Relationship Between Impaired Chronotropic Response, Cardiac Output During Exercise, and Exercise Tolerance in Patients with Chronic Heart Failure

Abstract

The present study was undertaken to investigate the relationship between the extent of impaired chronotropic response and cardiac output during exercise, and exercise tolerance in patients with chronic heart failure. The subjects consisted of 24 patients (mean 60.1 ± 14.0 years) who had mild chronotropic incompetence. Cardiopulmonary exercise testing was performed in all patients, and heart rate (HR), anaerobic threshold (AT), maximum oxygen uptake (peak VO₂), slope of the regression line relating the ventilatory equivalent to carbon dioxide output (VE/VCO₂ slope), and exercise time were measured. Cardiac output (CO) was measured by a thoracic bioimpedance method and cardiac index (CI) was calculated. Plasma norepinephrine (NE) was measured at rest and immediately after the exercise test. The changes in HR, NE, and CI from the resting state to immediately after exercise were calculated as ΔHR, ΔNE, and ΔCI, respectively. The ΔNE was converted to a logarithmic scale and ΔHR/log ΔNE was used as a parameter of HR response to sympathetic nerve stimulation. The results were as follows: HR and NE in the resting state had no correlation with AT and with peak VO₂. ΔHR/logΔNE correlated positively with both AT and peak VO₂, and negatively with the VE/VCO₂ slope. ΔHR/logΔNE correlated positively with peak CI, %ΔCI, and ΔCI/exercise time. The data suggest that one of the mechanisms of low exercise tolerance in chronic heart failure patients was due to an inadequate increase in CO response against exercise caused by an impaired HR response to increased NE.