Extra Topic 1.1 -- Anesthesia Machine

Question 1

How would you perform a proper machine check?

(Monday morning you enter the OR to begin your day.)

Answer 1

I would start by turning on the anesthesia machine and all monitors;

verifying the presence of emergency ventilation equipment; and

calibrating or setting the capnometer, pulse oximeter, O2 analyzer, and pressure monitors and alarms.

I would then check the high-pressure system by opening each E-cylinder to ensure adequate gas pressure (the O2 cylinder should be at least half full about 1000 psig), verifying that the central pipeline supply hoses are properly connected, and confirming that the pipeline gauges read about 50 psig.

After verifying the integrity of the high-pressure system, I would check the low-pressure system by ensuring filled vaporizers, checking for leaks (the method of testing is machine dependent), and testing the flowmeters.

I would then check the scavenging system, calibrate the O2 monitor, ensure proper ventilation function, check the integrity of the unidirectional valves, inspect the circuit, verify adequate CO2 absorbent, and ensure the availability of airway equipment and suctioning.

Question 2

How do you check for leaks in the low-pressure system?

(Monday morning you enter the OR to begin your day.)

Answer 2

The low-pressure leak test is used to verify the integrity of the machine from the flow control valves to the common outlet.

Since there is no longer a universal test that can be applied to every anesthesia machine, I would verify the proper method of testing for the workstation I was using.

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For machines without a check valve near the common outlet (i.e. Drager), I could perform the traditional positive-pressure leak test.

For machines with a check valve near the common outlet (i.e Datex-Ohmeda), I would perform a negative-pressure leak test by turning off the anesthesia machine, attaching a suction bulb to the common gas outlet, squeezing the bulb until it collapsed, and then observing that it stayed collapsed for at least 10 seconds.
This test would be repeated with each vaporizer opened one at a time.

Question 3

What mechanisms protect against delivery of a hypoxic mixture?

(Monday morning you enter the OR to begin your day.)

Answer 3

Anesthesia machines are equipped with safety devices such as –

• the fail safe alarm, which sounds if the pressure in the O2 pipeline falls below 30 psig, and
• oxygen failure cut off valves, which decrease or discontinue the flow of other gases when oxygen pressure decreases below a certain threshold.

However, these safety devices do not definitively prevent the delivery of a hypoxic gas mixture; and

the best method of prevention is vigilance and proper monitoring of the oxygen analyzer.

Question 4

What is the difference between a Desflurane and Sevoflurane vaporizer?

(Monday morning you enter the OR to begin your day.)

Answer 4

The Sevoflurane vaporizer is a variable-bypass vaporizer, in which a variable amount of gas is directed into a vaporizing chamber where it mixes with volatile agent before returning to mix with the rest of the carrier gas that was directed to bypass the chamber.

The relatively high vapor pressure and heat of vaporization of Desflurane, however, requires a special vaporizer.

Unlike the variable-bypass vaporizer, this vaporizer does not direct fresh gas flows into the vaporizing chamber (an extremely high fresh gas flow rate would be required to adequately dilute the carrier gas).

Rather, a reservoir of desflurane is electrically heated to create a vapor pressure of 2 atmospheres and then pure Desflurane vapor is mixed with the fresh gas prior to exiting the vaporizer.