Tracheal size is a determinant of the bronchoconstrictive response to inhaled methacholine

We hypothesized that the size of the large airways is a determinant of bronchial responsiveness to inhaled methacholine. This was investigated by measuring the relationship of tracheal size and branching angle to the bronchoconstrictive response to methacholine, in 169 male construction insulators, aged 20-50 yrs, as part of a workforce-based, cross-sectional survey of respiratory health. Bronchial responsiveness was expressed as the concentration of methacholine, inhaled for 2 min of tidal breathing, which provoked a 15% fall in forced expiratory volume in one second (FEV1). Tracheal size was assessed from tracings of standard posteroanterior chest radiographs, at full inspiration. After accounting for the effect of airway calibre (FEV1/forced vital capacity (FVC)), age, height, and pack-years of cigarette smoking, on airways responsiveness, there was a significant association (p < 0.05) between tracheal length, diameter, or surface area and the degree of bronchoconstriction obtained by inhaling methacholine. The increase in airway responsiveness with decreasing tracheal size may reflect increased deposition of methacholine, secondary to smaller cross-sectional area and greater linear velocity of air in the trachea and main bronchi.