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

This study examined the consistency between three indexes of cerebral blood flow (CBF) obtained by using transcranial Doppler ultrasound in eight huma n volunteers. Each subject undertook three sessions of graded exercise, con sisting of 6 min of rest, 6 min at 20% of maximal oxygen uptake ((V)over do tO(2max)), 6 min at 40% (V)over dotO(2max), and 6 min of recovery. Values w ere obtained every 10 ms for the velocity associated with the maximal frequ ency of the Doppler shift (V-P), the intensity-weighted mean velocity (V-IW M), and total signal power (P). Beat-by-beat averages for three indexes ((V ) over bar(P), (V) over bar(IWM), (P.V) over bar(IWM)) provided significant ly different results for the percent changes in CBF with exercise. At 20% o f (V)over dotO(2max), (V) over bar P and (V) over bar(IWM) showed significa nt (P < 0.05) increases of 8 and 6%, respectively, whereas (P.V) over bar(I WM) showed a nonsignificant increase of 3%. At 40% of (V)over dot O-2max, ( V) over bar(P) and (V) over bar(IWM) showed significant (P < 0.05) increase s of 14 and 8%, respectively, whereas (P.V) over bar(IWM) showed a nonsigni ficant increase of 4%. Our results suggest that the increase in CBF with ex ercise that has been reported with transcranial Doppler ultrasound needs to be treated with caution, as much of the response could arise as an artifac t from the increase in amplitude and frequency of the arterial pressure wav eform.