The Anesthesia Gas Machine

Michael P. Dosch CRNA PhD, Darin Tharp CRNA MS
University of Detroit Mercy Graduate Program in Nurse Anesthesiology
This site is

Revised March 2016


Processing: Fail-safe, Flowmeters, Hypoxic guard

Fail-safe system

What happens inside the workstation when oxygen pipeline pressure drops?

The fail safe device ensures that whenever oxygen pressure is reduced and until flow ceases, the set oxygen concentration shall not decrease at the common gas outlet. In addition, the loss of oxygen pressure results in alarms, audible and visible, at 30 psi pipeline pressure.

Fail-safe systems don't prevent hypoxic mixtures. For example, as long as there is pressure in the oxygen line, nothing in the fail safe system prevents you from turning on a gas mixture of 100% nitrous oxide (however, this should be prevented by the hypoxic guard system, see below) or 100% helium (which wouldn’t be prevented by the hypoxic guard, since the hypoxic guard only connects oxygen and nitrous oxide flowmeters).

Older nomenclature:

Newer Dräger machines (Fabius, Apollo) use a Sensitive Oxygen Ratio Controller (S-ORC). Its fail-safe component shuts off nitrous oxide if the oxygen flow is less than 200 mL/min, or if the oxygen fresh gas valve is closed. Audible and visible alarms sound if pipeline pressure is less than 1.38 + 0.27 bar (20 +4 psi). In case of complete oxygen pipeline failure, the machine will supply pipeline air so that some oxygen and agent can still be supplied to the patient.

In the newest workstations (Aisys, Avance, Perseus), the gas mixer prevents hypoxic breathing mixtures through electronic sensing and control of gas flow, pressure, and concentration.

What should you do if you lose oxygen pipeline pressure? Just like a crossover,

  1. Open the emergency oxygen cylinder fully (not just the three or four quick turns used for checking)
  2. Disconnect the pipeline connection at the wall
  3. Ventilate by hand with the anesthesia breathing circuit, rather than with the mechanical ventilator (which uses cylinder oxygen for the driving gas if the pipeline is unavailable) or a bag-valve-mask (Ambu), because staying with the anesthesia breathing circuit means you can still deliver volatile agent.


One can conceive of flowmeters by how they work:

  Control Display Example
Needle valve
Glass tube
Aestiva, Aespire
Transitional (hybrid)
Needle valve
Apollo, Fabius
Aisys, Avance, Perseus

Or one can conceive of flowmeters by function: patient breathing gas, scavenger, common gas outlet, or auxiliary oxygen flow.

Of course, we also measure flow (and volume, and composition) of inhaled and exhaled gases in the breathing circuit.

Traditional flowmeters

Glass flowmeter diagramDiagram of a glass flowmeter. Click on the thumbnail, or on the underlined text, to see the larger version.

In a traditional flowmeter, flow is mechanically controlled (needle valve) and displayed (glass tube with bobbin). A Thorpe tube is an older term for flowmeters. The components are- needle valve, indicator float, knobs, valve stops. Flow increases when the knob is turned counterclockwise (same as traditional vaporizers). At low flows, the annular-shaped orifice around the float is (relatively) tubular so (according to Poiseuille's Law) flow is governed by viscosity. At high flows (indicated on the wider top part of the float tube), the annular opening is more like an orifice, and density governs flows.

Regular mechanical needle valves and glass flowtubes are utilized in the Excel, Modulus, Aestiva; and Narkomed 2, 3, 4, and 6000.

Transitional (hybrid) flowmeters

Apollo flowmeters Apollo flowmeters are on the lower-left of the display. Notice the common gas outlet flowmeter, to the right of the flowmeter knobs. Click on the thumbnail, or on the underlined text, to see the larger version.
Fabius GS flowmeters Fabius GS flowmeters (with common gas outlet flowmeter on the left). Click on the thumbnail, or on the underlined text, to see the larger version (29 KB).

In a transitional flowmeter, flow is mechanically controlled (needle valve) and electronically displayed (bar graph on computer screen).Transitional flowmeters have no glass tubes. The flow rate is indicated with a bar graph on a monitor screen. There is a needle valve (so flow can be generated even without electric power) in the Fabius GS and Apollo. Flows are captured and displayed electronically. Thus, transitional (and electronic) flowmeters allow automated anesthesia record-keepers to chart fresh gas flows.

Electronic flowmeters

Aisys flowmeter controls Aisys electronic flowmeters. Click on the thumbnail, or on the underlined text, to see the larger version (178 KB).

Setting fresh gas flow on the Aisys, Avance, and Perseus is different. The gas mixer electronically controls flow of all gas and vapor to the patient, and these are displayed on a monitor screen. The user sets

It's an odd way to do it for anesthetists who are used to setting a process variable ("I'll use 2 L nitrous oxide + 2 L oxygen, which will give me 4 L/min FGF at an FIO2 = 0.5") rather than setting the desired outcome and letting the process be taken care of by the machine ("I want a total flow of 4 L/min at an FIO2 of 0.5 in nitrous oxide") .

Auxiliary oxygen, common gas outlet, and scavenging flowmeters

Besides the flowmeters which deliver gas to the breathing ciruit, flowmeters are used in three other ways in the gas machine.

Auxiliary oxygen flowmeters are an optional accessory currently offered on most new gas machines. They are useful for attaching a nasal cannula or other supplemental oxygen delivery devices. They are advantageous because the breathing circuit and gas delivery hose (between the common gas outlet and breathing circuit) remain intact while supplemental oxygen is delivered to a spontaneously breathing patient. Thus, if the anesthetist desires to switch from a nasal cannula to the circle breathing system during a case, he or she can accomplish this instantaneously, and without the possibility of forgetting to reconfigure the breathing circuit properly. Another advantage is that an oxygen source is readily available for the Ambu bag if the patient needs to be ventilated manually for any reason during a case (for example, breathing circuit failure). One disadvantage is that the auxiliary flowmeter becomes unavailable if the pipeline supply has lost pressure or has been contaminated; this is because the auxiliary flowmeter is supplied by the same wall outlet and hose connection that supplies the main oxygen flowmeter. If users do not realize this, then time could be wasted while they attempt to utilize this potential oxygen source (Anesth Analg. 2010;110(5):1292).

Common gas outlet flowmeters are used as a backup on some gas machines with transitional flowmeters (Apollo, Fabius GS, ADU). If optional, they are strongly recommended, as they are the only indication of oxygen flow if the computer display fails, or in a power failure situation after battery backup is exhausted.

Scavenging flowmeters: Many new machines use open scavenging interfaces. An indication that suction is adequate is mandatory with these systems in order to avoid exposure to waste anesthesia gases (see below Disposal: Scavenging and Waste gases). Unfortunately, the suction indicator may be occult (behind or beneath the bellows in Aisys or Apollo; within the back cabinet behind the E cylinders in the Fabius).

Using flowmeters

Choosing an appropriate fresh gas flow rate is covered below in Delivery: Using breathing circuits and ventilators. Flowmeters on some machines have a minimum oxygen flow of 200-300 mL/min. Some (especially newer) machines have minimum oxygen flows as low as 50 mL (or no minimum oxygen flow at all). Supply pressure is 50 psi; (older) Ohmeda may have a second-stage regulator which supllies oxygen at 14 psi, and nitrous oxide at approximately 26 psi (this is a component in the fail-safe or hypoxic guard systems in older machines).

Safety features - The oxygen flow control knob is touch-coded. If a gas has two glass flowtubes, they are connected in series. All gas flows first through fine, then through coarse flowtubes, controlled by a single flow-control knob. It is customary in the US for the oxygen flow tube to be on the right of the others, on the left in the UK. In either case, oxygen always enters the common manifold downstream of other gases.

Care of flowmeters includes ensuring that:

Processing- Hypoxic Guard System

"Proportioning Systems" is the board exam terminology for the hypoxic guard system. These systems link nitrous oxide and oxygen flows (mechanically, pneumatically, or electronically) to prevent final inspired oxygen concentration less than 0.25.

Link-25Photograph of the (older) Link-25 system. Click on the thumbnail, or on the underlined text, to see the larger version (63 KB).

The older Ohmeda Link 25 is a purely mechanical system: A chain links nitrous oxide and oxygen flow control knobs, allows either to be adjusted independently, yet automatically intercedes to maintain a minimum 1:3 ratio of oxygen to nitrous oxide. Also, older Ohmeda machines (Modulus, Excel) supply nitrous oxide to its flow control valve at 26 psi, via a second-stage pressure regulator. Therefore, the system has pneumatic and mechanical components in its control of gas mixture. See Anesth Analg 2001;92:913-4 for a report of failure of the chain-link mechanism.

S-ORCDiagram of the S-ORC. Click on the thumbnail, or on the underlined text, to see the larger version.

 Dräger S-ORC (newer hypoxic guard system as found on Fabius GS). Resistors generate back pressure on a control diaphragm, in proportion to oxygen and nitrous oxide flows.  S-ORC guarantees a minimum FIO2 of 21% by limiting nitrous oxide flow.

Newest workstations with electronic gas mixers (Aisys, Avance, Perseus) supply a minimum FIO2 of 25% with nitrous oxide, but will allow 21% if the carrier gas is air only. The hypoxic guard system includes desflurane on workstations with electronic gas mixers. These sytems usually include a backup knob and glass flowtube, because they will not function if electric power fails.

Key points

Return to the top of this page.
Return to Table of Contents.
Go to the next page