Fractional delivery of fresh gas: A new index of the efficiency of semi-closed breathing systems

In earlier clinical studies, we have found a significant difference in the fractional utilization of fresh gas (Fu) when using an enclosed afferent reservoir breathing system (EAR) for adult patients compared with children This difference was explained by a large arterial to end-tidal carbon dioxide difference in the adults, reflecting a larger alveolar deadspace. In the present study, a new index of breathing system efficiency, fractional delivery of fresh gas (Fd) is proposed, which is independent of alveolar deadspace. In order to demonstrate this, values of Fd were calculated for the EAR during controlled ventilation of adults, children and a lung model. There were no significant differences between the groups. A maximum efficiency of 0.94 for the EAR was close to the theoretical limit of 1.0 predicted by the results at minute volume ventilation to fresh gas flow ratio (VE:VF) values greater than 2.0. For adult patients, the values of Fd were shown to be significantly greater than the values of Fu at the same VE:VF ratio (Fd = 0.91, Fu = 0.72 at VE:VF = 2.0 (P <0.05)).