The significance of volume-adjusting the maximal midexpiratory flow in assessing the response to a bronchodilator drug.

Forced expiratory spirograms were obtained before and for six hours after 25 subjects ingested ephedrine and placebo in a double-blind crossover study. Significant changes in the forced vital capacity (FVC) and the forced expiratory volume in one second (FEV1.0) were noted on days when ephedrine was administrated, while the mean forced expiratory flow during the middle half of the FVC (FEF25-75%) failed to indicate significant bronchodilation. When FVC increases after therapy with a bronchodilator drug, one is no longer measuring flow during the same volume segment and driving pressure (static transpulmonary pressure [Pst]) as before administration of the bronchodilator drug. Volume-adjusting the FEF25-75% after therapy to the same volume and Pst over which flow is being measured in the tracings before bronchodilator therapy yielded highly significant increases in flow after administration of the bronchodilator drug.