The Anesthesia Machine Flashcards
Describe the gas inlets of an anesthesia machine.
They are agent specific for all medical gasses whether they are from cylinders (pin-index safety system) or the hospital pipeline supple (diameter-index safety system).
What is the fail-safe of the anesthesia machine?
A device located downstream from the N20 supply source that proportionately decreases or completely discontinues the supply of N20 in response to decrease in the pressure from the O2 supply source. The term fail safe is a misnomer since the device only responds to changes in O2 pressure. A central pipeline crossover could still supply normal pressure from the O2 yet deliver a hypoxic mixture.
What is the oxygen supply failure alarm of the anesthesia machine do?
Alarms if O2 pressure drops below 30 PSIG
What do the pressure regulators in anesthesia machine circuits do?
They reduce gas pressure from cylinders or wall.
What does the second stage regulator of an anesthesia machine do?
Found in most modern Ohmeda machines, they reduce pressure before entering the flowmeters to 14 PSIG
What do the flow control valves of the anesthesia machine do?
They allow adjustment of gas flow; upstream from this point is the high-pressure circuit and downstream from this point is the low-pressure circuit.
Describe the flowmeters of the anesthesia machine.
They are constant-pressure (across both sides of bobbin, variable orifice flowmeters. A leak downstream from the O2 flowmeter may deliver a hypoxic mixture thus the O2 flowmeter is positioned most downstream although it does not protect against a leak in the O2 flowmeter itself.
Describe the flowmeter proportioning systems.
Ohmeda’s is the Link-25 system that is a chain that links N20 and O2 flow control valves.
Drager’s O2 ratio monitor control is a pneumatic interlock consisting of specific resistors located downstream from the O2 and N20 flow control valves that regulate a N20 slave-control valve.
Describe a standard anesthesia vaporizer.
A standard anesthesia vaporizer is variable bypss, flow-over, temperature-compensated, agent-specific, and out of circuit.
How does a standard anesthesia vaporizer compensate for changes in the ambient temperature of the room in which it exists?
Vaporizers are constructed of materials that do not readily change temperature but are able to conduct heat from the environment as necessary to allow vaporization. Ohmeda uses a bimetallic strip and drager uses an expansion element to direct more flow to the vaporizing chamber if low vaporizer temperatures were to develop.
Describe the effect of intermittent back-pressure on a standard anesthesia vaporizer.
The compression of gas molecules in both the bypass and vaporizing pressure may result in the retrograde flow of gas containing vapor to enter the low resistance bypass chamber inadvertently increasing vaporizer output. Ohmeda made a smaller vaporizing chamber, added a baffle system and a one-way check valve just before the common gas outlet and drager added a high resistance inlet to the vaporizing chamber.
How do flow rates affect anesthesia vaporizers?
low flow rates < 250 cc/min reduce vaporizer outputs because insufficient turbulence is created to advance the dense vapor molecules. High flow rates > 15L/min also reduce vaporizer outputs because of incomplete mixing with and saturation of the carrier gas in the vaporizing chamber.
What is the effect of suddenly switching from O2 to high N20 concentrations on anesthesia vaporizer outputs?
There is a sudden temporary decrease in vapor output due to the greater solubility of N2O in halogenated liquids.
Describe the desflurane vaporizer.
Electrically heated gas-vapor blender that heats to 39 degrees celsius creating a vapor pressure of 2 atm and no variable bypass.
What problems would arise if desflurane were administered using a standard anesthesia vaporizer?
Desflurane’s high vapor pressure requires a large amount of supplied heat otherwise the temperature of the vaporizer would drop and would require extensive fresh gas flow through the variable bypass chamber to dilute the carrier gas sufficiently to allow for clinically useful concentrations.
Describe an open breathing system.
No bags, rebreathing, CO2 resorption or unidirectional valves
Describe a semi-open breathing system.
Mapleson, Bain circuits are semi-open. They have a reservoir bag and 1 valve but no rebreathing or CO2 resorption.
Describe a semi-closed breathing system.
Reservoir bag, 3 valves (pop-off, inspiratory, expiratory and partial rebreathing with CO2 resorption.
Describe a closed breathing system.
Reservoir bag, 3 valves (pop-off, inspiratory, expiratory) and total rebreathing with CO2 resorption.
What Mapleson systems are best for spontaneous ventilation?
All > Dogs > Can > Bite. A is best beacuse elimination of expired gases occurs through the open pop-off valve near the patient.
What Mapleson systems are best for controlled ventilation?
Dog > Bites > Can > Ache. D is best for controlled ventilation because elimination of expired gases can occur during inspiration through the partially closed pop-off valve away from the patient.
What are the components of a circle system?
In order:
- Fresh gas flow
- Inspiratory and expiratory unidirectional valves
- Corrugated tubes
- Y-piece connector
- Adjustable pressure limiting valve
- Reservoir bag
- Canister with CO2 absorber.
Why are the components of a circle system arranged the way they are?
Unidirectional valves located as close to patient as possible to prevent backflow into inspiratory limb during expiration.
FGF should enter between absorber and inspiratory valve to allow pt to receive most of FGF without its dilution or scavenging.
Pressure relief valve should be located immediately before absorber to conserve soda lime and minimize venting of fresh gas.
Breathing bag should be located in expiratory limb to decrease resistance to exhalation.
Rebreathing can occur if either inspiratory or expiratory valves become stuck
Compare the Mapleson systems to the Circle system in terms of advantages and disadvantages.
The circle system allows conservtion of gas, heat, humidity, closed-circuit anesthesia but is cumbersome and has many connections predisposing to disconnection.
The Mapleson system is simple, light-weight, portable, allows rapid changes in inspired gas concentrations. Is wasteful of gases and does not conserve heat and humidity.
