The Anesthesia Gas Machine
Michael P. Dosch CRNA PhD
University of Detroit Mercy - Nurse Anesthesia
This site is https://healthprofessions.udmercy.edu/academics/na/agm/index.htm.
The Anesthesia Gas Machine
Michael P. Dosch CRNA PhD
University of Detroit Mercy - Nurse Anesthesia
This site is https://healthprofessions.udmercy.edu/academics/na/agm/index.htm.
Revised May 2025
The hospital medical gas pipeline is the primary gas 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 downstream from the 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, or when pipeline hoses are disconnected.
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.
|
Gas |
Color |
Service Pressure |
Capacity |
Pin Position |
|
Oxygen |
green (white) |
1,900 |
660 |
2-5 |
|
Nitrous Oxide |
blue (blue) |
745 |
1,590 |
3-5 |
|
Air |
yellow |
1,900 |
625 |
1-5 |
Calculating the time remaining for a cylinder If the pipeline oxygen supply fails, it s an emergency, and the patient only has the contents of the E size oxygen cylinder on the machine to sustain them (before switching to room air and bag-valve-mask as a very last resort).
Since the timeline for restoration of pipeline supplies is unpredictable, and the supply of E-sized oxygen cylinders is finite, you must have a means to calculate how long the oxygen cylinder on the machine will last. It s a perennial Board-type question- but it has real clinical importance. Use this equation to solve for minutes your E tank will last.
| Minutes remaining = (0.3 x Gauge pressure psi) ÷ Oxygen Flow L/min |
Example: Your oxygen tank pressure gauge reads 1000 psi. How long can an O2 flow of 2 L/min be maintained? (The tank will last approximately 150 min, at an oxygen flow of 2 L/min.) See footnote 1.
Cylinder valve - is the most fragile part, so protect it during transport. Consists of
The safety relief device is composed of at least one of these: frangible disc (bursts under extreme pressure), fusible plug (Wood's metal, which has a low melting point), or safety relief valve (opens at extreme pressure).
The hanger yoke:
The check valve in the cylinder yoke functions to:
For more detail, download rules from Oklahoma State University for compressed gases- safe handling or the Compressed gas safety guide from Stony Brook SUNY.
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.
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. See footnote 2.
One should avoid plugging devices into these convenience receptacles which turn electrical power into heat (e.g. 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 on the machine 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.
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 (see footnote 3) 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 through dark hallways.
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 (or more)- but perhaps (depending on the model) without patient monitors or mechanical ventilation. For example, GE 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. New electronic gas mixers (e.g. Aisys, Perseus) require a backup pneumatic/mechanical flowmeter ("Alternate/ Emergency O2" flow control). Glass-tube flowmeters with digital display of flows often have a backup common gas outlet flow meter which indicates total fresh gas flow (Fabius GS, Apollo).
New gas machines which retain pneumatic-mechanical (knob needle valve & glass tube) flowmeters and traditional variable bypass vaporizers (e.g. Aespire, Aestiva, Penlon/Paragon) 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?
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. Also see the excellent article, How do I prepare for OR power failure? from the APSF Newsletter (2016).
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, as recently as 2024, involving the death of an 8-year old after a three-minute biopsy (footnote 4). Anesthetists' responses to oxygen pipeline failure (and crossover) were inadequate when these events were studied via simulation (footnote 5).
For a crossover, one must
Crossover means a switch in gas supply lines, such that there is a non-oxygen gas (e.g. nitrous oxide or nitrogen) flowing from the oxygen pipeline. The principal sign of crossover is a declining FIO2 in spite of apparently adequate oxygen flow. You must disconnect the pipeline supply, since 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).
If oxygen pipeline pressure is lost entirely, the principal signs are a low FIO2 alarm, and activation of the fail safe system (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. We need to open the cylinder only partially to check it, but must open it fully when we are using it (else it may not empty completely). Second, although it is not strictly necessary, I advocate disconnecting the pipeline supply if pipeline pressure is lost for two reasons:
It is recommended to ventilate manually when pipeline oxygen is unavailable in machines which use oxygen in whole or part as the driving gas (gas that compresses the ventilator bellows). It is better to manually ventilate with the breathing circuit on the machine if you can (not with a bag-valve-mask). Using the breathing circuit allows you to administer volatile agent. 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 (footnote 7).
This admonition to ventilate manually if pipeline oxygen supply pressure is lost) 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 for the ventilator. A second exception may 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.