Oxygen consumption in the heart, hepatomesenteric bed, and brain in young and elderly human subjects, and accompanying sympathetic nervous activity

Although the reduction in whole-body energy expenditure with aging has been well documented, there is little information about the changes that individual organs undergo. We therefore measured oxygen consumption in the heart, hepatomesenteric bed, and brain in elderly subjects and young controls, using central venous catheter techniques and the application of Fick's principle. We also measured whole-body, cardiac, and hepatomesenteric sympathetic nervous activity using isotope dilution methodology. Cardiac, hepatomesenteric, and cerebral oxygen consumption was similar in both groups. Whole-body and hepatomesenteric sympathetic nervous activity was also similar in the study groups, whereas cardiac norepinephrine (NE) spillover was significantly higher in the elderly. In contrast to the young, cardiac sympathetic nervous activity as assessed from NE spillover was not related to either cardiac oxygen consumption or cardiac work in the elderly. The data suggest that although oxygen consumption in the heart, hepatomesenteric bed, and brain are not different between young and elderly individuals, the relationship between sympathetic nervous activity and oxygen consumption in individual organs may alter with aging.