Predicting cerebral blood flow response to orthostatic stress from restingdynamics: effects of healthy aging

The transfer function relating arterial pressure (AP) to cerebral blood flo w velocity (CBFV) during resting conditions has been used to predict the CB FV response to hypotension. We hypothesized that this approach could predic t the CBFV response to posture change in elderly individuals if impaired au toregulation allowed changes in AP to be passively transferred to CBFV. AP (Finapres) and CBFV (middle cerebral artery transcranial Doppler) were meas ured in 10 healthy young (age 24 +/- 1 yr) and 10 healthy elderly (age 72 /- 3 yr) subjects during 5 min of quiet sitting and 1 min of active standin g while breathing was paced at 0.25 Hz. Transfer functions between AP and C BFV changes during sitting were estimated from each fall waveform in both l ow-frequency (LF; 0.05-0.2 Hz) and heartbeat-frequency (HBF; 0.7-1.4 Hz) ra nges. The impulse-response function was used to compute changes in CBFV dur ing posture change. The LF transfer function did not predict orthostatic ch anges in CBFV in either group, suggesting normal cerebral autoregulation. I n the HBF range, the prediction was high in elderly (R = 0.65 +/- 0.23) but not young subjects (R = 0.19 +/- 0.35; P < 0.003, young vs. elderly). Thus rapidly acting regulatory mechanisms that reduce the transmission of beat- to-beat changes in AP to CBFV may be engaged during posture change in young but not elderly subjects.