Mechanisms maintaining cerebral perfusion during systemic hypotension are impaired in elderly adults.

Postural hypotension abruptly lowers cerebral perfusion, producing unsteadiness which worsens with aging. This study addressed the hypothesis that maintenance of cerebral perfusion weakens in the elderly due to less effective cerebrovascular autoregulation and systemic cardiovascular responses to hypotension. In healthy elderly (n = 13, 68 ± 1 years) and young (n = 13, 26 ± 1 years) adults, systemic hypotension was induced by rapid deflation of bilateral thigh cuffs after 3-min suprasystolic occlusion, while heart rate (HR), mean arterial pressure (MAP), and blood flow velocity of the middle cerebral artery (V_MCA) were recorded. V_MCA/MAP indexed cerebrovascular conductance (CVC). Durations and rates of recovery of MAP and V_MCA from their respective postdeflation nadirs were compared between the groups. Thigh-cuff deflation elicited similar hypotension and cerebral hypoperfusion in the elderly and young adults. However, the time elapsed (T_Δ) from cuff deflation to the nadirs of MAP and V_MCA, and the time for full recovery (T_R) from nadirs to baselines were significantly prolonged in the elderly subjects. The response rates of HR (ΔHR, i.e. cardiac factor), MAP (ΔMAP, i.e. vasomotor factor), and CVC following cuff deflation were significantly slower in the elderly. Collectively, the response rates of the cardiac, vasomotor, and CVC factors largely explained T_RVMCA. However, the T_RVMCA/ΔMAP slope (-3.0 ± 0.9) was steeper (P = 0.046) than the T_RVMCA/ΔHR slope (-1.1 ± 0.4). The T_RVMCA/ΔCVC slope (-2.4 ± 0.6) was greater (P = 0.072) than the T_RVMCA/ΔHR slope, but did not differ from the T_RVMCA/ΔMAP slope (P = 0.52). Both cerebrovascular autoregulatory and systemic mechanisms contributed to cerebral perfusion recovery during systemic hypotension, and the vasomotor factor was predominant over the cardiac factor. Recovery from cerebral hypoperfusion was slower in the elderly adults because of the age-diminished rates of the CVC response and cardiovascular reflex regulation. Systemic vasoconstriction predominated over increased HR for restoring cerebral perfusion after abrupt onset of systemic hypotension.