Internal carotid and vertebral artery blood flow is evaluated unilaterally on the neck using duplex ultrasound. Evaluation is in the longitudinal section 1-2 cm distal to the carotid bifurcation and the vertebral artery is evaluated between the transverse processes of C2-5 with the head turned approximately 30⁰ to the contralateral side. In order to limit the influence of ventilation, two recordings of approximately 20 s of both arteries are conducted at each level of MAP and the mean is reported. A frequency of 8-12 megahertz is used and gain is set as high as possible while vessel lumen is echo-free. Pulsed-wave Doppler at a stable angle ≤ 60º determined the angle-corrected time averaged maximum flow velocity (TAVMAX), that corresponds to twice the mean blood velocity. Diameter is assessed using automatic software to track the vessel wall and blood flow is: 0.125 * 60 * TAVMAX * π * diameter^2 and CBF is the sum of unilateral internal carotid and vertebral artery blood flow.