Postural changes in cardiac volumes in men in relation to adult age.

Cardiac volumes by equilibrium gated cardiac blood pool scans and heart rate were measured in the supine and sitting positions in 64 male volunteer subjects (age 25-80 yrs) who had been rigorously screened to exclude cardiovascular disease. After the upright position was assumed, the average cardiac output of all subjects was unchanged but heart rate increased and stroke volume decreased due to a decrease in end diastolic volume. Neither the supine or sitting cardiac output nor the average postural change in cardiac output, cardiac volumes or heart rate was age-related. While the average cardiac output among the subjects was unaltered with a change in posture, in some individuals it increased slightly while in others it decreased. The postural change in cardiac output among the individuals correlated by linear regression analysis with a change in heart rate only in younger subjects and with a change in stroke volume in all age groups, but the slope of this relationship was greater in older than in younger subjects. The postural change in stroke volume was strongly correlated with a change in end diastolic volume and this relationship did not vary with age. Thus, although the average postural change in cardiac output among healthy subjects is not age-related, a given change in cardiac output with posture in an older individual depends more on a change in stroke volume and less on a heart rate change than in a younger one. This result, like the response to vigorous upright exercise previously demonstrated to occur with aging, indicates a greater reliance in the elderly on the Frank-Starling mechanism than on heart rate for a given change in cardiac output in response to perturbations from the basal supine state.