Cerebral autoregulation and CO₂ reactivity in anterior and posterior cerebral circulation during sevoflurane anesthesia

The purpose of the study was to compare cerebral autoregulation (CA) and CO₂ reactivity (CO₂R) between the anterior and posterior circulation under sevoflurane anesthesia. We studied 9 adult ASA physical status I patients (22-47 yr) scheduled for elective orthopedic surgery. Blood flow velocity in the middle cerebral artery (Vmca) and in the basilar artery (Vba) were measured using transcranial Doppler ultrasonography. For CA testing, arterial blood pressure was increased using phenylephrine infusion. CA was quantified with the autoregulatory index (ARI). CO₂R was investigated at PaCO₂ of 30 ± 2.8 mm Hg, 39.4 ± 2.6 mm Hg, and 48.7 ± 2.8 mm Hg. Linear regression analysis was used for CO₂R. We found ARI was preserved in both arteries: ARImca (middle cerebral artery) = 0.72 ± 0.2; ARIba (basilar artery) = 0.66 ± 0.2; P = 0.5. With regard to CO₂R, Vmca increased with slope of 1.7 cm/s/mm Hg PaCO ₂, Vba increased with slope of 1.5 cm/s/mm Hg PaCO₂; P = 0.83. Absolute Vmca was higher compared with Vba; P < 0.05. We conclude that in healthy individuals under 0.5 MAC of sevoflurane and small-dose remifentanil: 1) mean flow velocities of BA are less than those of MCA; 2) autoregulation and CO₂R are preserved in the basilar artery and are similar to those of MCA.