Changes in respiratory drive account for the magnitude of dyspnoea during bronchoconstriction in asthmatics

To evaluate whether the interindividual differences in dyspnoea perceived by asthmatic subjects for the same level of airway narrowing could depend on different changes in respiratory drive, we assessed the relationship between changes in airway calibre, changes in neuromuscular output, and dyspnoea rate during progressive bronchoconstriction induced by methacholine. We studied 18 asymptomatic asthmatic subjects (aged 18-36 yrs; 11 males and 7 females) with normal lung function. Dyspnoea (Borg scale), mouth occlusion pressure (P0.1), and forced expiratory volume in one second (FEV1) were measured at baseline and after inhalation of aerosols of doubling concentrations of methacholine (MCh). The progressive bronchoconstriction induced by MCh was associated with a progressive increase both of P0.1 and dyspnoea. Dyspnoea score was linearly related either to the fall in FEV1, or to the increase in P0.1. However, the slope values of the relationship between dyspnoea score and the corresponding percent fall in FEV1 showed a large interindividual variability (0.05-0.32; coefficient of variability (CoV) 43%). By contrast, the slope values of the relationship between dyspnoea score and the corresponding percent increase in P0.1 ranged 0.02-0.05 (CoV = 14%), indicating a more homogeneous response to dyspnoea for the same change in P0.1. At the highest MCh concentration, the dyspnoea score was linearly related to the corresponding change in P0.1 (r = 0.91; p < 0.01), but not to the corresponding percentage fall in FEV1 (r = 0.28). These results show that the interindividual differences in dyspnoea perceived by asthmatic subjects for the same level of airway narrowing are associated with different changes in respiratory drive during bronchoconstriction.