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In pulmonary arterial hypertension (PAH), the exercise-induced increase in stroke volume (SV) is limited by the increase in pulmonary artery pressure. In left heart failure (LHF), systemic arterial pressure increases little during exercise, and the SV increase is limited by the left ventricle itself. These differences might be reflected by a dissimilar SV and heart rate (HR) response to exercise, which could have important therapeutic implications, for example in beta-blocker therapy. Therefore, we tested the hypothesis that SV and HR responses during exercise are different between PAH and LHF patients.We included 28 PAH and 18 LHF patients (recruited from the heart failure unit) matched on a maximal oxygen uptake of <15 mL/kg/min, who were referred to our Pulmonary Function Department between 2000 and 2008 for a maximal cardio-pulmonary exercise test. Only patients who had not been exposed to beta-blockers were included. Pulmonary arterial hypertension and LHF patient groups had equally impaired exercise tolerance (about 42% of predicted) with a maximal oxygen uptake of 0.80 ± 0.29 and 0.86 ± 0.19 L/min. The peak SV response to exercise was significantly lower in PAH patients (−14 mL, P = 0.01); this was compensated by a steeper slope of HR relating to oxygen uptake (0.03 beats/mL, P = 0.001).We conclude that PAH patients have a smaller SV response, but a larger HR response than LHF patients.