The validity of using transcranial Doppler (TCD) measurement of cerebral blood flow velocity (CBFV) to assess cerebral autoregulation (CA) still is a concern. This study measured CBFV in the middle cerebral artery (MCA) using TCD and volumetric cerebral blood flow (CBF) in the internal carotid artery (ICA) using color-coded duplex ultrasonography to assess CA during steady-state changes in mean arterial pressure (MAP). Twenty-one healthy adults participated. MAP was changed stepwise by intravenous infusion of sodium nitroprusside and phenylephrine. Changes in CBFV, CBF, cerebrovascular resistance (CVR = MAP/CBF) or resistance index (CVRi = MAP/CBFV) were measured to assess CA by linear regression analysis. The relationship between changes in ICA diameter and MAP was assessed. All values were normalized as percentage changes from baseline. Drug induced changes in MAP were from −26% to 31%. Changes in CBFV and CVRi in response to MAP were linear and the regresssion slopes were similar between MCA and ICA. However, CBF in ICA remained unchanged despite large changes in MAP. Consistently, a steeper slope of changes in CVR relative to CVRi was observed (0.991 vs. 0.804, P < 0.05). The ICA diameter changed inversely in response to MAP (r = −0.418, P < 0.05). These findings indicate that CA can be assessed with TCD mesurements of CBFV and CVRi in MCA. However, it is likely to be underestimated when compared to the measurements of CBF and CVR in ICA. The inverse relationship between changes in ICA diameter and MAP suggests that large cerebral arteries are involved in CA.
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