Simplified functional diagram of Fabius, Tiro, or Apollo anesthesia machine

The diagram depicts the machine during the inspiratory phase of a mechanical breath (dashed blue arrows, direction of FGF; dotted red arrows, gas flow generated by the ventilator) 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 mechanical controls with electronic flowmeters, and then flows through a conventional mechanical vaporizer (except for desflurane, which is electronic). A fresh gas decoupling valve directs the FGF during the inspiratory phase to the reservoir bag, and once that is filled, to the exhaust valve. During the prior expiratory phase, the electronically driven piston ventilator would have filled with gas from the reservoir bag, and if the reservoir bag were empty, the ventilator's negative pressure relief valve would have opened to entrain room air. On inspiration, tidal volume from the piston ventilator is delivered directly to the patient without contribution of FGF. The ventilator-controlled PEEP/Pmax valve is closed during inspiration so that only the patient side of the breathing circuit is pressurized. During exhalation excess gas is vented from the circuit through an exhaust valve that contributes negligible PEEP.