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

With a computer technique variability of the configuration of maximum expiratory flow-volume (MEFV) curves was studied in terms of slope ratio, SR. SR = dV/dV divided by V/V, where V is the instantaneous flow and V is the volume increment above residual volume.) Approximately four SR-V curves, each based on three to five smoothed and averaged MEFV curves, were derived for each of 20 normal subjects (aged 23-55 yr) on a single occasion, and again at least 1 wk later. Individual curves were largely reproducible, the maximum difference in SR at most volumes being 0.3-1 U, but frequently showed substantial yet reproducible fluctuations with volume. These corresponeded to hitherto unrecognized irregularities of maximum expiratory flow that may reflect sudden changes in the location of flow limitation.