Anesthetic Equipment: Machines, Circuits and Airways
Core knowledge
Understand the anaesthetic machine as a set of subsystems: gas supply (cylinders, pipeline, pin index safety system), pressure regulators, flowmeters (Thorpe tube - read at the top of the float), vaporisers (variable bypass, plenum, agent-specific, outside the circuit to avoid concentration changes), and the breathing circuit. Know the circle system components (unidirectional valves, CO2 absorber, reservoir bag, APL valve, scavenging) and the Mapleson classifications (A–F) with their behaviour during spontaneous vs controlled ventilation. Know the safety features: oxygen failure alarm, nitrous oxide–oxygen ratio cut-off, and that cylinders are colour-coded (oxygen white, nitrous oxide blue in many systems).
Airway equipment: supraglottic devices, laryngeal mask seal pressures, endotracheal tube sizing and cuff principles, and capnography as the standard monitor.
Traps
- Confusing Mapleson A (good for spontaneous, poor for controlled) with Mapleson D (good for controlled).
- Forgetting that vaporisers are agent-specific and should be locked/filled correctly; misfilling is a classic safety stem.
- Misreading a flowmeter float or assuming a pipeline failure is impossible - always check the back-up cylinder.
- Assuming low circuit volume means low fresh gas flow is safe - rebreathing and CO2 accumulation can occur.
Rule of thumb
For any equipment question, ask: does this device control fresh gas delivery, gas concentration, or gas removal? Vaporisers control concentration; circuits and APL valves control removal; flowmeters control delivery.
Worked example
A capnograph shows a rising baseline CO2. In a circle system this points to exhausted absorbent or a faulty unidirectional valve - check the absorber and valves before blaming the patient.