Cyclic perfusion of the lung by dense gas breathing may reduce the (A-a)DO2

The decrease in the alveolar-arterial O2 difference (A-a)Do2 in dense atmosphere could be the result of cyclic lung perfusion due to large swing in pleural pressure during breathing cycles /4/. Using a mathematical model of the lung to calculate (A-a)Do2, a slight decrease was demonstrated in (A-a)Do2 as perfusion changed from steady to cyclic. The present study incorporates variable vertical partitioning of perfusion in this model, as a function of instantaneous total blood flow. This demonstrated a reduction of the Po2 difference between the apex and the base of the lung, and a reduction of (A-a)Do2, when perfusion was assumed to be cyclic, of 4.1 to 5.0 torr when blood flow was continuous throughout the breath, and of 5.0 to 5.7 torr when perfusion was assumed to be pulsatile. The results agree with experimental findings: reduction of (A-a)Do2 when breathing dense gas and improved apical perfusion in pulsatile blood flow. Calculation of the spatial VA/Q inequality suggests there is no correlation with (A-a)Do2 reduction. The decrease in (A-a)Do2 is attributed to the mixing of lung capillary blood flows having different O2 saturations.