Anesthesia Equipment, Instrumentation, and Technology
Map the machine by pressure zone
High pressure: cylinders, regulators, pipeline inlets. Pipeline supply is about 50 psig; a full oxygen E-cylinder reads roughly 1900–2200 psig and holds about 660 L; a nitrous oxide cylinder reads about 745 psig and stays there until the liquid is exhausted, so pressure says nothing about remaining volume. Intermediate: flowmeters (Thorpe tubes, read at the top of the float). Low pressure: vaporizers, breathing circuit, ventilator.
Know the checkout: oxygen supply and fail-safe, oxygen ratio monitor, vaporizer interlock, circle-system leak test, unidirectional valves, CO2 absorbent, APL valve and scavenger, bellows, and alarm limits. Vaporizers are agent-specific variable-bypass devices mounted out of circuit; desflurane requires a heated, pressurized vaporizer. Dry absorbent degrades sevoflurane to Compound A and desflurane to carbon monoxide.
Traps: cylinder colors differ by country; a fail-safe device prevents a hypoxic mixture but does not itself deliver oxygen; Mapleson A is efficient for spontaneous ventilation while B and D suit controlled ventilation; pulse oximetry lags and fails with poor perfusion, methemoglobin, and carboxyhemoglobin.
Worked example: a full E-cylinder holds about 660 L. At 5 L/min it lasts 660 / 5 = 132 minutes, roughly 2.2 hours. Below about 200 psig, remaining oxygen is proportional to gauge pressure, so time is estimated from pressure rather than the full cylinder figure.