The Anesthesia Gas Machine
Michael P. Dosch CRNA PhD
University of Detroit Mercy Graduate Program in Nurse Anesthesiology
This site is http://www.udmercy.edu/crna/agm/.
Revised July 2012
ANESTHESIA GAS MACHINE> COMPONENTS & SYSTEMS> PROCESSING> FAIL-SAFE, FLOWMETERS, HYPOXIC GUARD
What happens if you lose oxygen pipeline pressure?
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) or 100% helium (which wouldn’t be prevented by the hypoxic guard, since the hypoxic guard only connects oxygen and nitrous oxide flowmeters).
The newest Dräger machine, the Fabius GS, uses 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 Aisys, gas flow, pressure, and concentration are sensed and controlled via electronic mechanisms.
What should you do if you lose oxygen pipeline pressure?
One can conceive of flowmeters by how they work:
|Aestiva, Narkomed 6400|
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.
Diagram of a glass flowmeter. Click on the thumbnail, or on the underlined text, to see the larger version (51 KB).
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.
|ADU flowmeters are on the lower-left of the left hand display. Click on the thumbnail, or on the underlined text, to see the larger version (77 KB).|
|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).|
Gas machines with 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 ADU and Fabius GS. Flows are captured electronically as follows: flow from the needle valve is conducted to a small chamber of known volume and held there momentarily by a solenoid valve until the transduced pressure within the chamber reaches a preset limit. This gives a known mass of gas. This cycle is repeated sufficiently often for the desired flow rate to occur, and the number of times the solenoid opens is sensed and can be related to flow. Thus, transitional (and electronic) flowmeters allow automated anesthesia record-keepers to chart fresh gas flows.
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 and Avance is different. The machine controls flow of all gas and vapor to the patient electronically. The controls and numeric display are on the lower-left of the screen. The display is numeric, with an optional bar-graph display. 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") .
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 many models of 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.
Common gas outlet flowmeters are used as a backup on some gas machines that electronically capture and display flows on a computer screen (Aisys, 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 the bellows in Aisys or ADU; within the back cabinet behind the E cylinders in the Fabius) or not included with the basic package, only as an optional accessory.
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 tubes, they are connected in series. All gas flows first through fine, then through coarse flowtubes, controlled by a single flow-control knob. The alternative, (very much) older, less safe arrangement is two tubes in parallel with two knobs. 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:
"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.
Photograph of the Link-25 system. Click on the thumbnail, or on the underlined text, to see the larger version (63 KB).
Ohmeda Link 25 is a 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.
The hypoxic guard system includes desflurane on the ADU. However, when using air and desflurane on the ADU (as well as on older machines) it is possible to deliver less than 21% oxygen. The hypoxic guard system does not prevent this (again, it only links oxygen and nitrous oxide). Of course, the oxygen concentration monitor (in the gas analysis module of the monitor) as well as the ADU itself (the left screen) should alarm under these conditions. The left (gas machine) side screen will alert the user to the fact that they have created a hypoxic mixture.
Dräger ORMC is a pneumatic interlock designed to keep fresh gas flow (FGF) at least 25 + 3% oxygen, by limiting nitrous oxide flow (unlike the Link-25, which increases oxygen flow as nitrous oxide flow is turned on). The ORMC also rings alarms (it has an electronic component) which are inactivated in "All gases" mode (a switch found on older machines).
Dräger S-ORC (newest hypoxic guard system as found on Fabius GS) guarantees a minimum FIO2 of 23%.
In the Aisys, the electronic gas mixer supplies a minimum FIO2 of 25%, but will allow 21% if the fresh gas flow is air only.