Responses of cerebral arterioles to increased venous pressure

Cerebral venous collaterals: A new fort for fighting ischemic stroke?

Stroke therapy has entered a new era highlighted by the use of endovascular therapy in addition to intravenous thrombolysis. However, the efficacy of current therapeutic regimens might be reduced by their associated adverse events. For example, over-reperfusion and futile recanalization may lead to large infarct, brain swelling, hemorrhagic complication and neurological deterioration. The traditional pathophysiological understanding on ischemic stroke can hardly address these occurrences. Accumulating evidence suggests that a functional cerebral venous drainage, the major blood reservoir and drainage system in brain, may be as critical as arterial infusion for stroke evolution and clinical sequelae. Further exploration of the multi-faceted function of cerebral venous system may add new implications for stroke outcome prediction and future therapeutic decision-making. In this review, we emphasize the anatomical and functional characteristics of the cerebral venous system and illustrate its necessity in facilitating the arterial infusion and maintaining the cerebral perfusion in the pathological stroke content. We then summarize the recent critical clinical studies that underscore the associations between cerebral venous collateral and outcome of ischemic stroke with advanced imaging techniques. A novel three-level venous system classification is proposed to demonstrate the distinct characteristics of venous collaterals in the setting of ischemic stroke. Finally, we discuss the current directions for assessment of cerebral venous collaterals and provide future challenges and opportunities for therapeutic strategies in the light of these new concepts.

Non-Invasive Respiratory Impedance Enhances Cerebral Perfusion in Healthy Adults

Optimization of cerebral blood flow (CBF) is the cornerstone of clinical management in a number of neurologic diseases, most notably ischemic stroke. Intrathoracic pressure influences cardiac output and has the potential to impact CBF. Here, we aim to quantify cerebral hemodynamic changes in response to increased respiratory impedance (RI) using a non-invasive respiratory device. We measured cerebral perfusion under varying levels of RI (6 cm H₂O, 9 cm H₂O, and 12 cm H₂O) in 20 healthy volunteers. Simultaneous measurements of microvascular CBF and middle cerebral artery mean flow velocity (MFV), respectively, were performed with optical diffuse correlation spectroscopy and transcranial Doppler ultrasound. At a high level of RI, MFV increased by 6.4% compared to baseline (p = 0.004), but changes in cortical CBF were non-significant. In a multivariable linear regression model accounting for end-tidal CO₂, RI was associated with increases in both MFV (coefficient: 0.49, p < 0.001) and cortical CBF (coefficient: 0.13, p < 0.001), although the magnitude of the effect was small. Manipulating intrathoracic pressure via non-invasive RI was well tolerated and produced a small but measurable increase in cerebral perfusion in healthy individuals. Future studies in acute ischemic stroke patients with impaired cerebral autoregulation are warranted in order to assess whether RI is feasible as a novel non-invasive therapy for stroke.

Hemodynamic effects of developmental venous anomalies with and without cavernous malformations

BACKGROUND AND PURPOSE

Association between developmental venous anomalies is well known, but remains unexplained. Our aim was to study possible hemodynamic differences around developmental venous anomalies with and without cavernous malformations.

MATERIALS AND METHODS

In this prospective study approved by the institutional review board, PWI was performed in 24 patients with 25 DVAs (10 with and 15 without CMs) who consented to participate. We calculated relative cerebral blood volume, relative cerebral blood flow, and relative mean transit time for the brain surrounding the DVA tributaries in reference to contralateral mirror image locations. Corresponding control values (cCBV, cCBF, and cMTT) were generated in a similar fashion for remote ipsilateral regions with normal venous drainage, also in reference to contralateral mirror image locations. Perfusion parameters for DVAs and control regions were tested for differences between groups with the t test for independent or paired samples (or the nonparametric equivalents). Similar testing was done for perfusion parameters for DVAs with and without CMs.

RESULTS

Normal-appearing brain surrounding DVAs showed increased rCBV (median = 2.98; range = 1.39-6.61), increased rCBF (median = 2.00, range = 0.79-4.43), and increased rMTT (mean = 1.46; 95% confidence interval, 1.32-1.59). These were significantly higher than median cCBV (0.99; 95% confidence interval, 0.89-1.06; P < .01), median cCBF (1.00; 95% confidence interval, 0.94-1.27; P < .01), and mean cMTT (1.00; 95% confidence interval, 0.98-1.02; P < .01), respectively. Mean rMTT (1.70; 95% confidence interval, 1.46-1.93) for DVAs with CMs was higher than mean rMTT (1.29; 95% confidence interval, 1.19-1.40; P < .01) for DVAs without CMs.

CONCLUSIONS

DVAs are strongly associated with altered hemodynamics. Significant differences in these hemodynamic alterations for DVAs with and without CMs suggest their possible role in the formation of CMs.

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.

Pial arteriole dilation during somatosensory stimulation is not mediated by an increase in CSF metabolites

Pial arterioles supplying the hindlimb somatosensory cortex dilate in response to contralateral sciatic nerve stimulation. The mechanism of this pial vasodilation is not well understood. One possibility is that vasoactive metabolites released during brain activation may diffuse to subarachnoid cerebrospinal fluid (CSF) to dilate pial vessels. To test this hypothesis, we implanted closed cranial windows in rats and measured pial arteriolar dilation to sciatic nerve stimulation during constant rate superfusion of the pial surface with artificial CSF. We reason that flushing the pial surface with CSF should quickly dissipate vasoactive substances and prevent these substances from dilating pial arterioles. CSF flow (1 and 1.5 ml/min) significantly reduced pial arteriole dilation induced by 5% CO2 inhalation, but the same flow rates did not affect dilator responses to sciatic nerve stimulation. We conclude that brain-to-CSF diffusion of vasoactive metabolites does not play a significant role in the dilation of pial arterioles during somatosensory activity.

Cerebral Doppler and misrepresentation of flow changes

To determine whether cerebral blood flow velocity (CBFV) measurements were representative of cerebral blood flow (CBF) changes in pathological flow situations five newborn piglets were investigated. They underwent measurements of CBF by electromagnetic flowmetry on a modified common carotid artery where extracerbral branches were tied off simultaneously with Doppler recording either from the same precerebral or an intracerebral artery. The two methods agreed well within moderate carbon dioxide and blood pressure changes. During severe hypotension and hypertension Doppler overestimated CBF by 25-100%. During transfusion of infected or incompatible blood the two methods differed in opposite directions with Doppler reading from 30-200% of CBF. Transfusion of chilled blood caused CBFV to overestimate 15% and heated blood caused 20% underestimation. These results could be explained by diameter changes in response to variation in myogenic tone or vasoactive substances. CBFV measurements could be seriously misleading in severe clinical derangements where neonatal brain damage might occur.

A bedside test for cerebral autoregulation using transcranial Doppler ultrasound

Although disorders of cerebral autoregulation are commonly seen in neurosurgical disease, there is currently no test of autoregulation in widespread use that may be performed safely at the bedside. The presence of autoregulation, however, can be seen in the brief hyperemic response in the middle cerebral artery distribution following a transient manual carotid artery compression in the neck. This transient hyperemic response (THR) is readily measured with transcranial Doppler techniques, and therefore might serve as a qualitative marker of cerebral autoregulation. To evaluate the THR as a clinical tool, carotid compressions were performed during 172 TCD studies on 79 patients with neurosurgical disorders and on 10 patients without cerebral disease. The results were correlated with clinical status (e.g., Hunt-Hess Grade for subarachnoid hemorrhage and Glasgow Coma Score for trauma). There were no complications arising from the compressions. A separate assessment of autoregulation was made from TCD recordings obtained intraoperatively during 16 procedures and correlated to the pre-operative THRs. Autoregulation was further assessed in 4 patients during a hypotensive challenge, and again compared to the THRs. A strong correlation was seen between the THR results and clinical status. The THR was also strongly correlated with the intraoperative assessments, and all 4 patients receiving hypotensive challenges had abnormal THRs and demonstrated evidence of poor autoregulation during the challenge. None of the control patients had abnormal THRs. The THR arising from transient artery compression is readily detected with TCD techniques and correlates well with clinical status and other indicators of autoregulatory ability.(ABSTRACT TRUNCATED AT 250 WORDS)