Anaesthetic Machine

Quick Answer

Modern anaesthetic machines integrate gas delivery, vaporization, breathing systems, and monitoring with multiple safety features. Gas supply: Central pipeline (oxygen 400 kPa, air, nitrous oxide) or cylinders (oxygen 137 bar, air 137 bar, nitrous oxide 52 bar) regulated to 400 kPa via pressure regulators. Flowmeters: Thorpe-type variable orifice flowmeters (oxygen always downstream to prevent hypoxic mixtures if leak), electronic flow sensors in modern machines. Vaporizers: Temperature-compensated, variable bypass, agent-specific (sevoflurane, isoflurane, desflurane) with keyed fillers to prevent wrong agent; desflurane requires heated (39°C) Tec 6/7 vaporizer due to high boiling point (22.8°C). Safety features: Oxygen failure alarm (whistle if pressure <200 kPa), oxygen ratio monitor/proportioning system (prevents N₂O/O₂ ratios >3:1), pin index system (gas-specific yoke connections), non-interchangeable screw thread (NIST) for hoses, color coding, pressure relief valves (7 kPa). Breathing systems: Circle system with CO₂ absorber (soda lime or baralyme), adjustable pressure limiting (APL) valve (0-60 cm H₂O), reservoir bag (1-3 L). Scavenging: Active or passive removal of waste anaesthetic gases to atmosphere, mandatory in Australia to reduce occupational exposure (N₂O levels <25 ppm, volatiles <2 ppm). Monitoring: ANZCA PS41 mandates continuous monitoring of inspired oxygen, airway pressure, expired CO₂, and volatile agent concentration. Pre-use check: Mandatory before each case (ANZCA PS55) including gas supply check, vaporizer check, breathing system check, leak test, and monitoring calibration. Indigenous considerations: Equipment function identical; cultural considerations around equipment use and informed consent remain important. [1-10]