The Anesthesia Machine

Question 1

Describe the gas inlets of an anesthesia machine.

Answer 1

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).

Question 2

What is the fail-safe of the anesthesia machine?

Answer 2

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.

Question 3

What is the oxygen supply failure alarm of the anesthesia machine do?

Answer 3

Alarms if O2 pressure drops below 30 PSIG

Question 4

What do the pressure regulators in anesthesia machine circuits do?

Answer 4

They reduce gas pressure from cylinders or wall.

Question 5

What does the second stage regulator of an anesthesia machine do?

Answer 5

Found in most modern Ohmeda machines, they reduce pressure before entering the flowmeters to 14 PSIG

Question 6

What do the flow control valves of the anesthesia machine do?

Answer 6

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.

Question 7

Describe the flowmeters of the anesthesia machine.

Answer 7

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.

Question 8

Describe the flowmeter proportioning systems.

Answer 8

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.

Question 9

Describe a standard anesthesia vaporizer.

Answer 9

A standard anesthesia vaporizer is variable bypss, flow-over, temperature-compensated, agent-specific, and out of circuit.

Question 10

How does a standard anesthesia vaporizer compensate for changes in the ambient temperature of the room in which it exists?

Answer 10

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.

Question 11

Describe the effect of intermittent back-pressure on a standard anesthesia vaporizer.

Answer 11

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.

Question 12

How do flow rates affect anesthesia vaporizers?

Answer 12

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.

Question 13

What is the effect of suddenly switching from O2 to high N20 concentrations on anesthesia vaporizer outputs?

Answer 13

There is a sudden temporary decrease in vapor output due to the greater solubility of N2O in halogenated liquids.

Question 14

Describe the desflurane vaporizer.

Answer 14

Electrically heated gas-vapor blender that heats to 39 degrees celsius creating a vapor pressure of 2 atm and no variable bypass.

Question 15

What problems would arise if desflurane were administered using a standard anesthesia vaporizer?

Answer 15

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.

Question 16

Describe an open breathing system.

Answer 16

No bags, rebreathing, CO2 resorption or unidirectional valves

Question 17

Describe a semi-open breathing system.

Answer 17

Mapleson, Bain circuits are semi-open. They have a reservoir bag and 1 valve but no rebreathing or CO2 resorption.

Question 18

Describe a semi-closed breathing system.

Answer 18

Reservoir bag, 3 valves (pop-off, inspiratory, expiratory and partial rebreathing with CO2 resorption.

Question 19

Describe a closed breathing system.

Answer 19

Reservoir bag, 3 valves (pop-off, inspiratory, expiratory) and total rebreathing with CO2 resorption.

Question 20

What Mapleson systems are best for spontaneous ventilation?

Answer 20

All > Dogs > Can > Bite. A is best beacuse elimination of expired gases occurs through the open pop-off valve near the patient.

Question 21

What Mapleson systems are best for controlled ventilation?

Answer 21

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.

Question 22

What are the components of a circle system?

Answer 22

In order:

1. Fresh gas flow
2. Inspiratory and expiratory unidirectional valves
3. Corrugated tubes
4. Y-piece connector
5. Adjustable pressure limiting valve
6. Reservoir bag
7. Canister with CO2 absorber.

Question 23

Why are the components of a circle system arranged the way they are?

Answer 23

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

Question 24

Compare the Mapleson systems to the Circle system in terms of advantages and disadvantages.

Answer 24

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.

Question 25

What types of CO2 absorbers are there?

Answer 25

Soda lime and baralyme

Question 26

What is the absorptive capacity of CO2 absorbers?

Answer 26

26L CO2 / 100 g absorbant but actually 10 - 20 L due to channeling.

Question 27

What is the common site for leaks in circle systems?

Answer 27

CO2 absorbant usually following CO2 canisters exchange

Question 28

What conditions predispose to compound A production?

Answer 28

Requires use of sevoflurane with:
Low flows
closed circuit anesthesia
use of baralyme
high concentration of sevoflurane
high absorbant temperatures
fresh absorbant

Question 29

What alarms should be set to check for ventilator malfunction?

Answer 29

1. Low pressure - threshold set 5 cm H20 below PIP
2. Volume
3. Expired CO2
4. Inspired O2 - may be used to detect a hole in the bellows of Ohmeda ventilator which uses 100% O2 or Drager which uses an air/O2 mixture.

Question 30

What does Ohmeda use to drive its bellows?

Answer 30

100% FiO2

Question 31

What does Drager use to drive its bellows?

Answer 31

A mixture of air and O2.

Question 32

Why do we use scavenging systems with our anesthesia machines?

Answer 32

Because the amount of gas used for anesthesia usually exceeds patient’s needs to avoid environmental contamination.

Question 33

What is the most important component of a scavenger?

Answer 33

The interface because it protects the breathing circuit or ventilator from excess positive or negative pressures.

Question 34

What are the problems encountered with scavenging interfaces?

Answer 34

Excess positive pressure
Excess negative pressure resulting in hypoventilation due to negative pressure in the circuit or counterintuitively by causing collapse of the APL valve causing it close and causing excess positive pressure and potential barotrauma to the patient.

Question 35

What are the FDA recommendations for an anesthesia machine check?

Answer 35

1. Emergency ventilation equipment

HIGH PRESSURE SYSTEM

2. O2 cylinders - open to at least 1/2 full (1000 PSIG), then close
3. Pipeline gases - check hoses and that 50 PSIG delivered

LOW PRESSURE SYSTEM
Vaporizers - fill, tighten
Negative pressure leak test
Flowmeters - attempt to create hypoxic mix

SCAVENGING SYSTEM
check connections between scavenging and APL
Waste gas vacuum - adjust if possible
Negative pressure relief valve - with APL open and minimal O2 flow allow scavenger reservoir to collapse (absorber pressure gauge should read 0)
Positive pressure relief valve - with APL open and flusing O2, the scavenger reservoir bag should distend fully (absorber pressure gauge should read no greater than 10 cm H20)

BREATHING SYSTEM
O2 monitor - calibrate
Breathing system - attach circuit
Positive pressure leak test - set flows to 0, close APL, occlude Y-piece, flush O2 until pressure is 30 cm H20, stop flush (pressure should be sustained for 10 sec), open APL (pressure should go to 0)

MANUAL AND AUTOMATIC VENTILATION
2nd bag on Y piece check ability of bellows to deliver breaths, then check ability to bag ventilate manually

MONITORS
Check, calibrate, set. capnometer, pulse ox, O2 analyzer, respiratory volume monitor, pressure limits

Question 36

How do you do a negative pressure leak test?

Answer 36

It checks the low pressure system.
With the machine off, attache suction bulb to common gas outlet, squeeze bulb until collapsed, bulb should stay collapsed for 10 seconds (repeat with each vaporizer open), then turn machine back on.

Question 37

What do most people argue is the most important anesthesia machine monitor?

Answer 37

The O2 analyzer because only it can detect the delivery of a hypoxic mixture. All others respond to pressure not gas concentration and are upstream from the flow control valves.

Question 38

How much N20 is left in the N20 cylinder if it reads 745 PSIG?

Answer 38

Impossible to tell without weighing the tank. N20 is mainly liquid with 1590L in a full tank. The pressure will remain 745 PSIG until the tank is 3/4 empty.

Question 39

If the regulator is properly functioning, is there a concern that cylinder left in the open position will gradually be emptied with time?

Answer 39

Yes because the pressure within the machine routinely drops below 45 PSIG throughout the day due to flushing or decreases in the central O2 supply pressure.

Question 40

How do you test a Bain circuit?

Answer 40

A kinked or disconnected inner gas hose is easily unrecognized, leading to inadequate FGF and or hypoventilation. To test the hose, the elbow-connector should be occluded, reservoir bag should be filled by flushing the circuit with O2, the occlusion over the connector should be released while flushing the circuit with O2 and the bag should be observed for immediate collapse from the negative pressure Venturi effect.

Question 41

What are the advantages of a Mapleson system over a circle system for ventilation of a neonate?

Answer 41

The purported advantages of a Mapleson system include its low resistance, ability to change inspired gas concentration more quickly, simple design. But practically all these offer little real benefit over the circle system (which is adequate even for premature infants).

Question 42

What fresh gas flow rate do you need for use of a Jackson-Rees modification of Mapleson D?

Answer 42

2 - 3 x the minute ventilation to prevent rebreathing.

Question 43

Why do you need a negative pressure leak test? Wouldn’t a positive pressure lead test do the same?

Answer 43

No. The negative pressure leak test checks integrity of machine from flow control valves to common gas outlet. It is essential for Ohmeda machines that possess a unidirectional machine outlet check valve. It also works on all machines, even those without unidirectional check valves and is more sensitive than the positive pressure leak tests (30cc/min leak vs 250cc/min leak).

Question 44

Would proceed with an elective case if the FiO2 analyzer is malfunctioning?

Answer 44

No. Albeit rare, detection of an error central pipeline O2 delivery could not otherwise be detected. It would also be a violation of FDA recommended standards.