Importance of the time interval between FEV1 measurements in a methacholine provocation test.

We examined the hypothesis that a forced expiratory volume in one second (FEV1) manoeuvre (and the preceding deep inhalation) before inhalation of methacholine might influence FEV1 measured after methacholine, if the time between measurements was short. Six to nine healthy subjects inhaled a single dose of methacholine, known to cause about 20% decrease in FEV1, on different days in different test protocols. If an FEV1 manoeuvre was performed immediately before methacholine, the first FEV1 measured 3 min after provocation was higher (77% of basal FEV1) than if a pre-methacholine FEV1 manoeuvre was not performed (64%). This effect of a pre-methacholine FEV1 manoeuvre was also demonstrated at 2, 4 and 6, but not at 10 min after the start of methacholine inhalation. If an FEV1 manoeuvre was not performed before methacholine, the second and subsequent FEV1 measured in constricted airways was higher than the first, and of similar magnitude to the first FEV1 in tests where a pre-challenge FEV1 manoeuvre was performed. In another trial, 10 healthy subjects performed two stepwise methacholine tests, with either 6 or 3 min between dose steps. The percentage decrease in FEV1 per mg of inhaled methacholine decreased from 2.6 (1.9-5.2) to 1.7 (0.8-2.3) (median, interquartile-range) when the time interval was shortened. The results suggest that the deep inhalation associated with the FEV1 manoeuvre decreases the bronchial tone in airways constricted by methacholine for up to 6 min, possibly due to yielding of cross-links in airway smooth muscles.