Difference In The Responses Of Cerebral Vascular Resistance Index, Critical Closing Pressure And Pulstatility Index To Orthostatic Stress

The single-parameter model of the cerebral vascular resistance index (CVRi), which relies on systemic blood pressure rather than cerebral perfusion pressure (CPP), may not accurately reflect the intricate pressure-flow/velocity relationships within the cerebral circulation. Despite this, CVRi is often used, overlooking the complexity of cerebral hemodynamics. PURPOSE: This study aims to examine the validity of changes in CVRi during orthostatic stress. METHODS: In 20 young participants, heart rate, arterial blood pressure, and middle cerebral artery blood velocity (MCA V) were measured in both supine and 50° head-up tilt (HUT) positions to assess critical closing pressure (CrCP), pulsatility index (PI), and CVRi. Estimated CPP was calculated as mean arterial pressure minus hydrostatic pressure, determined using the formula: head-to-heart distance (cm) × sin (50°) × 0.735. CVRi and CrCP were then normalized to this estimated CPP. RESULTS: Both CVRi (P < 0.001) and CrCP (P = 0.001) increased during HUT, while PI remained unchanged (P = 0.516). However, the relationship between the changes in CVRi and CrCP or PI was not significant (P = 0.279 or P = 0.499, respectively). Interestingly, when both CVRi and CrCP were normalized by estimated CPP, they decreased during HUT, and the relationship between them was significant (P = 0.030). CONCLUSIONS: These findings suggest that CVRi may not accurately reflect cerebral vascular tone during HUT. The mechanisms underlying these inconsistencies remain unclear; however, one possible explanation is that the arterial blood pressure at the heart level, used for calculating CVRi, does not fully represent CPP. Supported by: JSPS KAKENHI #22H03470