Fresh gas flow in coaxial Mapleson A and D circuits during spontaneous breathing

In a lung model simulating spontaneously breathing halothane anaesthesia, the rebreathing characteristics of the coaxial Mapleson A (Lack circuit) and D (Bain circuit) systems were tested. Using decreasing fresh gas flows (VF), the end-tidal carbon dioxide fraction (FACO2) was monitored and the point of rebreathing (R.P.) detected. The effects of changes in minute volume (VE), dead-space to tidal volume ratio (VD/VT) and carbon dioxide elimination (VCO2) were studied. The effect of increased tidal volumes (VT) on FACO2 was investigated for some different fresh gas flows (VF). The VF/VE ratio for R.P. in the Bain circuit was approximately 2 and in the Lack circuit 0.88. In both circuits an increase in VE and a decrease in the VD/VT ratio resulted in higher demands on VF if rebreathing was to be avoided. The latter effect was much more pronounced in the Lack circuit. In neither system did any changes in VCO2 affect the rebreathing characteristics. The conclusion was drawn that the Lack system is a much better choice concerning the fresh gas flows for anaesthesia with spontaneous breathing than the Bain system. It was also concluded that the fresh gas flows recommended by Humphrey for the Lack system (i.e. 51 ml X min-1 X kg b.w.-1) and by the manufacturers for the Bain system (i.e. 100 ml X min-1 X kg b.w.-1) are inadequate and should be increased if a considerable degree of rebreathing is to be avoided.