Rebreathing, resistance and external work of breathing in three different coaxial Mapleson D systems

Using a lung model, rebreathing characteristics, resistance against gas flow and the external work of breathing were tested in three different coaxial Mapleson D systems: the Medicvent D system, the Bain original system and the Coax-II system. The rebreathing characteristics were found to be similar in all systems in both spontaneous and controlled ventilation. The Bain system was found to have the lowest resistance and work of breathing and the Coax-II system the highest. The differences were small and clinically insignificant. Both the resistance and the work of breathing increased with fresh gas flow. The resistance against expiration was found to be in the range 135-160 Pa at a total gas flow of 31 1.min-1, which is well within the acceptable level. The resulting end-expiratory pressure was never above 100 Pa (1 cmH2O) in any system. We concluded that there was no clinically significant difference among the three systems despite differences in design. The coaxial Mapleson D systems can also be used safely with high fresh gas flows with regard to resistance and end-expiratory pressures.