CHRONIC EXERTIONAL DYSPNEA AND RESPIRATORY MUSCLE FUNCTION IN PATIENTS WITH CHRONIC OBSTRUCTIVE PULMONARY-DISEASE

The symptom of breathlessness is an important outcome measure in the m anagement of patients with chronic obstructive pulmonary disease (COPD ). Clinical ratings of dyspnea and routine lung function are weakly re lated to each other. However, in the clinical setting breathlessness i n COPD is encountered under conditions of increased respiratory effort , impeded respiratory muscle action, or functional weakness. Thus, the present study was carried out to determine whether and to what extent clinical ratings of dyspnea and respiratory muscle dysfunction relate to each other. In 21 patients with COPD two methods were used to rate dyspnea: a modified Medical Research Council Scale (MRC) and the Base line Dyspnea Index (BDI), which is a multidimensional instrument for m easuring dyspnea based on three components: magnitude of task, magnitu de of effort, and functional impairment. A baseline focal score was ob tained as the sum of the three components. Measures were: pulmonary vo lumes; arterial blood gases; maximal voluntary ventilation (MVV); maxi mal inspiratory and expiratory pressures (MIP and MEP, respectively); and breathing patterns ventilation (V-E), tidal volume (V-T), and resp iratory frequency (Rf). In 15 patients pleural pressure was also measu red during both quiet breathing (Ppl(sw)) and maximal inspiratory snif f maneuver at FRC (Pplsn). BDI and MRC ratings related to each other a nd showed comparable weak associations with standard parameters (FEV1, Paco(2), V-T), MIP, and MEP. In contrast, MVV closely and similarly r elated to both ratings. Ppl(sw) (%Ppl(sn)), a measure of respiratory e ffort, and Ppl(sw) (%Ppl(sn))/V-T(%VC), an index of neuroventilatory d issociation, related significantly to both the BDI (r(2) = -0.77 and r (2) = -0.75, respectively) and the MRC (r(2) = 0.81 and r(2) = 0.74, r espectively). Using MVV, Ppl(sw) (%Ppl(sn)), and Ppl(sw) (%Ppl(sn))/V- T(%VC) in a stepwise multiple regression as independent variables with BDI rating as dependent variable, MVV explained an additional 14.5% o f the variance of the BDI over the 67.8% predicted by Ppl(sw) (%Ppl(sn )). Our results demonstrate that the level of chronic exertional dyspn ea in COPD increases as the ventilatory muscle derangement increases. The level of the relationships among dyspnea ratings and MVV and respi ratory effort helps to explain some of the mechanisms of chronic dyspn ea of COPD. These measures should be considered for therapeutic interv ention to reduce dyspnea.