Blood flow in internal carotid and vertebral arteries during orthostatic stress.

It remains unclear whether orthostatic stress evokes regional differences in cerebral blood flow. The present study compared blood flow in the internal carotid (ICA) and vertebral arteries (VA) during orthostatic stress (60 deg head-up tilt; HUT) in six healthy young men. The ICA and VA blood flow were measured using Doppler ultrasonography. Dynamic cerebral autoregulation was also determined during supine (Supine) and HUT conditions, from the rate of regulation (RoR) in cerebrovascular conductance of the ICA and VA during acute hypotension induced by the release of bilateral thigh-cuffs. The HUT decreased ICA blood flow by -9.4 ± 1.7% (P < 0.01 versus Supine), leaving ICA conductance unchanged. In contrast, there was no significant difference in VA blood flow between Supine and HUT, and VA conductance increased (+12.9 ± 0.8%, P < 0.01). In addition, dynamic cerebral autoregulation in both the ICA and VA was attenuated during HUT, and the magnitude of the attenuation in RoR was greater in the VA [0.25 ± 0.03 s(-1) Supine versus 0.16 ± 0.02 s(-1) HUT (-33.9 ± 5.8%); P < 0.05] compared with the ICA [0.23 ± 0.02 s(-1) Supine versus 0.20 ± 0.03 s(-1) HUT (-10.6 ± 13.4%); P > 0.05]. These data indicate that orthostatic stress evokes regional differences in cerebral blood flow and possible differences in dynamic cerebral autoregulation between two main brain vascular areas in response to an acute change in blood pressure during orthostatic stress.