Regulation of regional cerebral blood flow during graded reflex-mediated sympathetic activation via lower body negative pressure

Abstract

The role of the sympathetic nervous system in cerebral blood flow (CBF) regulation remains unclear. Previous studies have primarily measured middle cerebral artery blood velocity to assess CBF. Recently, there has been a transition towards measuring internal carotid artery (ICA) and vertebral artery (VA) blood flow using duplex Doppler ultrasound. Given that the VA supplies autonomic control centers in the brainstem, we hypothesized that graded sympathetic activation via lower body negative pressure (LBNP) would reduce ICA but not VA blood flow. ICA and VA blood flow were measured during two protocols: Protocol-1, low-to-moderate LBNP (-10, -20, -30, -40 Torr) and Protocol-2, moderate-to-high LBNP (-30, -50, -70 Torr). ICA and VA blood flow, diameter, and blood velocity were unaffected up to -40 LBNP. However, -50 and -70 LBNP evoked reductions in ICA and VA blood flow (e.g., -70 LBNP: %∆VA-baseline= -27.6±3.0) that were mediated by decreases in both diameter and velocity (e.g., -70 LBNP: %∆VA-baseline diameter= -7.5±1.9 and %∆VA-baseline velocity= -13.6±1.7), which were comparable between vessels. Since hyperventilation during -70 LBNP reduced PCO, this decrease in PCO was matched via voluntary hyperventilation. Reductions in ICA and VA blood flow during hyperventilation alone were significantly smaller than during -70 LBNP and were primarily mediated by decreases in velocity (%∆VA-baseline velocity= -8.6±2.4; %∆VA-baseline diameter= -0.05±0.56). These data demonstrate that both ICA and VA were unaffected by low-to-moderate sympathetic activation, whereas robust reflex-mediated sympatho-excitation caused similar magnitudes of vasoconstriction in both arteries. Thus, contrary to our hypothesis, the ICA was not preferentially vasoconstricted by sympathetic activation.

Gravitational effects on intracranial pressure and blood flow regulation in young men: a potential shunting role for the external carotid artery

“…In the present study, increases in IOP (P Ͻ 0.001; Table 1), an indirect index of ICP, might have been expected to decrease cerebral perfusion pressure and therefore perfusion during HDT. However, both ICA and VA blood flows were well maintained during mild body position change, as found in previous studies (12,26,41), thus suggesting that cerebral perfusion pressure was well maintained. Similarly, the previous study of Petersen et al (32) demonstrated that regulation of ABP maintained cerebral perfusion pressure regardless of body position.…”

Specific effect of hypobaria on cerebrovascular hypercapnic responses in hypoxia

“…MCAv resting values during baseline were only significantly increased in 5,500 m HH when compared to NN and 3,000 m HH (Table ). MCAv was logically decreased during hyperventilation due to the hypocapnia‐induced vasoconstriction (Kaur et al, ). Then, hypercapnia triggers cerebral vasodilation, which induces an increase in MCAv.…”

Exercise intensity and middle cerebral artery dynamics in humans

“…Exclusion criteria were: 1) inability of study staff to acquire signal of the MCA using transcranial Doppler ultrasound, 2) inability to perform the alternating leg movements on the seated recumbent stepper 3) diagnosis of Parkinson's disease, mild cognitive impairment, Alzheimer's disease or multiple sclerosis, 4) pulmonary disease or dependency on supplemental oxygen or 5) diagnosis of myocardial infarction or heart failure. Before reporting to the laboratory at the University of Kansas Medical Center, participants were asked to abstain from food for two hours (Fisher et al 2008, Kaur et al 2018, Ogoh et al 2005, caffeine for a minimum of six hours, (Ainslie et al 2008) and vigorous exercise for twelve hours prior to testing. (Ogoh et al 2005, Billinger, Craig, et al 2017 All female participants were verbally questioned regarding menstrual status.…”