Detection of Airway Obstruction in Exercise-Induced Asthma

Pulmonary function test results were evaluated in 24 bronchodilator-dependent asthmatic boys before treadmill exercise (baseline) and at 7 and 30 minutes afterward. The following pulmonary function parameters were measured: forced vital capacity, forced expiratory volume in one second, maximal midexpiratory flow, peak expiratory flow rate, thoracic gas volume, airway resistance, specific airway conductance, and closing volume. A clinical scoring system based on auscultatory findings was also used to determine the degree of wheezing and respiratory effort before, during, and after exercise. Results showed that specific airway conductance, maximal midexpiratory flow, and closing volume were the most sensitive parameters reflecting changes in airway caliber. Less significant changes also appeared in the forced expiratory volume in one second and in the forced vital capacity. Determination of closing volume, although a sensitive technique, could be achieved in only a minority of severely asthmatic children (25 percent of our patients). It was also evident that a normal peak flow reading and normal auscultatory findings did not necessarily imply a normally functioning lung. It was observed that the degree of exercise-induced asthma (EIA) depended on workload. Judged by the changes in maximal midexpiratory flow rate, airway resistance, and specific conductance, the incidence of EIA was 75 percent for workload 1, corresponding to 1.26 watts/kg of body weight and 100 percent for work load 2, corresponding to 1.75 watts/kg of body weight. Correlation coefficients calculated from absolute values between individual pulmonary function tests were lower for workload 1 but markedly higher in the case of workload 2.