Effects of age on ventricular performance during graded supine exercise.

To assess the effects of age on ventricular performance, graded supine exercise tests with equilibrium radionuclide ventriculography were performed in six normal subjects of mean age 37 +/- 4 years and in eight normal subjects with a mean age of 59 +/- 2 years. At a standard submaximal work load, older subjects had a similar heart rate (older: 126 +/- 10, younger: 128 +/- 5 bpm) and systolic blood pressure responses (older: 198 +/- 24, younger: 202 +/- 24 mm Hg). Cardiac output counts increased appropriately in both groups during submaximal exercise. However, when expressed as percent change from resting values, the increases in cardiac output (older: 125 +/- 14, younger: 75 +/- 10 L/min; p less than 0.05) were greater for the older subjects. The percent change in end-diastolic counts (older: 8.4 +/- 5, younger: -2.8 +/- 4), stroke counts (older: 26 +/- 6, younger: 8.6 +/- 4), and ejection fraction (older: 18 +/- 3, younger: 11 +/- 1%) in proceeding from rest to exercise Stage III (600 kg-m/min) was greater for the older subjects. Age-related differences in each of these measurements were significant at p less than 0.05. These findings suggest that cardiac output during exercise is maintained by an increased heart rate in younger subjects, and by a combination of increased heart rate and the Frank-Starling mechanism in older individuals. Since the heart rate and mean blood pressure response to exercise were similar in both groups, the use of the Frank-Starling mechanism during exercise in older subjects suggests that age-related differences in ventricular preload are important in modulating the performance of the aging left ventricle.