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The anaesthesia machine, gases and operating room fire
The anaesthesia machine is built with layers of protection against a hypoxic mixture, but only the oxygen analyser in the inspiratory limb detects every failure — and next to it a self-inflating bag and an oxygen cylinder must always be at hand.
Why it matters in the exam
The exam asks about the machine's safety systems (pin index, hypoxic guard, fail-safe valve), the difference between an oxygen cylinder and an N2O cylinder (gas versus liquid), a failure in the oxygen pipeline and what to do, the CO2 absorbent and the signs that it is exhausted, Mapleson circuits, the heated vaporiser for desflurane, and airway fire — the order of actions.
In an oxygen cylinder the gas is compressed, so pressure falls in direct proportion to the amount left. N2O is held in the cylinder as a liquid, and the pressure stays constant until the liquid is used up — so the amount is known only by weighing. In the circle system there are one-way valves and a CO2 absorbent; when the absorbent is exhausted CO2 appears in the inspired gas and the capnography baseline does not return to zero. Dry absorbent with desflurane can produce CO, and with sevoflurane compound A forms and, with dry absorbent, heat as well.
An operating room fire needs three components: an oxidiser (oxygen or N2O), fuel (drapes, the tracheal tube, alcohol-based skin preparation) and an ignition source (electrosurgery or laser). In head and neck surgery the oxygen concentration is kept as low as possible, the skin preparation is allowed to dry, and for airway laser surgery a laser-resistant tube with a saline-filled cuff is used. In an airway fire the tube is removed, gases are stopped, saline is poured, and then the airway is re-established and checked by bronchoscopy.
Key points
- Only the inspiratory oxygen analyser detects a hypoxic mixture from any cause.
- Oxygen in a cylinder — gas; N2O — liquid, content known by weighing.
- Inspired CO2 = exhausted absorbent or a faulty valve.
- Desflurane needs a heated vaporiser.
- Fire: oxidiser, fuel and ignition; in the airway — remove the tube, stop gases, saline.
Summary table
| Issue | Main point | Exam point |
|---|---|---|
| Machine protections | Pin index for cylinders, unique pipeline connectors, fail-safe, hypoxic guard | Only the oxygen analyser detects a pipeline crossover |
| Cylinders | Oxygen — gas, pressure reflects content; N2O — liquid, constant pressure until the end | N2O content is known by weighing |
| Circuit and absorbent | One-way valves, CO2 absorbent changes colour when exhausted | Inspired CO2 = exhausted absorbent or a faulty valve |
| Vaporisers | Agent-specific and temperature-compensated; desflurane — heated, pressurised vaporiser | High vapour pressure and boiling point near room temperature |
| Airway fire | Remove the tube, stop gases, saline, re-ventilate, bronchoscopy | Prevention: low oxygen, dry skin preparation, laser tube |
A free exam question
בבדיקת מכונה לפני כל מקרה הרדמה, מה המרדים מחויב לבדוק על פי המלצות ASA ? (the question as examined, in Hebrew)
- א. לבדוק שהלחץ בבלון החמצן הרזרבי גבוה מ- 1000 PSIG
- ב. לבדוק שהלחץ בצינורות הגזים המגיעים לחדר גבוה מ- 50 PSIG
- ג. לבדוק את הלחץ במערכת ההנשמה ומבחן דליפה
- ד. לעשות קליברציה לסנסור החמצן
In the full version for subscribers
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Sources
- Miller's Anesthesia, 9th edition (2020) — Inhaled Anesthetic Delivery Systems
- ASA Practice Advisory for the Prevention and Management of Operating Room Fires, 2013 — Fire prevention and management
- Morgan & Mikhail's Clinical Anesthesiology, 7th edition (2022) — The Anesthesia Workstation; Breathing Systems
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.