Hemodynamic basis of the reduced oxygen uptake relative to work rate during incremental exercise in patients with chronic heart failure

Background: The ratio of the increase in oxygen uptake to the increase in work rate (ΔVO2/ΔWR) during incremental exercise is reduced in patients with severe chronic heart failure (CHF). However, the pathophysiological basis of the reduced O2 uptake relative to work rate has not been elucidated. Methods: To elucidate the hemodynamic basis of the reduced ratio of ΔVO2/ΔWR during exercise in severe CHF, 48 patients with CHF (15 patients in class I, 21 in class II and 12 in class III) performed maximal ergometer exercise with respiratory gas analysis. Cardiac output and systemic O2 extraction were measured at 1-min intervals during exercise. Results: Both peak VO2 and peak cardiac output decreased as the severity of CHF advanced. Patients in class III showed significantly reduced ΔVO2/ΔWR than those in class I (8.2±0.9 vs. 9.8±1.5 ml/min/W, P<0.01). Cardiac output at rest was significantly lower, and O2 extraction at rest was significantly higher in class III than class I. The ratio of the increase in cardiac output to the increase in work rate (ΔCO/ΔWR) was significantly lower in class III than class I (42.5±14.5 vs. 60.6±10.3 ml/min/W), and the ratio of the increase in O2 extraction to the increase in work rate (ΔO2 extraction/ΔWR) was significantly higher in class III than class I (0.45±0.13 vs. 0.34±0.08%/W). The ΔVO2/ΔWR was significantly correlated with the ΔCO/ΔWR (r=0.67, P<0.01), and the ΔCO/ΔWR was inversely correlated with ΔO2 extraction/ΔWR (r=−0.65, P<0.01). Conclusions: Decreased O2 supply due to reduced cardiac output was not fully compensated by the increased O2 extraction. Reduced ratio of ΔVO2/ΔWR in advanced CHF reflected the severely attenuated cardiac output response to exercise.