Provision of endogenous and exogenous humidity for the Bain breathing circuit

The endogenous humidity produced by a partial rebreathing system, the Bain Breathing Circuit, was measured clinically and experimentally. The addition of humidification by interposing a Garthur Vapor Condenser (HME) or a Bennett Cascade heated humidifier into the system was also studied. Without supplemental humidity, the Bain Breathing Circuit probably meets marginal requirements for preservation of ciliary morphology and function during endotracheal anaesthesia. However, additional humidity is needed to prevent alteration in pulmonary mechanics. The HME is a beneficial adjunct to the Bain Breathing Circuit, raising humidity to near acceptable levels, especially if ambient temperatures range from 24 degrees to 26 degrees C. However, the deadspace of the HME is 17 ml, which might limit its use in children. The Bennett Cascade humidifier plus the Bain Breathing Circuit satisfies minimal suggested humidity requirements at ambient reservoir water temperatures of 24 degrees to 26 degrees C. Heating the reservoir water to maximum heat capacity increased humidity but caused marked water condensation along the inner fresh gas tube of the Bain Breathing Circuit.