Differences in Physiological Response to Exercise in Patients With Different COPD Severity

BACKGROUND: Patients with COPD have reduced exercise tolerance associated with dyspnea. This exercise intolerance is primarily due to impaired ventilatory mechanics, but it is also associated with a combination of factors, including inefficient gas exchange, lactic acidosis at a low work rate, and exercise-induced hypoxemia. The survival prognosis of COPD patients with severely reduced exercise capacity is extremely poor, but the pathophysiology of these patients during exercise remains to be accurately established. The present study aimed to characterize life-threatening factors such as hypoxemia, acidosis, and sympathetic activation during exercise in these patients. METHODS: We monitored changes in life-threatening factors and compared these factors among quartile groups, defined according to their peak oxygen uptake status. Ninety-one COPD subjects (82 males, 9 females, average age 69.7 +/- 6.8 y) consecutively underwent incremental cardiopulmonary exercise testing using a cycle ergometer. Arterial blood gases, lactate, and catecholamines were measured during cardiopulmonary exercise testing. RESULTS: The pathophysiology of the COPD differed among the 4 subject groups. Subjects with the most severely reduced exercise capacity (peak oxygen uptake <= 623 mL/min) were characterized by exercise-induced steep decrease in P-aO2 slope (-78 +/- 70 mm Hg/L/min), rapid progression of respiratory acidosis, little change in lactic acidosis, and sympathetic activation at low-intensity work load (plasma norepinephrine 1.41 +/- 0.94 ng/mL at 20 watts work load), in addition to the limitation of increase in ventilation and impaired gas exchange. CONCLUSIONS: The mechanisms of exercise intolerance in COPD patients significantly differed among subjects with different exercise capacities. Subjects with the most severely reduced exercise capacity had the characteristics of exercise-induced hypoxemia, sympathetic overactivity, and progressive respiratory acidosis at low-intensity exercise. These life-threatening pathophysiological conditions could be improved by medication and/or pulmonary rehabilitation.