The Anesthesia Gas Machine

Michael P. Dosch CRNA PhD, Darin Tharp CRNA MS
University of Detroit Mercy Graduate Program in Nurse Anesthesiology
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Revised March 2016


Supply system: Gases & electricity

Gas sources


The hospital medical gas pipeline source is the primary source for the anesthesia gas machine. Oxygen is produced by fractional distillation of liquid air. It is stored as a liquid at -150 to -175 degrees C in a large flask (because the liquid occupies 1/860 of the space the gas would occupy). Safety systems and regulators send oxygen to the hospital pipeline at approximately 50 psi; which is therefore the "normal working pressure" of the anesthesia delivery system.

Nitrous oxide is stored as a liquid, at ambient temperature, in large tanks (745 psi- H tank) connected to a manifold which regulates the pipeline pressure to approximately 50 psi.

Pipeline inlets (near the yoke blocks for cylinders) are connected with DISS (diameter index safety system) non-interchangeable connections. The check valve, located down stream from the pipeline inlet, prevents reverse flow of gases (from machine to pipeline, or to atmosphere), which allows use of the gas machine when pipeline gas sources are unavailable.

Cylinder source


Standards for cylinders are written by the U.S. Department of Transportation (DOT), the Compressed Gas Association, the National Fire Protection Association, and the American Society of Mechanical Engineers. US DOT regulations have the force of law, as do Food and Drug Administration (FDA) regulations on the quality and purity of the medical gas contents.

Capacity, color, markings of cylinders

(Figures from different sources vary slightly; the table below is based on CGA Pamphlet P-2)



US (internat'l)

Service Pressure


Pin Position


green (white)




Nitrous Oxide

blue (blue)





(black & white)




Cylinder component parts

Cylinder valve - is the most fragile part, so protect during transport. Consists of

The safety relief device is composed of at least one of

The hanger yoke:

  1. orients cylinders,
  2. provides unidirectional flow, and
  3. ensures gas-tight seal.

The check valve in the cylinder yoke functions to:

  1. minimize trans-filling,
  2. allow change of cylinders during use, and
  3. minimize leaks to atmosphere if a yoke is empty.

Storage, handling and installation


For more detail, download rules from Oklahoma State University for compressed gases- safe handling or the Compressed gas safety guide from Stony Brook SUNY.

Medical gases

Nitrous oxide is manufactured by thermal decomposition of (NH4)2NO3. It is non flammable but supports combustion (same as oxygen). Oxygen is produced by fractional distillation of liquid air. Impurities are permitted in medical gases as long as they do not exceed small amounts of known contaminates.


Reserve tanks are present on the gas machine for emergency use. Marked & color-coded. (Beware if you practice overseas- there are US and global color standards, which differ.) PISS (pin-index safety system) prevents misconnection of a cylinder to the wrong yoke. Keep cylinders closed except when checking, or while in use. The cylinder pressure regulator converts high, variable cylinder pressure to a constant pressure of approximately 45 psi downstream of the regulator. This is intentionally slightly less than pipeline pressure, to prevent silent depletion of cylinder contents if a cylinder is inadvertently left open after checking its pressure. Cylinder pressure gauge indicates pressure in the higher-pressure cylinder only (if two are opened simultaneously).

Electrical power supply

Main electrical power is supplied to the gas machine through a single power cord which can become dislodged. Because of this possibility, as well as the possibility of main electrical power loss, new gas machines must be equipped with battery backup sufficient for at least 30 minutes of limited operation. What functions remain powered during this period is device-specific, so one must familiarize oneself with the characteristics of each model. For example, if you disconnect electrical power from the ADU, it loses monitors (right screen), but gas delivery and ventilation continue during the period when you are relying on battery backup.

Convenience receptacles are usually found on the back of the machine so that monitors or other equipment can be plugged in. These convenience receptacles are protected by circuit breakers or fuses. In theory, blowing a fuse in one of these circuits should not affect the operation of the rest of the machine. However, loss of monitors is a risk. In view of the easy availability of electrical receptacles in the OR, one should never allow any electrical devices to be plugged into the back of the anesthesia machine (Anaesthesia 2002;57(11):1134).

It is a mistake to plug devices into these convenience receptacles which turn electrical power into heat (air or water warming blankets, intravenous fluid warmers, fiberoptic light sources) for several reasons. First, these devices draw a lot of amperage (relative to other electrical devices), so they are more likely to cause a circuit breaker to open. Second, the circuit breakers are in non-standard locations (so check for their location before your first case). If a circuit breaker opens, all devices (monitors, perhaps the mechanical ventilator) which receive their power there may cease to function. If you are not familiar with the circuit breaker location, valuable time may be lost while a search is conducted. Finally, in some workstations, the circuits are protected by fuses. If a fuse blows it cannot be reset, and the machine must be taken out of service until a replacement fuse can be installed.

Failures and faults

Loss of main electrical power

Devices (or techniques) which do not rely on wall outlet electrical power include:

Devices which require wall outlet electrical power include:

Generally, hospitals have emergency generators that will supply operating room electrical outlets in the event power is lost. But these backup generators are not completely reliable. Troianos (Anesthesiology 1995;82:298) reports on a 90 minute interruption in power during cardiopulmonary bypass, complicated by almost immediate failure of the hospital generators. One unanticipated hazard was injuries to personnel as they went to fetch lights and equipment.

In electrical failure, loss of room illumination, mechanical ventilation and physiologic monitors are the principal problems. In general, current gas machines have battery backup sufficient for 30 minutes of operation- but perhaps (depending on the model) without patient monitors or mechanical ventilation. New flowmeters that are entirely electronic (e.g. Aisys, Perseus) require a backup pneumatic/mechanical flowmeter ("Alternate O2" flow control). Mechanical flowmeters with digital display of flows have a backup glass flow tube which indicates total fresh gas flow (Fabius GS, Apollo).

New gas machines which retain mechanical needle valve-controlled flowmeters and traditional pneumatic variable bypass vaporizers (e.g. Apollo, Fabius GS, Aespire) have an advantage in that delivery of gases and agent can continue indefinitely- but how long do you want to continue surgery by flashlight, with anesthesia monitored by the five senses? The Aisys provides gas and vapor delivery and all monitors (oxygen, volume and pressure, gas monitoring) for at least 30 minutes if main electrical power is lost.

It remains critical to understand and anticipate how each particular anesthesia gas machine type functions (what parts and for how long) when main electrical power is lost. The best place to find this information is in the operator's manual.

Failure of pipeline oxygen supply

Pipeline sources are not trouble free: contamination (particles, bacteria, viral, moisture), inadequate pressure, excessive pressures, and accidental crossover (switch between oxygen and some other gas such as nitrous oxide or nitrogen) are all reported. These are not theoretical problems. Intraoperative hypoxemia related to pipeline gas contamination continues to be reported in the US (Anesth Analg 1997;84:225, Anesth Analg 2000;91:242). Anesthetists' responses to oxygen pipeline failure (and crossover) were inadequate when these events were studied via simulation (Anaesthesia. 2007;62(2):122, Anesth Analg. 2010;110(5):1292, Anesth Analg. 2006;102(3):865).

For a crossover, one must

  1. turn on backup oxygen cylinder, and
  2. disconnect oxygen pipeline supply hose from the wall.
"Crossover" means a switch in gas supply such that there is a non-oxygen gas (e.g. nitrous oxide or nitrogen) flowing from the oxygen pipeline. Gas will flow from whichever source is at a higher pressure- the contaminated pipeline (at 50 psi) or the emergency tank supply of oxygen (supplied to the machine at 45 psi). So you must disconnect the pipeline supply.

In contrast, if oxygen pressure is lost entirely, a low oxygen supply alarm will sound, and the fail safe system will activate (see next section). Similar to a crossover, first you must open the backup oxygen cylinder fully. Anesthetists are not in the habit of doing this- usually we need to open the cylinder only partially to check its pressure. the oxygen cylinder must be opened fully when using it as the oxygen source, or it may not empty completely. Second, although it is not strictly necessary, I advocate disconnecting the pipeline supply if it fails for two reasons:

  1. The oxygen supply system has failed. Until notified that it is functional and free of impurities, why use it? If the pressure is restored, but with inappropriate (non-oxygen) contents, the pipeline (wrong) gas will flow if its pressure exceeds the regulated cylinder pressure (45 psi).
  2. It is mandatory to disconnect the pipeline in case of a crossover. Is it wise to memorize two different strategies for two similar problems, when disconnecting from the pipeline supply is a safe response for both situations? See:

It is recommended to ventilate manually (not with an Ambu,; use the breathing circuit on the machine so you can still administer volatile agent) when pipeline oxygen is unavailable in machines which use oxygen in whole or part as the driving gas that compresses the ventilator bellows. Maintaining mechanical ventilation in the absence of pipeline oxygen can use an entire E cylinder of oxygen (approximately 600 L) in an hour or less (Anesth Analg 2002;95:148-50).

This admonition applies to almost all gas machines. The exceptions are piston ventilators, which do not use driving gas or bellows at all (Fabius GS, Apollo), or turbine ventilators (Perseus). They only require electrical power and fresh gas flow. A second exception would be the Aisys, which can sense the loss of oxygen and switch to piped air as the driving gas (if it is available), which would also tend to preserve the cylinder oxygen for the fresh gas flow.

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