Quality of dyspnea in bronchoconstriction differs from external resistive loads

To test the hypothesis that patients perceive the same quality of dyspnea d uring mild bronchoconstriction and external resistive loads, we studied sub jects with asthma under two conditions: (I) during methacholine bronchoprov ocation to mimic the bronchospasm of mild asthma and (2) while breathing on a circuit to which was added a range of external resistors to mimic the me chanical load of mild asthma. During each of these stimuli, respiratory var iables, overall dyspnea intensity on a modified Borg scale, and the qualita tive descriptors of breathlessness from a 19-item questionnaire were assess ed. The "chest tightness" and "constriction" responses were significantly m ore frequent in the methacholine trials as compared with the external load trials (p < 0.0001). The "chest tightness" or "constriction" response was c hosen during 92% of the 26 trials of methacholine bronchoconstriction compa red with 3% of the 72 trials of breathing against the external resistors, C hanges in functional residual capacity were not significantly different bet ween the two conditions. We conclude that in mild asthma, the sensation of chest tightness is distinct from the sensation of work and effort and is no t attributable to the mechanical load imposed on the respiratory system.