Relative Contributions of Systemic Hemodynamic Variables to Cerebral Autoregulation during Orthostatic Stress

Postural changes impair the ability of the cerebrovasculature to buffer against dynamic pressure fluctuations, but the mechanisms underlying this impairment have not been elucidated. We hypothesized that autoregulatory impairment may reflect the impact of static central volume shifts on hemodynamic factors other than arterial pressure (AP). In 14 young volunteers, we assessed the relation of fluctuations in cerebral blood flow (CBF) to those in AP, cardiac output, and CO2, during oscillatory lower body pressure (LBP) (±20 mmHg at 0.01 and 0.06 Hz) at three static levels (-20, 0, and +20 mmHg). Static and dynamic changes in AP, cardiac output, and CO2 explained over 70% of the variation in CBF fluctuations. However, their contributions were different across frequencies and levels: dynamic AP changes explained a substantial proportion of the variation in faster CBF fluctuations (partial R²=0.75, standardized β=0.83, p<0.01), whereas those in CO₂ explained the largest portion of the variation in slow fluctuations (partial R²=0.43, β=0.51, p<0.01). There was, however, a major contribution of slow dynamic AP changes during negative (β=0.43), but not neutral (β=0.05) or positive (β=-0.07) LBP. This highlights the differences in contributions of systemic variables to dynamic and static autoregulation, and has important implications for understanding orthostatic intolerance.

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