We compared relative changes in middle cerebral artery velocity and internal carotid artery flow during autoregulation testing to test the validity of using transcranial Doppler recordings of middle cerebral artery velocity to evaluate cerebral autoregulation in humans.

Seven human volunteers had dynamic autoregulation tested during surgical procedures that included exposure of the internal carotid artery. The mean arterial blood pressure and middle cerebral artery velocity spectral outline (Vmax), using transcranial Doppler, and ipsilateral internal carotid artery flow, using an electromagnetic flowmeter, were continuously and simultaneously recorded during transient sharp decreases in blood pressure that were induced by rapid deflation of thigh blood pressure cuffs. The resulting responses of velocity in the middle cerebral artery and flow in the internal carotid artery were compared.

Moderate decreases in blood pressure evoked responses in cerebral autoregulation. There were no significant (P = .97) differences between the responses in middle cerebral artery velocity and internal carotid artery flow to the blood pressure decreases.

Relative changes in Vmax accurately reflect relative changes in internal carotid artery flow during dynamic autoregulation testing in humans. Therefore, alterations in middle cerebral artery diameter do not occur to the extent that they introduce a significant error in making these comparisons.