Simplified functional diagram of an Aisys anesthesia machine

The diagram depicts the machine during the inspiratory phase of a mechanical breath (long-dashed blue arrows, direction of FGF; short-dashed red arrows, gas flow generated by the ventilator; dotted green arrows, drive gas flow) with the inspiratory valve open and the expiratory valve closed. Lightning bolts indicate sites of electronic control. During spontaneous or manual ventilation, excess gas is vented out of the breathing circuit through the APL valve, which is excluded from the system during mechanical ventilation. From the wall outlets or cylinders, pressurized gases (O₂, green; air, yellow; N₂O, blue) pass through pressure regulators on entrance of the anesthesia machine. Fresh gas is metered by electronic controls with electronic flowmeters, and the mixture then flows through an electronically controlled variable bypass vaporizer. The resulting anesthetic gas mixture enters the circle system proximal to the inspiratory valve. Compressed oxygen from the wall outlet is electronically metered (compensating, over sequential breaths, for the continuous inflow of fresh gas) to drive the ventilator bellows. The entire breathing circuit is pressurized during inspiration. During exhalation excess gas is vented from the bellows through an exhaust valve that contributes an obligatory PEEP of 3 cmH₂O.