Processing Flashcards
What is considered the processing aspects of the anesthesia machine?
The areas distal to where the gases enter the gas machine and extends to the area proximal to the common gas outlet
What is the only gas that does not pass via fail-safe mechanism
Oxygen, always added last
Once the gases enter the common manifold, where do they go next within the machine?
Vaporizers to pick up the anesthetic agent
After picking up anesthetic agents from the vaporizer, the new mixture enters which part of the gas machine?
Common gas outlet (Dalton’s Law)
If you forget to close the oxygen tank after checking it why wouldn’t there be flow from the cylinder?
One way check valves would be closed by higher pressures in pipeline supply
What are the five tasks of oxygen?
- Powers the fail safe valve (keeps the valve open) O2 is the driving gas that will turn off all other gases
- Drives the anesthesia ventilator
- Travels from the needle valve to the flow meters themselves
- Provides pressure to prevent the low pressure alarm
- Oxygen flush valve
This system was designed to prevent unintentional delivery of a hypoxic mixture to the flow control valves
Fail-safe Design system
How would a hypoxic mixture be possible under the Fail safe design system?
If the gas holding open the pressure valve is not oxygen a hypoxic mixture is possible
What does the Fail-safe design system do if the O2 supply pressure falls below threshold setting?
O2 pressurizes and holds open a pressure sensor shut off valve that reduces or interrupts supply of N2O and other gases to their flow meters
How is N2O typically released into the circuit?
O2 pressure forces a piston up on the spring that will open a valve (fail-safe) and allow N2O to flow
What type of valve is the fail-safe valve?
Ball and spring type valve, all or nothing arrangement that is open at O2 pressure > 20 psi and closed at lower pressures
When will the Drager and Ohmeda gas machines low pressure alarm go off?
Ohmeda less than 28psi
Drager less than 37psi
What should be done if the low pressure alarm goes off when using pipeline supply?
Immediately switch to emergency cylinder supply, unplug the pipeline supply, switch to manual ventilation mode and low flows
What are the five components of a flowmeter?
Knob Needle valve Valve stops Flow tube Indicator float
What is another term used for flowmeters?
Rotameters
What is the basal metabolic rate/consumption of a patient?
200-300mL/min this is the same default of minimum O2 flows as the anesthetic gas machine
What is the purpose of the needle valve?
It controls gas flow, by turning the needle valve counter clockwise the orifice becomes larger, increasing flow of O2
Device that prevents damage to the needle valve from excess force?
Valve stops
What safety features are implemented on the flowmeter control knobs?
Color and touch (O2 fluted knob)
What is the accuracy of a Thorpe tube at room temperature?
Accuracy 2-3% at room temperature and 760mHg
Why aren’t Thorpe tubes interchangeable?
The diameter is gas specific
What represents the variable orifice?
The area between outside of the bobbin and inside of the glass tube
What causes the ball/bobbin to float in a flowmeter?
A certain pressure difference across the bobbin/ball
What indicates the flow of gas in a flowmeter?
The ball or bobbin rotating
Where do you read the ball and the bobbin?
The ball is read in the middle and the bobbin is read at the top
Why are two flowmeters presents for each gas?
Allows accurate measurement at high and low flows
Low flowmeter up to 1Lpm
High flowmeters up to 10-12Lpm
Why is oxygen positioned closest to the common gas outlet?
A leak in any other flowmeter tube least likely to affect oxygen supply to patient, ensures accidental decrease in delivered O2 (hypoxic mixture)
Electronic flowmeter are considered how much more accurate than flow-tubes?
5 times more accurate
What is considered a hypoxic mixture?
less than 21% O2 in any total concentration
How do you calculate total concentration?
Take the liters of the gas and divide it by the total liters in the mixture
EX: 2L O2, 3L N2O
2/5 = 0.4 40% O2
What is the hypoxic guard system?
Links N2O and O2 in order to prevent hypoxic mixture, it ensures final breathing mixture > 25% +/- 4% O2
What are the two types of proportioning systems?
Mechanical link
Pneumatic link
In the pneumatic link, what is the role of the oxygen ratio monitor controller?
Shuts off N2O when oxygen pressure less than 10 psi
In the pneumatic link, what is the role of the sensitive oxygen ratio controller?
Shuts off N2O if the flow is less than 200mL/min, or if the oxygen fresh gas valve is closed
What are four causes of a hypoxic mixture?
Wrong supply gas in O2 pipeline or cylinder
Defective pneumatics or mechanics
Leak downstream of control valves
Use of third inert gas (helium)
The gaseous phase of a substance that is liquid at room temperature
Vapor
What is the purpose of the vaporizer?
Blend combined flow of fresh gases from flowmeters with sufficient vapor to form clinically useful concentrations
How does temperature effect the rate of evaporation?
Increased temperature, increased rate of evaporation
What three factors determine the rate of vaporization?
Temperature
Vapor pressure of liquid
Partial pressure of vapor above liquid
How does temperature affect vapor pressure?
Vapor pressure increases with temperature
What is the vapor pressure of Halothane?
243mmHg at 20°C
What is the vapor pressure of Isoflurane?
239mmHg at 20°C
What is the vapor pressure of Enflurane?
175mmHg at 20°C
What is the vapor pressure of Sevoflurane?
170mmHg at 20°C
What is the vapor pressure of Desflurane?
669mmHg at 20°C
How do you calculate the volumes percent of an anesthetic agent?
volume % = partial pressure of anesthetic agent / total ambient pressure x 100
Why do we need to know the vapor pressure of the anesthetics at 20°C?
This is the temperature in a typical OR
What is the term used to describe the number of calories needed to convert 1g of liquid to vapor, without temperature change in the remaining liquid
Latent heat of vaporization
What term is used to describe the number of calories needed to increase the temperature of 1g of substance by 1 degree celsius?
Specific heat
What measures how fast a substance transmits heat?
Thermal conductivity
What is typically the metal of choice for the construction of vaporizers?
Copper, to minimize temperature changes associated with vaporization
What are the two different types of vaporizers?
Variable bypass Measured flow
What are the major characteristics of a measured flow vaporizer?
You have to determine the carrier gas split based on calculations
Bubbles gas up through the liquid anesthetic
You have to determine temperature
Can utilize multiple anesthetic agents
What are the major characteristics of a variable bypass vaporizer?
Predominately used in the US
Determines gas split
Gas flows over the liquid vapor interface
Automatically determines temperatureAgent specific
What is the commonality of the two types of vaporizers?
Both are out of the circuit
How is the flow determined in a measured flow vaporizer?
Flow through vaporizer determines anesthetic concentration, measured with dedicated Thorpe tube
How to calculate concentration using a measured flow vaporizer?
1% iso with total gas flow of 5L/min = 5000mL1%
5000 = 50
50mL/31% XmL/69% x = 111mL/min
111mL/min of flow via vaporizer flowmeter will produce 1% iso at 20C in a total gas flow of 5L/min
What two factors affect flow rate that influences the output of the vaporizer?
Rates less than 250mL/min or greater than 15L/min result in agent output that is less than dial setting
How does temperature affect flow into a vaporizer?
As temperature increases, greater portion of gas flow directed through the bypass chamber
What determines how much gas is allowed into the vaporizer based on temperature?
Bimetallic strip, as temp decreases the metal folds and the valve opens, as temp increases metal pushes the valve shut
What can happen if you forget to put the cap back on the vaporizer and run it?
The liquid will spray out of the opening
What is the purpose of the vaporizer interlock system?
Ensures only one vaporizer is turned on at a time
What is the consequence of tipping a vaporizer greater than 45 degrees?
Liquid agent can enter bypass chamber and increase output concentration
How can the vaporizer interlock system be defeated?
Removing the center vaporizer
What can occur if you place an agent with a higher vapor pressure in a vaporizer designated for an agents with a lower vapor pressure?
Agent delivered will be higher than dialed setting
What is the name of the vaporizer used for Desflurane?
Tec 6 Vaporizer
What type of vaporizer is a Tech 6?
Gas/Vapor blender, NOT variable bypass
What mechanism in the Tech 6 vaporizer ensures the flow of desflurane out of the vaporizing chamber is proportional to the carrier gas flow?
The pressure is sensed by a differential pressure transducer it adjusts some resisters specific at R1
On the Tech 6, what color must the light be in order to turn the control dial?
Green, cannot turn dial in warm-up mode (Amber)
What can we do to test the battery power for the alarms on a Tech 6 Vaporizer?
Turn on the vaporizer and then unplug the electrical connection, a “no output” alarm should ring within seconds
What protective feature is used on a bottle of Desflurane to prevent it from going everywhere if the bottle shatters?
Clear coating on the bottle, if the bottle breaks, the agent is self contained in skin of the bottle
What type of valve is the O2 flush valve?
Ball and spring type valve (all or nothing)
What is the standard delivery of the O2 flush valve?
35-75L/min at 50 psi, proceeds directly from supply to common gas outlet
