Changes in Oxygen Uptake-Work Rate Relationship as a Compensatory Mechanism in Patients with Heart Failure

Abstract

To assess the compensatory change in oxygen uptake (VO2) kinetics during exercise in patients with heart failure, we performed cardiopulmonary exercise testing using a cycle ergometer in 29 cardiac patients and 18 normal subjects. The work rate increased linearly (1w/6sec) after a 4min, 20w warm-up. The ratio of increase in VO2 to increase in work rate (ΔVO2/ΔWR) was determined by linear regression of VO2 plots. Cardiac output by dye dilution method and plasma norepinephrine concentration (NE) were measured at rest and 20w warm-up. VO2 at rest and at 20w warm-up and ΔVO2/ΔWR decreased significantly with increasing severity in NYHA functional classification. ΔVO2/ΔWR showed significant negative correlation to the difference in NE levels at rest and at 20w (r= -0.69, p<0.001). The delta values of arteriovenous O2 content difference, calculated using Fick's principle, and cardiac index from rest to 20w warm-up failed to show significant relationship to ΔVO2/ΔWR. These results suggest that the decrease in VO2 requirement in heart failure patients is due probably to blood redistribution during exercise as a compensatory mechanism for exercise intolerance. © 1992, The Japanese Circulation Society. All rights reserved.