Estimation of effective parabronchial gas volume during intermittent ventilatory flow: theory and application in the duck.

The difference in gas exchange performance between continuous and intermittent ventilatory flow is theoretically studied in the alveolar lung model. When measurements obtained with intermittent flow are analyzed assuming continuous flow, an apparent diffusing capacity. Dapp, results which is an underestimate of the true value, D. This true value, D, may be assessed from measurements at continuous flow. With decreasing effective lung gas volume, Veff, Dapp increasingly deviates D. The dependence of Dapp/D on Veff in the parabronchial lung seems to be similar to that in the alveolar lung. Experimental data of Dapp/D (Scheid et al., 1977) are used to assess Veff for the duck lung. The average value of Veff thus obtained, 93 ml, exceeds the anatomical estimate of parabronchial gas volume. Gas transfer across the open parabronchial ends may contribute in enlarging the parabronchial gas volume to the volume, Veff, that is effective as gas capacity in conditions of non-steady ventilatory flow.