The Role of Regional Cerebral Blood Flow on Tolerance to Central Hypovolemia

Tolerance to central hypovolemia is highly variable, and accumulating evidence suggests that protection of anterior cerebral blood flow (CBF) may not be the underlying mechanism. The posterior cerebral circulation supplies blood to the autonomic and respiratory control centers in the medulla, so may be associated with tolerance to central hypovolemia. We hypothesized that individuals with high tolerance to central hypovolemia would exhibit prolonged preservation of CBF in the posterior versus anterior cerebral circulation. 20 subjects (8M/12F) completed a presyncopal-limited lower body negative pressure (LBNP) protocol (3 mmHg/min onset rate) simulating hemorrhage. Middle cerebral artery velocity (MCAv), posterior cerebral artery (PCAv) (both via transcranial Doppler ultrasound), and arterial pressure (via finger photoplethysmography) were measured continuously. Subjects who completed ≥70 mmHg LBNP were classified as high tolerant (HT; N=9), and low tolerant (LT; N=11) if they completed ≤60 mmHg LBNP. The minimum difference in LBNP tolerance between groups was 214 s. Mean MCAv decreased by a similar magnitude in both groups throughout LBNP (P≥0.15 between groups), but HT subjects exhibited greater decreases at presyncope (P=0.02). In comparison, mean PCAv decreased below baseline from -30 mmHg LBNP in LT subjects (P=0.01), but remained unchanged in HT subjects until -60 mmHg (P≥0.89). We conclude that individuals with higher tolerance to central hypovolemia exhibit prolonged preservation of CBF in the posterior cerebral circulation, but not in the anterior circulation, thus delaying the onset of presyncope.