Baroreflex control of the heart rate during central hypovolemic stress in young and elderly men.

To test the hypothesis that tolerance to central hypovolemic stress is diminished with a reduced sensitivity of cardiac baroreflex in young and old men, twenty-four healthy elderly male subjects (67.5+/-0.9 yrs) and 24 young male subjects (22.4+/-0.4 yrs) underwent a 21 min bout of ramped lower body negative pressure (LBNP) (0 to -60 mmHg, 10 mmHg each for 3 min). Heart rate(HR), blood pressure, thoracic impedance (Z0) and calf circumference were measured throughout the experimental period. The sensitivity (BRS) of arterial baroreflex control of the cardiac interval was calculated from spontaneous changes in beat-to-beat arterial pressure and the RR interval during the LBNP test. The occurrence of presyncope episodes during LBNP was 16.7% in elderly subjects and 37.5% in young subjects. The changes of HR, blood pressure, Z0 and calf circumference in the young (Young fainter group) and the elderly (Elderly fainter group) subjects who showed presyncope during LBNP did not differ from those in the subjects (Young and Elderly nonfainter groups) who did not show presyncope. The steady-state HR responses to the changes of Z0 during LBNP did not differ among the four groups. In young and elderly subjects, the baseline values of BRS in the fainter groups tended to be smaller than those in the nonfainter groups, and the difference in the young group was significant (P<0.05). No LBNP-related change of BRS was observed in elderly subjects, whereas a gradual LBNP-related decrease was observed in young subjects. These findings suggest that 1) aging does not increase intolerance to central hypovolemic stress induced by a given LBNP, 2) the reduced vagal BRS is related partly to low LBNP tolerance in young and elderly men.