Anaesthesia Machine Checks
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Direct answer
A machine fault discovered before induction is an inconvenience; the same fault discovered after the patient is anaesthetised is a catastrophe. The pre-use check walks the gas pathway end to end — pipeline and cylinder supply, flowmeters, vaporisers, the breathing system with a formal leak test, the ventilator, scavenging, suction and monitors — before the first case and after any circuit change. Modern workstations run automated self-tests, but the checklist still demands human eyes on what a self-test cannot see: the state of the soda lime, the presence of a self-inflating resuscitation bag, and the oxygen cylinder kept in reserve for transport.
What you must remember
- Gas supply: pipeline gauge reads about 4 bar (roughly 50 psi); the back-up E-size oxygen cylinder on the yoke is kept at least half full — a full E cylinder holds about 680 litres at roughly 137 bar.
- Pin index safety system: oxygen fits only positions 2 and 5, nitrous oxide 3 and 5, air 1 and 5 — physically preventing a wrong-gas yoke.
- Flowmeter checks: bobbins must spin freely without sticking; verify the oxygen–nitrous oxide hypoxic guard link and the oxygen failure alarm by running N2O with oxygen turned off.
- Vaporisers: correct agent via keyed filler, adequate liquid level, securely seated, interlock allowing only one vaporiser open at a time.
- Leak test: occlude the Y-piece, close the APL valve and pressurise the breathing system to 30 cmH2O using the oxygen flow — pressure must hold; then confirm both circle unidirectional valves move during ventilation.
- Soda lime: ethyl violet indicator turns purple when exhausted, but fades back towards white on resting, so log hours of use rather than trusting colour alone.
- Scavenging and suction: the scavenger connects to the APL valve and ventilator relief port; suction is a separate wall system tested independently.
- Back-up kit: a self-inflating (Ambu) bag and a full oxygen cylinder — the two items most often missing when the machine fails.
Walking the check on a Monday morning
Start outside the machine: pipeline hose connected, wall outlet pressure near 4 bar, cylinders present with the oxygen reserve valve closed until needed. Turn the master switch on, run the electronic self-test where fitted, and watch it complete rather than pressing "override" through a failed step. At the flowmeter block, run each gas and confirm the bobbin rises and falls smoothly.
Now the breathing system: assemble it as it will be used, squeeze the reservoir bag, perform the 30 cmH2O leak test with the APL closed, then open the APL and confirm the bag empties. Set the ventilator, attach a test lung or bag to the Y-piece and watch the bellows deliver a tidal volume with the inspiratory valve fluttering one way and the expiratory the other. Glance at the soda lime canister — an absorbent that ran dry over the weekend can degrade desflurane or isoflurane into carbon monoxide, the classic "Monday morning" hazard when gas flows are left on unattended. Finish with scavenging connected, suction pulling at the catheter end, monitors with alarm limits set, and the Ambu bag visible. Record the check with a signature — in NABH-accredited Indian theatres that documented check is an audit item, not a formality.
Where students slip
The oxygen failure device and the hypoxic guard get swapped in vivas. The failure device (the Bosun whistle) cuts the flow of other gases and sounds when oxygen supply pressure falls; the hypoxic guard (the Link-25 ratio system) physically couples the oxygen and nitrous oxide bobbins so the delivered mixture never falls below roughly a quarter oxygen. Two separate mechanisms, two separate exam answers. The second slip is believing an automated self-test replaces the manual check — self-tests verify internal electronics and leaks, not whether the soda lime is fresh, the scavenger tubing is actually connected to the wall, or the Ambu bag exists. Third: the leak test performed with the APL valve open proves nothing, because gas escapes through the valve you forgot to close.
Frequently asked questions
What pressure should the pipeline gauge read on a checked machine?
About 4 bar (roughly 45–55 psi); a much lower reading means a pipeline, hose or area-valve problem that must be sorted before induction.
How is the breathing system leak test performed?
Occlude the Y-piece, close the APL valve, fill the system to 30 cmH2O with oxygen and confirm the pressure holds; a fall indicates a leak to be traced before use.
Why should soda lime be checked even if it looks white?
The ethyl violet indicator fades back towards white when the absorbent rests, so colour understates exhaustion — track hours of use and change on schedule.
Which pin index positions accept an oxygen cylinder?
Positions 2 and 5; nitrous oxide uses 3 and 5 and air 1 and 5, making cross-fitting physically impossible.
Which items does an automated self-test not cover?
Soda lime condition, physical scavenging connections, the self-inflating resuscitation bag and the transport cylinder — all human-check items on every checklist.