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

Processing: Fail-safe, Flowmeters, Hypoxic guard

• Fail-safe system
• Flowmeters
  • How they work
    • Glass flowmeters
    • Hybrids (needle valve controls and electronic capture and display)
    • Gas Mixers (electronic flow control, capture, and display)
    • EtC (end-tidal) Control
  • Classified by what they do (flowmeters for FGF, auxiliary oxygen, scavenging, and common gas outlet)
  • Using flowmeters safely
• Proportioning systems (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 (the point where gas prepared within the machine enters the breathing circuit). In addition, the loss of oxygen pressure results in high-priority alarms, audible and visible.

Fail-safe systems don't prevent hypoxic mixtures in every possible circumstance. 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% of a third gas (e.g. helium if present)(which wouldn’t be prevented by the hypoxic guard, since the hypoxic guard only connects oxygen and nitrous oxide flowmeters).

Newer Drager machines (e.g. 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 (normally ~50 psi) is less than 20 + 4 psi. In case of complete oxygen pipeline failure, the machine will supply pipeline air so that oxygen 21% 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 flows, pressures, and concentration.

What should you do if you lose oxygen pipeline pressure?

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

Why? Something is wrong with the oxygen pipeline. What if the supply problem evolves into a crossover (non-oxygen gas in the oxygen pipeline)? Ifn that case, pipeline flow will continue (delivered at ~50 psi) rather than from your oxygen cylinder source (down-regulated to 45 psi). And unlike oxygen supply failure, which causes immediate, high priority audible and visible alarms, only the low F_IO₂ alarm will sound.

This is one reason you must check the oxygen alarm each day- even if you are only doing sedation; general anesthesia is always plan B. Starting a case with only a pulse oximeter but without an oxygen analyzer is like driving a car with a busted fuel gauge. You won t get an indication of a fuel problem until you are out of gas entirely. It s much smarter to hear an alarm at the beginning of an oxygen delivery problem (oxygen analyzer) than at the end (low saturation on the pulse oximeter), when hypoxemia is well established and the lung and your delivery system are completely devoid of oxygen.

So disconnect the pipeline connection at the wall if oxygen pipeline pressure is lost, or a crossover is occurring. It's easier to remember one strategy which works for any problem with the pipeline, than to remember that sometimes you must, and sometimes it is optional, to disconnect. And use that oxygen analyzer always!

Finally, 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.

Flowmeters

Flowmeters differ in how they control and display gas flow. One can conceive of flowmeters by how they work:

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 glass flowmeters

Diagram 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 older machines, but only a few current machines (Aespire, Aestiva, Paragon).

Hybrid 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.
	Mindray A5 flowmeters (with common gas outlet flowmeter between the pressure gauges and below and to right of the oxygen flow knob). Click on the thumbnail, or on the underlined text, to see the larger version.

A hybrid flowmeter has characteristics that are transitional between glass flowmeters and fully electronic gas mixers. Flow is mechanically controlled (needle valve) and electronically displayed (bar graph on computer screen). Thus, hybrid flowmeters have no glass tubes or float bobbins. 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 Apollo and Mindray A5. Flows are captured and displayed electronically. Thus, transitional (and electronic) flowmeters allow automated anesthesia record-keepers to chart fresh gas flows. To allow measurement of total fresh gas flow in the event of monitor screen failure or electrical power failure, these designs usually incorporate a common gas outlet flowmeter, which measures the total flow of all gases (oxygen plus carrier gas-either air or nitrous oxide).The common gas outlet flowmeter is visible in the first picture above, to the right of the Apollo needle valves.

Gas mixers

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

• Carrier gas (nitrous oxide, air)
• F_IO₂ desired
• Total FGF -fresh gas flow L/min

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").

EtC (end-tidal) Control

Et Control . Click on the thumbnail, or on the underlined text, to see the larger version.

Electronic control of gas mix and fresh gas flow allows "Et Control" (in the GE Aisys CS2). Et control automatically adjusts fresh gas flow and dialed anesthetic agent (AA) to maintain expired oxygen (EtO2) and agent (EtAA) targets. The user sets the desired outcome (patient's end tidal oxygen & agent); the process of getting there (changing gas flows and agent concentration) is manipulated by the machine to produce the desired outcome quickly, and without further user intervention. This has the following advantages:

• Fast: Reach 90% of target EtAA within an average of 90 seconds and maintain the target at minimal flow rates (footnote 1).
• Reduced workload: One study has shown it can reduce the number of key presses by more than 50%, simplifying adoption of low-flow strategies by staff (footnote 2).
• Enhanced vigilance: more accurate in maintaining the set EtO2 and EtAA regardless of patient status.
• Eco-friendly: significant potential decline in the rate of greenhouse gas emissions.
• Cost savings: can reduce anesthetic agent consumption significantly.

At the end of the case, one can emerge the patient withe EtC aid. There are two choices depending on the rapidity necessary. Select "Purge" and fresh gas flow is changed to 10 L/min with the vaporizer off, to flush agent and complete emergence as rapidly as possible. Selecting "Off" lets agent decrease gradually by stopping any further delivery of agent to the circuit (e.g. closes the vaporizer), while leaving fresh gas flows low. This results in a slower process whereby the patient "coasts" upwards towards readiness for extubation.

Auxiliary oxygen, common gas outlet, and scavenging flowmeters

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

1. 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 remains 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. (Beware of forgetting to switch the CO₂ sampling line back to the breathing circuit!) 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 pipeline (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 (footnote 3).
2. Common gas outlet flowmeters are used as a backup on some gas machines with transitional flowmeters (Apollo, Fabius, Mindray). 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.
3. 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.

Safety features - The oxygen flow control knob (if present) 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 with glass flowtubes includes ensuring that:

• floats spin freely,
• qualified service personnel regularly maintain gas machines,
• an oxygen analyzer used always (of course, the readings are erroneous during the use of a nasal cannula unless you are sampling from the airway),
• one never adjusts a flowmeter without looking at it,
• one includes flowmeters in visual monitoring sweeps, and
• one turns glass flowmeters off before opening cylinders, connecting pipelines, or turning machine "on".

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) which is meant to prevent oxygen concentration less than 0.25 from being delivered to the breathing circuit when nitrous oxide is in use.

The potential flaws?

1. These systems do not sense the presence of oxygen in the oxygen pipeline; only the pressure within it. Thus, in a crossover (where some non-oxygen gas is present in the oxygen pipeline), hypoxic gas mixtures are possible without malfunction in the hypoxic guard system. Crossovers are, thankfully, rare. But their consequences can be fatal. And they continue to occur.

• In the US, as recently as 2024, death of an 8-year old after a three-minute biopsy occurred due to crossed pipelines (footnote 4)
• In 2019, nitrogen contamination of the oxygen pipeline to 3 operating rooms in one suite, caused severe hypoxemia and required resuscitation (footnote 5).
• “In a European study from 2004-2006, the investigators found media reports of six deaths from nitrous oxide across Germany, Switzerland and Austria, none of which were reported scientifically. As recently as 2016, there was one death and one case of neurological damage from a Sydney hospital due to cross connection of oxygen and nitrous oxide supplies to a hospital theatre" (footnote 6).

2. A more common reason for a hypoxic breathing mixture is combining low fresh gas flow with lower F_IO₂ with air as the carrier gas. The flaw which prevents the hypoxic guard system from intervening here is that hypoxic guard systems regulate the delivered O₂/N₂O ratio, not inspired oxygen concentration.

• Current hypoxic guard systems do not reliably prevent a hypoxic F_IO₂ with O₂/N₂O and O₂/air mixtures, particularly at low fresh gas flows (0.7 to 3 L/min) footnote 7.
• Lethal hypoxemia will develop, and rapidly, given particular combinations of fresh gas flow and delivered O₂ concentration settings (e.g. 1 L/min air results in the rapid formation of an inspired O₂ concentration of 12% over the course of 2 min). This is because the patient uses oxygen from the breathing circuit faster than it is being replenished in the fresh gas flow (footnote 8). A hypoxic guard system that regulates the delivered ratio of O₂/N₂O fails to prevent drops in inspired oxygen from this cause.
• An alternative is seen on the Maquet Flow-i, which uses an active hypoxic guard. This active system intervenes and overrules operator settings when inspired F_IO₂ is too low. This contrasts with conventional hypoxic guard systems on other machines, in which the hypoxic guard only sets a ratio of O₂:N₂O delivered to the breathing circuit. The Maquet Flow-i automated gas control targets desired outcomes (inspired O₂ and expired agent concentration) and maintains them at any set level, by altering fresh gas flows (footnote 9). Aisys CS2 has a similar active hypoxic guard function when Et Control is used; the desired outcome is set (etO₂, etAA, and minimum fresh gas flow). This type of automated gas control (an active delivered or inspired hypoxic guard) rapidly reverses and limits the duration of inspired hypoxic episodes when a conventional hypoxic guard would fail to do so (footnote 10).

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

The older Ohmeda Link 25 is a purely mechanical system: A chain links nitrous oxide and oxygen flow control knobs, allowing 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.

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

 Dräger S-ORC (hypoxic guard system as found on Fabius). Resistors generate back pressure on a control diaphragm, in proportion to oxygen and nitrous oxide flows.  Like Link-25 and other older systems, S-ORC only guarantees a minimum delivered F_IO₂ of 21% (by limiting nitrous oxide flow).

Newest workstations with electronic gas mixers (Aisys, Avance, Perseus) supply a minimum delivered F_IO₂ 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 systems usually include an emergency backup knob and glass flowtube at the common gas outlet to indicate total FGF, because the gas mixer ceases to function if electric power fails. Automated gas control (as found on Maquet Flow-i workstation, or the Drager Zeus workstation) regulates inspired oxygen and expired agent concentration (as discussed above).

Key points

• remember the alternate term proportioning system, and
• hypoxic guard systems CAN permit hypoxic breathing mixtures IF:
  1. O2 delivery is less than consumption by the patient (e.g. 1 L/min air in the fresh gas flow)
  2. wrong supply gas in oxygen pipeline or cylinder,
  3. defective pneumatic, mechanical, or electronic components,
  4. leaks exist downstream of flow control valves, or
  5. if third inert gas (such as helium) is used (not true of electronic gas mixers).

Footnotes
1. Br J Anaesth 2013;110:561
2. Anaesth Intensive Care 2013;41:95
3. Anesth Analg 2010;110:1292
4. AHRQ 2024 at https://psnet.ahrq.gov/web-mm/hypoxic-gas-supply-cross-connected-pipelines
5. APSF Newsletter June 2019
6. Anaesth Intens Care 2017; Historical Suppl: 21-8 at doi 10.1177/0310057X170450S104
7. J Clin Monit Comput 2015;29:491 at doi 10.1007/s10877-014-9626-y. Note video content (scroll down).
8. J Clin Monit Comput 2016;30:251 at doi 10.1007/s10877-015-9710-y
9. J Clin Monit Comput 2016; 30:341 at doi 10.1007/s10877-015-9723-6
10. J Clin Monit Comput 2016;30:63 at doi 10.1007/s10877-015-9684-9