Closed-circuit anesthesia refers to a technique in which, during the delivery of an inhalation anesthetic agent using an anesthesia work station (machine) with a circle system, the adjustable pressure relief valve is closed and the fresh gas inflow is adjusted such that it is just sufficient to match the amount of inhaled anesthetic gas and O₂ that is taken up by the patient and the amount of gas removed by any side-stream monitors (for measurement of anesthetic gas concentration, CO₂, and O₂) (Box 13-1). To prevent hypercapnia, a CO₂ absorber must be used; all exhaled gases except CO₂ are rebreathed when this technique is used. One must ensure that the circuit is free of any leaks (more than 20 potential sites exist for gases to leak from the circuit/system) (Box 13-2).

Whereas, during administration of most inhalation anesthetics, one monitors only the fractional inspired O₂ concentration (FIO₂