Anesthesia Equipment and Instrumentation
The machine as a chain
The anesthesia machine is a sequence: gas supply → regulators → fail-safe and proportioning devices → flowmeters → vaporizers → common gas outlet → breathing circuit → scavenger. Questions test whether you know which component protects against which failure. The pin index safety system (PISS) is cylinder-specific; the diameter index safety system (DISS) prevents incorrect hose-to-wall connections. Do not interchange them in an answer.
Pipeline oxygen arrives at roughly 50–55 psi. A full E-cylinder of oxygen holds about 660 L at ~1900 psi; a nitrous E-cylinder holds about 1590 L and maintains a constant ~745 psi until the liquid is gone, so nitrous cylinder pressure never tells you how much remains. The fail-safe valve reduces or cuts nitrous flow when oxygen pressure falls, but it does not guarantee a 25% minimum oxygen concentration - that is the proportioning system's job. The oxygen flush delivers 35–75 L/min directly to the common gas outlet, bypassing the vaporizers.
The pre-use checkout
Follow an organized sequence: emergency ventilation equipment, gas supply and cylinders, machine controls and alarms, vaporizers, breathing-system leak test, unidirectional valves, absorber, scavenger, and monitors. The low-pressure leak test uses negative pressure with the vaporizer off. Soda lime changes color with exhaustion, but inspired CO2 - not color - is the reliable sign.
Rule of thumb: before the case the machine must hold a positive-pressure leak at 30 cmH2O for at least 10 seconds, and an oxygen analyzer must read about 21% in room air.
Trap: assuming the fail-safe valve prevents a hypoxic mixture; it does not.