J. Clark Venable, M.D.

As the complexity of the anesthesia delivery apparatus increased, so did the possibility of equipment failure and misuse. This led to the development of a checklist which was to be performed at the beginning of each day to insure that the main systems of the anesthesia machine were functioning properly.

Although malfunctions in the anesthesia machine are rare, they do occur, and can have serious consequences for the patient. Of 3,791 malpractice claims studied by the American Society of Anesthesiologists, two percent involved the gas delivery equipment. Most commonly the end result was brain damage or death.

In an attempt to reduce or eliminate causes of complications in the operating room, much time has been devoted to the study of how errors happen and what things predispose to them. Usually, an adverse event is the result of several factors which in and of themselves pose no risk to patients, but when taken together can result in patient injury. For example, a leak in the yoke of an oxygen cylinder on an anesthesia machine (which results in the tank losing all its oxygen) does not harm the patient by itself. Only if the wall source oxygen fails will the empty oxygen tank pose a serious threat to the patient. Even though individual faults may seem unimportant when taken in isolation, it is important to seek them out and eliminate them to minimize the risk they will combine with another fault to cause patient injury.

II. The FDA-approved Anesthesia Machine Check

The latest incarnation of the machine check was published in 1993 by the Food and Drug Administration. It tests each component of the anesthesia machine as follows.

Emergency Ventilation System

1. Verify Backup Ventilation Equipment is Available and Functioning.

It is essential, and potentially life saving, that a means to ventilate the patient other than the anesthesia machine be available within each operating room in case of a machine failure.

High Pressure System

2. Check Oxygen Cylinder Supply

Open O₂cylinder and verify at least half full (about 1000 psi) Close cylinder

3. Check Central Pipeline Supplies

Check that hoses are connected and pipeline gauges read about 50 psi.

In order to accurately know the oxygen cylinder pressure, the wall source must first be disconnected. This is because a check valve prevents oxygen from flowing from the tank as long as oxygen pressure exists in the pipeline.

Low Pressure System

4. Check Initial Status of Low Pressure System

Close flow control valves and turn vaporizers off. Check fill level and tighten vaporizers’ filler caps.

Now is the time to add agent to any vaporizers that are running low.

5. Perform Leak Check of Machine Low Pressure System

Verify that the machine master switch and flow control valves are off. Attach suction bulb to common (fresh) gas outlet.

The suction bulb is necessary because of the outlet check valve. This valve prevents gas from flowing backward toward the vaporizers which may create unpredictable levels of inhaled anesthetic concentration in the fresh gas outflow. The reason we do a negative pressure leak test is to open the outlet check valve by applying suction, thus allowing us the test the more proximal portions of the low pressure system for leaks.

Squeeze bulb repeatedly until fully collapsed.

Verify bulb stays fully collapsed for at least 10 seconds

This step would not work if the machine were on since there is always a minimum oxygen flow present and the bulb would never stay deflated.

Open one vaporizer at a time and repeat ‘c’ and ‘d’ above.

By opening one vaporizer at a time, we are testing just that part of the system. If the bulb does not remain deflated after opening the Halothane vaporizer, we know that the leak is somewhere in the Halothane vaporizer or its attachment to the system.

Remove suction bulb and replace fresh gas hose.

6. Turn on Machine Master Switch

and all other necessary equipment

7. Test Flowmeters

a. Adjust flow of all gases through their full range, checking for smooth operation of floats and undamaged flow tubes.

b. Attempt to create a hypoxic O₂/NO₂ mixture and verify correct changes in flow and/or alarm.

The purpose here is to make sure no floats are stuck down (or up), or that the flow of air was not mistakenly left on. Both of these have happened in our OR recently.

Scavenging System

Adjust and Check Scavenging System

a. Ensure proper connections between the scavenging system and both APL (pop-off) valve and ventilator relief valve.

b. Adjust waste gas vacuum (if possible)

c. Fully open APL valve and occlude Y-piece.

d. With the O₂ flush activated allow the scavenger reservoir bag to distend fully, and then verify that absorber pressure gauge reads < 10 cm H₂O.

To insure that pressure will not build up in the system and cause patient injury, this step makes sure that the gas can be vented to the scavenging system.

Breathing System

9. Calibrate O₂ monitor

a. Remove the O₂ sensor from the circuit, allow it to equilibrate with room air, and press the calibration button.

a. Ensure monitor reads 21% on room air.

b. Verify low O2 alarm is enabled and functioning

c. reinstall sensor in circuit and flush breathing system with O₂.

The purpose of the flush is two-fold. First, to allow the next step in the check to be done. Second, to get the O2 concentration above the low oxygen alarm limit of 30%.

d. Verify that monitor new reads greater than 90%.

The only monitor which prevents the delivery of a hypoxic mixture of gas to the patient is the O2 sensor. For this reason, of all the checks done, this one is among the most important.

10. Check Initial Status of Breathing System

a. Set selector swith to ‘bag’ mode.

b. Check that breathing circuit is complete, undamaged and unobstructed.

c. Verify that CO₂ absorbent is adequate.

d. Install breathing circuit accessory equipment (e.g. humidifier, PEEP valve) to be used during the case.

11. Perform Leak Check of the Breathing System

a. Set all gas flows to zero (or minimum).

b. Close APL (‘pop-off’) valve and occlude Y-piece.

c. Pressurize breathing system to 30 cm H₂O with O2 flush.

d. Ensure that pressure remains fixed for at least 10 seconds.

e. Open APL (pop-off) valve and ensure pressure decreases.

Do not release the pressure by taking your thumb off the elbow adapter. Release it by opening the APL valve. This step insures that positive pressure can be delivered to the patient. It rules out the possibility that the fresh gas hose was not reconnected, that the O2 sensor was not reattached properly, that there is a significant leak in the system, and so on.

Manual and Automatic Ventilation System

12. Test Ventilation System and Unidirectional Valves

a. Place breathing bag on Y-piece.

b. Set appropriate ventilator parameters for next patient.

c. Switch to automatic ventilation (Ventilator) mode.

d. Fill bellows and breathing bag with O₂ flush and then turn ventilator on.

At this point, I like to set the respiratory rate to an appropriate number (such as 8), check the tidal volume to make sure it is not too high or too low (I like 750 cc), and check the inspiratory flow and I:E ratio.

e. Set O2 flow to minimum, other gas flows to zero.

f. Verify that during inspiration bellows delivers appropriate tidal volume and that during expiration bellows fill completely.

Turning the fresh gas flow up insure that the ventilator pressure relief valve is functioning. If it were not, the high pressure alarm would bed activated.

h. Verify that the ventilator bellows and simulated lungs fill and empty appropriately without sustained pressure at end expiration.

i. Check for proper action of unidirectional valves.

You should check to see that the valves are indeed present, and that they move the way they are supposed to. If a valve is stuck in the closed or open position, it should be apparent at this point.

j. Exercise breathing circuit accessories to ensure proper function.

k. Turn ventilator OFF and switch to manual ventilation (Bag/APL) mode.

About fifteen seconds after turning off the ventilator, the apnea alarm will sound. To stop the alarm, you must press the proper buttons to disable the apnea and volume alarms on the front of the machine.

Final Positions

13. Check Final Status of Machine

a. Vaporizers off

b. APL valve open

c. Selector switch to ‘Bag’

d. All flowmeters to zero

e. Patient suction level adequate

f. Breathing system ready to use

III. Conclusion

The complex nature of anesthesia machines requires that this equipment be checked prior to use to insure proper functioning. The performance of the FDA Machine Checkout Procedure insures that the systems important for delivering oxygen and removing CO2 have been checked and shown to be functional prior to patient use.