Locus of symptom limitation and exercise response to bronchodilation in chronic obstructive pulmonary disease.

PURPOSE To evaluate the concordance between subjective and objective indices of muscle fatigue during exercise and to assess the significance of the perception of dyspnea and leg fatigue for the exercise response to bronchodilation in chronic obstructive pulmonary disease (COPD). METHODS Sixty-eight patients with COPD performed either 2 constant work-rate cycling exercises or 2 endurance shuttle walking tests. These tests were preceded by nebulization of placebo or 500 mug of ipratropium bromide. Changes in forced expiratory volume in 1 second and in endurance time with bronchodilation were measured. Changes in quadriceps twitch force after exercise were evaluated. In addition, the locus of symptom limitation was assessed. RESULTS Patients who stopped exercising because of leg fatigue showed a larger fall in quadriceps twitch force in comparison with patients who stopped for dyspnea. The proportion of patients who developed contractile fatigue of the quadriceps (postexercise fall in quadriceps twitch force >15% resting value) was substantially smaller in patients stopping exercise because of dyspnea than in those stopping because of leg fatigue or a combination of the 2 symptoms. The locus of symptom limitation modulated the exercise response to bronchodilation; patients stopping exercise because of leg fatigue or a combination of dyspnea/leg fatigue showed a smaller improvement in endurance time to constant work-rate exercise with bronchodilation compared with those stopping because of dyspnea. CONCLUSION Patients with COPD reporting leg fatigue as the main exercise-limiting symptom had a smaller increase in endurance time to constant work-rate exercise after bronchodilation compared with those reporting dyspnea as the main limiting symptom.