An index of cerebral vascular resistance during orthostatic stress and exercise

Background: The single-parameter model of the cerebral vascular resistance index (CVRi), which is based on systemic blood pressure rather than cerebral perfusion pressure (CPP), is commonly 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. Purpose: This study aimed to evaluate the validity of CVRi during orthostatic stress (head-up tilt, HUT) and aerobic exercise (EX), both of which independently alter CPP without necessarily affecting arterial blood pressure at heart level or CBF. Methods: Vascular tone indices, including CVRi, critical closing pressure (CrCP), and pulsatility index (PI), were measured during HUT (20 participants; 23 ± 3 years) and EX (14 participants; 24 ± 3 years). CVRi was calculated as mean arterial pressure divided by mean middle cerebral artery blood velocity (MCA V). Cerebral PI was calculated using the formula: (systolic MCA V – diastolic MCA V) / mean MCA V. CrCP was estimated using the regression line between the systolic and diastolic values of arterial blood pressure and MCA V. Results: HUT increased CVRi and CrCP (P ≤ 0.001) but did not alter PI (P = 0.516). Conversely, EX did not alter CVRi (P = 0.926), but it decreased CrCP (P = 0.039) and increased PI (P = 0.002). Correlation analysis of between changes in CrCP and CVRi, and between changes in PI and CVRi (calculated as differences between HUT and supine or between EX and baseline) revealed no significant relationships during HUT (r = 0.189, P = 0.425 for CrCP; r = 0.701, P = 0.091, for PI) or EX (r = −0.075, P = 0.838 for CrCP; r = 0.317, P = 0.373 for PI). Conclusion: These findings underscore the limitations of CVRi as an index, as it disregards deviations in CPP from systemic blood pressure. This oversight may result in inaccuracies when estimating cerebrovascular resistance, particularly during conditions that independently alter CPP. This study was supported by JSPS KAKENHI #23K24727. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.