Measurements A catheter is placed in the radial or brachial artery on the non-dominant arm for evaluation of arterial pressure and gas variables. Total volume of blood sampled is less than 25 ml. A long line is placed in a cubital vein and advanced to the subclavian vein for infusion of sodium nitroprusside. The arterial pressure measurement is used for evaluation of stroke volume, cardiac output, and total peripheral resistance by modified pulse contour analysis. Heart rate is evaluated by electrocardiogram lead II. Forehead skin oxygenation and blood flow is evaluated close to the hairline using Laser Doppler flowmetry with an integrated oxygenation sensor at a depth of 1-2 mm in an area of approximately 9 mm^2. Cerebral and biceps muscle oxygenation are evaluated using near-infrared spectroscopy . Transcranial Doppler is used to evaluate MCA Vmean. At a constant diameter of the MCA, changes in blood velocity reflect those of regional CBF, but the diameter may be affected by changes in MAP and PaCO2.