Abstract
PURPOSE: Cerebral blood flow (CBF) increases ~20% during whole body exercise although a Kety-Schmidt-determined CBF is reported to remain stable; a discrepancy that could reflect evaluation of arterial vs. internal jugular venous (IJV) flow and/or that CBF is influenced by posture. Here we test the hypothesis that IJV flow, as determined by retrograde thermodilution increases during exercise when body position is maintained.
METHODS: Introducing retrograde thermodilution, IJV flow was measured in 8 healthy humans at supine and upright rest and during exercise in normoxia and hypoxia with results compared to changes in ultrasound-derived IJV flow and middle cerebral artery mean velocity (MCA Vmean).
RESULTS: Thermodilution determined IJV flow was in reasonable agreement with values established in a phantom (R=0.59, p<0.0001) and correlated to the ultrasound-derived IJV flow (n=7; Kendall's tau, 0.28, p= 0.036). When subjects stood up, IJV blood flow decreased by 9±13 (mean±SD)% (219±57 to 191±73 ml·min, p<0.0001) and the influence of body position was maintained during exercise (p<0.0001). Exercise increased both IJV flow and MCA Vmean (p=0.019 and p=0.012, respectively) and the two responses were similar (p=0.50). During hypoxia, however, only MCA Vmean responded with a further increase (p<0.0001).
CONCLUSION: As determined by retrograde thermodilution, IJV flow seems little sensitive to hypoxia, but does demonstrate the about 15% reduction in CBF when humans are upright and, provided that body position is maintained, also the increase in CBF during whole body exercise.
Rasmussen, Peter