Physiologic correlates of the heart rate response to upright isotonic exercise: Relevance to rate-responsive pacemakers

Abstract

Rate-responsive cardiac pacing requires a sensitive physiologic variable that is closely correlated with the heart rate-oxygen uptake relation, particularly in patients with heart failure whose cardiac output response to exercise is more dependent on heart rate. Accordingly, the heart rate response to upright exercise was measured in 81 patients with heart failure or hypertension, or both, and in 27 normal subjects. Oxygen uptake (V̇O2), minute ventilation (V̇e), cardiac output, right heart pressures and the mixed venous temperature, oxygen saturation (SV̇O2) and pH were analyzed throughout exercise.Linear regression analysis or these variables with heart rate revealed the following: 1) There was a highly linear heart rate-V̇O2relation in each subject (the average slope of this relation was greater [p < 0.05] in patients with more severe failure). 2) V̇ewas highly correlated with exercise heart rate, and its slope was not different between normal subjects and patients. 3) Mixed venous temperature and pH were poor predictors of exercise heart rate, particularly at low or moderate levels of work; however, SV̇O2was highly correlated with heart rate for all levels of work.Thus, in normal subjects and patients with heart failure or hypertension, or both, heart rate increases linearly with isotonic leg exercise. Minute ventilation and mixed venous oxygen saturation are highly correlated with this response and may serve as potential sensors for rate-responsive pacemakers.