Study objectives: The American Thoracic Society guidelines for methacholine-induced airway hyperresponsiveness include a ≥ 20% reduction in FEV1 or a ≥ 40% reduction in specific airway conductance (sGaw). The objectives of the current study are to assess the concordance between these two criteria and to characterize the pulmonary function and respiratory symptoms of patients with different patterns of methacholine hyperresponsiveness.
Study design: A prospective study of 248 consecutive patients referred for methacholine bronchoprovocation testing.
Results: Positive methacholine hyperresponsiveness was noted in 179 patients; 139 patients (78%) had a ≥ 20% reduction in FEV1, whereas 40 patients (22%) had a ≥ 40% reduction in sGaw alone without a significant change in FEV1. The former group had the following: (1) higher baseline lung volumes, (2) lower baseline values of FEV1 and sGaw, (3) forced expiratory flow between 25% and 75% of vital capacity (FEF25–75)/FVC ratios compared to patients with a reduction in sGaw alone (0.72 ± 0.26 vs 0.97 ± 0.28, mean ± SD; p < 0.0001), and (4) more frequent presence of wheezing and chest tightness (p < 0.05).
Conclusions: First, a substantial number of patients have a reduction in SGaw alone in response to methacholine, and secondly, this response is seen in patients with a higher FEF25–75/FVC ratio. Since the FEF25–75/FVC ratio is thought to be an index of airway size relative to lung size, we speculate that the larger intrinsic airway size relative to lung size may explain the differences in baseline parameters and patterns of methacholine hyperresponsiveness.