Factors influencing capnography in the Bain circuit

The Bain circuit provides continuous fresh gas flow near the airway. The potential mixing of this fresh gas with expired gas may prevent reliable analysis of expired gas. We therefore investigated the interaction of sampling site, fresh gas flow rate, expiratory flow rate, and sampling flow rate on expiratory capnography. Sampling near the fresh gas outlet yielded inaccurate results under several of these conditions. The magnitude of the error was related to the fresh gas and expiratory flow rates. A reliable sampling region near the endotracheal tube was identified.