Autonomic Neural Control of the Cerebral Vasculature

Abstract

Background and Purpose-The effect of antihypertensive drugs on autonomic neural control of the cerebral circulation remains unclear. This study was designed to compare middle cerebral artery mean blood velocity responses to acute hypotension with and without ␣ 1 -adrenoreceptor blockade (Prazosin) in young, healthy humans. Methods-Acute hypotension was induced nonpharmacologically in 6 healthy subjects (meanϮSE; 28Ϯ2 years) by releasing bilateral thigh cuffs after 9 minutes of suprasystolic resting ischemia before and after an oral dose of Prazosin (1 mg/20 kg body weight). Results-Prazosin had no effect on thigh cuff release-induced reductions in mean arterial pressure and middle cerebral artery mean blood velocity. However, Prazosin attenuated the amount of peripheral vasoconstriction through the arterial baroreflex as evidenced by a slower return of mean arterial pressure to baseline (Pϭ0.03). Immediately after cuff release, cerebral vascular conductance index increased through cerebral autoregulation and returned to resting values as a result of an increased perfusion pressure mediated through arterial baroreflex mechanisms. The rate of regulation, an index of cerebral autoregulation, was attenuated with Prazosin (control versus Prazosin; rate of regulationϭ0.204Ϯ0.020 versus 0.006Ϯ0.053/s, Pϭ0.037). In addition, as mean arterial pressure was returning to resting values, the rate of change in cerebral vascular conductance index was decreased with Prazosin (0.005Ϯ0.006/s) compared with control (0.025Ϯ0.005/s; Pϭ0.010). Conclusions-These data suggest that during recovery from acute hypotension, decreases in cerebral vascular conductance index were mediated by increases in arterial blood pressure and sympathetically mediated cerebral vasoconstriction.

Neurovascular coupling in humans: Physiology, methodological advances and clinical implications

“…4,20 This is in contrast to the vast majority of systemic vasculature where the parasympathetic nervous system does not actively regulate tone. Despite extensive autonomic innervation in the brain, findings in humans support only a modest and somewhat frequency-dependent role of sympathetic [21][22][23] and parasympathetic activity 24 in influencing dynamic cerebral autoregulation. The details of the complex interactions involved in human CBF regulation have recently been reviewed.…”

Assessment of cerebral autoregulation: the quandary of quantification

“…For the determination of RoR, the CVC index (CVCi) was calculated by dividing MCAv mean by MAP. MAP, MCAvmean, and CVCi values were determined relative to control values, defined as the mean of the 4 s immediately before thigh-cuff release (1,30). The RoR index was taken as follows:…”

Influence of Changes in Blood Pressure on Cerebral Perfusion and Oxygenation

“…Maintenance of adequate cerebral perfusion during normal physiological challenges, such as assumption of upright posture from a supine, squatting, or sitting position, requires the integrated control of CBF and systemic blood pressure via CA and the arterial baroreflex. 48 In our experimental design, we effectively override the input from one of these components (ie, the baroreflex) by pharmacologically maintaining an increased or decreased MAP. Although speculative, it would seem possible that some of the variation in the CA response between participants could be explained by between-individual variability in the CA-baroreflex relationship; that is, those with a greater input from the baroreflex under "normal conditions" would show the greater reductions in CA in this experimental setting.…”