Increased venous pressure causes myogenic constriction of cerebral arterioles during local hyperoxia

Cerebral Pathophysiology in Extracorporeal Membrane Oxygenation: Pitfalls in Daily Clinical Management

Extracorporeal membrane oxygenation (ECMO) is a life-saving technique that is widely being used in centers throughout the world. However, there is a paucity of literature surrounding the mechanisms affecting cerebral physiology while on ECMO. Studies have shown alterations in cerebral blood flow characteristics and subsequently autoregulation. Furthermore, the mechanical aspects of the ECMO circuit itself may affect cerebral circulation. The nature of these physiological/pathophysiological changes can lead to profound neurological complications. This review aims at describing the changes to normal cerebral autoregulation during ECMO, illustrating the various neuromonitoring tools available to assess markers of cerebral autoregulation, and finally discussing potential neurological complications that are associated with ECMO.

Effect of Internal Jugular Vein Compression on Intracranial Hemorrhage in a Porcine Controlled Cortical Impact Model

Internal jugular vein (IJV) compression has been shown to reduce axonal injury in pre-clinical traumatic brain injury (TBI) models and clinical concussion studies. However, this novel approach to prophylactically mitigating TBI through venous congestion raises concerns of increasing the propensity for hemorrhage and hemorrhagic propagation. This study aims to test the safety of IJV compression in a large animal controlled cortical impact (CCI) injury model and the resultant effects on hemorrhage. Twelve swine were randomized to placement of a bilateral IJV compression collar (CCI+collar) or control/no collar (CCI) prior to CCI injury. A histological grading of the extent of hemorrhage, both subarachnoid (SAH) and intraparenchymal (IPH), was conducted in a blinded manner by two neuropathologists. Other various measures of TBI histology were also analyzed including: β-amyloid precursor protein (β-APP) expression, presence of degenerating neurons, extent of cerebral edema, and inflammatory infiltrates. Euthanized 5 h after injury, the CCI+collar animals exhibited a significant reduction in total SAH (p = 0.024-0.026) and IPH scores (p = 0.03-0.05) compared with the CCI animals. There was no statistically significant difference in scoring for the other markers of TBI (β-APP, neuronal degeneration, cerebral edema, or inflammatory infiltration). In conclusion, IJV compression was shown to reduce hemorrhage (SAH and IPH) in the porcine CCI model when applied prior to injury. These results suggest the role of IJV compression for mitigation of not only axonal, but also hemorrhagic injury following TBI.

Myogenic and metabolic feedback in cerebral autoregulation: Putative involvement of arachidonic acid-dependent pathways

The present paper presents a mechanistic model of cerebral autoregulation, in which the dual effects of the arachidonic acid metabolites 20-hydroxyeicosatetraenoic acid (20-HETE) and epoxyeicosatrienoic acids (EETs) on vascular smooth muscle mediate the cerebrovascular adjustments to a change in cerebral perfusion pressure (CPP). 20-HETE signalling in vascular smooth muscle mediates myogenic feedback to changes in vessel wall stretch, which may be modulated by metabolic feedback through EETs released from astrocytes and endothelial cells in response to changes in brain tissue oxygen tension. The metabolic feedback pathway is much faster than 20-HETE-dependent myogenic feedback, and the former thus initiates the cerebral autoregulatory response, while myogenic feedback comprises a relatively slower mechanism that functions to set the basal cerebrovascular tone. Therefore, assessments of dynamic cerebral autoregulation, which may provide information on the response time of the cerebrovasculature, may specifically be used to yield information on metabolic feedback mechanisms, while data based on assessments of static cerebral autoregulation represent the integrated functionality of myogenic and metabolic feedback.

Prognostic Impact of Cerebral Small Vessel Disease on Stroke Outcome

Cerebral small vessel disease (SVD), which includes white matter hyperintensities (WMHs), silent brain infarction (SBI), and cerebral microbleeds (CMBs), develops in a conjunction of cumulated injuries to cerebral microvascular beds, increased permeability of blood-brain barriers, and chronic oligemia. SVD is easily detected by routine neuroimaging modalities such as brain computed tomography or magnetic resonance imaging. Research has revealed that the presence of SVD markers may increase the risk of future vascular events as well as deteriorate functional recovery and neurocognitive trajectories after stroke, and such an association could also be applied to hemorrhagic stroke survivors. Currently, the specific mechanistic processes leading to the development and manifestation of SVD risk factors are unknown, and further studies with novel methodological tools are warranted. In this review, recent studies regarding the prognostic impact of WMHs, SBI, and CMBs on stroke survivors and briefly summarize the pathophysiological concepts underlying the manifestation of cerebral SVD.

Influence of acute jugular vein compression on the cerebral blood flow velocity, pial artery pulsation and width of subarachnoid space in humans

Purpose

The aim of this study was to assess the effect of acute bilateral jugular vein compression on: (1) pial artery pulsation (cc-TQ); (2) cerebral blood flow velocity (CBFV); (3) peripheral blood pressure; and (4) possible relations between mentioned parameters.

Methods

Experiments were performed on a group of 32 healthy 19–30 years old male subjects. cc-TQ and the subarachnoid width (sas-TQ) were measured using near-infrared transillumination/backscattering sounding (NIR-T/BSS), CBFV in the left anterior cerebral artery using transcranial Doppler, blood pressure was measured using Finapres, while end-tidal CO₂ was measured using medical gas analyser. Bilateral jugular vein compression was achieved with the use of a sphygmomanometer held on the neck of the participant and pumped at the pressure of 40 mmHg, and was performed in the bend-over (BOPT) and swayed to the back (initial) position.

Results

In the first group (n = 10) during BOPT, sas-TQ and pulse pressure (PP) decreased (−17.6% and −17.9%, respectively) and CBFV increased (+35.0%), while cc-TQ did not change (+1.91%). In the second group, in the initial position (n = 22) cc-TQ and CBFV increased (106.6% and 20.1%, respectively), while sas-TQ and PP decreases were not statistically significant (−15.5% and −9.0%, respectively). End-tidal CO₂ remained stable during BOPT and venous compression in both groups. Significant interdependence between changes in cc-TQ and PP after bilateral jugular vein compression in the initial position was found (r = −0.74).

Conclusions

Acute bilateral jugular venous insufficiency leads to hyperkinetic cerebral circulation characterised by augmented pial artery pulsation and CBFV and direct transmission of PP into the brain microcirculation. The Windkessel effect with impaired jugular outflow and more likely increased intracranial pressure is described. This study clarifies the potential mechanism linking jugular outflow insufficiency with arterial small vessel cerebral disease.

Carotid compression: investigation of cerebral autoregulative reserve in rats

Easy-to-perform, reversible techniques to analyse cerebral autoregulation are still missing in animal research. The carotid compression technique has been established to investigate dynamic cerebral autoregulation in humans. Adapting the carotid compression technique, we compared data from the new application with that of a classical exsanguination method. Compressing the ipsilateral carotid artery with a non-traumatic clip device for 10s modulated cerebral perfusion pressure. After clip release, the peaking laser-Doppler flow velocity increase over the somatosensory cortex allowed calculation of the transient hyperaemic response ratio (THRR) in relation to baseline. Modulating blood-pressure levels maintenance of cerebral blood-flow velocity was compared with THRR responses. With decreasing blood-pressure levels, the THRR first increased (29+/-16% at 95+/-10 mmHg to 39+/-13% at 75+/-10 mmHg) before it returned to baseline values at 54+/-10 mmHg (27+/-14%). THRR significantly dropped to 11+/-12% at 34+/-11 mmHg when resting cerebral blood-flow velocity levels also started to decline. Based on the close correlation between blood-flow velocity levels and THRR responses, we have concluded that carotid compression is an alternative technique that can be used to assess cerebral autoregulation in rats. The technique allows less invasive and reversible testing of dynamic autoregulation to be performed, and the technique can easily be applied in conjunction with functional tests to potentially allow deeper insights into cerebral vasoregulative mechanisms.

Dynamic autoregulatory response after severe head injury

OBJECT

The purpose of this study was to evaluate the extent and timing of impairment of cerebral pressure autoregulation after severe head injury.

METHODS

In a prospective study of 122 patients with severe head trauma (median Glasgow Coma Scale Score 6), dynamic tests of pressure autoregulation were performed every 12 hours during the first 5 days postinjury and daily during the next 5 days. The autoregulatory index ([ARI] normal value 5 +/- 1.1) was calculated for each test. The changes in the ARI over time were examined and compared with other physiological variables. The ARI averaged 2.8 +/- 1.9 during the first 12 hours postinjury, and continued to decrease to a nadir of 1.7 +/- 1.1 at 36 to 48 hours postinjury. At this nadir, in 87% of the patients the value was less than 2.8. This continued deterioration in the ARI during the first 36 to 48 hours postinjury occurred despite an increase in cerebral blood flow ([CBF], p < 0.05) and in middle cerebral artery blood flow velocity ([BFV], p < 0.001), and could not be explained by changes in cerebral perfusion pressure, end-tidal CO2, or cerebral metabolic rate of O2. A marked decrease in cerebrovascular resistance ([CVR], p < 0.001) accompanied this deterioration in the ARI. Patients with a relatively higher BFV on Day 1 had a lower CVR (p < 0.05) and more impaired pressure autoregulation than those with a lower BFV.

CONCLUSIONS

The inability of cerebral vessels to regulate CBF normally may play a role in the vulnerability of the injured brain to secondary ischemic insults. These studies indicate that this vulnerability continues and even increases beyond the first 24 hours postinjury. Local factors affecting cerebrovascular tone may be responsible for these findings.

Autoregulatory vasodilation of parenchymal vessels is impaired during cerebral vasospasm

Impaired CBF autoregulation during vasospasm after aneurysmal subarachnoid hemorrhage (SAH) could reflect impaired capacity of distal vessels to dilate in response to reduced local perfusion pressure or simply indicate that the perfusion pressure distal to large arteries in spasm is so low that vessels are already maximally dilated. Autoregulatory vasodilation can be detected in vivo as an increase in the parenchymal cerebral blood volume (CBV). Regional CBV, CBF, and oxygen extraction fraction in regions with and without angiographic vasospasm obtained from 29 positron emission tomography studies performed after intracranial aneurysm rupture were compared with data from 19 normal volunteers and five patients with carotid artery occlusion. Regional CBF was reduced compared to normal in regions from SAH patients with and without vasospasm as well as with ipsilateral carotid occlusion (P < .0001). Regional oxygen extraction fraction was higher during vasospasm and distal to carotid occlusion than both normal and SAH without vasospasm (P < .0001). Regional CBV was reduced compared to normal in regions with and without spasm, whereas it was increased ipsilateral to carotid occlusion (P < .0001). These findings of reduced parenchymal CBV during vasospasm under similar conditions of tissue hypoxia that produce increased CBV in patients with carotid occlusion provide evidence that parenchymal vessels distal to arteries with angiographic spasm after SAH do not show normal autoregulatory vasodilation.

Rapid autoregulation of cerebral blood flow: a laser-Doppler flowmetry study

The mechanisms underlying autoregulation of CBF were studied in 19 rabbits using laser-Doppler flowmetry. A cranial plexiglas window was chronically inserted in the skull with dental cement under general anesthesia. The animals then were reanesthetized 5-7 days later and subjected to aortic bleeding while CBF was measured with the probe placed on the window. In the first set of experiments, MABP was decreased (from 90 to 30 mm Hg) and was maintained constant for 1 min. During the first seconds, CBF followed the steep decrease of MABP. Then, CBF increased and reached a plateau within 3-13 s, depending on the severity of hypotension. Hyperemia occurred when blood was restored, and the CBF recovered from this posthypotensive hyperemia with a rapid phase (within 2 s) and a slow phase (total recovery within 1 min). The lower limit of autoregulation was found to be 40 mm Hg. An increase in CBF due to papaverine showed that vasodilation was not maximal below this limit. In the second set of experiments, the rabbits were subjected to four episodes of hypotension at 40 mm Hg each but of different durations (from 2-3 to 60 s). The posthypotensive hyperemia was not influenced by the duration of hypotension, but the time of the total recovery phase increased with the duration of hypotension. We conclude that there exist rapid adaptive mechanisms leading to autoregulation and that the vasodilation is not dependent upon the duration of hypotension.