Effects of Alpha Adrenergic Blockade on Autoregulation and Chemical Vasomotor Control of CBF in Stroke

Abstract

Autoregulation and chemical vasomotor control of cerebral blood flow (CBF) were quantitatively tested in 19 patients with various types and sites of cerebral ischemia and infarction. The effect of alpha adrenergic blockade of cerebral vessels by phenoxybenzamine (PBZ) also was evaluated in more than half of the patients. No correlation was found between the degree of cerebral dysautoregulation and impairment of chemical vasomotor control of CBF. Patients with brainstem ischemia and infarction showed normal vasomotor reactivity of CBF to changes in Pa CO 2 , despite the impairment of cerebral autoregulation during both induced hypotension and hyper ension. Following intracarotid injection of 10 mg PBZ, the degree of cerebral dysautoregulation lessened during induced hypotension but increased during induced hypertension. Chemical vasomotor control of CBF in response to changes in P CO 2 , was not significantly altered by the PBZ injection. Changes in autoregulation index (a quantitative measure of cerebral autoregulation) after the infusion were compared with changes in chemical regulation index (a quantitative measure of cerebral vasomotor reactivity). Induced hypotension and hypercapnia yielded a proportional correlation between these two indexes, while induced hypotension and hypocapnia revealed a significant inverse correlation. This study demonstrates that cerebral autoregulation is influenced by the autonomic innervation of cerebral vessels. Alpha adrenergic blockade improves impaired autoregulation when cerebral perfusion pressure (CPP) is lowered but worsens it when CPP is raised, indicating that PBZ impairs the ability of cerebral vessels to constrict during induced hypertension and improves their ability to dilate during induced hypotension. This study also supports the view that chemical vasomotor control is regulated not directly by autonomic innervations of cerebral vessels but by a local controlling mechanism.

Neurogenic Control of Cerebral Blood Flow in the Baboon. Effects of Alpha Adrenergic Blockade With Phenoxybenzamine on Cerebral Autoregulation and Vasomotor Reactivity to Changes in Pa CO 2

“…In these experiments autoregulatory response when CPP was decreased was not affected by PBZ injection, indicating alpha adrenergic innervation is more concerned with vasoconstrictor tonus as reported in man. 37 The present data are in agreement with those reported in man 37 ' 55 and with the view that when CPP is raised, autoregulation is mainly mediated by the alpha adrenergic innervation of cerebral arteries. 56 Let us now consider the possible physiology of the adrenergic neurogenic control of cerebral vessels.…”

Influence of adrenergic receptor blockade on circulatory and metabolic effects of disordered neurotransmitter function in stroke patients.

“…Since intracarotid infusion of either alpha (PBZ) or beta (PPL) adrenergic blocking agents has been found in separate studies to produce some beneficial effects on CBF, cerebral autoregulation, metabolism, EEG and neurological deficit in patients with recent stroke, 6 " 8 the present study was undertaken using PBZ and PPL combined in the hope of obtaining further improvement in the above parameters in the series of patients reported here. However, neither clinical nor EEG benefit was noted in the present study and slight transient worsening of the existing neurological deficit occurred in four patients.…”

Cerebral Autoregulation in Stroke

“…5,6,13 Obviously, TCD can be used to study CBFV responses to steady-state changes in BP (static pressure autoregulation) 14,15 and the autoregulatory reserve and adaptability after, for example, CO 2 inhalation or breathing maneuvers (chemical vasomotor autoregulation or cerebrovascular reactivity). 12 Although Stirling Meyer 16 found no correlation between the degree of static pressure dysautoregulation and impairment of chemical vasomotor autoregulation in stroke patients, many authors today interpret the isolated assessment of chemical vasomotor autoregulation as a steady-state method (ie, measurement at 2 static levels of CO 2 ). 17 Contrasting with the classical static autoregulation, no uniform method exists to provoke, measure, analyze, and report dynamic or chemical vasomotor autoregulation.…”