THE EFFECT OF DEEP INSPIRATION ON METHACHOLINE DOSE-RESPONSE CURVES IN NORMAL SUBJECTS

Normal subjects develop exaggerated airway narrowing when deep inspira tion (DI) is voluntarily suppressed during methacholine challenge. Fai lure of periodic inflation may interfere with the bronchodilating effe ct of DI, acid this may be fundamental to the difference in bronchodil ation caused by DL in asthmatics and normal subjects. To determine whe ther repeated exhalations to residual volume (RV) and/or incomplete in spiration to baseline total lung capacity (TLC) could contribute to ex aggerated narrowing during challenge, we tested 10 subjects an three s eparate days using modified methacholine challenge protocols, On Day 1 , partial and complete flow volume curves were obtained after. each do se. On Day 2, DI was prohibited, but partial curves were performed. On Day 3, DI and exhalation to RV were prohibited, Tit was measured pre- and post-challenge on eack day, After comparable doses of methacholine , there was a greater change in FEV1 on Day 2 (27 +/- 15) and Day 3 (3 8 +/- 17) than on Day 1 (14 +/- 8) (p < 0.05). There were no differenc es 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 norma l subjects when DI is prohibited and that this effect is not due to re peated expiration to RV, nor due to an artifact caused by a failure to inhale to TLC.