Changes in cerebral blood flow velocity after release of intraoperative tourniquets in humans: a transcranial Doppler study. The effect of release of intraoperative thigh tourniquets on velocity of blood flow in the middle cerebral artery was examined in five patients given general anesthesia with controlled ventilation for lower extremity orthopedic procedures using transcranial Doppler sonography. Middle cerebral artery blood flow velocity increased significantly from 52 +/- 6 (SEM) to 82 +/- 24 cm/s (an increase of 58% +/- 13%) within 4 +/- 1 min after tourniquet release and remained significantly elevated for 7 min. A positive linear correlation was found between middle cerebral artery blood flow velocity and PETCO2 on each occasion (0.97 greater than or equal to r greater than or equal to 0.84, 0.001 greater than P greater than 0.0001) after tourniquet deflation. Assuming a linear relationship between flow velocity and flow, these findings suggest that significant increase in cerebral blood flow can occur after intraoperative tourniquet release and that this increase appears to be mostly CO2-dependent. 