Assessments of flow by transcranial Doppler ultrasound in the middle cerebral artery during exercise in humans

University Laboratory of Physiology, University of Oxford, Oxford OX1 3PT, United Kingdom This study examined the consistency between three indexes of cerebral blood flow (CBF) obtained by using transcranial Doppler ultrasound in eight human volunteers. Each subject undertook three sessions of graded exercise, consisting of 6 min of rest, 6 min at 20% of maximal oxygen uptake ( O 2 max ), 6 min at 40% O 2 max , and 6 min of recovery. Values were obtained every 10 ms for the velocity associated with the maximal frequency of the Doppler shift ( V P ), the intensity-weighted mean velocity ( V IWM ), and total signal power ( P ). Beat-by-beat averages for three indexes ( P , IWM , provided significantly different results for the percent changes in CBF with exercise. At 20% of O 2 max , P and IWM showed significant ( P