Humidity of the Bain and Circle Systems Reassessed

The humidity outputs of the Bain circuit, a traditional non-coaxial Mapleson D system, and a circle system with and without a soda lime absorber were evaluated in a laboratory model simulating a 70-kg subject. The breathing systems were tested with tidal volumes of 0.35 and 0.70 L and frequencies of 10-20 breaths/min to maintain an end-expiratory CO2 of 4.5%. There were small differences in inspiratory gas humidity between the Bain and the non-coaxial Mapleson D system. With a fresh gas flow of 5 L/min in the Bain circuit, the humidity was 9.8-16.5 mg H2O/L depending on tidal volume and respiratory frequency. The humidity output of the circle absorber system with fresh gas flows of 0.5 or 2 L/min was 21.6-25.2 mg H2O/L at 60 min. Therefore, this study does not support previous studies, which propose that the Bain circuit has superior humidifying properties compared with those of the circle absorber system.