Cerebral Perfusion Mismatch: Challenging the Validity of Cerebral Vascular Resistance index in Orthostatic Stress.

The cerebral vascular resistance index (CVRi), a single-parameter model based on systemic blood pressure rather than cerebral perfusion pressure (CPP), is widely used to assess cerebrovascular resistance. However, it does not accurately reflect the pressure-flow/velocity relationships in the cerebral circulation, as cerebral blood flow (CBF) is primarily regulated by CPP. This study evaluated the validity of CVRi during orthostatic stress induced by head-up tilt (HUT), which alters CPP independently of arterial blood pressure at heart level and CBF. Twenty young healthy participants (aged 23 ± 3 years) were included, and vascular tone indices, CVRi, critical closing pressure (CrCP), resistance-area product (RAP), and pulsatility index (PI) were measured during HUT. HUT significantly increased CVRi and CrCP (P ≤ 0.003) but did not affect RAP and PI (P ≥ 0.277). However, correlation analysis between changes in CVRi and other vascular indices (i.e., CrCP, RAP, and PI), calculated as the difference between HUT and supine positions, revealed no significant relationships (all r = 0.189-0.701; all P = 0.090-0.425). Moreover, the CVRi response to orthostatic stress differed from that of the corrected CVRi during HUT, which accounts for changes in hydrostatic pressure. Specifically, CVRi increased during HUT (P = 0.003), whereas corrected CVRi, adjusted for changes in hydrostatic pressure, significantly decreased (P < 0.001). These findings highlight the limitations of CVRi, which fails to account for deviations in CPP from systemic blood pressure or for other physiological factors (e.g., intracranial pressure), potentially leading to inaccuracies in estimating cerebral vascular resistance.