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Patients treated with a Thoratec HeartMate II left ventricular assist device (LVAD) are supported at a fixed pump speed. It is uncertain whether pump speed has a significant effect on exercise capacity. We investigated the relationship between pump speed and exercise capacity and the influence of residual LV functionWe exercised 30 patients 6 months after HeartMate II implantation at clinical pump speed (typically 9000 r.p.m.) and again at the lowest speed available (6000 r.p.m.). Overall, peak oxygen uptake (pkVO2) positively correlated with LV ejection fraction (LVEF) both at the clinical pump speed (r = 0.41, P = 0.03) and after pump speed reduction (r = 0.50, P = 0.01). We divided the patients into two groups; those with higher LVEF (LVEF ≥40%) and those with lower LVEF (LVEF <40%) at the time of exercise testing. The response to speed change was different between the two groups. In the higher LVEF group, the impact of LVAD pump speed reduction was minimal (pkVO2 21.4 ± 4.8 mL/kg/min vs. 20.8 ± 5.5 mL/kg/min, P = 0.38). In the lower LVEF group, the pkVO2 was lower at both speeds; 17.2 ± 5.3 and 14.7 ± 5.9 mL/kg/min, respectively. In the lower LVEF group, the pkVO2 decreased by 2.5 mL/kg/min (P = 0.02) with speed reduction.HeartMate II patients with lower residual LV function had a lower pkVO2 and were more sensitive to pump speed reduction. This suggests that modulation of LVAD speed during exercise could be of benefit to this group of patients.