Forced Expiratory Time and Bronchial Hyperresponsiveness to Methacholine

Abstract

Pulmonary function tests (PFTs) are normally performed prior to methacholine inhalation challenges (MICs). In contrast to normal baseline spirometry (FEV1, FEF25%–27%, FVC), we have observed patients with positive MICs having shortened forced expiratory times (FET100%) in the baseline pre-MIC PFT. We prospectively evaluated the correlation of abnormalities in baseline pre-MIC FET100% in patients who have positive vs. negative MICs. Prospective analysis of baseline pre-MIC FET100% and MIC results in suspected asthmatics with normal lung exams, spirometry and chest x-rays. Using a PC20 FEV1 of ≤ 8 mg/ml methacholine, there were 115 positive and 69 negative MICs. The mean (±1 SD) FET100% in the positive MIC group was 3.57 ± 1.68 sec vs. 4.73 ± 1.60 sec in the negative group. The difference in these means was statistically significant (p < 0.0001). There was a statistically significant difference in the incidence of FET100% < 4 sec in the positive (55.65%) vs. the negative (30.43%) MIC group, p < 0.001. There was also a statistically significant difference in the incidence of positive MIC in FET100% < 4 sec (75.29%) vs. FET100% ≥ 4 sec (51.52%), p < 0.001. Our results suggest that in our highly selected, well-characterized population, FET100% < 6 sec is common and FET100% < 4 sec correlates with an increased likelihood of having a positive MIC.