Anaesthesia III Week 1-4 Flashcards
What are the NZ cylinder sizes?
A-J
A is our standard size
How is oxygen stored in cylinders?
As a gas
13700kPa
How is nitrous oxide stored in a cylinder?
As liquid with vapour on top
4400kPa
It is only partially filled to reduce excessive pressure with changing temperature
What is a filling ratio?
The weight of the fluid in the cylinder divided by the weight of water required to fill the cylinder.
In warmer climates the ratio is less.
0.75 filling ratio in NZ
Why might ice form on a cylinder?
At high use when plenty of liquid is being converted to a gas and using latent heat of vaporisation leaving the cylinder extremely cold
What is required on a cylinder label?
Name Chemical symbol Form ID and batch number Hazard info Size Maximum pressure (bars) Content capacity (L) Fill date and expiry date Directions Handling instructions
What are safety features of cylinder?
Molybdenum steel Thread on neck melts at temperature to let gas escape and prevent built up pressure Label Colour code Pin index system valves Regularly checked - annual
What is the storage criteria for cylinders?
Purpose built area, fire proof, well ventilated, away from heat, grease, oil, dampness and chemicals, keep empty away from full.
What is a bodok seal?
Rubber (neoprene) with brass rim to prevent a leak between cylinder and yoke
Describe the pin index and colour system for the five main gases
Oxygen: White shoulder, white body, 2,5
Nitrous: ultramarine shoulder, white body, 3,5
Air: White/black quarters, white body, 1,5
Carbon dioxide: green shoulder, white body, 1,6
Entonox: White/ultramarine quarters, white body, 7
Describe the pipelines
400kPa
Copper (reduce microbe, degradation) and brass fittings
Colour coded
Sleeve index system prevents incorrect bollard attachment
Non-interchangeable screw thread prevents incorrect machine attachment
What is the cylinder manifold?
The nitrous, entonox and back up oxygen cylinder bank
Large cylinders in 2 groups. Connected to common pipe with non return valves
Connect to pipeline via regulator
In either group in use, all valves are opened so cylinders drain evenly and auto switch to other group when empty
Describe the components of a medical gas cylinder.
Aluminium case
Neck at top where valve attaches
Detailed label
What is a VIE?
Vacuum insulated evaporator
Stores liquid oxygen
Double walled steel tank with perlite in a vacuum space - insulation
Safety valve to release excess pressure
If more O2 needed a valve allows liquid O2 to pass over insulated copper and it evaporates
-150 to -170 degrees and 1000kPa
Latent heat allows VIE to keep cool
How is medical air stored?
Compressor plant
Room air is drawn in, filtered, compressed and stored for use.
Compare and contrast bulk gas supply and cylinder supply.
Amount which is stored: VIE huge, cylinder less
13700 cylinder: 1000 VIE
VIE expensive but cheap oxygen while cylinders are cheap but expensive oxygen
VIE very cold
Both colour coded
Both hazardous
Both pressure regulated
What is a pressure regulator reducing valve?
Changes a high pressure to a low pressure
Protects machine from surges
Allows fine gas control
Reduces consequences of gas leak
Balance between high pressure and diaphragm on spring maintains a constant pressure
Relief valves in case of failure
List the 10 features of anaesthetic machine safety.
O2 failure alarm N2O cut out Anti hypoxic mechanism Back up battery Cover over on/off Recessed O2 flush button 22mm/15mm CGO Individual P gauges visible at front Anti static wheels Pressure regulator (NIST, O2 cylinder, pin index, colour coding, oxygen analyser, ventilator alarms, auto vapour compensation)
Describe a pressure gauge.
Pressure in cylinder or pipeline display
Flexible, coiled tube connected to a pointer at one end and gas supply at other. Pressure changes causes coil/uncoil so needle moves
Colour coded
Calibrated per each gas
Draw a VIE
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How is the pressure managed from pipeline to machine?
A regulator achieves 100-200kPa
What is a one way/back flow check valve?
Next to inlet yoke
Prevents leaks from empty yokes when cylinder not attached
What is a flow meter?
Measure flow rate of gas
Calibrated to each gas at room temp and atm pressure
+/-2.5% accuracy
Flows > 1L/min measured in L/min, less than this is on 100ml/min
Comprised of control valve, conical tube and bobbin
200ml/min oxygen always flow
Control valves colour coded, ridged
O2 sits more proud, last gas, ridges
2 gears connect O2/N2O to prevent hypoxia mix
What are the features of bobbins?
Bobbin rises with gas
Fluted sides causing spin which can be seen by a dot
Measure from top of bobbin or middle of round bobbin
What are the components of the suction system?
Pump/power source continuously generate -500mmHg Suction controlled with filter Collection vessel Tubing Yankeur
What are the ideal features of suction?
-53kPa should be maintained
Should withstand 40L/min flow
Take less than 10s to generate
Reservoir large to hold but not too big to slow start up
Tubing must resist collapse, be clear, sufficient diameter and length
Yankeur should b tapered, clear, smooth and multi holes to prevent blockage
How is suction created?
Electric motor and pneumatic pumps generate negative pressure by Venturi principle
Amount of vacuum can be manually adjusted
Describe the oxygen flush and draw it.
Non locking recessed button Self close valve Delivers pure oxygen by bypassing vaporisers and flow meters 35-75L/min and 400kPa Risk of barotrauma and diluting agent
What are the 10 features of oxygen failure alarm?
Depend on O2 pressure Require no battery/mains Specific audible signal for sufficient duration and volume Warn of impending failure and actual failure Interrupt flow of other gases Prevent cancel/use until fixed Located on reduced pressure side Tamper proof Not affected by back pressures Cuts other gases (fail safe)
What is the anti hypoxic mechanism?
Mechanical: chain links O2 with N2O control valves so that an increase in N2O results in a proportional increase in O2
Paramagnetic O2 analyser: continuously measures O2 and the nitrous is auto switched off when O2 is less than 25%
Pneumatic: pressure sensitive diaphragm
Describe the common gas outlet.
Where all gas exits
22mm male and 15mm female connection
What happens at the neck of the cylinder when it gets too hot?
The neck melts to reduce explosion risk
What is a vaporiser?
Changes liquid agent to a vapour
This gets expresses as a percent of saturated vapour added to gas flow
Adds a controlled amount of agent to the fresh gas flow
What are the ideal characteristics of vaporisers?
Performance not affected by changes in FGF, volume of agent, temperature or pressure Low resistance to flow Light weight Economic and safe Corrosion and solvent resistant
What does the interlock system do? (I.e select-a-tec)
Locks vaporiser to back bar Ensure correctly mounted (Compromised agent delivery) Cannot dial on unless correctly engaged Cannot use more than one at a time FGF only enters when correctly engaged and on Trace vapour when off is minimal
Define vapour
Gaseous phase of a substance which is usually a liquid or solid. It is at a temperature less than critical point so is easily condensed.
What is the saturated vapour pressure of the main agents?
Sevo: 157mmHg
Des: 669mmHg
Iso: 238mmHg
At 20 degrees
How does the plenum vaporiser work?
Calibrated to agent spec
FGF is split as it enters: bypass and vapour chamber
Chamber fully saturates gas with vapour
Concentration adjusted by dial which alters amount of gas flowing through the chamber and bypass
Cooling occurs due to latent heat
Bimetallic strip automatically adjusts splitting ratio to compensate for this so when temperature is low more gas flows to the chamber
How does saturation of the FGF get achieved in the plenum?
High surface area on contact between the FGF and agent
Done with wicks saturated with liquid agent or a series of baffles
Define SVP?
Pressure of the vapour in equilibrium with its liquid phase. ie there is an equal amount of molecules condensing as there are evaporating. The maximum amount of evaporation has been reached
What are the safety features of the plenum 5/7?
Anti spill means agent can't enter bypass when tipped - up to 180• Interlock safety features Pressure relief valve downstream Keyes fillers Low filling port Colour and label Dial always increases anti-clockwise Release button at rear to turn dial Vaporiser ID Agent specific Liquid level indicator Anti-overfill Temperature compensatory mechs 0 on dial most obvious, extra markings for less than 1%
What are the hazards of the plenum 5/7?
Interlock uses o-ring increases leak risk
Pumping effect
Preservatives may interfere with wicks or bimetallic strip
Over tip
Simultaneous inhaled agent
Electronic failure
Incorrect agent
Describe the filling devices.
Agent specific
Geometric and colour coded
Cannot overfill
Anti pollution cap stays on bottle to prevent vaporisation
What is Mac?
Minimum alveolar concentration The concentration of vapour in lungs needed to prevent response in 50% of subjects in response to surgical stimulus Compares strength of agents Values assume no other hypnotics used Some gases are additive eg N2O
What factors increase mac?
Children Hyperthermia Hyperthyroid Alcoholism Drug use
What factors decrease mac?
Old age and neonates Hypothermia Hypoxia CNS depression N2O use Alpha-2-agonists Opioids TIVA Intoxication
What are the macs of the agents?
Sev: 2
Iso: 1.15
Des: 6
Hal: 0.75
What is the pumping effect?
A back pressure coming from CGO causing already agented gas to re-enter the vaporiser where more agent is therefore added. This results in higher than expected (toxic) levels.
Non return pressure relief safety valve prevents this
What makes the Aladdin different to other variable bypass vaporises?
It is a plenum with electronic controls which regulate the vapour concentration with flow valves
Consist of electronic controls in machine and transportable casette
Casette is the liquid sump
Agent concentration is adjusted by changes in the FGF through the casette
Pressure relief valve to scavenging
Define latent heat of vaporisation.
Amount of energy to convert 1g of liquid to a vapour without a change in temperature. The affect of this causes cooling of the chamber
Define specific heat capacity
Amount of energy to increase temperature of 1g of substance by 1degree Celsius.
Vaporiser should be high specific heat capacity to resist changes in temperature
What is splitting ratio?
How much gas goes through the bypass vs chamber Variable bypass Measured flow Dual circuit (means it's not split at all)
Why is an indirect regulator used?
When cylinder connected to a system
Ensure that pressure remains same even though cylinder is emptying thereby constant flow
What does plenum refer to?
Gas is under positive pressure to function the vaporiser
Unidirectional flow only
There is a higher resistance to flow than the draw over types
Always a VOC
What is a VOC?
Out of circuit
Between flow meter and system
Positive pressure used to overcome resistance
No water vapour can enter
What is a VIC?
Uncommon Within the circle system on either limb Resistance must be low Patient breathing drives it Needs frequent cleaning and drying from exhaled condensation
How does temperature affect concentration?
Amount of vaporisation decreases with temperature. This is made worse by latent heat. Temperature compensatory mechanisms built in to counteract this
How does flow affect concentration?
High flows cause inadequate contact time with wicks therefore reducing concentration
Low flows means resistance increases and more gas goes through the bypass therefore reducing concentration
How does back pressure affect concentration of vapour?
This is the pumping effect
When already saturated vapour is forced back into the vaporiser and gets more agent added very high concentrations result
(Check valve prevents this)
What are 4 other minor affects to vapour concentration?
Liquid level: too much liquid means less room for vapour
Additives: may stick to wicks
Carrier gas: calibrated for use with O2 so other carriers may change the concentration due to density difference
Altitude: calibrated at sea level however partial pressure stays the same so no chemical change
What should you do if the vaporiser is accidentally inverted?
Wash out at 5L/min for 5 minutes on 5%
Describe the standard Aladdin casette.
Output is sensed and altered every 200ms Electronic controls and bypass in machine, casette is sump Electronic control of flow valve considers temp, P, FGF and carrier Fill indicator level and port Lock handle keeps inserted correctly Magnetic sensor read by machine Temperature sensors next to magnet One way check valve inlet Flow control valve outlet Chamber has wicks and baffles Automatic warming fan underneath Agent monitoring alerts to overdose Can't overfill, anti-tip protect Scavenging if pressure high Easy-Fil sevo; Saf-T-Fil des 250ml sump
Describe the desflurane Aladdin cassette.
Vaporisers freely in cassette
Below 22.8 degrees bypass gas enters
Above this the one way valve closes and vapour is directly Injected. Otherwise sump pressure become too high.
Fan warms between 17-20 degrees
The system considers % when calculating FiO2 and can alert if hypoxic
What are the characteristics of desflurane that require specialised vaporisers?
Highly volatile SVP 664mmHg BP 22.8 degrees Due to SVP, high volumes entrained by carrier gas so large FGF needed for relevant concentration: doesn't work! This also leads to excessive cooling
How does the tec 6 work?
Mounted with interlock
Electrically heated chamber (5min warm up to 39 degrees) then shut off valve will open to allow operation
FGF never enters chamber; meets with vapour downstream and mixes
Differential pressure transducer adjusts pressure reducing valve at chamber outlet to ensure the pressure of the vapour is equal to FGF
Dial controls flow of vapour into FGF and therefore the output concentration
Fixed restriction in FGF pathway ensures pressure of carrier gas is proportional to gas flow
Electronic controls compensate for changes by maintaining a pressure balance
What are some specific safety features of tec 6?
Shut off valve won't open until warmed up Back up battery Electronic compensation Malfunction alarms Extra button when dialling above 12% Performs self test
How does select-a-tec work?
Pair of port valves for each position
Has o-rings
Each vaporiser has mounting bracket which contains 2 plungers which fit over port valves and seal with o-ring
Locking lever
When turned on, plungers move down and open port valves allowing FGF
How does the interlock work?
When one dial is on it causes a rod to move making other dials inoperable
What are the hazards and safety of the Aladdin?
Tilt in any direction Anti overfill Audible fill level alarm Self check Automatic compensation for flow/temp Magnetic auto ID; colour code Liquid level visual Monitored every 200ms Check valves open/close with removal Leaks - oring Require power
What are the characteristics of an ideal ventilator?
Simple, portable, economical and robust Some gas wastage expected Supply TV1500, RR60, variable I:E, deliver gas and vapour, have PEEP Monitor airway factors Provide humidification Alarms for disconnection, high pressure and power failure Provide various modes Easy to clean/sterilise
What are the methods of cycling?
(How inspiration changes to expiration) Volume: TV is reached Time: inspiratory duration reached Pressure: pressure reached Flow: flow is reached
What are the available sources of power for ventilators?
Electrical
Pneumatic
What is the mechanism of a bag in bottle ventilator?
Time cycled
Compressed air or oxygen is the driving gas which enters the chamber and forces the bellows down pushing out FGF
The volume of driving gas = FGF
Closed system which recycles gas
Contains solenoid valves
Scavenging valve underneath is forced open when pressure reached
What are some safety features of the bag in bottle ventilator?
Positive pressure in bellows creates 2-4H2O peep
Bellows collapse to empty and remains there if there’s a leak/disconnect
Ascending bellows may continue to move as usual in a leak/disconnect
What is VCV?
Time cycled
Supplies set TV
Calculates flow based on TV and inspiration time
Can adjust by measuring volumes at flow sensors
Compensate for compliance
What is PCV?
Time cycled
Supplies constant pressure during inspiration
Calculates time from frequency and I:E
High initial flow pressurises circuit then drops to maintain set pressure
Sensors can measure pressure and adjust flow
This mode gives a decelerating flow
Can compensate for a leak
What are some indications for PCV?
If danger of high paw eg LMA, COPD, neonate
Low compliance eg pregnancy, obesity, laparoscopy
Compensate for leak!!
What’s is PCV-VG?
Ventilator delivers TV using decelerating flow and constant pressure
Adjusts each breath so lowest pressure used
Efficient and compensates for lung changes
Delivers initial TV to establish the inspiratory pressure of the patient
What is SIMV?
Provide assured RR/TV that supplements patients spontaneous efforts. Permits spont breathing between mechanical breaths while providing mandatory breaths if RR falls.
Within synchronised window the ventilator will assist spont breaths
Can be VCV, PCV or PCV-VG driven
What is PSV PRO?
Patient triggered: applies pressure when inspiratory effort sensed.
Improves gas exchange and reduces effort.
Apnoea backup if patient doesn’t breath long enough: switches to SIMV PCV
Switches back to mode when patient takes a number of breaths
What is CPAP PSV?
For a spontaneously breathing patient
Constant support on inspiration
What is I:E?
Inspiratory vs expiratory time
Work of breathing is minimal with short inspiration time and small TV
GA reduces FRC so a longer inspiration helps generate all alveoli
A high inspiration increases O2 and a high expiration rids more CO2
(1:1 for stuff lungs, 1:3 for CO2 retention)
How does the machine adjust flows?
Samples information at flow sensors
2 tubes from each sensor connect to transducer that measures changes in pressure and flow. A third transducer measures airway pressure from inspiratory sensor.
Proportional flow valve which can deliver a variety of flow rates takes the settings and adjusts flow to match
Inside machine
Describe mechanical inspiration.
Pilot pressure closes expiration valve
Driving gas pushes down bellows
FGF flows from bellows, through absorber and through the inspiratory check valve to patient
Describe mechanical expiration.
Drive gas flow stops and exhalation valve opens. Gas flows from patient through valve and into bellows
Any residual driving gas goes to scavenging
If PEEP is on, static pressure on pilot port of exhalation valve does this
During exhalation the FGF flows backward through absorber and into expiratory limb
What is the ventilator (or bellows) relief valve?
Beneath bellows
Exhausts excess breathing gas to scavenging
Opens at 2-4cmH20 therefore maintaining a minimum of 2cmH20 in the system at all times to assist refill
Acts to prevent barotrauma during expiration and pause
Draw the pressure, flow and volume waves for VCV and PCV.
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What are the affects of ventilation?
Reduced venous return Reduced cardiac output Increased intraabdominal pressure Barotrauma Pneumonia Increased intracranial pressure
What are the benefits of ventilation?
Increased gas exchange
Reduced work of breathing
Reduced distress
Avoid complications
Describe natural respiration.
Contraction of diaphragm and intercostal muscles change the volume of the thoracic cavity and create a negative pressure which sucks air in. The muscle then relax and the pressure returns to normal so air escapes
What does pneumatic mean?
Operated by air or gas under pressure Bellows are driven by compressed gas Uses more gas Doesn't require power More portable
Compare and contrast PCV with VCV
PCV:
+: less barotrauma, higher gas exchange from decelerating flow, compensate for leaks
-: can’t adjust for lung compliance = risk hypoventilation
VCV:
+: maintain constant TV, precise control of PaCO2, compensate for compliance
-: potential for barotrauma, can’t compensate for leaks
Describe the classifications of our modern ventilator.
Function: Dual circuit Time cycled Ascending bellows Electric and pneumatic Various operation method and inspiratory phase gas control
What are the four classifications of ventilator mechanisms of action?
Mechanical thumbs (T-piece) MV divider (all driving gas is delivered as MV) Bag squeezer Intermittent blower (jet ventilator)
What is fresh gas decoupling?
Normally gas to patient is sum of gas from bellows plus FGF from inlet. In decoupling FGF is diverted into reservoir by decoupling valve until expiration where valve opens and it is allowed to refill the bellows. Excess goes to scavenging. This allows decreased barotrauma. Risk or room air entrainment.
What are the classifications of Tec 5,7 and Aladdin?
Variable bypass Flow-over Automatic temp compensation Calibrated VOC
What are the classifications of the Tec 6?
Dual circuit Gas/vapour blender Thermostat control Calibrated VOC
What is used to measure cylinder filling pressure?
Bourdon gauge
What’s the equation to work out the amount of nitrous left in the cylinder?
(Weight-tare) / density
How are cylinders tested?
Tested 5 yearly
Recorded on disk at neck
Internal endoscopic External check for corrosion, impact and distortion Pressure test at 22000kPa One cylinder from batch gets: Flat/bend/impact test per batch Tensile test where strips are stretched
What is the yield point?
Point where substance is permanently deformed
What valves do large cylinders have?
No pin index
Various thread: internal and external, left or right handed
Bull nose at end of cylinder
What is special about storage of entonox and N2O?
At temperatures below 0 the contents might separate. This would cause high nitrous to be delivered on its own.
Must invert a few times or rewarm the cylinder prior to use
Why might a flow meter stop working?
Dirt, dust, grease
Static
Bank not level
Flow meter off
Why should cylinders be opened slowly?
Avoid a sudden surge in pressure which could cause a sudden increase in heat which could ignite particles
Also damage internal parts
Why is nitrous oxide given a filling ratio and why isn’t the pressure gauge a good measure of the content?
Filling ratio due to room for evaporation to occur
The vapour/gas exerts the pressure. As vapour is used, more is produced so the pressure remains the same until all the liquid is used up and no more vaporisation can occur. At this point the pressure readily drops.
What is the weight and capacity of a size A cylinder?
- 79 Kg
2. 8 L
What are the states of the other main gas cylinders?
Air:
Gas 13700 kPa
Carbon dioxide:
Liq/vap 5000 kPa
Entonox:
Gas 13700 kPa
What does the Pmax alarm setting relate to?
At what pressure the driving gas is vented out at.
Normally 40 cmH2O
How does the Aladdin cassette work (not desflurane)?
The CPU collects information from the dial setting, sump P, temp, flow through bypass and flow through chamber and modulates the flow control valve to change the amount of gas coming into the chamber.
Vaporisation happens freely in chamber and contains wicks and baffles for increased surface area
Why is O2 the last gas on a flow meter bank?
If there is a crack in a flowmeter a hypoxic mixture could occur so O2 is delivered last
