The use of transcranial doppler to assess cerebral blood flow during exercise in humans
Poulin MJ., Syed RJ., Robbins PA.
There are conflicting reports concerning the changes in cerebral blood flow (CBF) during dynamic exercise. This is related, in part, to whether global, cortical or regional changes in CBF are measured. Transcranial Doppler ultrasound (TCD) studies have given fairly consistent results, suggesting modest increases during light to moderate exercise. However, all of these relate to beat averages of the velocities associated with the maximal frequencies of the Doppler shift (V̄P). The use of V̄P as a measure of CBF requires that the maximal velocity is proportional to the mean velocity of the blood flow in the vessel and that the cross-sectional area of the vessel remains unchanged. Thus, this study examined the degree of consistency between V̄P and two other indices of CBF obtained using TCD in humans (n=8; 22.3±2.4 yrs, mean±SD) during submaximal dynamic exercise. Each subject undertook three (24 min) sessions of graded exercise consisting of 6 min rest, 6 min at 20% maximal oxygen uptake (V̇O2max), 6 min at 40% V̇O2max and 6 min recovery. Values were obtained every 10 ms for VP, the intensity-weighted mean velocity (VIWM) and signal power (P). Beat-by-beat averages for three indices of flow (V̄P, V̄IWM, P·VIWM) provided significantly different results (ANOVA) for the changes in CBF with exercise. At 40% V̇O2max, V̄P and V̄IWM showed significant increases of 14.1±10.4% and 8.2±9.8%, respectively, while P·VIWM showed a non-significant increase of 4.3±9.8%. Our results suggest that the increase in CBF with exercise previously reported with TCD needs to be treated with caution, as much of the response could arise as an artefact, with a likely source being the increased pulsatility of blood flow.