Relative sensitivity of different pulmonary function tests in the evaluation of exercise-induced asthma

Sixty asthmatic children were exercised on a bicycle ergomete rand had pulmonary function tests perormed before and repeatedly after exercise. Pulmonary function measurements included airway resistance (Raw), specific airway conductance (SGaw), functional residual capacity (FRC), peak expiratory flow rate (PEFR), maximum mid-expiratory flow (MMEF), forced expiratory volume during first second of expiration (FEV1), and forced vital capacity (FVC). At any one time during the post-exercise observation period decreases in SGaw were greater than changes in any other pulmonary function test, making SGaw the most sensitive test for the detection of exercise-induced airway obstruction in asthmatics. Beyond five minutes after exercise PEFR and MMEF were reduced by exercise approximately equally, but somewhat less often and less markedly than SGaw. Exercise-induced reductions in FEV1 were less marked and less frequent than decreases in PEFR and MMEF, and reductions in FVC were the least severe and least often observed abnormality. Decreases in SGaw were significantly, but not linearly correlated with decreases in PEFR, MMEF, FEV1, FVC, and FEV1/FVC. There were statistically significant linear correlations between exercise-induced increases in FRC and decreases in FVC and between increases in Raw and FRC. If we accept that increases in Raw and FRC indicate increases in large and small airway obstruction respectively, exercise-induced decreases in FVC may indirectly suggest acute hyperinflation and thus small airway obstruction. Although the positive correlation between Raw and FRC indicated that both large and small airway obstruction developed after exercise in many of our asthmatics, increases in Rae were usually greater than increases in FRC, suggesting that large airway obstruction tends to be greater than small airway obstruction in exercise-induced asthma.