Study of intrapulmonary gas mixing in man by simultaneous wash-out of helium and sulfur hexafluoride☆

In order to assess the role played by stratified inhomogeneity in limiting alveolar gas exchange, simultaneous wash-out of He and SF6 was measured in normal subjects. From the end-expired concentrations of He and SF6 of two successive breaths a separation index S.I., was calculated as a quantitative measure of test gas segregation. Typically wash-out of He was faster than that of SF6 (positive S.I.). About one half of the separation effect could be attributed to single breath dead space being higher for SF6than for He (mean difference 17 ml). The remaining part of S.I. was ascribed to stratification in alveolar space. With certain assumptions concerning the nature of the intrapulmonary gas mixing process (simple diffusion or diffusion combined with convection) the stratificational gas transfer conductance could be estimated. The stratificational conductance for O2 was estimated at 3 to 7 mmol · min−1 · torr−, resulting in stratificational PO2 (and PCO2) differences of 4 to 2 torr in resting conditions. The analytical approach and the experimental results are compared with previous studies.