Colour coding is one of the safety features used in the use and delivery of gases in medical practice. In the UK, white is for oxygen, blue for nitrous oxide and black for medical air.
E. False. Oxygen is stored as a gas in the cylinder hence it obeys the gas laws. The pressure changes in an oxygen cylinder accurately reflect the contents. However, nitrous oxide is stored as a liquid and vapour so it does not obey Boyle’s gas law. This means that the pressure changes in a nitrous oxide cylinder do not accurately reflect the contents of the cylinder.
The following statements relate to safety features in flowmeter design.
A. Modern anaesthetic machines are designed to deliver nitrous oxide with a minimum percentage of oxygen
B. The delivery of a hypoxic mixture to the patient is
prevented by separating the oxygen and nitrous oxide delivery systems
C. The Quantiflex® anaesthetic machine has two flowmeters; one for oxygen and one for nitrous oxide, with a control knob for each
D. The Quantiflex® anaesthetic machine delivers no more than a 30% oxygen concentration
E. Bobbin stops and an illuminated flowmeter bank ensure that the bobbin remains visible at very high or very low flows
A. True. The design of modern anaesthetic machines make it impossible for nitrous oxide to be delivered without the addition of a minimum percentage of oxygen. European Standard EN 740 requires anaesthetic machines to prevent the delivery of a gas mixture with an oxygen concentration below 25%.
B. False. In modern anaesthetic machines, oxygen and nitrous oxide flows may have separate flow control knobs but they are linked mechanically or pneumatically, so that it is impossible to deliver less than a specified oxygen concentration (typically 25% to 30% oxygen).
C. False. The Quantiflex ® anaesthetic machine has two flowmeters; one for oxygen and one for nitrous oxide, but with one control knob for both flowmeters.
D. False. The Quantiflex ® anaesthetic machine is designed to deliver relative concentrations of oxygen and nitrous oxide. The oxygen concentration can be adjusted in 10% steps from 30% to 100%, thus preventing the delivery of hypoxic mixtures.
E. True.
When working with flowmeters, it is important to note that the oxygen control knob is situated to the left (in the UK) whereas in the USA and Canada, it is situated to the right.
This may be due to historical reasons but is a cause of a potential problem if there is a crack in an adjacent downward flowmeter that may result in a hypoxic mixture.
In such cases, the oxygen will flow via the least resistant path, i.e. the cracked flowmeter, so potentially producing a hypoxic mixture (Fig 1).
To avoid this issue, oxygen is the last gas to be added to the mixture delivered to the back bar (Fig 2).
With this design, the oxygen flowmeter is retained on the left but oxygen is added last to the mixture.
In November 2000, a 3-year-old girl died in the Accident and Emergency Department of Newham Hospital, London. She was mistakenly given pure nitrous oxide gas instead of oxygen. In the urgency of the moment - the need to resuscitate a seriously ill child - a doctor mistakenly administered nitrous oxide only.
The European Standard for anaesthetic machines (EN 740) requires them to have the means to prevent the delivery of a gas mixture with an oxygen concentration below 25%.
In modern anaesthetic machines it is impossible for nitrous oxide to be delivered without the addition of a fixed percentage of oxygen. This is achieved by using interactive oxygen and nitrous oxide controls. This helps to prevent the possibility of delivering a hypoxic mixture to the patient.
Oxygen and nitrous oxide have separate flow control knobs, but they are linked mechanically or pneumatically, so that it is impossible to deliver less than a specified oxygen concentration (typically 25% to 30% oxygen).
Give an overview of Vaporizers, Oxygen Flush and Alarms, and the relevance to anaesthetics.
Describe the key features, functionality and safety features of vaporizers and their filling devices
Describe the key features, functionality and safety features of the emergency oxygen flush and oxygen supply failure alarm
Explain the function of the compressed oxygen outlets and common gas outlet of the anaesthetic machine
List the key features, functionality and safety features of the Triservice anaesthetic apparatus
Vaporizers are made of copper as copper is a good heat sink material. They consist of a bypass channel and vaporization chamber. The latter has wicks to increase the surface area available for vaporization. The gas leaving the vaporizing chamber is fully saturated
Vaporizers have a temperature-sensitive valve that controls the splitting ratio. They have colour- and geometrically-coded filling devices
Vaporizers used for desflurane have different design modifications due to desflurane’s unique physical properties
Emergency oxygen flush delivers 100% oxygen at flows of 35-75 L/min. Inappropriate use can cause barotrauma and potential awareness
Oxygen supply failure alarms are activated by the oxygen pressure itself with no other power supply source. They also allow the supply of ambient air
The Triservice apparatus has two Oxford miniature vaporizers (OMVs), a self-inflating bag and a non-rebreathing valve. It is suitable for both spontaneous and controlled breathing. The OMV is a draw-over vaporizer with no temperature compensation. It has a heat sink and can be used with different inhalational agents
Vaporizers need to incorporate several important design features in order to accommodate the physical properties of volatile inhalational agents and to control the transfer of heat caused by their vaporization.
Question: With these factors in mind, what would you expect the functional characteristics of the ideal vaporizer to be?
The ideal vaporizer’s performance would be unaffected by:
Changes in fresh gas flow
The volume of the liquid agent
Ambient temperature and pressure
Decrease in temperature due to vaporization
Pressure fluctuation due to the mode of respiration
In addition, it should:
Have a low resistance to the flow of gas
Be lightweight and have a small liquid requirement
Incorporate safety features to prevent accidental delivery of excessively high concentrations of the inhalational agent
Be economical in use, with minimal servicing requirements
Have a corrosion- and solvent-resistant construction
A vaporizer is a device used to add a specific, controlled and predictable concentration of an inhalational agent, in the form of a vapour, to the fresh gas flow (FGF) before it is delivered to the patient. The amount delivered is expressed as a percentage of saturated vapour added to the gas flow.
Fig 1 shows the basic design of a vaporizer with a vaporizing chamber containing the liquid anaesthetic agent, and a bypass channel. FGF passing through the vaporizing chamber picks up the anaesthetic vapour. This is then mixed with the anaesthetic- free gas bypassing the chamber.
The proportion of vapour-containing gas and bypass gas is controlled by a dial.
As already noted, vaporizers present numerous design challenges. One such challenge is the loss of latent heat of vaporization.
Question: Why is this a significant challenge?
As the inhalational agent evaporates, its temperature decreases due to the loss of latent heat of vaporization. A cold liquid is less volatile than a hot one so lowering the temperature of the inhalational agent makes it less volatile and the concentration carried by the FGF decreases.
Modern vaporizers incorporate design features that overcome this challenge and these are examined in the following pages.
The material used in the vaporizer needs to offer:
High density
High specific heat capacity
Very high thermal conductivity
Question: What material provides these characteristics?
Copper.
Copper acts as a heat sink, readily giving heat to the anaesthetic inhalational agent and maintaining its temperature. The vaporizer shown in Fig 1 is a copper-made design.
A temperature-sensitive valve within the body of the vaporizer automatically adjusts the splitting ratio of FGF and inhalation agent. Temperature valves incorporate either a bellows design or a bimetallic strip.
Bellows design
The bellows design allows more flow into the vaporizing chamber as the temperature decreases. As the temperature decreases, the bellows contract, restricting the flow of fresh gas through the narrowed valve channel, thus allowing more flow through the vaporizing chamber (Fig 1).
Bimetallic strip
The bimetallic strip (Fig 2) is made of two strips of metal with different coefficients of thermal expansion bonded together. As the vaporizer is used, the temperature of the inhalational agent decreases. The strip bends, so allowing more flow into the vaporizing chamber to maintain the full saturation of the gas leaving it
During use, the desflurane vaporiser is mounted on the Selectatec system. The vaporiser incorporates auditory and visual malfunction alarms and some designs have a back-up 9-volt battery in case of a mains failure. Key components of the desflurane vaporiser are explained here.
Fresh gas flow
Unlike the vaporisers that have been described so far, the fresh gas flow (FGF) does not enter the vaporisation chamber. Instead, the FGF enters the path of the regulated concentration of desflurane vapour before the resulting gas mixture is delivered to the patient.
Vaporising chamber
The desflurane vaporisation chamber is electrically heated and requires a warm-up period of 50 to 10 minutes to reach its operating temperature of 39°C, i.e. above its boiling point, and an SVP of more than 1550 mmHg. The vaporiser will not function below this temperature and pressure.
Differential pressure transducer
The FGF pressure is measured by a differential pressure transducer which adjusts a pressure-regulating resistor (R1) at the outlet of the vaporisation chamber. The pressure transducer senses pressure at the orifice on one side and the pressure of desflurane vapour upstream to the pressure-regulating valve on the other side.
Pressure-regulating resistors
Pressure-regulating resistor (R1) is located at the outlet of the vaporisation chamber. This is adjusted by the differential pressure transducer and regulates FGF pressure. The percentage control dial with a rotary valve adjusts a second resistor (R2) which controls the flow of desflurane vapour into the FGF, and thus the output concentration. The dial calibration is from 0-18%, with 1% graduation from 0-10% and 2% graduation from 10-18%.
Orifice
The FGF is restricted by a fixed orifice so that the pressure of the carrier gas within the vaporiser is proportional to gas flow. The transducer ensures that the pressure of desflurane vapour upstream of the resistor equals the pressure of FGF at the orifice. This means that the flow of desflurane out of the vaporising chamber is proportional to the FGF, so enabling the output concentration to be made independent of FGF rate.
Identify the components of the vaporizer shown below.
An anaesthetist induces anaesthesia intravenously. He starts the maintenance of anaesthesia by administering the inhalational agent.
He tries to turn on the percentage dial of the vaporizer. However, the dial does not move and no inhalational agent is administered.
Question: What is the most likely cause of this problem?
The anaesthetist was not able to turn on the dial because the locking lever of the Selectatec system was not engaged. The locking lever has to be engaged before the percentage control dial can be moved.
The most common cause of this is a failure to switch off the control dial of the other vaporizer.
An error of this kind should have been detected before the commencement of anaesthesia as part of the routine anaesthetic equipment check-up.
This indispensible alarm warns of a failure of the oxygen supply (Fig 1). In such situations, the nitrous oxide supply is automatically switched off as a safety measure, and air (with 21% oxygen) is delivered to the patient.
Question: What characteristics would you demand of the ideal warning device?
Despite the many variations in design, the characteristics of the ideal warning device are that:
- Activation depends only on the pressure of oxygen
- It requires no batteries or mains power
- To attract attention, it gives a distinctive, audible signal of sufficient duration (at least 7 seconds) and volume (more than 60 db at 1m distance from the front of the machine)
- It gives a warning of impending failure and a further alarm that failure has occurred
- It has pressure-linked controls which interrupt the flow of all other gases when it operates. Atmospheric air is allowed to be delivered to the patient without carbon dioxide accumulation. It should be impossible to resume anaesthesia until the oxygen supply has been restored
- The alarm is positioned on the reduced-pressure side of the oxygen supply line
- It should be tamper-proof. The alarm can be switched off only by restoring the oxygen supply
- It is not affected by back-pressure from the anaesthetic ventilator
Identify the safety features of a modern anaesthetic machine.
A. An anti-spill mechanism in the vaporizer
B. A pressure relief valve
C. An oxygen-failure alarm
D. Vaporizer downstream flow restrictors
E. Interlocks between the vaporizers
A. Correct. The vaporizer’s anti-spill mechanism prevents the liquid anaesthetic agent from entering the bypass channel, even if the vaporizer is tipped upside down. This avoids the accidental delivery of dangerously high concentrations of the inhalational agent.
B. Correct. A pressure relief valve, set to open at 35-40 kPa is designed to protect the machine (not the patient) from high pressures. The patient should be protected from excessive pressures by relief valves within the breathing circuit.
C. Correct. This essential alarm warns of a failure of the oxygen supply.
D. Correct. Downstream flow restrictors are used to maintain the vaporizer at a pressure greater than any pressure required to operate commonly used ventilators.
E. Correct. Interlocks between the vaporizers prevent inadvertent administration of more than one volatile agent concurrently.
Consider the following statements regarding the oxygen emergency flush on an anaesthetic machine.
A. Its use poses no risk to the patient
B. Its button is designed to minimize accidental activation
C. It increases the risk of awareness during anaesthesia
D. It can be safely used with a minute volume divider ventilator
E. It delivers an oxygen flow rate of 20 L/min
A. False. The inappropriate use of the oxygen flush during anaesthesia increases the risk of awareness (100% oxygen can be delivered) and barotrauma to the patient (due to the high flows delivered).
B. True. The button is recessed in a housing to prevent accidental depression.
C. True. This can happen by diluting the anaesthetic mixture; see ‘A’.
D. False. Due to the high FGF (35-70 L/min), the minute volume divider ventilator does not function appropriately.
E. False. 35 to 75 L/min can be delivered by activating the oxygen emergency flush on the anaesthetic machine.
The emergency oxygen flush, when activated, supplies pure oxygen from the outlet of the anaesthetic machine. The flow bypasses the flowmeters and the vaporizers. A flow of about 35-75 L/min at a pressure of about 400 kPa is expected.
The emergency oxygen flush is usually activated by a non-locking button and using a self-closing valve (Fig 1). It is designed to minimize unintended and accidental operation by staff or other equipment.
The button is recessed in a housing to prevent accidental depression (Fig 2).
Excessive use of the emergency oxygen flush can put the patient at a higher risk of barotrauma due to the high operating pressure and flow of the oxygen flush.
Inappropriate use can lead to the dilution of the anaesthetic gases mixture and possible awareness. It should not be activated while ventilating a patient using a minute volume divider ventilator.
Consider the following statements concerning modern plenum vaporizers.
A. The bimetallic strip valve in Tec Mk5 is in the
vaporizing chamber
B. Gas flow emerging from the vaporizing chamber should be fully saturated with the inhalational agent
C. The inhalational agent concentration delivered to the patient gradually decreases the longer the vaporizer is used due to the cooling of the agent
D. The inhalational agent concentration supplied by the vaporizer is dependent on the fresh gas flow
E. Vaporizers are made of copper due to the material’s high density, high specific heat capacity and its very high thermal conductivity
A. False. The bimetallic strip valve in the Tec Mk5 is in the bypass chamber. This design has been in use since Tec Mk3 to avoid corrosion of the strip in a mixture of oxygen and inhalational agent when positioned in the vaporizing chamber.
B. True. This can be achieved by increasing the surface area of contact between the carrier gas and the anaesthetic agent. Full saturation should be achieved despite changes in fresh gas flow. The final concentration is delivered to the patient after mixing with the FGF from the bypass channel.
C. False. The concentration delivered to the patient stays constant due to temperature compensating mechanisms.
D. False. The inhalational agent concentration supplied by the vaporizer is virtually independent of the FGFs between 0.5 to 15 L/min.
E. True. Copper acts as a heat sink so helping to maintain the temperature of the liquid anaesthetic agent.
The following statements concern the Triservice anaesthetic apparatus.
A. It requires a reliable supply of compressed gases and vapours
B. An oxygen supplement can be connected to a cylinder downstream of the vaporizers
C. The apparatus can be used both for spontaneous and controlled ventilation
D. It is suitable for prolonged use at high gas flows
E. The calibration scales on the Oxford miniature vaporizers can be detached allowing the use of different inhalational agents
A. False. The apparatus is suitable for use in remote areas where the supply of compressed gases and vapours is unreliable.
B. False. The oxygen supplement cylinder is connected upstream of the vaporizers.
C. True.
D. False. The Triservice anaesthetic apparatus is equipped with two OMVs that are not suitable for prolonged use at high gas flows. Their design has no temperature compensation features which means that vapour concentration decreases as the temperature of the inhalational agent decreases.
E. True.
The Triservice anaesthetic apparatus name derives from the three military services: Army, Navy and Air Force. The apparatus is suitable for use in remote areas where the supply of compressed gases and vapours is unreliable (Fig 1).
The apparatus consists of the following components (Fig 2):
A face mask with an integral non-rebreathing valve
A self-inflating breathing bag
Two OMVs
An oxygen supplement that can be connected to a cylinder upstream of the vaporizers
A length of tubing that acts as an oxygen reservoir during expiration
The Triservice apparatus can be used both for spontaneous and controlled ventilation:
The patient can draw air through the vaporizers
The exhaled gases are vented out via the non-rebreathing valve
The self-inflating bag can be used for controlled or assisted ventilation
Give an overview of pollution and scavenging, and the relevance to anaesthetics.
Describe current knowledge of the risks of occupational pollution in the operating theatre and the acceptable levels of waste gases
List the various methods used in reducing the pollution in the operating theatre
Describe the functionality and safety features of passive scavenging
Describe the functionality and safety features of active scavenging
There is no proven association between occupational exposure to trace levels of waste anaesthetic vapours in scavenged operating theatres and adverse health effects. However, it is desirable to vent out the exhaled anaesthetic vapours and maintain a vapour-free theatre environment
Methods used to decrease theatre pollution include: theatre ventilation, regional anaesthesia, total intravenous anaesthesia, circle breathing system, the careful filling of vaporizers and scavenging
Scavenging systems can be either passive or active. Passive systems are simple to construct and cheap to maintain but are relatively inefficient. The exhaled gases are driven by either the patient’s respiratory efforts or the ventilator. Active systems, in which a vacuum drives the gases through the system, are more efficient
The scavenging system should not affect the ventilation and oxygenation of the patient and should not affect the dynamics of the breathing system
In the light of the potential occupational hazard facing operating theatre staff from exposure to trace levels of waste anaesthetic agents, many countries, including the UK, have produced a list of the maximum accepted concentrations detected in the operating theatre.
In 1996, the UK authorities recommended maximum accepted concentrations (when calculated over an 8-hour, time-weighted average). It should be noted, however, that these were not universally agreed upon.
The maximum accepted concentrations in parts per million (ppm) are shown below for four inhalational agents. Use your knowledge of the agents to match them to their UK maximum accepted concentrations.
It should be noted that the maximum accepted concentration for isoflurane is also 50 ppm.
Scavenging systems collect waste inhalation anaesthetic gases from the breathing system and dispose of them safely. The scavenging system should not affect the ventilation and oxygenation of the patient and should not affect the dynamics of the breathing system.
Question: Why is a scavenging system needed?
There needs to be an adequate and reliable system for scavenging waste anaesthetic gases in any location in which inhalation anaesthetics are administered to prevent any possible harmful effects of the waste gases. For example, un-scavenged operating theatres can show N2O levels of 400-3000 ppm. This is significant because long- term exposure to N2O has been found to affect the bone marrow.
The key features of a scavenging system are:
A collecting device for gases from the breathing system/ventilator at the site of overflow
A ventilation system to carry waste anaesthetic gases from the operating theatre
A method for limiting both positive and negative pressure variations in the breathing system
Monitoring the performance of the scavenging system should be part of the anaesthetic machine check.
Question: Considering the functional requirements of a scavenging system, what properties would you wish to see in an ideal design?
The ideal scavenging system:
Is easy and simple to install, and is reliable and cheap to run.
Does not affect the oxygenation and ventilation of the patient.
Does not affect the dynamics of the breathing system.
Is attached with components incompatible with those used in the patient’s breathing system, so avoiding any risk of accidental connection to the patient.
In case of a fault, prevents adverse events such as barotrauma or, an unacceptable increase in the apparatus dead space. It should also incorporate a mechanism to protect the patient against excessively high or low pressures in the breathing system.
Is capable of dealing with the various expiratory flow rates generated by patient or ventilator.
Disposes of waste anaesthetic gases without causing pollution of other parts of the theatre or hospital.
Performs without being affected by external factors such as the direction of the wind.
Uses a dedicated vacuum system to dispose of the waste gases that is separate from the suction system used in the hospital.
The following statements concern the reduction of pollution in the operating theatre:
A. Scavenging removes all anaesthetic agents except N2O
B. Low flow anaesthesia using the circle breathing system can reduce pollution
C. Unventilated theatres are 10 times more contaminated than properly ventilated theatres
D. Vaporizers should be filled in fume cupboards to prevent accidental spillage of inhalational agent
E. Cardiff Aldasorber absorbs all inhalational agents
A. False. Scavenging removes all anaesthetic agents including N2O. In any location in which inhalation anaesthetics are administered, there should be an adequate and reliable system for scavenging waste anaesthetic gases.
B. True.
C. False. Unventilated theatres are four times more contaminated than properly ventilated theatres. One of the most important factors in reducing pollution is adequate theatre ventilation, where the circulating air is changed 15-20 times per hour.
D. False. Modern vaporizers use agent-specific filling keys which limit spillage.
E. False. The Cardiff Aldasorber absorbs all inhalational agents but not N2O.
There are numerous ways to descrease pollution in the operating theatre (Fig 1). Six common methods are described here.
- Adequate theatre ventilation and air conditioning
Adequate theatre ventilation and air conditioning, with frequent and rapid changing of the circulating air (15-20 times per hour), is one of the most important factors in reducing pollution (Fig 2). Theatres that are unventilated are four times as contaminated with anaesthetic gases and vapours as those with proper ventilation. A non-recirculating ventilation system is usually used to avoid contaminating other parts of the operating theatre suite. In labour wards, where anaesthetic agents including entonox are used, rooms should be well ventilated with a minimum of five air changes per hour. - The circle breathing system
‘This breathing system recycles the exhaled anaesthetic vapours, absorbing carbon dioxide (CO2) (Fig 3). It requires a very low fresh gas flow, so reducing the amount of inhalational agents used. This decreases the level of theatre environment contamination. - Total intravenous anaesthesia
This anaesthetic technique using only the administration of intravenous drugs can reduce or eliminate the hazard of exposure to the waste anaesthetic vapours. Simple or sophisticated intravenous pumps with specialist software can be used - Regional anaesthesia
With the use of regional anaesthesia, the risk of pollution is reduced or eliminated. Regional anaesthesia can be ideal for limb and lower abdominal surgery (Fig 5). Intravenous sedation can be used in addition. - Avoidance of spillage during vaporizer filling
Filling the vaporizers used to be a significant contributor to the hazard of pollution in the operating theatre. Fig 6 shows an older design of vaporizer that used a screw top inlet. This design increased the risks of spillage during vaporizer filling. The absence of an agent-specific filler also increased the risk of adding the wrong agent to the wrong vaporizer.
Modern vaporizers use special agent-specific filling devices as a safety feature that also reduces spillage and pollution. - Scavenging
Modern operating theatres and anaesthetic machines use scavenging systems to reduce the amount of pollution in theatre (Fig 7). The following pages will provide more information about scavenging.
The following statements concern pollution in the operating theatre by inhalational agents:
A. There is an international convention for the maximum accepted concentration of inhalational agents in the operating theatre
B. In the UK, monitoring the concentration of inhalational agents in the operating theatre is carried out annually
C. In the UK, an un-scavenged operating theatre would have less than 100 ppm of N2O
D. In the USA, N2O, when used as the sole anaesthetic agent and when measured at 8-hour, time-weighted average concentrations, should be less than 25 ppm during the administration of an anaesthetic
E. A T-piece paediatric breathing system can cause theatre pollution
A. False. There is no international standard for the concentrations of trace inhalational agents in the operating theatre environment. This is mainly due to unavailability of adequate data. Different countries set their own standards but there is an agreement on the importance of maintaining a vapour-free environment in the operating theatre.
B. True. Monitoring the concentration of inhalational agents in the operating theatre is done annually in the UK, and on a quarterly basis in the USA in each location where anaesthesia is administered.
C. False. An un-scavenged operating theatre would have 400-3000 ppm of nitrous oxide. In the UK, the recommended maximum accepted concentration over an 8-hour, time-weighted average is 100 ppm of N2O.
D. True.
E. True. A T-piece paediatric breathing system can cause theatre pollution due to the open-ended reservoir. A modified version has an APL valve allowing scavenging of the anaesthetic vapours.
The anaesthetist notices the sudden collapse of the reservoir bag of the breathing in a spontaneously breathing anaesthetized patient connected to a scavenging system. He checks the breathing system for any disconnections and finds none. What other possible causes might there be?
A. Possible distal obstruction in the breathing system
B. There may be a failure of fresh gas flow into the breathing system
C. There may be excessive negative pressure in the scavenging system
D. It might be the result of excessive fresh gas flow
E. The reservoir bag might be ruptured
A. Incorrect. Distal obstruction in the breathing system will not lead to collapse of the reservoir bag.
B. Correct. In rare instances, reservoir bag collapse can be due to a failure of fresh gas flow into the breathing system.
C. Correct. Excessive negative pressure in a faulty scavenging system can lead to collapse of the reservoir bag.
D. Incorrect. Excessive fresh gas flow does not lead to collapse of reservoir bag.
E. Correct. This is possible but highly unlikely if routine checks of the anaesthetic machine have been carried out. Options B and C are the most probable causes in this situation.
During an anaesthetic in a spontaneously breathing patient connected to a scavenging system, the anaesthetist hears a sudden hissing sound nearby the anaesthetic machine. What could be the cause?
A. There may be a failure of fresh gas flow into the breathing system
B. It might be due to excessive fresh gas flow
C. There could be a distal obstruction in the receiver component of the scavenging system
D. There may be excessive negative pressure in the scavenging system
E. All of the above
A. Incorrect. Fresh gas flow failure does not lead to a hissing sound.
B. Incorrect. Excessive fresh gas flow does not lead to a hissing sound.
C. Correct. The hissing sound can be due the release of the gases from the receiver component of the scavenging system, due to a distal obstruction. This can occur in both passive and active scavenging systems.
D. Incorrect. Excessive negative pressure in a faulty scavenging system can lead to collapse of the reservoir bag.
E. Incorrect.
Give an overview of the Checking of Anaesthetic Equipment.
Describe the tests performed in checking the anaesthetic equipment
Identify potential equipment malfunctions in anaesthetic practice
Recognize the design modifications of modern anaesthetic equipment to prevent such malfunction
Routine checking of equipment is essential in the safe delivery of anaesthetic care
Ensure that you have turned on the anaesthetic machine after connecting it to the mains supply
Check the gas supply; both piped gases and cylinders
Make sure that the monitoring equipment is working adequately
Ensure that the various components of the anaesthetic machine are functioning correctly - flowmeters, vaporizers, oxygen emergency flush, scavenging system and suction system. Also check for leaks
Check the breathing system and its components
Ensure the availability of different airway management devices
When possible, use single-use devices
In cases of anaesthetic machine failure, have available means of ventilation and administering oxygen
In order to obtain an accurate reading, an appropriately sized blood pressure cuff should be used.
Fig 1 illustrates different sizes of blood pressure cuffs - (from left) small adult, medium adult and large adult.
Here’s a guide to the correct width of the cuff:
3 cm: Infant
6 cm: Child
9 cm: Small adult
12 cm: Standard
15 cm: Large adult cuff
Question: How do you identify the correct size of cuff to use?
To identify the correct size of cuff, estimate the circumference of the upper arm at mid point between shoulder and elbow. The bladder inside the cuff should encircle at least 80 % of the arm circumference, as shown in Fig 2. The width of the cuff should be 20 % more than the diameter of the arm. The cuff should be placed so that the midline of the bladder is over the arterial pulsation. Also, adjust the frequency of measurements according to the clinical condition of the patient and the type of surgery. This is usually done every 3 or 5 min.
Reserve gas cylinders should be available to ensure a continuous supply of medical gases, even in case of central gas supply failure.
Which reserve gas cylinders would you normally attach to an anaesthetic machine?
A. Oxygen
B. Carbon dioxide
C. Air
D. Nitrous oxide
Normally, oxygen, nitrous oxide and air cylinders are attached to the anaesthetic machine as a reserve supply.
Carbon dioxide cylinders should not normally be present on the anaesthetic machine due to risk of administering high concentrations of CO2.
After you have ensured that each pipeline is correctly inserted into the appropriate gas supply terminal, check the gauge pressures on the anaesthetic machine.
Question 1: What should the readings on the pipeline pressure gauges be?
Question 2: What should the readings on the full O2, N2O and Medical Air cylinder pressure gauges be?
The pressure gauges for pipelines connected to the anaesthetic machine should indicate 400–500 kPa (kilo Pascal).
The full cylinder pressure readings should be:
Oxygen - 13 700 kPa (137 bars)
Nitrous oxide - 4400 kPa (44 bars)
Medical air – 13 700 kPa (137 bars)
The emergency oxygen bypass control should be checked next. When the bypass button is pressed, the oxygen should flow without significant decrease in the pipeline supply pressure. It is important to check that the control valve shuts off when the button is released.
Question 1: When using the emergency oxygen by-pass control, what is the flow and pressure generated?
Question 2: Fig 1 shows the emergency oxygen bypass control. What safety feature is incorporated in its design?
Question 3: Fig 1 shows a chest x-ray of a patient exposed to high pressure oxygen flows from an emergency oxygen flush. Can you detect any abnormalities? What are the risks to the patient of excessive use of the emergency oxygen flush?
Question 4: What are the possible effects on the anaesthetic gases of inappropriate use of the oxygen flush?
Question 5: The emergency oxygen flush should not be activated while using a minute volume divider ventilator (see Fig 3). Why not?
The emergency oxygen flush, when activated, supplies pure oxygen with a flow of about 35-75 L/min at a pressure of about 400 kPa.
As a safety feature, the button is recessed in a housing to prevent accidental activation.
Excessive use of the emergency oxygen flush can put the patient at a higher risk of barotrauma due to the high operating pressure and flow of the oxygen flush. Fig 2 shows left tension pneumothorax with shift of the mediastinum to the right.
Inappropriate use can lead to dilution of the anaesthetic gases and possible awareness.
A minute volume divider ventilator delivers to the patient the fresh gas flow from the anaesthetic machine. It is inappropriate to deliver flows in excess of 35 L/min to the patient.
Fig 1 shows the Selectatec interlocking system commonly found on vaporizers in UK hospitals.
The commonest causes of leaks due to vaporizers are:
Incorrectly engaged vaporizers on the back bar
Loss of one of the O-rings on the mounts where the vaporizer is positioned. As the vaporizers are changed, the O-rings can accidentally adhere to the vaporizer, so causing a leak when another vaporizer is positioned
Question: Leaking vaporizers are a common cause of critical incidents. What makes a leak such a potentially dangerous fault?
A leak can lead to the delivery of incorrect proportions of anaesthetic gases.
As vaporizers Fig 1 are used to deliver the vapour of the volatile agents, it is important to make sure that:
The vaporizing chambers are adequately filled, as shown in Fig 2, and that the filling port(s) is/are tightly closed
The vaporizers are correctly fitted with a fully engaged back bar locking mechanism
The control knobs rotate fully throughout their full range
As we have seen, the consequences of any leakage of volatile agents can be serious. It is therefore essential that a leak test is carried out whenever a vaporizer is used or changed.
To test for leaks:
Turn off the vaporizer
Set a flow of oxygen of 5 L/min, as shown in Fig 1
Temporarily occlude the common gas outlet of the anaesthetic machine, as illustrated in Fig 2
Check for leak
There is no leak if the flow meter bobbin (if present) dips, as shown in Fig 3
Turn on the vaporizer and repeat this test
There should be no leak of liquid from the filling port
After testing, ensure that the vaporizers and flow meters are turned off
Co-axial breathing systems, such as Bain-type and circle co-axial, need to be tested for the patency of the inner tube. This can be done by performing an occlusion test on the inner tube and checking that the APL valve, where fitted, can be fully opened and closed.
Question: Why do you think it is important to the check the patency of the Bain’s breathing system inner tube?
In the Bain’s breathing system, the inner tube delivers the fresh gas flow (FGF). Occlusion or obstruction of the inner tube will prevent FGF from being delivered to the patient. Disconnection of the inner tube will result in a significant increase in the apparatus dead space.
How do you test for leaks due to a fault in the vaporizer?
What are the commonest causes of leaks in vaporizers?
A. Incorrectly engaged vaporizers
B. Partially filled vaporizer
C. Loss of one of the O-rings
D. The filling port(s) is/are partially closed
E. Tilting the vaporizer
Submit
A. Correct. Vaporizers may be incorrectly engaged on the back bar of the machine.
B. Incorrect. Although you need to ensure that the vaporizer is full before you start the anaesthetic, a partially filled vaporizer does not lead to leakage.
C. Correct. As the vaporizers are changed, these O-rings can accidentally adhere to the vaporizer, so causing a leak when another vaporizer is positioned.
D. Correct. The filling port(s) should be tightly closed to prevent leakage.
E. Incorrect. Tilting the vaporizer can lead to spilling of the liquid agent into the by-pass channel but should not lead to leakage.
The following statements relate to checking anaesthetic equipment.
A. The tug test is used to ensure that gas cylinders are secured and turned on
B. A suction unit should take no longer than 10 s to generate a vacuum of 500 mm Hg
C. Anaesthetic machine anti-hypoxia system is designed to prevent the administration of less than 18 %
oxygen
D. Single-use devices can be used more than once with different patients if adequately cleaned
E. In the scavenging system, a 22 mm connector is usually used
A. False. The tug test is used to confirm that each pipeline is correctly inserted and engaged into the appropriate gas supply terminal. Inadequately inserted sockets can appear to stay attached to the terminal even when hanging vertically.
B. True.
C. False. The European Standard for anaesthetic machines (EN740) requires them to have the means to prevent the delivery of a gas mixture with an oxygen concentration below 25 %.
D. False. Any device that is designated ‘single-use’ must be used for one patient only, and not re-used.
E. False. In the scavenging system, a 30 mm connector is used as a safety measure in order to prevent accidental misconnection to other ports of the breathing system.
Give an overview of the circle breathing system.
Describe a suitable arrangement of a circle and low flow breathing system
Describe the advantages of low flow anaesthesia
Explain the principles of carbon dioxide absorption
Identify the need for monitoring expired gases
The circle system allows economical use of anaesthetic vapours by allowing rebreathing and use of an absorber to remove carbon dioxide
The absorption reaction forms heat and water, which can humidify inspired gases and therefore heat is conserved
Unidirectional valves ensure that gas flow is in the correct direction, but the circle is a more complex design compared to other breathing systems
As the FGFs are decreased, less anaesthetic vapour is wasted, but this leads to a discrepancy between dialled vaporizer concentration and true concentration
To achieve rapid change in anaesthetic concentration, it is better to alter the flow rate, not the vaporizer setting
To check your knowledge of breathing systems, answer the following questions:
Question: In the Mapleson breathing system, high gas flows are often employed. What are the two disadvantages of high gas flows in breathing systems?
Question: Decreasing gas flows lead to less vapour vented and therefore greater economy of anaesthetic vapours. What else could happen if gas flows are decreased?
- They are wasteful of anaesthetic gases. High flows result in the venting of expired gas, which contains anaesthetic vapour. This is then lost and cannot be used again
- The compressed gases used are dry with no water vapour. In order to humidify the gases, the respiratory tract dries out and heat loss occurs due to evaporation
Answer: Rebreathing becomes possible with CO2 retention. The circle system incorporates a method of absorbing this CO2 so that rebreathing of gases becomes possible. Rebreathing also leads to an economy of anaesthetic vapours.
Vaporizer output can become less predictable. This is explained in the section on vaporizers in the circle system where gas monitoring becomes more important at low flows.
When the flows decrease to less than minute volume, it is called low flow. However, the term often applies to flows of less than 1 L/min.
Question: What three advantages can you think of for using the circle breathing system?
Question: What six potential disadvantages can you think of for using the circle breathing system?
- Recycling of the anaesthetic agents is economical and minimizes pollution in the operating theatre
- As the expired gases are humid and warm, heat loss (latent heat of vaporization) via the respiratory tract is reduced
- Its characteristics are similar for both spontaneous and controlled ventilation
The use of the circle breathing system has a number of potential disadvantages such as:
- The rebreathing of certain breakdown products of anaesthetic agents
- Potential formation of carbon monoxide
- Complex arrangement with more potential for errors
- Some components are difficult to clean
- Gas monitoring is required, as the inspired gas concentration becomes difficult to predict
- Gas leaks in the system are difficult to compensate for
Anaesthetic gases with the CHF2 side chain (enflurane, isoflurane and desflurane) can react with soda lime or barylime to form carbon monoxide. Desflurane has the greatest effect, followed by enflurane and then isoflurane.
This is only significant when the water content is less than 1.5% in soda lime.
Question: Can you think of two clinical situations when this would be likely to occur, and how carbon monoxide production could be prevented?
- When the circuit is left unused for a long length of time, e.g. overnight or at weekends
- When a small basal flow from the anaesthetic machine occurs
The association of strong alkalis such as KOH and NaOH to the production of carbon monoxide has led to the subsequent removal of KOH and reduction in amounts of NaOH used.
Some absorbers (e.g. Amsorb ® ) do not use strong alkalis at all.
Carbon monoxide production is prevented by the use of small absorber canisters that are changed regularly, disconnecting unused gas pipelines when possible and designing breathing systems so that the FGF from the anaesthetic machine bypasses the absorber. To sufficiently dry out soda lime requires high temperatures not associated with normal use.
Question: What four disadvantages are inherent in the Water canister system?
Four disadvantages of the Water canister system (Fig 1) are:
- The bulky canister is near the patient end. This is undesirable because the patient end must be accessible and any part of the breathing apparatus must be light and portable so that it does not cause traction at the airway
- A breathing filter is necessary to prevent inhalation of soda lime (which is very inflammatory to the lungs)
- Soda lime can form channels in which the expired gases can bypass, unless it is well-packed
- The granules nearest to the patient get exhausted first – increasing dead space
Regarding the circle system:
A. Gas monitoring is always necessary when using a circle system
B. Strong acids in the absorber are associated with carbon monoxide production
C. If the expiratory valve sticks in an open position the resistance to breathing will increase
D. Rebreathing is always associated with increases in inspired PaCO2
E. Small granules in the absorber are more efficient than larger granules
A. True. Gas monitoring is mandatory as volatile agent concentrations are not predictable in the circle system.
B. False. Strong alkalis are associated with carbon monoxide production.
C. False. This will lead to an increase in dead space. Increase in resistance occurs with valves stuck in a closed position.
D. False. The absorber will prevent an increase in carbon dioxide concentrations.
E. True. But the disadvantage is an increase in resistance.
Which of the following statements about the circle system are correct?
A. One unidirectional valve is sufficient
B. The APL valve is the same as those on other breathing systems
C. Its only advantage is in reducing the cost of the anaesthetic agents
D. Vaporizers placed outside the circle tend to give higher than dialled concentrations because of the recycling of the anaesthetic gases
E. The resistance to breathing in a circle system is greater than in a Mapleson D circuit
Submit
A. Incorrect. Two unidirectional valves are needed.
B. Correct. The principles are identical.
C. Incorrect. The circle system reduces heat loss and increases humidity in addition to reducing pollution.
D. Incorrect. The inspired vapour concentration is different from that dialled on the vaporizer due to the effect of addition of recirculated expired gas to the fresh gas mixture during low flows.
E. Incorrect. The resistance to breathing in Mapleson D is greater.
Which of the following statements about low flows are correct?
A. Low flows should be used throughout the anaesthetic period to save money
B. FGFs can be less than 200 ml/min
C. The FGF in a low flow system is less than alveolar ventilation
D. Low flows can be used in both spontaneous ventilation and controlled ventilation
E. CO2 absorption is unnecessary if FGFs exceed minute volume
A. Incorrect. In the beginning and end phases respectively, high flows should be set to wash in and out the volatile agents.
B. Incorrect. FGFs should never be less than basal oxygen consumption. Modern anaesthetic machines will not allow such low flows to be set.
C. Correct.
D. Correct. There are no problems in either mode of ventilation.
E. Correct.
The lower the flow, the greater the economy of anaesthetic vapour, but certain requirements must be met:
Fresh gas flow lower limit
The flow must be able to meet the oxygen requirements of the patient, which is approximately 200–250 ml/min in an adult (Fig 1). There must be no leaks from the system, and any gas analysed is returned to the patient. This is known as closed system anaesthesia.
Other essential requirements of low flow and closed systems
Accurate, low reading flow meters
Gas monitoring. This is because the vaporizer output becomes less predictable as the flows decrease
Time for sufficient denitrogenation. This may need a period of high flows
Low flow anaesthesia is usually not started at induction for the following reasons:
- High flows result in a more rapid onset of anaesthesia (Fig 1)
- Denitrogenation of the system
One reason for low flow anaesthesia not being started at induction is that the system needs to be denitrogenated. The air in the circuit at the beginning contains nitrogen and initial high flows can flush this air out. Also, nitrogen enters the system from the patient’s tissues and residual gas in the lungs. The use of air instead of nitrous oxide makes this less of a problem.
- Increased patient uptake of volatile agents
In addition, as the rapid intake of volatile agents and redistribution by the patient may lead to a decrease in volatile agent concentrations in the circuit, high flows are needed at the beginning to prime the system and to account for increased patient uptake.
Which of the following statements about soda lime are correct?
A. It has a high concentration of sodium hydroxide
B. It is highly irritant if inhaled
C. Heat is absorbed in the reaction with CO2
D. End-tidal CO2 can be a useful indicator of soda lime exhaustion
E. CO2 absorption causes soda lime to change colour
A. Incorrect. Soda lime has a high concentration of calcium hydroxide.
B. Correct. Extremely irritant to the lungs. This is mainly due to its very high pH.
C. Incorrect. Heat is released during the reaction with CO2 (exothermic reaction).
D. Correct. End-tidal CO2 will rise with soda lime exhaustion, and will also cause a rise in the inspired CO2.
E. Correct. It is the indicator that changes colour, not the soda lime. The colour changes depend on the indicator used.
There are various compounds used to absorb carbon dioxide, but the most frequently used compound in the UK is soda lime (Fig 1).
Soda lime consists of:
94% calcium hydroxide
2–5% sodium hydroxide
0.2% silica (to prevent disintegration of the granules)
A zeolite is added to maintain a higher pH for longer
Older preparations contained potassium hydroxide as a catalyst but modern soda lime does not need this.
Soda lime and its additives are made into granules to increase the surface area for absorption and to minimize resistance to gas flow. The granule size is measured in ‘mesh’. The usual size is 4–8 mesh. Too large a granule leads to an insufficient area for absorption, whereas too small a granule causes high resistance to gas flow.
100 g of soda lime can absorb up to 25 L of CO2.
During carbon dioxide absorption, a number of chemical reactions can occur. In principle, the exhaled gases are returned back to the canister, where the CO2 is absorbed and water and heat are produced.
This is an exothermic reaction, which alters the pH of the whole system. Dye indicators can be added to show when the soda lime has been exhausted (<10 pH ). The reaction also produces water. One mole of water is produced for each mole of CO2 absorbed.
Therefore, warmed and humidified gases are returned to the patient via the inspiratory tubing of the breathing system.
This effect is related to gas flow rates. At low rates, the humidifying and warming effect is increased. The opposite occurs at high flow rates. Also, low flow rates increase the consumption of soda lime. This is indicated by an increase in inspiratory end-tidal CO2.
Soda lime has a pH 13.5 (so it is very corrosive if inhaled), and a moisture content of 14–19%.
Give an overview of Face Masks and Oxygen Delivery Devices.
Define the key features and safety features of face masks and catheter mounts
Recognize the key features, functionality and safety features of different types of variable performance oxygen delivery devices
Recognize the important aspects and functionality of fixed performance oxygen delivery devices
It is important to note the following key points about face masks and oxygen delivery devices and their use:
Face masks are made of silicone rubber or plastic and are designed to ensure a snug fit over the face of the patient. They cause an increase in dead space (up to 200 ml in adults)
A catheter mount acts as an adapter between the tracheal tube or laryngeal mask and breathing system. They are usually made of plastic in different lengths and contribute to the apparatus dead space. Catheter mounts can be blocked by a foreign body
Variable performance oxygen delivery devices entrain ambient air. The inspired oxygen concentration depends on the oxygen flow rate, pattern and rate of ventilation, maximum inspiratory flow rate and how well the device fits the patient’s face
Fixed performance oxygen delivery devices usually use the Venturi principle to entrain ambient air. Changes in kinetic and potential energy during gas flow lead to negative pressure and air entrainment. There is no rebreathing or increase in dead space
Which of the following complications could be caused by an ill-fitting mask?
A. Injury to the branches of the facial or trigeminal nerves
B. Decreased apparatus dead space
C. Loss of air-tight seal over the face
D. Trauma to the eyes
E. Increased FiO2 delivered to patient
Always take care when applying the face mask:
A. True. Excessive pressure can cause injury to the branches of the facial or trigeminal nerves.
B. False. An ill-fitting mask can increase the apparatus dead space. This is of particular importance in paediatric anaesthesia.
C. True. For edentulous patients or those with a naso-gastric tube, it can prove difficult to maintain an air-tight seal over the face if the mask is ill-fitting.
D. True. Imprecise application of the mask on the face can cause trauma to the eyes.
E. False. It is very difficult to predict the FiO2 in an ill-fitting face mask as it depends on the proportion of ambient air entrainment.
Match the statements to the variable performance oxygen delivery device they describe.
Identify the factors that lead to the delivery of high and low FiO2 in variable performance oxygen delivery devices.
The following statements apply to Venturi oxygen face masks.
A. The side holes are used to entrain ambient air
B. The narrower the Venturi constriction, the higher the FiO2 achieved
C. There is no rebreathing
D. They can be used in adult patients only
E. They are useful in patients who are dependent on the hypoxic drive
A. False. The side holes are used to expel exhaled gases. Ambient air is entrained via the Venturi device itself.
B. False. The narrower the constriction is, the lower the pressure. Thus, the more entrainment of ambient air, the lower the FiO2.
C. True.
D. False. Venturi devices can be used for both adult and paediatric patients.
E. True.
Fixed performance oxygen delivery devices deliver a pre-determined oxygen concentration, regardless of the factors that affect the variable performance devices. Such factors might include inspiratory flow rate, respiratory rate and pattern.
The Venturi face mask (Fig 1) is an example of a fixed performance device. These are also known as High Air Flow Oxygen Enrichment (HAFOE) devices.
As with the variable performance face mask, the Venturi mask consists of a plastic body with side holes to vent expired gases. These holes are not used to entrain ambient air, as with variable performance face masks. The proximal end of the mask consists of a Venturi device. These are colour-coded, as shown in Fig 1. Each one is marked with the recommended oxygen flow rate to provide the desired oxygen concentration.
The colour coding of the Venturi devices denotes delivered FiO2, as shown in Fig 1.
The size of the constriction determines the final concentration of oxygen for a given gas flow. This is achieved, in spite of the patient’s respiratory pattern, by providing a higher gas flow than the peak inspiratory flow rate.
The concentration can be 0.24, 0.28, 0.31, 0.35, 0.4 or 0.6.
As the flow of oxygen passes through the constriction, a negative pressure is created. This causes the ambient air to be entrained and mixed with the oxygen flow.
As the gas flows through the Venturi device, the total energy of the flow consists of the sum of kinetic and potential energy. The kinetic energy is related to the velocity of the flow, whereas the potential energy is related to the pressure. When the flow of fresh oxygen passes through the constricted orifice into the larger chamber of the mask, the velocity of the gas increases distal to the orifice, causing the kinetic energy to increase.
As the total energy is constant, there is a decrease in the potential energy, resulting in negative pressure. This causes the ambient air to be entrained and mixed with the oxygen flow.
The FiO2 is dependent on the degree of air entrainment. Less entrainment ensures a higher FiO2 is delivered. This can be achieved by using smaller entrainment apertures or bigger ‘windows’ to entrain ambient air, as shown in Fig 1. The devices must be driven by the correct oxygen flow rate, calibrated for the aperture size if a predictable FiO2 is to be achieved.
As a result of the high fresh gas flow rate, the expired gases are rapidly flushed from the mask via its holes. Therefore, there is no rebreathing and no increase in dead space.
For example, a 24 % oxygen Venturi mask has an air:oxygen entrainment ratio of 25:1. This means an oxygen flow of 2 L/min which delivers a total air flow of about 50 L/min, well above the peak inspiratory flow rate, as shown in Graph 1.
These masks are recommended when a fixed oxygen concentration is desired in patients whose ventilation is dependent on the hypoxic drive.
Caution should be exercised, as it has been shown that the average FiO2 delivered in such masks was up to 5 % above the expected value.
Due to its being both noisy and bulky, patients often tend not to tolerate the mask with its Venturi device and the oxygen delivery tubing very well.
The following statements relate to oxygen delivery devices.
A. In an MC mask, the higher the inspiratory flow rate, the lower the FiO2
B. In a Venturi device, kinetic energy is related to pressure
C. Omitting the one-way valve in an MC mask with a reservoir bag, has no effect on its performance
D. A reservoir bag is usually added to a Venturi mask to improve its performance
E. The pattern of breathing has no effect on the FiO2 when using a MC mask
A. True.
B. False. Kinetic energy is related to velocity, whereas potential energy is related to pressure.
C. False. The one-way valve improves the performance by preventing rebreathing.
D. False. There is no need to add a reservoir bag to the Ventui mask. The Venturi mask can cope with any inspiratory flow rate generated by the patient.
E. False. In the MC mask, the length of the expiratory pause is important to expel all the expired gases and to fill the mask body with pure oxygen, ready for the next inspiration.
Give an overview of Oropharyngeal and Nasopharyngeal Airways
Demonstrate the design features of oropharyngeal and nasopharyngeal airways
Identify suitable opportunities for the use of oropharyngeal and nasopharyngeal airways
Employ the correct techniques for the sizing and insertion of oropharyngeal and nasopharyngeal airways
Oropharyngeal airways are designed to restore the normal pharyngeal anatomy found during consciousness and therefore help to maintain airway patency
The recommended methods for inserting the Guedel airway differ between adults and children
Bermann airways are designed to assist with oral fibreoptic intubation
Nasopharyngeal airways are better tolerated in the semi-conscious patient than oropharyngeal airways, and can be tolerated by awake patients
Nasopharyngeal airways are relatively contraindicated in patients on anticoagulants or those with bleeding diathesis, and are relatively contraindicated in those with base of skull fractures
Correctly identify the extraglottic airways shown below.
These statements concern fibre optics, laryngoscopes and their use in anaesthetic practice.
A. The Henderson laryngoscope is particularly useful with very obese patients and in obstetrics
B. For effective laryngoscopy with the Macintosh laryngoscope, the tip of the blade must be brought to lie firmly in the vallecula
C. In a flexible fibre optic scope, the light transmission and image bundles are essentially identical
D. The image quality of the fibre optic scope is generally superior to that of the digital model
E. Different designs of blade will require different insertion techniques for optimal performance, so it is more appropriate to be experienced with one device
A. False. The polio blade laryngoscope, not the Henderson blade, may be useful in such cases. The polio blade is a standard Macintosh size 3 blade mounted on an small wedge, thus altering the stand-off angle of the handle and allowing unobstructed manipulation of the laryngoscope.
B. True.
C. False. A coherent bundle is one in which the arrangement of fibres relative to one another is the same at both ends of the bundle. To carry light from the light source for illumination, a coherent bundle is not necessary.
D. False. The video or digital scope offers improved image quality over the coherent fibre optic bundle.
E. True.
Flexible fibre optic scopes (FFS) can follow the anatomical airway, more or less irrespective of its shape.
The FFS is so named because it transmits an image from the tip of the device to the eyepiece via a flexible glass fibre optic bundle which is encased in a series of flexible coverings.
A typical fibre optic bundle is composed of up to ten thousand individual glass fibres each with a diameter of around 10 µ m. Each fibre consists of a central glass core which is clad in a thin coating of a second glass of a different refractive index, as illustrated in Fig 2 (upper).
Light entering the fibre at the appropriate angle undergoes total internal reflection to emerge at the other end, as shown in Fig 2 (lower). The fibres are lubricated and can move against one another giving flexibility to the whole bundle.
As each fibre carries a tiny portion of the overall picture, it is essential for the clear transmission of an image that the arrangement of fibres is the same at both ends of the fibre optic cable, as illustrated in Fig 3.
A coherent bundle is one in which the arrangement of fibres relative to one another is the same at both ends of the bundle.
To carry light from the light source for illumination, a coherent bundle is not necessary.
Fig 4 shows the component parts of the FFS insertion tube. Click the zoom button to enlarge the image.
The following statements relate to the cuff of a tracheal tube:
A. It is there to prevent the tracheal tube sliding
B. It allows positive pressure ventilation
C. It helps protect the trachea from regurgitation of gastric contents
D. It may cause ischaemic damage to the tracheal wall
E. It may get damaged and leak
A. False. Sticky tape or a tie prevents tracheal tube movement.
B. True. The seal enables positive pressure ventilation.
C. True. Use of a cuff is mandatory if there is a risk of aspiration.
D. True. Avoid over inflating a cuff.
E. True. You can detect this by feeling a low pressure in the pilot balloon.
A cuff is an inflatable region at the patient end of a tracheal tube. Tracheal tubes may or may not have a cuff. The tracheal tube (top) does not have a cuff. The tracheal tube (middle) has a cuff that is deflated, and the tracheal tube (bottom) has an inflated cuff (Fig 1).
The inflated portion forms a seal against the tracheal wall (Fig 2). This seal prevents gases from leaking past the cuff and allows positive pressure ventilation. The seal also prevents matter such as regurgitated gastric contents going into the trachea.
After intubation, the cuff is inflated with air. This is done by attaching a syringe to the pilot balloon (Fig 1). The pilot balloon is connected to the cuff by a thin tube. As the syringe supplies pressurized air, the pilot balloon and cuff inflate.
Cuff inflation has to be done gently to prevent over-inflation and exerting too high a pressure on the tracheal mucosa. Continuous listening for an air leak should be used and the inflation should stop when the leak stops.
Once the cuff is inflated the syringe is removed. Air does not leak out as there is a one-way valve at the pilot balloon. By feeling the pilot balloon, the amount of pressure in the cuff can be estimated. If the cuff is leaking, e.g. due to damage by the surgeon during a thyroidectomy, the pilot balloon will collapse. Cuff pressure can also be checked using a specially designed pressure gauge.
Cuffs can either have a high volume with low pressure or have a low volume with high pressure (Fig 1).
High volume/low-pressure cuffs
Because of their large volume, these cuffs have a larger surface area in contact with the trachea. This means that they apply a lower pressure against the tracheal wall and have a lower incidence of tracheal wall ischaemia and necrosis. The seal is not as good as the seal in high-pressure cuffs because of the lower pressures and because the large cuff may develop wrinkles that allow material such as regurgitated gastric contents to pass by the cuff (Fig 2).
Low volume/high-pressure cuffs
These cuffs have a lower volume and the surface area in contact with the trachea is small. This results in a high-pressure seal that is more effective than the one created by high volume/low-pressure cuffs. However, this high pressure is more likely to cause tracheal ischaemia and necrosis if used for a prolonged period of time (Fig 3).
Regarding double lumen tracheal tubes:
A. Have three cuffs
B. The tracheal cuff is blue
C. Are easily malpositioned
D. Can be used to collapse one lung
E. Are available as ‘right‘- and ‘left‘-sided versions
A. False.
B. False. The bronchial cuff is blue.
C. True. You may need to confirm correct placement with a fibreoptic bronchoscope.
D. True. A collapsed lung can help the surgeon access structures in the thorax.
E. True. Left-sided double lumen tubes are used more often than right-sided double lumen tubes as they are easier to position correctly.
During thoracic surgery, there is usually a need for one lung to be deflated. There are special tracheal tubes called ‘double lumen tubes’ (DL tubes) to achieve this (Fig 1).
A DL tube can be thought of as two tracheal tubes sandwiched together. One tube is shorter and ends in the trachea and there is a cuff at this level called the tracheal cuff. The other tube extends further and enters a main bronchus and has its own cuff (bronchial cuff, usually coloured blue). Think ‘B’ for blue & bronchial.
DL tubes are inserted in a special way. Since they are easily malpositioned, the correct placement needs to be confirmed via various clinical tests and/or using a fibreoptic bronchoscope. DL tubes are available in specific right-sided or left-sided options depending on which main bronchus is to be intubated (Fig 2).
The right DL tube in Fig 2 has a specially formed bronchial cuff which is designed to prevent obstruction to the right upper lobe bronchus.
An alterative to using a double lumen tube to isolate one lung is to use a standard tracheal tube along with a special device called a ‘bronchial blocker’ (Fig 1). This is a thin tube with a cuff at one end (Fig 2). Here we describe its use to isolate the left lung.
First the trachea is intubated with a standard tracheal tube. As expected, it will now be ventilating the right and left lung (Fig 3).
Using a special connector, the bronchial blocker is inserted through the standard tracheal tube that was first inserted. It is guided into the lung to be blocked, which in this example, is the left one (Fig 4). A fibre optic scope (not shown) is often used for this purpose.
The diagram below illustrates the areas of tracheostomy tubes which can be adjusted to fit specific patient features. Some are adjustable on individual tubes and others vary between ‘off the shelf’ sizes and between manufacturers.
Large or obese patients will need a longer distance between the flange (at the skin) and the intra-tracheal portion. Ultrasound depth assessment at the time of tracheostomy may help judge this ‘neck’ distance. The position of the fenestrations needs to align with the laryngeal inlet.
The angle and length of the intra-tracheal portion should ensure a correct ‘fit’ in the trachea.
The outer diameter should be small enough to fit inside the trachea. The inner diameter determines the resistance to airflow and is important when ventilating patients.
The images in Fig 1a were taken with a fibre-optic endoscope and show the different tube tip positions when viewed from within the tube, and when viewed from above.
Question: Can you pair the images?
When a tracheostomy tube sits correctly wihin the trachea, the lowest resistance to ventilation and the best seal from the cuff will occur. The carina will appear ‘dead ahead’.
When performing or managing a cricothyrotomy, it is important to understand the relevant anatomy. Can you identify and label the surface anatomical structures in the figure below?
Cricothyrotomy refers to a group of techniques in which an airway is formed through the cricothyroid membrane.
Emergency cricothyrotomy is performed as a rescue procedure in ‘can’t intubate, can’t ventilate’ scenarios. As such, it is an essential skill for all anaesthetists.
Cricothyroid puncture may also be performed electively:
In the anticipated difficult airway to provide a temporary means of ventilation whilst the airway is secured
Via minitracheostomy tubes inserted through the cricothyroid membrane to allow tracheal toilet, but are not suitable for ventilation
Which of the following trachesotomy tubes would be suitable for use in a patient at risk of aspiration?
A. Cuffed tube with cuff inflated
B. Fenestrated tube
C. Uncuffed tube
D. Cuffed tube with cuff deflated
E. Minitracheostomy tube
A. Correct.
B. Incorrect.
C. Incorrect.
D. Incorrect.
E. Incorrect.
The tracheostomy tube in Fig 1 has a cuff. This seals off the upper airway. If the tube is correctly positioned within the trachea, gas can only move into and out of the lungs via the tracheostomy tube.
This means that:
The airway is ‘protected’ from aspiration, in the same way as when a cuffed endotracheal tube is inserted
Positive pressure ventilation can be effectively applied via the tracheostomy tube
If the tube becomes blocked, the patient has no other way of getting oxygen to the lungs (as the upper airway is sealed off)
Fenestrated tubes have one or more openings on the outer cannula, which allow air to pass through the patient’s oral/nasal pharynx as well as via the tracheostomy tube. The air movement allows the patient to speak and produces a more effective cough.
By using different inner cannulae (Fig 1) the outer hole can be blocked or remain patent.
Fenestrations increase the risk of oral or gastric contents entering the lungs. Patients who are at high risk of aspiration, or those receiving positive pressure ventilation should not have a fenestrated tube, unless a non-fenestrated inner cannula is used to block off the fenestrations.
Suctioning with a fenestrated tube should only be performed with the non-fenestrated inner cannula in situ, to ensure correct guidance of the suction catheter into the trachea.
Fig 1 shows a cuffed tracheostomy tube in situ. Airflow should only be through the tube to the lungs, allowing positive pressure ventilation if the tube is correctly sited.
Deflating the cuff, or better still, using an un-cuffed tube allows some airflow through the upper airway as in Fig 2.
Airflow can be increased by using a tube with a fenestration, marked at the angle of the tube in Fig 3.
Some patients benefit from the extra airflow through the larynx, which permits speech.
This method can also be used to ‘train’ the larynx by allowing air to flow through it again following a prolonged period of ventilation.
Move the labels on the right to their correct position on the graph.
Particles of a certain size, typically in the range 0.05 to 0.5 µ m, pass through the filter more easily than others. This size of particle is known as the most penetrating particle size. Particles of this size are too small to be directly intercepted by fibres and too large to undergo substantial Brownian motion.
The filtration efficiency as a function of particle aerodynamic diameter due to the different filtration mechanisms can then be plotted.
See Fig 1 opposite.
Note the minimum efficiency (maximum penetration) occurs at a particle diameter of about 0.3 µ m. This particle size is at the lower limit for the size of bacteria, but droplets of this size could contain many viruses, which tend to be less than 0.1 µ m in size.
Please select the circle that represents diffusion type of attraction
A. Interception
B. Inertial Impaction
C. Gravitational Settling
D. Diffusion
E. Electrostatic Attraction
Regarding ventilators:
A. A pressure-generator ventilator can compensate for changes in lung compliance
B. A flow-generator ventilator cannot compensate for leaks in the system
C. A time-cycling ventilator is affected by lung compliance
D. The duration of inspiration in a pressure-cycling ventilator is not affected by the compliance of the lungs
E. A pressure-generator ventilator can compensate, to a degree, for leaks in the system
A. False. A pressure-generator ventilator cannot compensate for changes in lung compliance. It cycles when the set pressure has been reached. This can be a larger or smaller tidal volume depending on lung compliance.
B. True. It will deliver the set flow whether there is a leak or not.
C. False. The ventilator will cycle with time regardless of lung compliance.
D. False. In a lung with low compliance, the duration of inspiration will be shorter because the set pressure level will be reached more quickly, leading the ventilator to cycle.
E. True. The ventilator will continue to deliver gases, despite the leak, until the pre-set pressure has been reached.
- Controlled mandatory ventilation
CMV allows for the complete replacement of a patient’s breathing. This is the mode used in the operating theatre.
- Assist control ventilation
In ACV, the patient’s own respiratory efforts (if they are of a sufficient magnitude) trigger inspiratory support from the ventilator. If spontaneous efforts are not detected, the ventilator functions as if in the CMV mode.
- Synchronized intermittent mandatory ventilation
SIMV allows the patient to breathe alone while remaining connected to the ventilator. The ventilator synchronizes its functioning with the patient’s own respiratory efforts so that the two do not overlap. This mode of ventilation support can aid the process of replacing ventilation with the patient’s own normal breathing (weaning).
- Pressure support ventilation
PSV is available on microprocessor-controlled ventilators. It is designed to augment the tidal volume of spontaneously-breathing patients by providing sufficient gas flow to maintain a predetermined pressure throughout inspiration. When the microprocessor detects a decrease in flow to a pre-set level, the machine cycles to the expiratory phase. PSV can be at a low level (5–15 cmH2O) when the patient has a good respiratory effort, or at higher levels to provide almost total ventilatory support.
Match the ventilator to the correct category by function.
- Minute volume dividers
This type of ventilator is most often found in the operating theatre. It divides the minute volume of the fresh gas flow from the anaesthetic machine into pre-set tidal volumes, thus determining the frequency of ventilation. An example of this type is the Manley ventilator.
- Bag squeezers
This type of ventilator can be regarded as replacing the hand ventilation of a Mapleson D (Bain) or a circle-breathing system.
The bag is in the form of bellows contained in a transparent container which is squeezed by increasing the pressure around it caused by the driving gas (Fig 2). The gas inside the bellows is the anaesthetic gas mixture. The driving gas may be compressed medical air or oxygen; it does not mix with the anaesthetic gas (Module 7e/Ventilators/Ventilators used in Operating Theatre and ICU has more information).
The ventilator driving the bag squeezer may be either electromagnetic or pneumatic.
- Electromagnetic ventilators
Here, a flow of compressed gas is controlled by electromagnetic valves under computerized control. This type of ventilator is found in the ICU and is capable of delivering a wide range of ventilation modes. An example is the Servo 900C ventilator
- Pneumatic ventilators
These are controlled only by pneumatic components. The VentiPAC ventilator is an example (Fig 4).
The pneumatic ventilator is mainly used as a transport ventilator. It is a flow generator, volume pre-set, time-cycled and pressure-limited ventilator, and it is MRI compatible.
Regarding the bag-valve device.
A. It provides the user with a tactile feedback regarding lung compliance
B. It is ideal for single-handed use
C. The FiO2 may be increased with the use of a reservoir bag
D. Uncontrolled ventilation may occur
E. The valve is difficult to dismantle for cleaning and sterilization
A. True.
B. False. A good mask seal is not always possible with a single-handed use. A two-handed mask grip requires a second person to squeeze the bag.
C. True.
D. True.
E. False. Although a single-use device, the valve can be easily dismantled for cleaning and sterilization.
The bag-valve device (also known as a self-reforming bag, Ambu bag or BVM) has a long history of use in anaesthesia and recovery (Fig 1) . It has a number of advantages. In addition to its relatively low cost, it is:
- Safe
Disposable designs for both adult and paediatric use
It provides the user with a tactile feedback regarding lung compliance
The shape of the self-inflating bag is automatically restored after compression. This allows fresh gas to be drawn from the reservoir - Adaptable
A reservoir bag for oxygen and an additional oxygen supply can be added to the bag-value device to increase the fractional inspired oxygen concentration (FiO2)
The device consists of a self-reforming bag of various sizes to fit different size patients - Easy to use
Widely used and relatively intuitive
Ready to use as it requires no power source
The bag-valve device has three main disadvantages when used in emergency ventilation:
- Mask seal difficult to achieve single-handed
A good mask seal is not always possible with a single-handed use. A two-handed mask grip requires a second person to squeeze the bag. - Entrained oxygen may limit FiO2
Entrained oxygen may not be enough to produce a desirable FiO2 of 1.0. - Uncontrolled ventilation may occur
Uncontrolled ventilation may occur with high tidal volumes and high peak pressures
High inflation pressures may cause harm by inflating the stomach as well as the lungs. This increases the chance of regurgitation, vomiting and inhalation of stomach contents, since the airway is not protected by intubation
In addition, high airway pressures may cause damage to the lung tissue
Regarding emergency and transport ventilation.
A. Emergency ventilation is routinely required for critically-ill but stable patients
B. In emergency ventilation, the minimum possible number of controls on the device is desirable
C. Transport ventilation may be required from one country to another
D. The VR1 is an example of a popular transport ventilator
A. False. Emergency ventilation is required where breathing has stopped and there is life-threatening hypoxia.
B. True.
C. True.
D. False. The VR1 is an example of an emergency ventilator.
Click on all the HME at various positions in the breathing circuit, and explain each.
Position 1: Correct. When placed at the Y connection the expiratory gases pass over the HME so warming it and depositing condensation on it. On inspiration fresh gas passes over the HME being warmed and humidified.
Position 2: Placing the HME on the inspiratory limb means that exhaled gases do not pass over the filter and it will not warm or humidify. It is not uncommon to see an extra HME/filter in this position to stop soda lime dust entering the breathing system.
Position 3: Placing the HME on the expiratory limb does not allow inspiratory gases to pass over the HME. It is not uncommon to see an additional HME/filter here in the breathing system to protect the machine from any infectious agent in the exhaled gases. If an HME is already in the correct position it is unnecessary. In ICU, it is common to see a HME/filter in this position as a hot water bath humidifier is used with no filter.
Position 4: Ideal position for heat and moisture exchange but risks blockage by patient secretions. Common positioning for paediatrics as it reduces dead space.
The image shows the different parts of a non invasive humidified continuous positive airway pressure (CPAP) circuit.
Describe each point.
- Patient temperature sensors maintain temperature and protect against airway burns.
- PEEP valve, provides desired end expiratory pressure.
- Sterile water for topping up heating chamber.
- HME being used as a filter to protect ventilator from contamination.
- Heating chamber to heat water to required temperature.
- Control panel, used to set required temperature and alarms.
Which of the following drugs can be delivered via airway?
A. Epinephrine
B. Bicarbonate Sodium
C. Atropine
D. Lidocaine
E. Diamorphine
F. Cocaine
Many drugs are delivered via the airway for systemic absorption or for local effects. During a cardiac arrest drugs can be delivered via the airway if iv access cannot be obtained.
A. True. During cardiac arrest, sometimes in haemoptysis.
B. False. Irritant to the airway.
C. True. In cardiac arrest.
D. True. As a local anaesthetic during awake fibreoptic intubation.
E. True. For analgesia.
F. True. For local anaesthetic effects and recreational use where it can have significant systemic side effects.
Many drugs are delivered directly to the airway, this can be for local effect or systemic absorption. In order to understand where drugs will be delivered to, it is important to have an understanding of particle size and the dimensions of the relevant parts of the airway. In spontaneously breathing or ventilated patients, many different medications can be delivered via the airway.
This picture shows a patient receiving humidified enriched air via a tracheotomy.
Match the subject or object to the statement to which it belongs.
Arrange the statements for High pass (HP) and Low Pass (LP) filters in Diagnostic and Monitoring modes.
As we have seen, there are a number of electrical factors that can interfere with the ECG signal recording and its quality. The main factors can be summarized as follows:
Capacitance coupling and electrostatic induction
These can be caused by stray capacitances between table, lights, monitors, patients and electrical cables. The energy is transferred via capacitance allowing AC signal but not DC voltage. To reduce this effect, the ECG cables are surrounded by copper screens. The induced signal is rejected as common mode.
Electromagnetic induction (inductive coupling)
This can be caused by any electrical cable or lighting. Such interference is reduced by using long, latticed ECG leads so rejecting the induced signal as common mode and also by using selective filters in amplifiers.
Radiofrequency interference (>150 Hz)
This is mainly caused by the use of the surgical diathermy. Such a high frequency current can enter the system via the mains supply, directly applied by the surgical probe or via radio-transmission via the probe and wire. This high frequency electrical interference can be reduced by using high frequency filters to clean up the signal, double screening the electronic components of the amplifiers and earthing the monitor’s outer screen.
What type of artifact is affecting the adjacent ECG record? How can it be reduced?
A. Radio frequency artifact
B. 50 Hz mains artifact
C. High pass filter
D. Low pass filter
E. Combination of high and low pass filter
A. False.
B. True. 50 Hz mains artifact is illustrated here.
C. False.
D. True. The 50 Hz mains artifact can be reduced by using a low pass filter.
E. False.
What type of artifact is affecting the adjacent ECG record? How can it be reduced?
A. Body and muscle artifact
B. Both silver and silver chloride electrodes should be used
C. Artifact caused by position of electrodes on bony areas
D. By using a Low Pass filter
E. With a combination of high and low pass filter
A. True. The illustration shows an example of body and muscle artifact caused, in this case, by the patient’s shivering.
B. False.
C. False.
D. False.
E. True. Body and muscle movement artifact can be reduced using high and low pass filters.
At which frequencies do the LEDs in a pulse oximeter probe emit light?
A. 250 nm
B. 660 nm
C. 600 nm
D. 750 nm
E. 940 nm
A. False.
B. True.
C. False.
D. False.
E. True.
Light is emitted at 660 nm (red) and 940 nm (infrared).
The pulse oximeter is an in-vivo monitor.
The co-oximeter is an in-vitro monitor which measures the concentration of different forms of haemoglobin (OxyHb, DeoxyHb, MetHb and COHb) in a haemolysed blood sample and can therefore derive oxygen saturation.
Both forms of oximeter function by using the absorption of light. In the case of the pulse oximeter the light is absorbed by whole human tissue, in the co-oximeter by blood alone.
The co-oximeter relies on the fact that there is a linear relationship between light absorbance and the concentration of an absorbing substance (see the Beer-Lambert Law illustrated in Fig 1) and that different haemoglobins absorb light at different frequencies.
With regard to the pulse oximeter components, which of the following statements are correct?
A. The probe LEDs transmit at red 660 nm and infrared 940 nm frequencies
B. Probes have a photodetector on the same side (transmission) which measures transmitted light
C. The photodetector can distinguish between red and infrared light
D. The computerized unit contains timing circuits which control the sequencing of the LEDs
E. The microprocessor processes filtered data from the probe
A. True.
B. False. Probes have a photodetector either on the opposite side – transmission, or the same side – reflectance, to measure the transmitted or reflected light.
C. False. The photodetector cannot distinguish between red and infrared light, but to accommodate this, the microprocessor turns each LED on and off in sequence many times per second.
D. True.
E. True. Yes, and it also allows the setting of alarms for pulse rate and saturation.
Probes have a photodetector either on the opposite side to the LEDs – transmission, or the same side as the LEDs – reflectance, to measure the transmitted or reflected light.
The photodetector produces current linearly proportional to the intensity of the light striking it.
It cannot distinguish between red and infrared light, but to accommodate this, the microprocessor turns each LED on and off in sequence, as shown in Fig 3.
When both LEDs are off, ambient light is measured so it can be extracted from the signal. This is because the photodetector can detect light over a wide range of frequencies, not just red and infrared.
By measuring how the two frequencies of light are absorbed across a pulsatile tissue bed, the oximeter can measure saturation and display a pulse waveform.
The block diagram shows the basic components of the computerized unit which includes:
Switching circuit
This controls the sequencing of the LEDs, as already discussed.
Automatic gain controllers
These control the intensity of the emitted light to compensate for thickness of finger and skin colouration.
Bypass filters
These filters extract the AC component and will be examined in more detail on the next page.
Analogue to digital signal converters
These convert analogue to a digital signal which can be processed by the microprocessor.
Microprocessor and monitor
The microprocessor processes the filtered data from the probe. It also allows the setting of alarms for pulse rate and saturation. The display shows pulse rate, saturation and pulse waveform (plethysmograph).
Regarding pulse oximeters, which of the following statements are true?
A. They all work by passing light through tissues
B. Red and infrared LEDs are never on at the same time
C. By having a period when both LEDs are off the oximeter can compensate for ambient light
D. Beer-Lambert Law describes the linear relationship between light absorbed by a sample and the concentration of absorbing substance in the sample
E. The pulsatile component is displayed on the monitor as the plethysmograph
A. False. Some work by reflection of light from tissue surfaces, not by transmission.
B. True. The LEDs are lit in sequence - never together.
C. True. The photodetector detects light over a wide frequency range and ambient light needs to be removed from the signal.
D. True. Beer Lambert Law states A = Log(Il / Io) = βlc.
E. True. The computerized unit can extract the pulsatile (AC) component, due to arterial pulsation, from the constant component (DC). This can be displayed on the monitor as the pulse waveform (plethysmograph).
Light from the LEDs is absorbed by many components in the finger. The photodetector generates a current proportional to the transmitted light.
The computerized unit can extract the pulsatile (AC) component, due to arterial pulsation, from the constant component (DC). This can be displayed on the monitor as the pulse waveform (plethysmograph).
The pulse rate is calculated from this waveform.
Regarding the absorption spectra, which of the following statements are true?
A. Oxyhaemoglobin absorbs more light at red frequencies than infrared
B. The isobestic point for oxyhaemoglobin and deoxyhaemoglobin is at 940 nm
C. Carboxyhaemoglobin mimics deoxyhaemoglobin at 940 nm
D. Methaemoglobin mimics deoxyhaemoglobin at 660 nm
E. The absorption of oxyhaemoglobin is half that of deoxyhaemoglobin at 660 nm
A. False. Absorption is much greater at the 940 nm than 660 nm frequency.
B. False. The isobestic point is at 810 nm.
C. False. Carboxyhaemoglobin absorbs greater at 660 nm - the same as oxyhaemoglobin.
D. True. The absorption is similar to deoxyhaemoglobin at 660 nm.
E. False. The absorption scale on the Y axis is logarithmic.
Oxyhaemoglobin: Absorption is much greater at the 940 nm than 660 nm frequency. Note the logarithmic absorption scale.
Deoxyhaemoglobin: Absorption is much greater at 660 nm than 940 nm. Note the logarithmic absorption scale.
Isobestic point: At 810 nm both forms of haemoglobin have the same absorption. This is the isobestic point.
Carboxyhaemoglobin: Note that this absorbs at 660 nm, the same as oxyhaemoglobin.
Methaemoglobin: The absorption is similar to deoxyhaemoglobin at 660 nm.
Identify the different absorption frequencies shown on the absorption spectogram.
Which of the following statements about pulse oximeters are correct?
A. Burns from LEDs may occur in poorly perfused patients
B. Pulse oximeter accuracy is +/-2 % between 100-90 % SpO2
C. Acute changes are not detectable
D. Pulse oximeters detect saturation and adequacy of ventilation
E. External fluorescent lighting can interfere with the readings of a pulse oximeter
A. True.
B. False. Accuracy is +/-2 % between 100-70 % SpO2. Below 70 % the calibration curve is extrapolated. Note that values below 90 % are usually acted on.
C. True. Acute changes are not detectable due to the sampling rate being up to 20 seconds. Remember that oximeters average out saturation readings over 5 to 20 seconds.
D. False. The pulse oximeter is only a detector of oxygen saturation, not adequacy of ventilation.
E. True. The following are also causes of error in pulse oximeters: excessive movement and poor positioning of the probe.
To determine the patient’s saturation, the processor calculates the ratio of absorption at 660 nm to 940 nm (R/IR ratio):
R/IR = ac660/dc660
__________
ac940/dc940
This is compared to a ‘look up’ table pre-programmed into the machine’s memory. Most oximeters average out saturation readings over 5 to 20 seconds.
Fig 1 is a typical curve obtained from studies in healthy volunteers. It shows oxygen saturation measured from blood gas analysis on the y axis against R/IR ratio measured by an oximeter on the x axis. The curve shows actual measurements at saturations down to 70 %. Below this the curve has to be extrapolated.
Hence the pulse oximeter is only truly accurate in normal physiological range with an accuracy of +/- 2 %.
Accuracy: +/-2 % between 100-70 % SpO2. Below 70 % the calibration curve is extrapolated. This is not really a problem clinically, as values below 90 % are usually acted on. Fig 1 shows a reading from an actual pulse oximeter (Y axis) where the red line indicates the departure from the ideal when compared to an in-vitro oximeter (X- axis).
Acute changes
Acute changes are not detectable due to the sampling rate being up to 20 seconds. (Most oximeters average out saturation readings over 5 to 20 seconds).
Detector of saturation
The pulse oximeter is only a detector of saturation, not adequacy of ventilation. As shown on the adjacent results, the arterial blood gas sample shows a patient who is well saturated but has gross type 2 respiratory failure.
Poor perfusion and low BP
Poor perfusion and low blood pressure limits the oximeter’s accuracy and ability to read SpO2, as the pulsatile (AC) component is harder to extract.
Carboxyhaemoglobin
Carboxyhaemoglobin in carbon monoxide poisoning and heavy smokers causes falsely high SpO2, as it resembles oxyhaemoglobin at 660 nm.
Methaemoglobin
Methaemoglobin caused by drugs, such as nitrates and local anaesthetics, can lead to misreading as it resembles deoxyhaemoglobin at 660 nm.
Causes of error:
Excessive movement
Venous pulsation
Venous pulsation e.g. Tricuspid regurgitation and impaired venous return may be misinterpreted as part of the pulsatile (AC component). Fig 2 shows three oximeter traces from the same patient. There is obvious venous pulsation in the trace from the oximeter on the forehead, which is not present from the other two sites.
Pressure sores
Burns
Nail polish
Nail polish can cause under-reading of the pulse oximeter. Blue and black polish produce greater decreases than purple and red. The degree of artifactual desaturation correlates with the difference in the polish’s absorbance at 660 nm and absorbance at 940 nm. Turning the oximeter through 90 ° can eliminate the problem.
How does capnography work?
Gases with molecules that contain at least two dissimilar atoms, e.g. carbon dioxide (contains carbon and oxygen atoms), absorb radiation in the infrared region of the spectrum.
CO2 absorbs the radiation at a wavelength of 4.3 micrometers. The amount of radiation absorbed is proportional to the number of CO2 molecules (partial pressure of CO2) present in gas mixture (Beer-Lambert law: the absorption of light depends on the properties of the material through which the light is travelling).
The various components of a capnograph include:
Light source - The infrared radiation is emitted by a hot wire or light source and the particular frequency required is obtained by passing the radiation through an interference filter. A microprocessor-controlled infrared source is used in order to produce a stable source with a constant output
Interference filter - It is an optical filter in which the wavelengths that are not transmitted are removed by interference phenomena rather than by absorption or scattering. In addition to being able to duplicate most of the spectral characteristics of absorption colour filters, these devices can be made to transmit a very narrow band of wavelengths
Sample chamber and reference chamber - The infrared radiation passes simultaneously through the sample and reference chambers. As glass absorbs infrared radiation, the sample chamber has windows made from a material which is transparent to infrared radiation, such as sodium chloride, silver bromide or sapphire. To avoid variations in the output, a beam of light passes through the reference chamber containing room air in the same way. The absorption detected from the sample chamber is compared to that in the reference chamber. This allows the calculation of CO2 values
Photodetector - After passing through the sample and reference chambers the radiation is focused on a photodetector. The greater the absorption of infrared radiation by the gas being measured in the sample, the less the radiation monitored by the detector and vice versa. Consequently, it is possible to process the detector output electronically to indicate the concentration of gas present
A beam of infrared light is passed across the gas sample to fall on to a thermopile detector or sensor. The presence of CO2 in the gas leads to a reduction in the amount of light falling on the detector. The thermopile detector produces heat which is measured by a temperature sensor and is proportional to the partial pressure of CO2 as present in the mixture in the sample chamber. This leads to changes in the electrical output and the voltage in a circuit. It is a rapid and accurate method to measure CO2 concentration throughout the respiratory cycle.
Fig 1 shows infrared spectrometry in action. Note that due to the large amount of infrared absorption in the sample chamber by CO2, little infrared finally reaches the detector.
Which of these statements describe the advantages of side stream capnopgraph and main stream capnopgraph?
In a side stream capnograph the sampling chamber is connected to the distal end of the breathing system via a sampling tube. The sampling tube is a 1.2 mm internal diameter tube that samples both inhaled and exhaled gases at a constant rate of about 150-200 ml/min and delivers them to the sample chamber. The tube is connected to a light weight adaptor near the patient’s end of the breathing system with a small increase in the dead space. The tube is made of teflon so it is impermeable to carbon dioxide and does not react with anaesthetic agents.
There is a moisture trap with an exhaust port, allowing gas to be vented to the atmosphere or returned to the breathing system. To measure end-tidal carbon dioxide accurately, the sampling tube should be positioned closer to the patient’s trachea. Other gases and vapours can also be analyzed from the same sample.
For clinical usefulness a capnograph needs a rapid response time, which has two components, the transit time and the rise time.
In the main stream version the sampling chamber is positioned within the patient’s gas stream, increasing the dead space. In order to prevent water vapour condensation on its windows, it is heated to about 41 ° C. Since there is no need for a sampling tube, there is no transport time delay in gas delivery to the sample chamber.
Other gases and vapours cannot be measured simultaneously in main stream version.
Which description matches each phase of a normal capnogram?
The output of both the main stream and side stream capnograph is a capnogram, a graphical plot of CO2 partial pressure (or percentage) versus time.
Each normal capnogram wave goes through four phases. The quick view of the four phases of normal capnography waveform is illustrated here:
Fig 1 This is a normal waveform of one respiratory cycle
Fig 2 This shows phase I where inhalation ends and exhalation begins, dead space air is eliminated first and no CO2 is present
Fig 3 This shows phase II in which alveolar gas begins to mix with dead space air, and a sharp upstroke is produced
Fig 4 This is phase III in which alveolar air predominates and the CO2 level plateaus as the exhalation continues. The end-tidal CO2 is noted at the end of exhalation
Fig 5 This is phase IV in which inhalation occurs, and CO2 level quickly returns to baseline
Which capnogram matches each condition?
Question:
What are the advantages of the von Recklinghausen oscillotonometer over the manual sphygmomanometer?
The von Recklinghausen oscillotonometer method does not rely on use of a stethoscope or the ability of the human ear to detect and distinguish differences in Korotkoff’s sounds.
This device enables monitoring of systolic, diastolic and mean blood pressure without using a stethoscope. However, one should carefully observe the oscillations on the aneroid gauge to record the pressure accurately. Again, this can give rise to operator error.
Using a manual sphygmomanometer, only systolic and diastolic blood pressures can be recorded, whereas the von Recklinghausen oscillotonometer can record systolic, diastolic and mean arterial blood pressure.
The von Recklinghausen oscillotonometer method does not rely on use of a stethoscope or the ability of the human ear to detect and distinguish differences in Korotkoff’s sounds.
This device enables monitoring of systolic, diastolic and mean blood pressure without using a stethoscope. However, one should carefully observe the oscillations on the aneroid gauge to record the pressure accurately. Again, this can give rise to operator error.
Using a manual sphygmomanometer, only systolic and diastolic blood pressures can be recorded, whereas the von Recklinghausen oscillotonometer can record systolic, diastolic and mean arterial blood pressure.
When using the von Recklinghausen oscillotonometer, the cuff is wrapped round the arm and inflated, following which the bleed valve is opened until the pressure starts to fall slowly. The control lever is pulled forward so that the needle jumps slightly in time with the pulse. When the needle starts to jump more vigorously, the control lever is released, and this displays systolic pressure. If the lever is pulled again, it can be released at the point of maximum oscillation to display the mean arterial pressure, and again at the point where oscillations reduce to give the diastolic pressure.
The device does not accurately measure the diastolic pressure. The systolic pressure is indicated by the onset of oscillation (as shown in Fig 1 - label 1). Maximum oscillations occur (Fig 1 - label 2) at mean arterial pressure. Diastolic pressure is the point at which oscillations reduce (Fig 1 - label 3).
Select the image to identify a correctly sized arm cuff.
A. The cuff is too small for the size of the arm. It is likely to over-read the actual blood pressure.
B. The cuff is too large for the size of the arm. It is likely to under-read the actual blood pressure.
C. Correct
Which of the equipment best matches the components used for detecting the pressure?
Question: What clinical information is provided by an arterial pressure waveform?
The slope of upstroke of the arterial wave indicates the inotropic component which relates to the myocardial contractility. The slope of the down stroke and position of the dicrotic notch gives an indication of peripheral vascular resistance. Reduced peripheral vascular resistance results in lowering of the position of the dicrotic notch. The area under the systolic component of the wave form (from the beginning of the upstroke to the dicrotic notch) is an index of stroke volume. The cardiac output can be calculated by multiplying the stroke volume by heart rate. The heart rate can be derived from the arterial waveform.
The myocardial oxygen demand is indicated by systolic time and myocardial oxygen supply is indicated by diastolic time.
The variation in the arterial pressure waveform, particularly in a ventilated patient, can indicate volume status of the patient. The hypovolaemic state results in an increased swing in the waveform during the respiratory cycle. This is mainly due to a greater decrease in blood pressure during the expiratory phase of positive pressure ventilation.
Question: Is there any difference in the waveform recorded by inserting a cannula into the aorta and one which is recorded by inserting a cannula into the radial artery?
The arterial pulse pressure wave undergoes a series of changes as it proceeds distally from the more central arteries; aorta through the arterial tree to the peripheral arteries (dorsalis pedis and radial artery). The high frequency components, such as the dicrotic notch, disappear the upstroke becomes steeper, systolic peak increases and diastolic trough decreases.
Peripheral arteries contain less elastic fibres as compared to the central arteries and therefore they are stiffer and less compliant. The arterial distensibility determines the amplitude and contour of the pressure waveform. In addition, the narrowing and bifurcation of arteries leads to impedance of forward blood flow, which results in backward reflection of the pressure wave.
Question: What clinical conditions may affect arterial waveform?
Certain clinical conditions such as hypotension, hypertension, arrhythmia and hypovolaemia can affect the arterial waveform.
