Analysis of the configuration of maximum expiratory flow-volume curves.

The ratios of the instantaneous tangent slopes to corresponding chord slopes are sensitive to curvatures of flow-volume curves; and these dimensionless slope ratios, SR, plotted against fractional volumes remaining to be expired, VCf, are sensitive to the shape of flow-volume curves but insenitive to flow or volume magnitudes. SR-VCf curves predicted for lungs which empty homogeneously differ substantially from those predicted for lungs which empty nonhomogeneously, the latter showing systematic increases in SR with VCf which may extend beyond the homogeneous range. SR-VCf curves for some 80 healthy subjects show systematic changes with age (range 6-64 yr), but all are consistent with advanced obstructive lung disease (chronic bronchitis, emphysema, asthma) are consistent with nonhomogeneous emptying, but it does not seem likely that SR-VCf curves will prove useful for detecting early stages of disease.