Diagram of the left hand display of the Datex-Ohmeda AS3 ADU. Click on the thumbnail, or on the underlined text, to see the larger version (77 KB).
The Anesthesia Gas Machine
Michael P. Dosch CRNA MS
University of Detroit Mercy Graduate Program in Nurse Anesthesiology
This site is http://www.udmercy.edu/crna/agm/.
ANESTHESIA GAS MACHINE> COMPONENTS & SYSTEMS> SUPPLY OF GASES & ELECTRICITY
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; the "normal working pressure" of the anesthesia gas machine.
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.
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.
(from 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 |
Cylinder valve - most fragile part, so protect during transport. Consists of
The hanger yoke:
The check valve in the cylinder yoke functions to:
For more detail, see Safety rules for handling compressed 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).
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 loss of main electrical power, new gas machines must be equipped with battery backup sufficient for 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 S/5 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 (usually) or fuses.
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) for two 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). In the Modulus SE they can only be reset by reaching from the front of the machine over the vaporizers(!). 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.
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;82:298-302) 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 older gas machines, loss of room illumination, mechanical ventilators and electronic monitors were the principal problems. In general, new gas machines have battery backup sufficient for 30 minutes of operation- also, however, without patient monitors or mechanical ventilation. New flowmeters that are entirely electronic (Julian) require a backup pneumatic/mechanical flowmeter ("Safety O2" flowmeter). Mechanical flowmeters with digital display of flows have a backup glass flow tube which indicates total fresh gas flow (S/5 ADU, Fabius GS). New gas machines with mechanical needle valve flowmeters and variable bypass vaporizers (ie Fabius GS) have an advantage in that delivery of gases and agent can continue indefinitely- but how long do you want to continue surgery by flashlight, and anesthesia monitored by the five senses? The Narkomed 6000 and Julian provide gas and vapor delivery and all monitors (oxygen, volume and pressure, gas monitoring) for 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.
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;84:225-7, Anesth Analg 2000;91:242-3).
For a crossover, one must
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 2 or 3 turns for checking its pressure. It 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:
It is recommended to ventilate manually 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 (Narkomed 6000, Fabius GS). They only require electrical power and fresh gas flow. A second exception would be the S/5 ADU, which can sense the loss of oxygen and switch to piped air as the driving gas, which would also tend to preserve the cylinder oxygen for the fresh gas flow.
Diagram of the left hand display of the Datex-Ohmeda AS3 ADU. Click on the thumbnail, or on the underlined text, to see the larger version (77 KB).
Users should exercise caution with regard to the displayed "calculated oxygen" concentration on the Datex-Ohmeda AS/3 ADU gas machine. The "Calc. 02%", optionally displayed in the mid-lower left of the primary machine status screen, is based on the flowmeter settings only-unlike the oxygen analysis results displayed in the lower center area. The danger arises in a crossover situation where the pipeline oxygen supply is replaced with another gas (for example, nitrogen). In this emergency, two sections of the same display will offer conflicting information. Because it is based on flowmeter settings, the "Calc. 02 %" will indicate the set oxygen concentration. The oxygen analyzer display will simultaneously alarm, accurately showing a dangerous hypoxic mixture. Although the manual clearly warns of this problem, the design may confuse providers in this rare emergency situation, and delay their response.