Pulmonary gas exchange during altered density gas breathing.

The alveolar-arterial O2 partial pressure difference (PAO2 - PaO2) has been shown to decrease as carrier-gas density increases. This study was designed to confirm or deny the hypothesis that the improvement in O2 exchange is a result of density-dependent changes in the alveolar ventilation-perfusion (VA/Q) distribution. On changing from heliox breathing 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 interaction 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 of gases inhaled and taken up by the blood.