Effect of inhaled methacholine on inspiratory flow

One hundred consecutive outpatients with symptoms suggestive of asthma who came to the Pulmonary Function Laboratory for a methacholine challenge test were studied. In addition to the forced expiratory maneuvers, forced inspiratory maneuvers were performed before and after the maximal response to methacholine. In 24 patients, the methacholine challenge suggested that they had asthma (forced expiratory volume in 1 s [FEV₁] decrease ≥20 percent). Six of these 24 patients also had a decrease in maximal forced inspiratory flow (FIFmax) ≥20 percent and nine had a decrease in forced inspiratory flow at 50 percent of vital capacity (FIF50) ≥20 percent, suggesting that bronchoconstriction can cause decreased inspiratory as well as expiratory flows. In 76 patients, the methacholine challenges were 'negative' (FEV₁ decrease ≤20 percent), suggesting that they did not have asthma. Nevertheless, in 11 of these 76 patients the FIFmax decrease was ≥20 percent, and in 14 patients the FIF50 decrease was ≥20 percent, suggesting that intermittent central airway obstruction is responsible for these patient's symptoms.