Historical picture

The Anesthesia Gas Machine

Michael P. Dosch PhD CRNA (retired)
University of Detroit Mercy - Nurse Anesthesia
This site is https://healthprofessions.udmercy.edu/academics/na/agm/index.htm.

Revised May 2025

Components and systems- Introduction

Introduction

Definition: The anesthesia gas machine is a device which delivers a precisely-known but variable gas mixture, including anesthetizing and life-sustaining gases. The anesthesia gas machine is also called the anesthesia workstation, or anesthesia delivery system.

The components and systems described in this document are typical for any anesthesia gas machine. The differences between older gas machines (Ohmeda Modulus, Excel, ADU, or Aestiva and the Drager 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, these older gas machines are becoming much less commonly used, as they pass the dates when the manufacturer's technical support and service are no longer available. The material on older machines is still valuable in a couple of settings. Older machines (if not discarded) often make their way to remote NORA (non-OR anesthesia) areas of the hospital, where general anesthesia is infrequent. Consequently, inadequate attention may be given to their functioning, stocking, and any unique checkout requirements. Or anesthetists may encounter them in low-resource environments on overseas missions.

Differences between models have more clinical impact with the latest generation of gas machines, because of the higher degree of computer-controlled systems, physiologic and machine functional monitoring, and electronic medical record integration. So the differences are more fully described here for new models from GE Healthcare (Aisys, Aespire, Avance) and Drager (Apollo, Perseus, 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 pipeline to common gas outlet (bypassing vaporizers and flowmeters); activation results in very high gas flow into the breathing circuit (35-75 L/min, approximately 1 liter/second).

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.

General features of all anesthesia workstations

The basic pneumatic-mechanical design of the anesthesia gas machine had become familiar to the last generation of providers. Since then, the basic design has been called upon to perform more complicated functions, with the advent of advanced ventilation and sophisticated monitors into the operating room, especially pulse oximetry, capnography, and gas analysis.

The integration of computer controls has yielded a new generation of anesthesia gas machines, which have a great deal of added functionality in a small package. These delivery systems are designed from the start to integrate monitoring, advanced ventilation, and the electronic anesthesia record. Examples of this new wave are the Aisys CS/2 and Perseus. These gas machines are being purchased because they

Required components of an anesthesia workstation

A former (withdrawn in 2014) anesthesia gas machine (workstation) standard was ASTM F1850 (by the American Society for Testing and Materials). With designs diverging more widely than in the past, no one standard can easily be applied to all workstations. But F1850 is still useful as a listing of desired systems.

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 Venticinque & Andrews (Miller), or Dorsch & Dorsch, or M Dosch in Nurse Anesthesia (Nagelhout & Elisha 2021). This is but one way to conceive of the machine- a better way might be the Supply, Processing, Delivery & Disposal model.

Supply

How do gases come to the anesthesia gas machine? (Site: back of the machine)

Processing

How does the anesthesia gas machine prepare gases before their delivery to the breathing circuit? (Site: within the machine, proximal to common gas outlet)

Delivery

How is the interaction of gases with the patient controlled and monitored? (Site: breathing circuit)

Disposal

How are gases disposed of? (Site: scavenger)
The machine components can also be logically conceptualized by the amount of pneumatic (gas) pressure they are exposed to:
  1. The High-pressure circuit consists of those parts which receive gas at cylinder pressure
    • cylinder hanger yokes (including filter and unidirectional valve)
    • cylinder pressure gauge and 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 (if present)
    • 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

Manufacturers

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

 


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