Measurement of lung emptying patterns during slow exhalations.

Five subjects slowly inhaled a 200-ml bolus of sulfur hexafluoride (SF6) from residual volume (RV) followed by an O2-Ar mixture to total lung capacity, then exhaled to RV, either slowly or as rapidly as possible. Larger amounts of SF6 and N2 were recovered in fast than in slow exhalations. We calculated the gas volumes of the apical and basal halves of the parenchymal mass as functions of exhaled volume during slow exhalations from 1) the difference between SF6 recovered in slow and fast exhalations and 2) an estimate of the apex-to-base concentration profile of SF6 in the lung after inspiration. The maximal volume difference, where the apex contained 600 ml more gas than the base, occurred when 70% of the vital capacity had been exhaled. The same calculation, but using N2 data, gave unrealistically large volume differences. Apparently SF6 delivered as a bolus results in an apex-to-base gradient that is large relative to intraregional gradients, but dilution of the resident N2 by a non-N2 gas results in sizable intraregional gradients.