INSPIRED GAS, REBREATHING AND APPARATUS DEADSPACE

Abstract

The relationships between mixed inspired gas, rebreathing and apparatus deadspace have been considered in relation to the conditions which may exist during anaesthesia. Different fractions of expired air may be re-inhaled according to the gas circuit employed, and it may be necessary to distinguish the gas drawn into the respiratory tract from the gas which actually enters the alveoli and so influences gas exchange. The effects of various degrees and patterns of rebreathing upon gas exchange may be determined by the use of a mixing equation analogous to Bohr's equation or by the use of a volume/concentration diagram. The effective mean concentration of a gas in mixed inspired air can only be derived when the instantaneous concentration is considered in relation to the volume inhaled rather than in relation to the time during which it is inhaled.