Different respiratory behaviors disclosed by induced bronchoconstriction in mild asthma patients

Respiratory responses to bronchoconstriction in asthma have been partially assessed and their significance is unclear. In 44 mild asthma patients we investigated respiratory responses during increasing levels of methacholine-induced bronchoconstriction. Inspiratory muscle activity, tidal volume, inspiratory and expiratory times were continuously monitored; breathing discomfort was rated. Mean inspiratory flow, respiratory frequency and ventilation were calculated. Lung function was assessed prior to and at maximum bronchoconstriction. Bronchoconstriction "dose-dependently" increased inspiratory muscle activity and breathing discomfort (P<0.01). In 37 patients (84.1%), the increase in inspiratory muscle activity was associated with increases in mean inspiratory flow and ventilation (P<0.01) because of selective rises in breathing depth (volume responders), or rate (frequency responders) or both (dual responders). In seven patients (15.9%) ventilation was unchanged. Individual respiratory responses were reproducible. With bronchoconstriction, frequency responders displayed greater hyperinflation and stronger breathing discomfort than volume responders (P<0.01). Analysis of the responses to induced bronchoconstriction disclosed distinctive and reproducible respiratory adjustments that may identify functionally different asthma subpopulations.