Time dependence of maximum flow as an index of nonuniform emptying

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Abstract
Nonhomogeneous human lungs are predicted to exhibit time dependence of maximal expiratory flow (.ovrhdot.Vmax): faster regions would contribute more flow early in the expiration, whatever the initial volume, resulting in different .ovrhdot.Vmax at a given total lung volume, depending on how long flow limitation has been operating. To test this concept, a new technique was developed that permits accurate superimposition of .ovrhdot.Vmax data over small volume intervals. When quick-release interrupted partial curves with similar volume history but different volume of initiation were compared over their late common-volume segments, the volume history effects could be differentiated from time dependence of the .ovrhdot.Vmax. Such time dependence was found in 7 of 7 bronchitics, 3 of 5 smokers and 0 of 14 nonsmokers. Emptying during the forced expirations is not uniform even in mild disease states and the time dependence of .ovrhdot.Vmax is a sensitive test of lung inhomogeneity.