Adaptation of human left ventricular volumes to the onset of supine exercise

The purpose of this study was to measure the changes and rates of adaptation of left ventricular volumes at the onset of exercise. Eight asymptomatic subjects, in whom intramyocardial markers had been implanted 3–6 years previously during aortocoronary bypass surgery, exercised in the supine position at a constant workload of 73.6 W for 5 min. Six also exercised first at 16.4 W, and then against a workload which progressively increased by 8.2 W every 15s. Cardiac volumes were measured by computer assisted analysis of the motion of the implanted markers. In the constant workload test, cardiac output increased rapidly from 5.7±1 min^-1 to 10.3±1.9 1 min^-1 by 2 min and then increased more slowly to 10.8±2.0 1 min^-1 by 5 min. The cardiac output increase was mainly due to an increase in heart rate from 68±12 beats min^-1 to 120±16 beats min^-1 with minimal changes in stroke volume. The time constant for the early increase in cardiac output was 45 s and for heart rate, 35 s. With progressively increasing workloads, there was an almost linear increase of heart rate and cardiac output, but these increased at a slower rate than during the early phase of the constant load exercise test. In conclusion: (i) rapid changes in cardiac output during supine exercise were produced by changes in heart rate; (ii) changes in stroke volume provided minor adjustments to cardiac output; (iii) the end-diastolic volume was almost constant.