The Anesthesia Gas Machine

Michael P. Dosch CRNA PhD
University of Detroit Mercy Graduate Program in Nurse Anesthesiology
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Revised July 2012


Components and systems- Introduction


The anesthesia gas machine is a device which delivers a precisely-known but variable gas mixture, including anesthetizing and life-sustaining gases.

The components and systems as described in this document are typical for a hypothetical generic anesthesia gas machine. The differences between older gas machines -such as the Ohmeda Modulus, Excel, or Aestiva and the Dräger Narkomed GS, Mobile, MRI, 2B, 2C, 3 or 4- are less than their similarities. Therefore only the differences with the most impact on clinical practice are described for this generation of machines. However, this generation of gas machines is becoming less common as they approach the time when they will no longer be supported by the manufacturer's technical support and service.

Differences between models have more clinical impact with the latest generation of gas machines because of the higher degree of computer-controlled basic systems, and monitor and electronic medical record integration. So the differences are more fully described here for new models from GE Healthcare (Aisys, Aespire, Avance, ADU) and Dräger (Apollo, Fabius GS).

Numbers to remember

The hospital pipeline is the primary gas source at 50 psi, which is the normal working pressure of most machines. Cylinders - Oxygen is supplied at around 2000 psi (regulated to approximately 45 psi after it enters the machine).

Oxygen flush is a "straight shot" from supply to delivery point, 35-75 L/min.

OSHA links on Waste Anesthetic Gases (WAGs) gives the NIOSH recommendation to OSHA - occupational exposure should be limited to (an eight hour time-weighted average of) not more than 2 ppm halogenated agents (0.5 ppm if nitrous oxide in use), and not more than 25 ppm nitrous oxide. Read the 1977 NIOSH recommendation on waste gases.

Tubing sizes- scavenger 19 or 30 mm, ETT or common gas outlet (CGO) 15 mm, breathing circuit 22 mm.

General features of all anesthesia gas machines

The basic pneumatic-mechanical design of the anesthesia gas machine has become familiar to a generation of providers. The basic design has been called upon to perform more complicated functions since 1990, with the advent of computer-controlled monitors into the operating room, especially pulse oximetry, capnography, and gas analysis. Our gas machines have become top-heavy with the monitors we have added to their basic design.

Now, the advent of the computer gives us a new generation of anesthesia gas machines, which have a great deal of added functionality in a small package, designed from the start to be microprocessor controlled. The prototypes of this new wave are the ADU, Aisys, Apollo, and Fabius GS. These gas machines are being purchased because they

Required components of an anesthesia workstation

The current anesthesia gas machine (workstation) standard is ASTM F1850 (a standard promulgated by American Society for Testing and Materials). The European standard is EN740.

F1850 specifies what is needed for an anesthesia workstation. The components are typically built into new gas machines, or they may be added to older machines. Required components include:

Path of gases within the machine

Oxygen has five "tasks" in the AGM; it powers the

  1. ventilator driving gas
  2. flush valve
  3. oxygen pressure failure alarm
  4. oxygen pressure sensor shut-off valve ("fail-safe")
  5. flowmeters.

It also plays a role in the hypoxic guard system, which maintains the correct proportion between flows of oxygen and nitrous oxide.

Five tasks of oxygen diagram

Diagram of the five tasks of oxygen. Click on the thumbnail, or on the underlined text, to see the larger version (26 KB).

The path of gases through the machine is illustrated in J Andrews (Miller), or Dorsch & Dorsch 4th ed (pg. 81), or M Dosch in Nurse Anesthesia (Nagelhout & Plaus 2010). This is but one way to conceive of the machine- a better way might be the Supply, Processing, Delivery & Disposal model.

    The machine components can also be logically conceptualized by the amount of pneumatic pressure they are exposed to:

  1. The High-pressure circuit consists of those parts which receive gas at cylinder pressure
    • hanger yoke (including filter and unidirectional valve)
    • yoke block
    • cylinder pressure gauge
    • cylinder pressure regulators
  2. The Intermediate pressure circuit receives gases at low, relatively constant pressures (37-55 psi, which is pipeline pressure, or the pressure downstream of a cylinder regulator)
    • pipeline inlets and pressure gauges
    • ventilator power inlet
    • Oxygen pressure-failure device (fail-safe) and alarm
    • flowmeter valves
    • oxygen and nitrous oxide second-stage regulators
    • oxygen flush valve
  3. The Low-pressure circuit includes components distal to the flowmeter needle
    • valves
    • flowmeter tubes
    • vaporizers
    • check valves (if present)
    • common gas outlet




There are two major manufacturers of anesthesia gas machines in the United States.

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