Early detection of "rebreathing" in afferent and efferent reservoir breathing systems using capnography.

Capnography was used to determine the onset of rebreathing in afferent (AR) and efferent (ER) reservoir breathing systems in a spontaneous ventilation lung model. In the case of the Lack and enclosed AR systems, the best sampling site was found to be in the exhaust limb of the systems, 5 cm from the Y connector. For the Magill system, fitted with a hooded scavenging valve, the best site was deep inside the hooded valve. In contrast, the best sampling site in an ER system (e.g. Bain system) was in the tracheal tube. For AR systems, the loss of a fresh gas elimination pattern (carbon dioxide trace failing to reach zero) was shown to occur at the onset of rebreathing. As the sampling site was moved distally into the exhaust limb, the same pattern was seen at greater flow rates--that is, before rebreathing was actually occurring. When sampling was within the tracheal tube, using ER systems, a typical "rebreathing wave" occurred at the onset of established rebreathing.