The association between baseline lung function and bronchial responsiveness to methacholine.

It has been reported that females show an increased frequency of bronchial hyperresponsiveness (BHR) compared to males and that this difference is abolished after taking into account differences in baseline FEV1. The aim of our study was to analyse how the distribution of BHR in males and females depends on the definition of BHR. Special emphasis was paid to the question whether the prevalence rates of BHR according to different definitions were related to baseline characteristics of the subjects and baseline lung function in the same manner. We analysed the data obtained within the European Community Respiratory Health Survey (ECRHS) in the Eastern German population sample of Erfurt aged 20-65 years (n = 931). In logistic regression analyses of different definitions of BHR, we used as parameters age, height, gender, smoking habits, baseline forced expiratory volume in one second (FEV1), forced vital capacity (FVC), and FEV1 as a percent of FVC (FEV1% FVC). Symptoms and reported diagnosis of asthma did not significantly depend on gender or age. When BHR was defined as the provocative dose causing a 20% fall in FEV1, BHR was more prevalent in females than in males (27.6% vs. 13.2%). Similar gender differences were found when defining BHR via a 10% fall in FEV1 or by using corresponding cut-off values of the linear dose-response slopes of the percent decline in FEV1 (DRS). Multiple linear regression analyses of various transformations of the DRS also indicated a higher degree of BHR in females. Independently of the definition chosen, however, the gender difference in the prevalence of BHR disappeared when height and FEV1 or FEV1% FVC or appropriate combinations were included in the model. The reciprocally transformed DRS showed the best resolution of the spectrum of bronchial responsiveness. These data are compatible with the hypotheses that (1) estimates of the distribution of BHR are distorted by differences in the methacholine dosage per lung size and that (2) airway geometry affects the measurement of BHR. It appears that these factors and not intrinsic differences in BHR between males and females contribute to the gender differences in the prevalence of BHR. Furthermore, our data support the superiority of the dose response slope for the analysis of bronchial responsiveness in epidemiologic surveys.