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

Assessments of ﬂow by transcranial Doppler ultrasound in the middle cerebral artery during exercise in humans. J. Appl. Physiol. 86(5): 1632–1637, 1999.—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 (V ˙ O 2max ), 6 min at 40% V ˙ O 2max , 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 ( V P , V IWM , P · V IWM ) provided signiﬁcantly different results for the percent changes in CBF with exercise. At 20% of V ˙ O 2max , V P and V IWM showed signiﬁcant ( P , 0.05) increases of 8 and 6%, respectively, whereas P · V IWM showed a nonsigniﬁcant increase of 3%. At 40% of V ˙ O 2max , V P and V IWM showed signiﬁcant ( P , 0.05) increases of 14 and 8%, respectively, whereas P · V IWM showed a nonsigniﬁcant increase of 4%. Our results suggest that the increase in CBF with exercise that has been reported with transcranial Doppler ultrasound needs to be treated with cau-tion, as much of the response could arise as an artifact from the increase in amplitude and frequency of the arterial pressure waveform.