The effect of deep inspiration on methacholine dose-response curves in normal subjects

Normal subjects develop exaggerated airway narrowing when deep inspiration (DI) is voluntarily suppressed during methacholine challenge. Failure of periodic inflation may interfere with the bronchodilating effect of DI, and this may be fundamental to the difference in bronchodilation caused by DI in asthmatics and normal subjects. To determine whether repeated exhalations to residual volume (RV) and/or incomplete inspiration to baseline total lung capacity (TLC) could contribute to exaggerated narrowing during challenge, we tested 10 subjects on three separate days using modified methacholine challenge protocols. On Day 1, partial and complete flow volume curves were obtained after each dose. On Day 2, DI was prohibited, but partial curves were performed. On Day 3, DI and exhalation to RV were prohibited. TLC was measured pre- and post-challenge on each day. After comparable doses of methacholine, there was a greater change in FEV1 on Day 2 (27+/-15) and Day 3 (38+/-17) than on Day 1 (14+/-8) (p < 0.05). There were no differences in changes in FEV1 and FVC between Days 2 and 3, or in TLC between all 3 d. We conclude that exaggerated airway narrowing occurs in normal subjects when DI is prohibited and that this effect is not due to repeated expiration to RV, nor due to an artifact caused by a failure to inhale to TLC.