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Objective Chronic obstructive pulmonary disease (COPD) increases the risk of cardiovascular problem.The symptom of dyspnea on exertion may be associated with pulmonary dysfunction or heart failure, or both. The study objective was to determine whether cardiac dysfunction adds to the mechanism of dyspnea caused mainly by impaired lung function in patients with mild-to-moderate COPD. Methods Patients with COPD and healthy controls performed incremental and constant work rate exercise testing. Venous blood samples were collected in 19 COPD patients and 10 controls before and during constant work exercise for analysis of N-terminal-pro-BNP (NT-pro-BNP). Results Peak oxygen uptake and constant work exercise time (CWET) were significantly lower in COPD group than in control group (15.81±3.65 vs 19.19a±6.16 ml/min kg, P=0.035 and 7.78±6.53 rain vs 14.77±7.33 min, P=0.015, respectively). Anaerobic threshold, oxygen pulse and heart rate reserve were not statistically significant between COPD group and control group. The NT-pro-BNP levels both at rest and during constant work exercise were higher in COPD group compared to control group, but without statistical significance. The correlations between CWET and NT-proBNP at rest or during exercise in patients with COPD were not statistically significant. Conclusions Heart failure does not contribute to exercise intolerance in mild-to-moderate COPD.