M: Pulmonary gas exchange during altered density gas breathing

difference (PACT- PG,) has been shown to decrease as carrier-gas density increases. This study was designed to confum or deny the hypothesis that the improvement in 02 exchange is a result of density-dependent changes in the alveolar ventilation-perfusion (VA/Q) distribution. On changing from heliox breath-ing to air breathing, there was an improvement in oxygen exchange along with a slight worsening of VA/Q distribution. The conclusion is reached that changes in VA/Q distribution due to altered carrier-gas density are not responsible for changes in O2 exchange. A possible explanation is related to the inter-action of diffusion and convection on inspiration, which may cause inspired gas distribution to be different from overall ventilation distribution. The interesting implication is that gas exchange properties of gases eliminated from the blood and exhaled are not necessarily symmetrical to the properties o