Normal human aging and cerebral vasoconstrictive responses to hypocapnia.

Cerebral vasoconstrictive capacitance was measured during voluntary hyperventilation hypocapnia in 22 healthly normal volunteers aged 21--65 years by serial 133Xe inhalation estimates of rCBF by the initial slope index method of Risberg (ISI2) in the steady state followed by the hypocapnic state. End-tidal PCO2 was monitored by a capnograph. There was significant linear correlation between reduction of PECO2 and the ISI2 values. Significant reduction of cerebral vasoconstrictive response to hypocapnia was found with normal advancing age which is attributed to (1) minor atherosclerosis or loss of elasticity of cerebral vessels with advancing age, (2) the presence of an ischemic threshold during hyperventilation at which CBF tends to stabilize.