Pharmacological studies of total cerebral blood flow in man with comments on the possibility of improving regional cerebral blood flow by drugs

The cerebrovascular response to norepinephrine: A scoping systematic review of the animal and human literature

Intravenous norepinephrine (NE) is utilized commonly in critical care for cardiovascular support. NE's impact on cerebrovasculature is unclear and may carry important implications during states of critical neurological illness. The aim of the study was to perform a scoping review of the literature on the cerebrovascular/cerebral blood flow (CBF) effects of NE. A search of MEDLINE, BIOSIS, EMBASE, Global Health, SCOPUS, and Cochrane Library from inception to December 2019 was performed. All manuscripts pertaining to the administration of NE, in which the impact on CBF/cerebral vasculature was recorded, were included. We identified 62 animal studies and 26 human studies. Overall, there was a trend to a direct vasoconstriction effect of NE on the cerebral vasculature, with conflicting studies having demonstrated both increases and decreases in regional CBF (rCBF) or global CBF. Healthy animals and those undergoing cardiopulmonary resuscitation demonstrated a dose-dependent increase in CBF with NE administration. However, animal models and human patients with acquired brain injury had varied responses in CBF to NE administration. The animal models indicate an increase in cerebral vasoconstriction with NE administration through the alpha receptors in vessels. Global and rCBF during the injection of NE displays a wide variation depending on treatment and model/patient.

Effects of catecholamine infusions on cerebral blood flow and oxygen consumption of the isolated perfused dog brain

Our earlier studies revealed a weak alpha-adrenergic and beta-adrenergic activity of the cerebral vessels of the isolated perfused dog brain. The present investigations were undertaken to determine whether vascular adjustments occur in the cerebral circulation during longer periods of catecholamine infusions. The experiments were performed on six isolated canine brains cross perfused from donor dogs. Norepinephrine (2 µg per minute), epinephrine (2 µg per minute), and isoprenaline (0.2 µg per minute) were applied intra-arterially (i.a.) for a period of ten minutes. Total venous outflow, perfusion pressure in the circle of Willis, and venous O 2 saturation were monitored continuously. Cerebral vascular resistance (CVR) and cerebral O 2 consumption (CMRO 2 ) were calculated. Based on the pressure-flow relationship tested in each brain, the indirect effects of catecholamines on CVR caused by autoregulatory influences were calculated and eliminated. During norepinephrine and epinephrine infusions cerebral blood flow (CBF) was found to be decreased by 10.2 ± 6.0% and 4.1 ± 3.3%, respectively, whereas during isoprenaline infusion CBF increased by 9.3 ± 3.6% (mean values ± SD). The maximal changes of CBF were reached in the first or second minute of catecholamine infusion and persisted up to the end of infusion (P > 0.05). After elimination of the indirect effects of catecholamines on CVR, the direct effects on CVR were reduced to about 50% of the original values and remained constant at the level reached during the whole period of infusion. CMRO 2 was not changed (P > 0.05) during infusion of the different catecholamines. Based on these investigations it is assumed that no pronounced vascular adjustments occur in the cerebral circulation during catecholamine infusions.

Evidence for the direct effect of adrenergic drugs on the cerebral vascular bed of the unanesthetized goat

Despite considerable research, the question of whether adrenergic drugs exert direct effects on the cerebral circulation has remained unresolved. With the development of a method for monitoring continuously the entire blood flow to one hemisphere in the unanesthetized goat, we have been able to study this problem directly. The effects of epinephrine, norepinephrine, and isoproterenol administered by close intra-arterial injection were investigated in 15 goats in which an electromagnetic flowmeter had been implanted previously on the internal maxillary artery, which, in this animal, provides the sole blood supply to a hemisphere. Both epinephrine and norepinephrine (0.1 to 5.0 µg) produced dose-dependent reductions in cerebral blood flow, a decrease of 55 ± 3% (SEM) occurring with the highest dose. Alpha receptor blockade of the ipsilateral hemisphere with phenoxybenzamine totally or partially abolished this cerebral vasoconstriction. Isoproterenol (0.01 to 1.0 µg) produced dose-dependent increases in cerebral blood flow, an increment of 75 ± 6% occurring with the highest dose. Beta blockade with propranolol totally or partially abolished the cerebral vasodilation induced by isoproterenol. Thus, epinephrine, norepinephrine, and isoproterenol exert powerful direct effects on the cerebral circulation of the unanesthetized goat, and these effects appear to be mediated by alpha and beta receptors.

Cerebral vasodilator therapy in stroke

Cerebral vasodilating agents have been questioned as an effective form of therapy for the stroke patient and even have been considered by some to be harmful. Animal studies show that often vasodilating agents will cause an intracerebral steal, but such a reaction has rarely been demonstrated in stroke patients. Several studies measuring cerebral blood flow in man have shown that vasodilating agents will increase cerebral blood flow even in ischemic regions in some patients. Significant clinical studies have not been carried out to determine whether these agents will alter the natural history of the disease. On the basis of reported studies of the effect of vasodilating agents on the cerebral circulation, it is suggested that further laboratory and clinical studies be performed. These agents could potentially be an effective form of therapy in some patients with occlusive cerebrovascular disease.

Oxygen availability in ischemic brain following hypocarbia and hypercarbia. Polarographical depth electrode recordings in evolving and completed experimental stroke in the monkey

Acutely implanted oxygen-recording polarographical depth electrodes were used to appraise changes in oxygen availability (O 2 a) in the brain of the Rhesus monkey. Modifications of oxygen availability induced by hypocarbia and hypercarbia in the normal and the infarcted brain are described. The most consistent finding was a reversal of the expected effects of low and high Paco 2 values during the acutely ischemic period (evolving stroke) and an incomplete return to the normal response by five days (completed stroke).