The Anesthesia Gas Machine

Michael P. Dosch CRNA PhD, Darin Tharp CRNA MS
University of Detroit Mercy - Nurse Anesthesia
This site is https://healthprofessions.udmercy.edu/academics/na/agm/index.htm.

Revised Jan. 2024

ANESTHESIA GAS MACHINE> COMPONENTS & SYSTEMS> PROCESSING> FAIL-SAFE, FLOWMETERS, HYPOXIC GUARD

• Fail-safe system
• Flowmeters
  • How they work
    • Traditional glass flowmeters
    • Transitional (hybrids with needle valve controls and electronic capture and display)
    • Electronic flow control, capture, and display
    • Et (end-tidal) Control
  • Function (flowmeters for FGF, auxiliary oxygen, scavenging, and common gas outlet)
  • Using flowmeters safely
• Proportioning systems (Hypoxic guard)

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 (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 (at around 30 psi pipeline pressure in some models).

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% 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).

Older nomenclature:

• Datex-Ohmeda called their fail safe a "pressure sensor shut off valve”; if pressure in the oxygen line fell to 20 psi, the flow of all other gases were shut off. Also, Ohmeda used a second-stage O2 pressure regulator (ensures constant oxygen flowmeter input until supply pressure is less than 12-16 psi).
• Dräger's "oxygen failure protection device" (OFPD) threshold is proportional, unlike Ohmeda's which is off-or-on. The oxygen ratio monitor controller (Dräger) shuts off nitrous oxide when oxygen pressure is less than 10 psi.

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
   • Why? Something is wrong with the oxygen pipeline. What if the supply problem evolves into a non-oxygen gas in the oxygen pipeline? If so, pipeline will flow (pressure 50 psi) rather than your oxygen cylinder source (down-regulated to 45 psi).
     • If you are lucky, the oxygen alarm will sound to warn you of the change (you do leave your alarms enabled, don't you?).
     • If for some reason the oxygen analyzer does not warn of the crossover, the pulse oximeter will- but only after the oxygen has been washed out by ventilation of the patient's functional residual capacity and vessel-rich group.
   • So disconnect the pipeline connection at the wall if oxygen pipeline pressure is lost. It's also 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!
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.

Flowmeters

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 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).

Transitional (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.
	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 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)
• FIO2 -percent 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") .

Et (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 proportions to maintain EtO2 and EtAA targets: Set the desired outcome (patient's end tidal oxygen & agent) and gas flows and concentrations are 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. (Br J Anaesth 2013;110:561)
• 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 (Anaesth Intensive Care 2013;41:95).
• 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.

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.

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 (Anesth Analg 2010;110:1292).

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 supplies 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 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 2019, nitrogen contamination of the oxygen pipeline to 3 operating rooms in one suite, caused severe hypoxemia and required resuscitation. APSF Newsletter June 2019
• “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 Anaesth Intens Care 2017; Historical Suppl: 21-8 at doi 10.1177/0310057X170450S104

2. A more common reason for a hypoxic breathing mixture is combining low fresh gas flow with lower F_IO₂ in a carrier gas of either air or nitrous oxide. The flaw here which prevents the hypoxic guard system from intervening is that hypoxic guard systems regulate 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).J Clin Monit Comput 2015;29:491 at doi 10.1007/s10877-014-9626-y. Note video content (scroll down).
• Lethal hypoxemia will develop, and develops 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. J Clin Monit Comput 2016;30:251 at doi 10.1007/s10877-015-9710-y. 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, which intervenes and overrules operator settings when inspired FIO2 is too low. This contrasts with conventional hypoxic guard system on all other machines, in which the hypoxic guard only sets a ratio of N2O delivered to the breathing circuit. The Maquet Flow-i automated gas control targets desired outcomes (F_IO₂ and expired agent concentration) and maintains them at any set level, by altering fresh gas flows. (J Clin Monit Comput 2016; 30:341 at doi 10.1007/s10877-015-9723-6). This is similar to all-electronic flowmeters with Et Control(as seen on Aisys CS2) in which the desired outcome is set (F_IO₂ and total fresh gas flow) instead of setting process variables (how many liters of oxygen and air or nitrous oxide). This type of automated gas control (an active delivered or inspired hypoxic guard) rapidly reverses and limits the duration of inspired hypoxic episodes when the conventional hypoxic guard fails to do so (J Clin Monit Comput 2016;30:63 at doi 10.1007/s10877-015-9684-9).

Photograph 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 for a report of failure of the chain-link mechanism.

Diagram 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 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 a backup knob and glass flowtube, because they will not 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).