Cerebral blood flow regulation: vascular resistance adjustments in the circle of Willis

Intracranial Blood Flow Changes in Patients with High-Grade Severe Carotid Artery Stenosis After Stenting

OBJECTIVE

We investigated whether the cerebral hemodynamic changes of pre- and poststenting in patients with severe carotid stenosis differ by stenosis grades.

METHODS

We prospectively recruited patients who underwent carotid artery stenting (CAS) after acute ischemic stroke from June 2014 to December 2015. We compared the mean relative cerebral blood flow (rCBF) changes (measured by whole-brain computed tomography perfusion) pre- and poststenting in patients with high-grade severe stenosis (HGSS) (90%-99%) versus patients with low-grade severe stenosis (LGSS) (70%-89%).

RESULTS

Among 24 patients included in the study (mean age, 66.2 ± 7.2 years; 91.7% men), 62.5% (15/24) were in the HGSS group and 37.5% (9/24) were in the LGSS group. In the HGSS group, rCBF increased in territories of the anterior cerebral artery (P = 0.021), middle cerebral artery (P < 0.001), posterior cerebral artery (P = 0.001), and basil ganglia (P = 0.003) after stenting. Of the patients with HGSS, 53.3% (8/15) had collateral flow through anterior communicating artery (AcomA) prestenting. After stenting, all the AcomA collaterals in HGSS reverted to normal (P = 0.002).

CONCLUSIONS

The improvement of brain perfusion combined with the normalization of collateral flow through the circle of Willis after CAS was observed only in patients with HGSS.

New insights into coupling and uncoupling of cerebral blood flow and metabolism in the brain

The brain has high metabolic and energy needs and requires continuous cerebral blood flow (CBF), which is facilitated by a tight coupling between neuronal activity, CBF, and metabolism. Upon neuronal activation, there is an increase in energy demand, which is then met by a hemodynamic response that increases CBF. Such regional CBF increase in response to neuronal activation is observed using neuroimaging techniques such as functional magnetic resonance imaging and positron emission tomography. The mechanisms and mediators (eg, nitric oxide, astrocytes, and ion channels) that regulate CBF-metabolism coupling have been extensively studied. The neurovascular unit is a conceptual model encompassing the anatomical and metabolic interactions between the neurons, vascular components, and glial cells in the brain. It is compromised under disease states such as stroke, diabetes, hypertension, dementias, and with aging, all of which trigger a cascade of inflammatory responses that exacerbate brain damage. Hence, tight regulation and maintenance of neurovascular coupling is central for brain homeostasis. This review article also discusses the waste clearance pathways in the brain such as the glymphatic system. The glymphatic system is a functional waste clearance pathway that removes metabolic wastes and neurotoxins from the brain along paravascular channels. Disruption of the glymphatic system burdens the brain with accumulating waste and has been reported in aging as well as several neurological diseases.

Brain infarction developed in hypertensive and normotensive patients during hospitalization--hemodynamic factors

To determine the effects of changes in blood pressure (BP) on the development of ischemic stroke, 17 patients who developed acute brain infarction during hospitalization were analyzed in relation to the vascular risk factors such as hypertension, previous stroke, diabetes mellitus, and hyperlipidemia. Their BPs were retrospectively referred to the onset of stroke. The average values of mean arterial blood pressure (MABP) on admission were 137 mmHg in hypertensive patients (HT; n = 9) and 101 mmHg in normotensives (NT; n = 8). HT patients received antihypertensive treatment after admission and MABP decreased by 22 mmHg (14%) prior to the ictus (three to sixty-five days after admission). MABP in NT patients remained constant before the attack (-1.4 mmHg, four to one hundred ten days after admission). Similarly, MABP decreased by 17 mmHg (four to ninety-three days) before the ictus in patients with previous stroke (n = 9), whereas it changed only a little in patients without history of stroke (n = 8). BP in each group elevated immediately after the stroke and gradually decreased to 90% of the resting level after two weeks. An excessive reduction of BP may induce brain infarction in HT or chronic stroke patients or cause it to recur.

Contribution of the inflow arteries to alterations in total cerebrovascular resistance in the rabbit

The contribution made by the cerebral inflow arteries to total cerebrovascular resistance (CVR) and their importance in producing alterations in cerebral blood flow (CBF) (i.e., changes in CVR) were investigated. The arterial blood pressure at the circle of Willis was measured in 14 anesthetized rabbits via transorbital retrograde cannulation of the ophthalmic artery. CBF was measured in 21 rabbits under identical experimental conditions, using the hydrogen clearance technique. Inflow artery resistance was calculated from the measurements which were made at both normocapnia and hypercapnia throughout hemorrhagic hypotension. Under resting conditions, the inflow arteries made a relatively minor contribution to total CVR (7%). Hypercapnia resulted in a decrease in CVR and an increase in CBF; however, inflow artery resistance remained constant. Autoregulation and reductions in total CVR were observed as PP was reduced to 35 mm Hg. Inflow artery resistance remained constant at pressures greater than 45 mm Hg and increased slightly at PP less than 45 mm Hg. The relative contribution of inflow artery resistance to total CVR increased under the various conditions studied--increasing by a factor of 2 during hypercapnia, by a factor of 3 during hypotension, and by a factor of 4 during hypotension + hypercapnia. We concluded that the large inflow arteries do not participate in the autoregulatory or CO2 responses of the cerebrovasculature of the rabbit.

The effect of alpha-adrenergic innervation on caudate blood flow

A thermal diffusion probe, with cannulas for intracerebral microinfusion of drugs and an electrode to monitor electroencephalographic (EEG) activity, was used to examine the local effect of vasoactive amines in a 4 to 5 mm sphere of caudate nucleus in cats. The results demonstrated that it is possible to alter local cerebral blood flow (CBF) without causing any change in systemic blood pressure or heart rate, or in CBF and the EEG in the opposite caudate. One-microliter intracerebral injections, containing varying amounts of phenylephrine, increased local CBF in proportion to dose. The effect was blocked by intracerebral infusion of phentolamine. However, local alpha-adrenergic blockade did not inhibit vascular responses to blood pressure elevation or to metabolic influences on local blood flow.