Cerebral Compliance Changes With Sympathetic Activation During Cold Pressor Test

Abstract

The role of sympathetic activation on arterial compliance has yet to be examined. This study examined cerebral flow velocity and cerebral compliance in response to sympathetic activation.Healthy subjects (n=13) performed a cold pressor test in which their forearms were immersed in a bucket of ice water for up to 3 minutes. Cerebral flow velocity (CFV) (middle (MCA), anterior cerebral (ACA), vertebral arteries (VA), external carotid), blood pressure, and end‐tidal CO2 were measured. Data was collected at baseline, first and final 15 sec of immersion.Compared to initial immersion, CFV increased the most in the last 15 sec of the protocol: ACA (+9.5±3.2%, P<0.02), MCA (+10.0±2.8%, P<0.02), VA (+15.4±4.6%, P<0.02). At the end of the protocol, significant increases in blood pressure were seen (+15.5±2.5 mmHg, P<0.001). Pulse wave velocity (PWV) increased in the periphery from baseline and through each time point: ACA (7.2±0.4 to 8.5±0.8 to 8.5±0.7 m/s); MCA (7.2±0.4 to 9.9±1.6 to 9.2±0.6 m/s); VA (6.7±0.5 to 7.0±0.4 to 8.5±1.4 m/s).Findings suggest that increasing sympathetic activity does not cause cerebral vasoconstriction, but increases CFV while reducing arterial compliance. Sympathetically mediated reductions in compliance may be an adaptive mechanism to enhance pressure transmission to the brain when humans are upright and pressure at the brain is lower due to the hydrostatic gradient. Supported by NASA/NIH.