Cerebral autoregulation of blood velocity and volumetric flow during steady-state changes in arterial pressure

Abstract

The validity of using transcranial Doppler 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 using transcranial Doppler 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 cerebrovascular 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 regression slopes were similar between middle cerebral artery 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 versus 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 transcranial Doppler measurements of CBFV and CVRi in middle cerebral artery. However, it is likely to be underestimated when compared with 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.

Keywords: blood pressure; carotid artery, internal; middle cerebral artery; ultrasonography, Doppler, transcranial.

Figures

Figure 1

Representative continuous recordings of pulsatile changes in vessel diameters at the internal carotid artery (ICA) and cerebral blood flow velocity (CBFV) at the ICA and middle cerebral artery (MCA). A. Region of interest is selected for segmental diameter measurements on high-resolution B-mode video using a semiautomatic wall-tacking and edge-detection software (see text); B. Representative ICA diameter pulsatile changes recorded at 21 frames/sec (Hz); C. The time-averaged peak and mean velocities (TAPV and TAMV) were measured at ICA in color Doppler/B mode; D. Continuous recordings of transcranial Doppler derived peak velocity (upper curve in green) at the MCA and electrocardiogram (lower curve in red).

Figure 2

Scatter plots of pooled individual data and simple linear regressions of percentage changes in cerebral blood flow velocity (ΔCBFV%, A and B) and cerebral blood flow (ΔCBF%, C) (top row), and cerebrovascular resistance index (ΔCVRi%, D and E) and cerebrovascular resistance (ΔCVR%, F) (bottom row) relative to the baseline during steady-state changes in mean arterial pressure (ΔMAP%) measured at the middle cerebral artery (MCA, A and D) and internal carotid artery (ICA, B, C, E, and F) without adjustment of changes in end-tidal CO2. The solid lines represent regression slopes and the dotted lines represent 95% confidence intervals.

Figure 3

Box plots of linear regression slopes between percentage changes in cerebrovascular resistance indices (ΔCVRi%) and mean arterial pressure (ΔMAP%) at the middle cerebral artery (MCA) and internal carotid artery (ICA) (open boxes), and between percentage changes in cerebrovascular resistance (ΔCVR%) and ΔMAP% at the ICA (filled box). The horizontal dotted and solid lines within the box represent the mean and median, respectively. *P < 0.01.

Figure 4

Scatter plots of pooled individual data and simple linear regression of percentage changes of the internal carotid artery (ICA) diameter and the reciprocal of cross-sectional area (CSA−1) of ICA from the baseline during steady-state changes in mean arterial pressure (ΔMAP%). The solid lines represent regression slopes and the dotted lines represent 95% confidence intervals. These relationships were not influenced by changes in end-tidal CO2 (see text).