The anaesthesia machine and breathing circuits, in principle

What it is

The anaesthesia machine takes gases from the hospital pipelines or from cylinders, reduces them to a safe working pressure, mixes them in chosen proportions, adds an exact concentration of volatile anaesthetic and delivers the fresh gas to a breathing circuit connected to the patient. It can be described in three parts. The high-pressure system holds the cylinders, with pin-index connectors so that a cylinder cannot be fitted to the wrong yoke. The intermediate-pressure system takes pipeline gas through gas-specific, non-interchangeable connectors, and includes the oxygen failure alarm, the fail-safe valve that cuts other gases when oxygen pressure falls, and the oxygen flush. The low-pressure system holds the flowmeters, the hypoxic guard that links oxygen and nitrous oxide flows so that a hypoxic mixture cannot be set, and the agent-specific, temperature-compensated vaporisers. The fresh gas reaches the patient through a breathing circuit: most adults are anaesthetised with a circle system, in which exhaled gas passes through a carbon dioxide absorbent and back to the patient through one-way valves, so low fresh gas flows can be used and agent and heat are saved; open circuits without absorbent (Mapleson types and the T-piece used in children) need higher flows to wash out carbon dioxide. The circuit includes a reservoir bag, an adjustable pressure-limiting valve, a ventilator and scavenging of waste gas. A full machine check before each list, and a shorter one before each case, is mandatory.

Key points

• Gas pipelines and cylinders are colour-coded, but the codes differ between countries: always read the label.
• Vaporisers are agent-specific and filled with keyed fillers to prevent the wrong agent being added.
• Desflurane needs a heated, pressurised vaporiser because it boils near room temperature.
• Scavenging removes exhaled anaesthetic gases so the theatre staff are not exposed to them.
• Dry absorbent can react with volatile agents and produce carbon monoxide or other harmful compounds; change it as the protocol says.

Components

• Gas supply: pipelines with wall outlets and backup cylinders on the machine.
• Pressure regulators, oxygen failure alarm and fail-safe valve.
• Flowmeters with the hypoxic guard, and the oxygen flush.
• Vaporisers and the common gas outlet.
• Breathing circuit: inspiratory and expiratory limbs, one-way valves, absorbent, reservoir bag, adjustable pressure-limiting valve, Y-piece, filter.
• Ventilator, airway pressure and volume monitors, and scavenging.

How it works

• Gas flows from high pressure to low pressure: regulators protect the machine and the patient from pipeline and cylinder pressures.
• Part of the fresh gas passes through the vaporising chamber and picks up saturated vapour; the dial sets how much, so the output concentration is accurate.
• In the circle, one-way valves keep gas moving in one direction, so exhaled carbon dioxide passes the absorbent before it can be breathed again.
• The adjustable pressure-limiting valve lets excess gas out to scavenging during spontaneous or manual ventilation; the ventilator takes over this during mechanical ventilation.

Step by step

1. Check that a self-inflating bag and backup oxygen are present — they are the rescue if the machine fails.
2. Confirm pipeline connections by tugging and check cylinder contents — a wrong or empty supply is a classic cause of hypoxia.
3. Check flowmeters, the hypoxic guard and the oxygen analyser calibration — they stop a hypoxic mixture reaching the patient.
4. Check vaporisers are filled, seated and locked, with no leak — a loose vaporiser leaks agent and gas.
5. Leak-test the breathing circuit and test the one-way valves and absorbent — leaks and stuck valves cause awareness or rebreathing.
6. Test the ventilator, alarms, suction and scavenging, and record the check — each is needed during the case.

Machine safety features and what they prevent

Next step

• Never start an anaesthetic without a completed machine check and a self-inflating bag within reach.
• If you suspect machine failure, ventilate the patient with the self-inflating bag and independent oxygen, keep the patient anaesthetised intravenously and call for help.
• If inspired carbon dioxide rises, check the absorbent and the one-way valves, increase fresh gas flow and change the absorbent.
• Report any fault to the biomedical engineering team and label the machine out of service until repaired.

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Elsewhere in DurgiPedia

• Bag-mask ventilation
• Dead space and alveolar ventilation

Connections

← All of anaesthesia

Educational content; it does not replace supervised training, your institution's protocols or the anaesthetist's or intensivist's decision. It contains no doses and no ventilator or infusion settings.

Sources: Anesthesia Machine (StatPearls (NCBI Bookshelf), 2022).

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Representative model, for educational purposes only.