The CBET exam almost always has a question or two on anesthesia machines. The questions might ask you about basic functions or common problems associated with these units. Keep in mind, anesthesia machines may differ based on the manufacturer, but test questions are usually not manufacturer specific.

The anesthesia machine utilizes pipeline gases as a main source of pressure for operation. These pipeline gases are supplied at 50 psig. A secondary or backup source consists of gas cylinders (oxygen or O2, nitrous oxide or N2O, and air), which are regulated at 45 psig through cylinder pressure regulators. Pipeline pressure is higher than the cylinder pressure (50>45 psig), which is the reason why the anesthesia machine utilizes pipeline gases before cylinder gases. The cylinder gases are used when the pipeline pressure falls below 45 psig. You may also hear the term “drive gas” for the 50 psig pipeline gas source.

The cylinder pressure regulators have two functions: Reduce the cylinder pressure to a constant 45 psig, and close off the cylinder gas supply when pipeline gas supply exceeding 45 psig is present. This prevents usage and depletion of the backup cylinder gases when there is still an adequate pipeline gas supply.

To prevent mixing up the pipeline hoses, the noninterchangeable screw thread—NIST—system is used to prevent piped gases from the wall being accidentally connected to the wrong inlet on the machine. Another system called the pin index safety system, or PISS, is used on size E and smaller cylinders. The pins protruding from the cylinder yoke of a particular gas have a unique configuration that fits into corresponding holes in the cylinder valve. This prevents the misconnection of cylinders to the wrong yokes. So you cannot connect an oxygen cylinder in place of an air cylinder—or worse, connect a nitrous cylinder where air should be.

Safety Features

In today’s health care facility, all gas tanks are color coded, indicating which gas is in the tank. It is very possible CBET questions could come from these color codes. We strongly suggest you know your gas color codes: oxygen—green, nitrous oxide—blue, air—yellow, carbon dioxide—gray, suction—white/purple, nitrogen—black, and helium—brown.

Even though anesthesia machines differ, test questions are usually not manufacturer specific.

One of the safety features you will see on anesthesia machines is called a fail-safe device. This feature will shut off the nitrous oxide supply if there is a loss of oxygen supply pressure. Nitrous oxide is hazardous, and so this feature stops the amount of nitrous oxide delivered when the oxygen supply is disconnected or lost. In other words, if the anesthesia machine was leaking oxygen, then the flow of nitrous oxide would drop automatically to prevent harm to the patient.

Another feature is the hypoxic safeguard feature that links the nitrous oxide flow control valve to the oxygen flow control valve. This feature ensures that the ratio of O2 to N2O flow can never be less than 1:3. The percentage of oxygen within a mixture should always be at least 21%. The anesthesia machine prevents users from delivering a hypoxic mixture (O2 below 21%), which is harmful to the patient. So, the flow of nitrous oxide is automatically lowered when oxygen flow is decreased, but nitrous oxide flow remains unchanged, if oxygen flow is increased. An example would be if there was 9 L/min of nitrous oxide flowing through the system, then oxygen flow would have to be at least 3 L/min. Remember, the 1:3 ratio!

The oxygen flush button is used to flush the system with oxygen. During the inspiratory phase, continuously pressing this button will cause the lungs to overinflate. During expiration, if the oxygen flush button is pressed, the bellow will initially fill rapidly to its maximum capacity. After reaching maximum capacity, in mechanical mode, any pressure in excess of 2 to 4 cm H2O will be vented through the pressure relief valve. So, a question that you might see on the test might be about a patient who has excess positive pressure accumulation and the bellows won’t go down. A probable cause would be failure of the pressure relief valve, which is preventing the gas from exiting. This is confirmed if manual ventilation resolves the problem. If not, then the problem is most likely from the breathing circuit connected to the patient.

Questions on the exam may ask about the significance of the oxygen flush button found on the unit. Each examinee should understand what the O2 flush button actually does. By depressing this button the unit will flush out remnant anesthetic gases from the system. Another possible question could be at what rate does oxygen get flushed upon pressing the button? This varies slightly, but the flow rate should be in the neighborhood of 35 to 75 L/min.

Device Functions

The manual ventilation mode or bag mode is when the user manually bags the patient to ensure proper respiration. This mode consists of an adjustable pressure limiting (APL) valve that helps control excess pressure buildup and prevents harm to the patient’s lungs. Remember that APL is only used during manual ventilation. As its name indicates, the APL valve limits the amount of pressure buildup that can occur during manual ventilation. The APL valve can be adjusted to regulate different pressures. The pressure inside the breathing circuit must generate a force that exceeds the spring compression force for the APL valve to open. As pressure continues to build from the combination of fresh gas flow and manual compression of the breathing bag, the opening pressure of the APL valve will be exceeded and excess gas will be vented to the scavenging system.

The scavenging system functions as a way to remove waste anesthetic gases, or WAGs, to minimize staff exposure to harmful anesthetic gases. Expiration and anesthetic gases are suctioned out of the unit via a suction line connected to a scavenging system. You may have noticed that the scavenger system is located in the bottom of the anesthesia machine since anesthetic gases are heavier than air, which makes it easier to suction out. Also, remember that anesthetic gases used today are nonflammable and each anesthetic agent has a specific vaporizer that it is used in and, like gas tanks, are color-coded.

The anesthesia machine has one inspiratory and one expiratory unidirectional flow valve. The inspiratory valve makes sure that there is no backflow through the inspiratory limb during expiration. The expiratory valve prevents the backflow through the expiratory limb during inspiration. There is a Y-piece that extends from the circuit to the patient. This part of the breathing circuit contains dead space. So, a possible CBET question could ask about the main function of the unidirectional flow valves, and the answer would be that they help to minimize the dead space in the Y connector by ensuring that there is no backflow of gases.

When looking at an anesthesia machine, you will notice there is an absorber canister with small white granules. These granules are soda lime crystals. The function of soda lime is to absorb carbon dioxide from the exhaled gas before the patient breathes it back again. Soda lime crystals change color over time, from white to blue/purple, due to pH changes in the granules. Remember that CO2 is an acid, and that makes the pH decrease, thus contributing to the color-changing process of soda lime. You may hear these canisters referred to as scrubbers as they scrub the CO2 out of the exhaled patient gas. This works much like an oxygen concentrator that has a sieve bed that oxygen flows through while the sieve bed scrubs off all the gasses except oxygen, which generates oxygen levels above 21%—which is room air.

The most common problems associated with anesthesia machines are leaks. A leak may exist in the manual or mechanical modes of the unit, or possibly both. The first step before using an anesthesia machine is to perform a leak test. This will help reduce problems during cases and ensure proper function of the unit.

If a leak is prominent in both modes, then you would want to look at components that are common between these two. For instance, always check to see the soda lime canister is securely closed. Service requests have been made many times because canisters were opened to drain water out, and were not closed properly, leading to a gas leak. Another problem would be ensuring that the oxygen sensor is properly installed. Gases tend to leak out from the sides of the oxygen sensor if not properly installed.

Many problems are not actual faults of the anesthesia machine, but with the ancillary equipment, such as tubing etc. There are also instances where the patient might be the problem. An example would be having a leak in the endotracheal tube cuff connected to the patient. If asked about anesthesia service questions or servicing anesthesia units, always work your way from the gases coming from the wall or cylinders to the patient. If you have a problem, for instance, with suction, the first thing you do is check to make sure the pipeline suction hose is connected, and then check the tubing. Employing a logical approach and the “keep it simple” method will serve you well in repairing anesthesia equipment as well as in answering questions correctly on the CBET exam.

I hope this information is useful, and we wish you the very best of luck on your journey to becoming a CBET!

John Noblitt, MAEd, CBET, is the BMET program director at Caldwell Community College and Technical Institute, Hudson, NC; and Robert Hijazi, MS, MHA, CBET, is a clinical engineer at Palmetto Health Hospital, Columbia, SC. For more information, contact .

Review Questions

  1. What is the purpose of the O2 flush button on the anesthesia unit?
    1. Provide patient with bolus of oxygen
    2. Wake patient
    3. Flush anesthetic agents from the system
    4. All the above

    See the answer

  2. Soda lime crystals should be changed when they turn what color?
    1. Brown
    2. White
    3. Green/yellow
    4. Blue/purple

    See the answer

  3. The most common problem with anesthesia units is usually____?
    1. Leaks
    2. Improper operation
    3. Wrong anesthetic agent
    4. Bad O2 sensor

    See the answer

  4. Nitrous oxide backup tanks on anesthesia units are always what color?
    1. Gray
    2. Blue
    3. White
    4. Orange

    See the answer