Detection of excessive bronchoconstriction in asthma.

Airway hyperresponsiveness is easily assessed by measuring the concentration or dose of an inhaled agonist that produces a defined response, e.g., PC20 or PD20. However, this measure does not assess excessive bronchoconstriction. We report the results of analyzing bronchial dose-response curves by measuring percent fall in vital capacity (delta FVC%) as the response rather than the PC20. In our analysis, delta FVC% was measured at the PC20, and therefore it was the dependent variable, whereas the concentration of agonist was the independent variable, in contrast to the usual bronchoprovocation tests in which the response is the independent variable and the dose is the dependent variable. We reasoned that a dose-dependent increase in gas trapping with histamine would detect excessive bronchoconstriction as a decrease in FVC; in contrast, PC20 measures only the ease of bronchoconstriction. In 10 patients with mild asthma the reproducibility of delta FVC% when FEV1 fell by 20%, i.e., at the PC20 concentratio...