Cardiac output does not limit submaximal exercise capacity in patients with chronic heart failure

Mechanisms of exercise limitation in patients with chronic heart failure (CHF) are incompletely understood. During matched submaximal, fixed-rate exercise, oxygen uptake is similar in patients and healthy controls. However, the importance of cardiac output (CO) remains unresolved. We aimed to determine the effect of submaximal exercise on CO and other haemodynamic variables in patients with CHF using a validated non-invasive inert gas rebreathing system.

Seventy-two subjects with a mean age (+/-SD) of 68.2 (+/-8.1) years, performed fixed-rate exercise for 3 min at 15, 30, 45, and 60 W workloads on a cycle ergometer. Cardiac output/index (CI) and oxygen uptake (VO(2)) were determined at each stage by inert gas rebreathing. Subjects with systolic HF (n = 27) were compared with those without (n = 45). Cardiac index was lower in subjects with CHF at rest and throughout exercise. VO(2) was the same for both groups at rest and during exercise. There was no difference in the relative or absolute increase in CI from rest to 60 W (1.70 +/- 0.69 vs. 1.99 +/- 0.56 L/min/m(2), P = 0.102, respectively). Arterio-venous O(2) saturation difference at peak exercise was 75.4 +/- 10.4 vs. 63.0 +/- 12.1%, P = 0.001, for CHF and non-CHF subjects, respectively.

During submaximal exercise, patients with systolic heart failure are able to increase their CO to a similar extent as those without; with equal levels of oxygen consumption, but requiring a much greater degree of tissue oxygen extraction.