The total dead space in the patient is the physiological dead space. Anaesthetic circuits are designed to minimise this occuring as it may lead to serious elevations in blood CO 2 levels. Circuits may eliminate rebreathing either by ensuring an adequate flow of fresh gas which flushes the circuit clear of alveolar gas, or, in the case of a circle system by the use of sodalime which absorbs the CO 2 so that low fresh gas flows may be used. For each of the circuits described below, fresh gas flow rates that will ensure minimal rebreathing will be suggested. It does not however, include systems with carbon dioxide absorption.
Fresh gas enters the system at the fresh gas outlet of the anaesthesia machine.
During the expiratory pause more fresh gas enters the system driving any remaining alveolar gas back along the corrugated tubing and out through the valve. If the fresh gas flow is sufficiently high all the alveolar gas is vented from the circuit before the next inspiration and no rebreathing will take place. With careful adjustment the fresh gas flow can be reduced until there is only fresh gas and dead space gas in the breathing system at the start of inspiration. This is an efficient system for spontaneously breathing patients if carbon dioxide absorption is not available. During controlled ventilation the Magill circuit works in a different way and becomes wasteful and inefficient, requiring high fresh gas flows to prevent rebreathing.