Backgrounds Peak exercise cardiac output (CO) increase is associated with an increase of peak oxygen uptake (VO2), provided that arteriovenous O2 difference [Δ(Ca − Cv)O2] does not decrease. At anaerobic threshold, VO2, is related to CO. We tested the hypothesis that, in heart failure (HF) patients with left ventricular assistance device (LVAD), an acute increase of CO obtained through changes in LVAD pump speed is associated with peak exercise and anaerobic threshold VO2 increase. Methods Fifteen of 20 patients bearing LVAD (Jarvik 2000) enrolled in the study successfully performed peak exercise evaluation. All patients had severe HF as shown by clinical evaluation, laboratory tests, echocardiography, spirometry with alveolar-capillary diffusion, and maximal cardiopulmonary exercise testing (CPET). CPETs with non-invasive CO measurements at rest and peak exercise were done on 2 days at LVAD pump speed set randomly at 2 and 4. Results Increasing LVAD pump speed from 2 to 4 increased CO from 3.4 ± 0.9 to 3.8 ± 1.0 L/min (ΔCO 0.4 ± 0.6 L/min, p = 0.04) and from 5.3 ± 1.3 to 5.9 ± 1.4 L/min (ΔCO 0.6 ± 0.7 L/min, p