Reflex-Mediated Reduction in Human Cerebral Blood Volume

Adrenergic nerves innervate the human cerebrovasculature, yet the functional role of neurogenic influences on cerebral hemodynamics remains speculative. In the current study, regional cerebrovascular responses to sympathoexcitatory reflexes were evaluated. In eight volunteers, contrast-enhanced computed tomography was performed at baseline, -40 mmHg lower body negative pressure (LBNP), and a cold pressor test (CPT). Cerebral blood volume (CBV), mean transit time (MTT), and cerebral blood flow (CBF) were evaluated in cortical gray matter (GM), white matter (WM), and basal ganglia/thalamus (BGT) regions. Lower body negative pressure resulted in tachycardia and decreased central venous pressure while mean arterial pressure was maintained. Cold pressor test resulted in increased mean arterial pressure concomitant with tachycardia but no change in central venous pressure. Neither reflex altered end-tidal carbon dioxide. Cerebral blood volume was reduced in GM during both LBNP and CPT (P<0.05) but was unchanged in WM and BGT. Mean transit time was reduced in WM and GM during CPT (P<0.05). Cerebral blood flow was only modestly affected with either reflex (P<0.07). The combined reductions in GM CBV (approximately -25%) and MTT, both with and without any change in central venous pressure, with small CBF changes (approximately -11%), suggest that active venoconstriction contributed to the volume changes. These data demonstrate that CBV is reduced during engagement of sympathoexcitatory reflexes and that these cerebrovascular changes are heterogeneously distributed.