High resolution simulation of basilar artery infarct and flow within the circle of Willis

On a global scale, cerebro- and cardiovascular diseases have long been one of the leading causes of death and disability and their prevalence appears to be increasing in recent times. Understanding potential biomarkers and risk factors will help to identify individuals potentially at risk of suffering an ischemic stroke. However, the widely variable construction of the cerebral vasculature makes it difficult to provide a specific assessment without the knowledge of a patient's physiology. In this paper we use the 3D blood flow simulator HemeLB to study flow within three common structural variations of the circle of Willis during and in the moments after a blockage of the basilar artery. This tool, based on the lattice Boltzmann method, allows the 3D flow entering the basilar artery to be finely controlled to replicate the cessation of blood feeding this particular vessel-we demonstrate this with several examples including a sudden halt to flow and a gradual loss of flow over three heartbeat cycles. In this work we start with an individualised 3D representation of a full circle of Willis and then construct two further domains by removing the left or right posterior communicating arteries from this geometry. Our results indicate how, and how quickly, the circle of Willis is able to redistribute flow following such a stroke. Due to the choice of infarct, the greatest reduction in flow was observed in the posterior cerebral arteries where flow was reduced by up to 70% in some cases. The high resolution domains used in this study permit the velocity magnitude and wall shear stress to be analysed at key points during and following the stroke. The model we present here indicates how personalised vessels are required to provide the best insight into stroke risk for a given individual.

Study protocol of validating a numerical model to assess the blood flow in the circle of Willis

INTRODUCTION

We developed a zero-dimensional (0D) model to assess the patient-specific haemodynamics in the circle of Willis (CoW). Similar numerical models for simulating the cerebral blood flow (CBF) had only been validated qualitatively in healthy volunteers by magnetic resonance (MR) angiography and transcranial Doppler (TCD). This study aims to validate whether a numerical model can simulate patient-specific blood flow in the CoW under pathological conditions.

METHODS AND ANALYSIS

This study is a diagnostic accuracy study. We aim to collect data from a previously performed prospective study that involved patients with aneurysmal subarachnoid haemorrhage (aSAH) receiving both TCD and brain Computerd Tomography angiography (CTA) at the same day. The cerebral flow velocities are calculated by the 0D model, based on the vessel diameters measured on the CTA of each patient. In this study, TCD is considered the gold standard for measuring flow velocity in the CoW. The agreement will be analysed using Pearson correlation coefficients.

ETHICS AND DISSEMINATION

This study protocol has been approved by the Medical Ethics Review Board of the University Medical Center Groningen: METc2019/103. The results will be submitted to an international scientific journal for peer-reviewed publication.

TRIAL REGISTRATION NUMBER

NL8114.

Quantification of blood flow patterns in the cerebral arterial circulation of individual (human) subjects

There is a growing research interest in quantifying blood flow distribution for the entire cerebral circulation to sharpen diagnosis and improve treatment options for cerebrovascular disease of individual patients. We present a methodology to reconstruct subject-specific cerebral blood flow patterns in accordance with physiological and fluid mechanical principles and optimally informed by in vivo neuroimage data of cerebrovascular anatomy and arterial blood flow rates. We propose an inverse problem to infer blood flow distribution across the visible portion of the arterial network that best matches subject-specific anatomy and a given set of volumetric flow measurements. The optimization technique also mitigates the effect of uncertainties by reconciling incomplete flow data and by dissipating unavoidable acquisition errors associated with medical imaging data.

Autoregulating Cerebral Tissue Selfishly Exploits Collateral Flow Routes Through the Circle of Willis

OBJECTIVE

Ischemic stroke is a leading cause of death and disability. Autoregulation and collateral blood flow through the circle of Willis both play a role in preventing tissue infarction. A steady-state model of the cerebral arterial network was used to investigate the interaction of these mechanisms when autoregulation is impaired ipsilateral to an occluded artery.

MATERIALS AND METHODS

Twelve structural variants of the circle of Willis were modelled with left internal carotid artery occlusion and coupled with (1) a passive model of the cerebral vascular bed, (2) a steady-state model of an autoregulating cerebral vascular bed, and (3) a model in which the contralateral hemisphere autoregulates and the ipsilateral hemisphere does not.

RESULTS

Results showed that if the autoregulatory response is impaired ipsilaterally, then, in the autoregulating hemisphere, cerebral flows are preserved at the expense of those on the ipsilateral side.

CONCLUSIONS

Thus, although autoregulation is an essential facilitator of collateral flow through the circle of Willis, contralateral autoregulation can exacerbate flow reductions if not balanced by the same response in the vascular beds on the ipsilateral side. The status of the autoregulatory response in both hemispheres can strongly influence cerebral blood flows and tissue survival and should, therefore, be monitored in stroke.

Reversible Cerebral Vasoconstriction Syndrome

Reversible cerebral vasoconstriction syndrome (RCVS) is a disease characterized by thunderclap headache with severe vasospasm of middle sized vessels of circle of Willis or the extracranial circulation which spontaneously revert back. We report a middle aged female with severe headache and vasospasm of the vertebral arteries and vessels of circle of Willis causing multiple cerebral infarcts. The vasospasm resolved within 3 months.

Three-dimensional hemodynamics analysis of the circle of Willis in the patient-specific nonintegral arterial structures

The hemodynamic alteration in the cerebral circulation caused by the geometric variations in the cerebral circulation arterial network of the circle of Wills (CoW) can lead to fatal ischemic attacks in the brain. The geometric variations due to impairment in the arterial network result in incomplete cerebral arterial structure of CoW and inadequate blood supply to the brain. Therefore, it is of great importance to understand the hemodynamics of the CoW, for efficiently and precisely evaluating the status of blood supply to the brain. In this paper, three-dimensional computational fluid dynamics of the main CoW vasculature coupled with zero-dimensional lumped parameter model boundary condition for the CoW outflow boundaries is developed for analysis of the blood flow distribution in the incomplete CoW cerebral arterial structures. The geometric models in our study cover the arterial segments from the aorta to the cerebral arteries, which can allow us to take into account the innate patient-specific resistance of the arterial trees. Numerical simulations of the governing fluid mechanics are performed to determine the CoW arterial structural hemodynamics, for illustrating the redistribution of the blood flow in CoW due to the structural variations. We have evaluated our coupling methodology in five patient-specific cases that were diagnosed with the absence of efferent vessels or impairment in the connective arteries in their CoWs. The velocity profiles calculated by our approach in the segments of the patient-specific arterial structures are found to be very close to the Doppler ultrasound measurements. The accuracy and consistency of our hemodynamic results have been improved (to [Formula: see text] %) compared to that of the pure-resistance boundary conditions (of 43.5 [Formula: see text] 28 %). Based on our grouping of the five cases according to the occurrence of unilateral occlusion in vertebral arteries, the inter-comparison has shown that (i) the flow reduction in posterior cerebral arteries is the consequence of the unilateral vertebral arterial occlusion, and (ii) the flow rate in the anterior cerebral arteries is correlated with the posterior structural variations. This study shows that our coupling approach is capable of providing comprehensive information of the hemodynamic alterations in the pathological CoW arterial structures. The information generated by our methodology can enable evaluation of both the functional and structural status of the clinically significant symptoms, for assisting the treatment decision-making.

Recruitment Pattern in a Complete Cerebral Arterial Circle

Blood flow through a vessel depends upon compliance and resistance. Resistance changes dynamically due to vasoconstriction and vasodilation as a result of metabolic activity, thus allowing for more or less flow to a particular area. The structure responsible for directing blood to the different areas of the brain and supplying the increase flow is the cerebral arterial circle (CAC). A series of 1D equations were utilized to model propagating flow and pressure waves from the left ventricle of the heart to the CAC. The focus of the current research was to understand the collateral capability of the circle. This was done by decreasing the peripheral resistance in each of the efferent arteries, up to 10% both unilaterally and bilaterally. The collateral patterns were then analyzed. After the initial 60 simulations, it became apparent that flow could increase beyond the scope of a 10% reduction and still be within in vivo conditions. Simulations with higher percentage decreases were performed such that the same amount of flow increase would be induced through each of the efferent arteries separately, same flow tests (SFTs), as well as those that were found to allow for the maximum flow increase through the stimulated artery, maximum flow tests (MFTs). The collateral pattern depended upon which efferent artery was stimulation and if the stimulation was unilaterally or bilaterally induced. With the same amount of flow increase through each of the efferent arteries, the MCAs (middle cerebral arteries) had the largest impact on the collateral capability of the circle, both unilaterally and bilaterally.

Simulation of one-dimensional blood flow in networks of human vessels using a novel TVD scheme

An extension of a total variation diminishing (TVD) scheme to solve one-dimensional (1D) blood flow for human circulation is proposed. This method is simple as it involves only a few modifications to existing shock-capturing TVD schemes. We have applied the method to a wide range of test cases including a complete simulation of the human vascular network. Excellent solutions have been demonstrated for problems involving varying and discontinuous mechanical properties of blood vessels. For 1D network simulations, the method has been shown to agree well with the reported computational results. Finally, the method has been demonstrated to compare favorably with in vivo experiments set up to study the impact of circle of Willis anomalies on flow patterns in the cerebral arterial system.

Impedance boundary conditions for general transient hemodynamics

We discuss the implementation and calibration of a new generalized structured tree boundary condition for hemodynamics. The main idea is to approximate the impedance corresponding to the vessels downstream from a specific outlet. Unlike previous impedance conditions, the one considered here is applicable to general transient flows as opposed to periodic ones only. The physiological character of the approach significantly simplifies calibration. We also describe a novel way to incorporate autoregulation mechanisms in structured arterial trees at minimal computational cost. The strength of the approach is illustrated and validated on several examples through comparison with clinical data.

Morphometric, geographic, and territorial characterization of brain arterial trees

Morphometric information of the brain vascularization is valuable for a variety of clinical and scientific applications. In particular, this information is important when creating arterial tree models for imposing boundary conditions in numerical simulations of the brain hemodynamics. The purpose of this work is to provide quantitative descriptions of arterial branches, bifurcation patterns, shape, and geographical distribution of the arborization of the main cerebral arteries as well as estimations of the corresponding vascular territories. For this purpose, subject-specific digital reconstructions of the brain vascular network created from 3T magnetic resonance angiography images of healthy volunteers are used to derive population-averaged morphometric characteristics of the cerebral arterial trees. Copyri

Long term follow-up of unilateral occlusion of the internal carotid artery including repeated tests of vasomotor reactivity by transcranial Doppler

BACKGROUND

Early study on pathological flow pattern in the ophthalmic artery (OA), connected with impaired vasomotor reactivity (VMR) and low pulsatility index (PI) in the anterior cerebral artery (ACA) on the occluded side, suggested collateral exhaustion. We undertook this study to evaluate whether the occurence of new strokes is predicted by special haemodynamic features.

METHOD

A total of 27 patients (22 men and five women), aged 63+/-15 years (mean+/-SD) with longstanding occlusion of the internal carotid artery (ICA), confirmed by duplex scanning were studied. They had minimal neurological deficit and were followed-up for mean 4.3+/-1.8 (mean+/-SD) years by repeated clinical and 3-D transcranial Doppler (3-D TCD) examinations with azetazolamide test of vasomotor reactivity (VMR).

RESULTS

During follow-up, seven patients had new strokes (five minor strokes and two major ones), two ipsilateral and four contralateral to the ICA occlusion and one in the posterior circulation. Four patients died, All patients experiencing a new stroke had previous symptoms and pathological flow patterns in the OA, i.e retrograde or isoelectric flow were noted in six of them. One patient with contralateral stroke experienced occlusion of the ICA located above the origin of the OA with anterograde flow; otherwise none of 11 patients with anterograde flow had a new stroke (p<0.05, Fisher exact text). During the follow-up, the initial mean velocity (MV) in the middle cerebral artery (MCA) on the occluded side in six patients with a new stroke in the anterior circulation, was 26.83+/-10.50 cm/s, which was significantly different from that of patients without a new stroke (45.80+/-12.8 cm/s) (p<0.01). MV in the ICA on the non-occluded side at the last examination was greater than that at the first examination (p<0.05) and increased after the use of acetazolamide only on this side (p<0.05), while PI decreased bilateraly (p<0.001 and 0.05). Resting MV both in the MCA on the occluded and ACA on the non-occluded side slightly decreased, while MV in the posterior cerebral artery (PCA) increased on the occluded side (p<0.083) compared with that at the start of the follow-up. VMR in the ACA decreased slightly both on the non-occluded and occluded side (Delta-6.9 and Delta-5.3 respectively), while impaired VMR<or=11% was not significantly connected with new strokes.

CONCLUSION

During the follow up, new strokes had appeared on both sides and in vertebrobasilar territory and were connected with pathological flow pattern in the OA and low MV in the MCA at the first examination.

[Mathematical modeling of the response of a resistive vessel to pressure]

The response of small arterial vessels to internal pressure makes an essential contribution to autoregulation in the vascular bed. It is believed that intracellular cytosolic Ca2+ concentration plays a pivotal role in the regulation of smooth muscle contractility and hence of vascular lumen. A simple mathematical model of blood flow in a resistive vessel is suggested. The model is based on the experimental data obtained for cerebral arteries, but may be used for any other resistive vessel. The model not only describes the regulation of the vascular lumer by transmural pressure but also shows realistic behaviour of the radius and intracellular cytosolic Ca2+ concentration at different velocities of pressure change. Possible variations of the radius along the vessel as a consequence of the Bayliss effect are considered.

Comparison of two methods (left carotid artery and abdominal aorta) for surgical implantation of radiotelemetry devices in CD-1 mice

The goal of this study was to compare two surgical methods, the left carotid (LC) and the abdominal aorta (AA), for mouse instrumentation with telemetry devices, to determine the best method for measuring cardiovascular (CV) parameters by radiotelemetry in freely moving mice. Surgery success rate, postsurgical recovery rate, clinical parameters, CV data (baseline and response to nicotine) and circadian rhythm measurements were compared between these techniques. Brains of LC-implanted mice were evaluated for potential ischaemia by direct observation of the Circle of Willis anatomy and histopathology. For this purpose, a total of 31 CD-1 male mice were instrumented with PA C20 devices (10 with LC and 21 with AA). Mortality, morbidity, physical examination, body weight (BW), water and food consumption (W/FC), mean blood pressure (MBP) and heart rate (HR) were monitored daily during the recovery period (10 days). CV baseline data were recorded continuously during two periods of four days, and finally, both LC- and AA-implanted mice received an acute subcutaneous administration of 1 mg/kg nicotine; BP and HR were recorded during 5 h after nicotine administration. Results showed that, in LC-implanted mice, 80% survived surgery and recovered well. In contrast, only 57% of mice implanted with the AA technique survived surgery and some presented lethal complications. Both techniques had similar recovery times for BW and W/FC, comparable return to normal circadian rhythm (day 6 post-surgery) and similar CV baseline values. No significant differences were observed in CV response to nicotine between both groups of implanted CD-1 mice. No histopathological changes suggestive of ischaemia were noted in the brain of mice implanted in the LC. Six out of the eight LC-implanted mice remained in good health and had good pressure signal for at least 100 days post-surgery, while most of the AA-implanted mice lost the signal pressure within 14-49 days post-surgery. In conclusion, we believe that LC implantation in mice is superior to the AA technique and is more appropriate for long-term telemetry studies, especially for smaller (transgenic) animals.

Computation of hemodynamics in the circle of Willis

BACKGROUND AND PURPOSE

Wall shear stress (WSS) and pressure are important factors in the development of cerebral aneurysms. We aimed to develop a computational fluid dynamics simulator for flow in the complete circle of Willis to study the impact of variations in vessel radii and bifurcation angles on WSS and pressure on vessel walls.

METHODS

Blood flow was modeled with Navier-Stokes equations as an incompressible newtonian fluid within rigid vessel walls. A model of the circle of Willis geometry was approximated as a network of tubes around cubic curves. Pulsatile inlet flow rates and constant outlet pressure were used as boundary conditions.

RESULTS

The simulations confirmed that differences in vessel radii and asymmetric branch angles influence WSS magnitude and spatial distribution. High WSS occurred at locations where aneurysms are frequent and in anatomic variants known to be associated with an increased risk for aneurysm development.

CONCLUSIONS

Computational fluid dynamics analysis can be applied to the complete circle of Willis and should be used to study the pathophysiology of this complex vascular structure, including risk factors for aneurysm development. Further development of the method should include simulations with flexible vessel walls.

Optimized protocol to reduce variable outcomes for the bilateral common carotid artery occlusion model in mice

The pre-clinical global ischemia model transient bilateral common carotid artery occlusion addresses the unique cascade of events leading to delayed neuronal cell death. However, the inconsistent occurrence of posterior communicating arteries (PcomA) in mice might cause high outcome variability. To determine a means for reducing variability, CD1 mice were subjected to bilateral common carotid artery occlusion for 12-40 min. Occlusion duration> or =18 min was applied to mice with bilateral regional cerebral blood flow (rCBF)> or =10% of baseline at 2.5 min of ischemia. However, only groups with ischemic duration< or =18 min were used for statistical analysis because of the high mortality in the other groups. After 7 days, patency of PcomA and hippocampal neuronal loss in the CA1 subfield were evaluated. Outcome variability was reduced when hemispheres containing PcomA were excluded from analysis; ischemic outcome was not affected by the presence of a contralateral PcomA. Extending ischemic duration based on rCBF did not reduce outcome variability because the initial rCBF could not reliably predict PcomA. Therefore, after an optimal ischemic duration, evaluating hippocampal injury in each hemisphere independently according to the existence of PcomA is an effective and reliable method to obtain consistent results in this pre-clinical mouse model.

REM sleep related increase in brain temperature: a physiologic problem

The roles of metabolic heat production, arterial blood flow and temperature in the genesis of the brain temperature increase related to REM sleep occurrence in several mammalian species are discussed on the basis of available experimental evidence. The experimental data show that only changes in arterial blood flow and temperature consistently underlie the rise in brain temperature in presence (cat) or absence (rabbit) of the carotid rete. The alteration of cardiovascular regulation in REM sleep is the remote cause of such rise. The proximate causes are decrease in carotid blood supply and increase in vertebral blood supply to the brain and related depression of systemic and selective brain cooling.

Adaptation of the hypothalamic blood flow to chronic nitric oxide deficiency is independent of vasodilator prostanoids

The aim of our study was to investigate the adaptation of the hypothalamic circulation to chronic nitric oxide (NO) deficiency in rats. Hypothalamic blood flow (HBF) remained unaltered during chronic oral administration of the NO synthase (NOS) inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME, 1 mg/ml drinking water) although acute NOS blockade by intravenous l-NAME injection (50 mg/kg) induced a dramatic HBF decrease. In chronically NOS blocked animals, however, acute l-NAME administration failed to influence the HBF. Reversal of chronic NOS blockade by intravenous l-arginine infusion evoked significant hypothalamic hyperemia suggesting the appearance of a compensatory vasodilator mechanism in the absence of NO. In order to clarify the potential involvement of vasodilator prostanoids in this adaptation, cyclooxygenase (COX) mRNA and protein levels were determined in the hypothalamus, but none of the known isoenzymes (COX-1, COX-2, COX-3) showed upregulation after chronic NOS blockade. Furthermore, levels of vasodilator prostanoid (PGI(2), PGE(2) and PGD(2)) metabolites were also not elevated. Interestingly, however, hypothalamic levels of vasoconstrictor prostanoids (TXA(2) and PGF(2alpha)) decreased after chronic NOS blockade. COX inhibition by indomethacin but not by diclofenac decreased the HBF in control animals. However, neither indomethacin nor diclofenac induced an altered HBF-response after chronic l-NAME treatment. Although urinary excretion of PGI(2) and PGE(2) metabolites markedly increased during chronic NOS blockade, indicating COX activation in the systemic circulation, we conclude that the adaptation of the hypothalamic circulation to the reduction of NO synthesis is independent of vasodilator prostanoids. Reduced release of vasoconstrictor prostanoids, however, may contribute to the normalization of HBF after chronic loss of NO.

Simulation of cerebral hemodynamics for preoperative risk assessment

BACKGROUND

An important part of the medical treatment of many cerebrovascular diseases is the occlusion of brain supplying arteries. Until now, the risk of this intervention can only be estimated by invasive diagnostics including the risk of cerebrovascular accidents.

METHODS AND RESULTS

As a supporting tool, a computer model of the circle of Willis was designed. The model is based upon linear differential equations describing electrotechnical circuits extended non-linearly. By these means, time continuous simulations of different states and the online observation of all calculated state variables such as blood pressure and blood flow in every modeled vessel became feasible. For individual simulations, model parameters were determined by MR-angiography and boundary values by simultaneous Duplex-measurements in both carotid and vertebral arteries. State variables generated by the model behaved physiologically and the reaction of individual cerebrovascular systems in critical situations could be investigated by special scenarios. Inaccuracies concerning the determination of model parameters and boundary values of the used differential equations are likely to be resolved in the near future through a more careful and technically improved determination of these values.

CONCLUSIONS

Computer models of subjects were created taking in account the individual anatomical and non-linear physical properties of real vascular systems supplying the brain. Thereby information could be obtained concerning the hemodynamic effects of an iatrogenic vascular occlusion.

Relationship between variations in the circle of Willis and flow rates in internal carotid and basilar arteries determined by means of magnetic resonance imaging with semiautomated lumen segmentation: reference data from 125 healthy volunteers

BACKGROUND AND PURPOSE

Volume flow rates in the feeding arteries of the brain are measured to evaluate blood flow dynamics in vascular disease. Although these flow values are thought to be effected by anatomic variations in the circle of Willis, few reports have described the effect. This study reports on the relationship between variations in the circle of Willis and volume flow rates in the bilateral internal carotid and basilar arteries of normal volunteers.

METHODS

We prospectively examined 125 healthy volunteers by MR imaging. Variations in the circle of Willis were classified as "textbook" type, hypoplasia of the precommunicating segment of the anterior cerebral artery (A1), hypoplasia of the precommunicating segment of the posterior cerebral artery (P1), or "other." Volume flow rates were measured by 2D cine phase-contrast MR imaging. Lumen boundaries and volume flow rates were semiautomatically determined by pulsatility-based segmentation.

RESULTS

Of the 117 subjects (61 men, 56 women; mean age, 23.6 years) considered suitable for flow measurement, 105 showed textbook type, and 6 each showed A1 hypoplasia and P1 hypoplasia. Total flow rates for the 3 variations were 781 +/- 151 mL/min (mean +/- SD), 744 +/- 119, and 763 +/- 129, respectively. Relative contributions by flow rates of the internal carotid arteries and the basilar artery for the 3 variations were 39.8%:38.9%:21.3%, 31.8%:49.1%:19.0%, and 46.6%:41.6%:11.7%, respectively, showing statistically significant differences.

CONCLUSIONS

Variations in the circle of Willis correlate significantly with relative contributions by the flow rates of the bilateral internal carotid and basilar arteries.

Interrupted Aortic Arch and Aortic Atresia With Circle of Willis-Dependent Coronary Perfusion

We present a case of type B interrupted aortic arch and aortic atresia. This combination is usually incompatible with life. However, the presence of an aberrant right subclavian artery and enough blood flow through the circle of Willis allowed perfusion of the coronary arteries through retrograde carotid and aortic blood flow. A two-ventricular repair was successfully undertaken.

Granulocyte-macrophage colony-stimulating factor as an arteriogenic factor in the treatment of ischaemic stroke

Granulocyte-macrophage colony-stimulating factor (GM-CSF) induces arteriogenic growth of collateral vessels after occlusion of cardiac or peripheral arteries. Recently, evidence has been provided that arteriogenesis also occurs in the brain under conditions of reduced arterial blood supply. Hemispheric hypoperfusion induced by unilateral carotid and bilateral vertebral artery occlusion (three-vessel occlusion, [3-VO]) led to the growth of the anterior and posterior segments of the circle of Willis, which is the main collateral pathway between the origins of the anterior, middle and posterior cerebral arteries. GM-CSF applied subcutaneously at daily doses of 40 microg.kg(-1) resulted in the marked acceleration of this process. Within one week after the onset of treatment, the diameter of the posterior segment of the circle of Willis enlarged to 170% of control, blood flow and the haemodynamic reserve capacity of the brain returned to normal, and haemodynamic stroke, induced after 3-VO by systemic hypotension, was greatly alleviated. GM-CSF-induced stimulation of arteriogenesis in the hypoperfused brain thus provides powerful protection against ischaemic stroke.

Metabolic Model of Autoregulation in the Circle of Willis

The Circle of Willis (CoW) is a ringlike structure of blood vessels found at the base of the brain. Its main function is to distribute oxygen-rich arterial blood to the cerebral mass. In a previous study, a one-dimensional (1D) model of the CoW was created to simulate a series of possible clinical scenarios such as occlusions in afferent arteries, absent or stringlike circulus vessels, or arterial infarctions (Moorhead et al., 2004, Comput. Methods Biomech. Biomed. Eng., 7(3), pp. 121–130). The model captured cerebral haemodynamic autoregulation by using a proportional-integral-derivative (PID) controller to modify efferent artery resistances. Although some good results and correlations were achieved, the model was too simple to capture all the transient dynamics of autoregulation. Hence a more physiologically accurate model has been created that additionally includes the oxygen dynamics that drive the autoregulatory response. Results very closely match accepted physiological response and limited clinical data. In addition, a set of boundary conditions and geometry is presented for which the autoregulated system cannot provide sufficient perfusion, representing a condition with increased risk of stroke and highlighting the importance of modeling the haemodynamics of the CoW. The system model created is computationally simple so it can be used to identify at-risk cerebral arterial geometries and conditions prior to surgery or other clinical procedures.

One-dimensional and three-dimensional models of cerebrovascular flow

The Circle of Willis is a ring-like structure of blood vessels found beneath the hypothalamus at the base of the brain. Its main function is to distribute oxygen-rich arterial blood to the cerebral mass. One-dimensional (1D) and three-dimensional (3D) computational fluid dynamics (CFD) models of the Circle of Willis have been created to provide a simulation tool which can potentially be used to identify at-risk cerebral arterial geometries and conditions and replicate clinical scenarios, such as occlusions in afferent arteries and absent circulus vessels. Both models capture cerebral haemodynamic autoregulation using a proportional-integral (PI) controller to modify efferent artery resistances to maintain optimal efferent flow rates for a given circle geometry and afferent blood pressure. The models can be used to identify at-risk cerebral arterial geometries and conditions prior to surgery or other clinical procedures. The 1D model is particularly relevant in this instance, with its fast solution time suitable for real-time clinical decisions. Results show the excellent correlation between models for the transient efferent flux profile. The assumption of strictly Poiseuille flow in the 1D model allows more flow through the geometrically extreme communicating arteries than the 3D model. This discrepancy was overcome by increasing the resistance to flow in the anterior communicating artery in the 1D model to better match the resistance seen in the 3D results.

Intra-arterial thrombolysis of embolic middle cerebral artery using collateral pathways

BACKGROUND AND PURPOSE

Cervical internal carotid artery (ICA) occlusion associated with middle cerebral artery (MCA) embolic occlusion requires prompt revascularization to prevent devastating stroke. With the advent of endovascular techniques for chemical and mechanical thrombolysis, the clinical outcome of patients with major arterial occlusions will improve. Finding the most expedient pathway to the site of end organ occlusion for thrombolysis is important.

METHODS

We present two cases of acute stroke secondary to thrombotic occlusion of the cervical ICA associated with MCA embolic occlusion treated with intra-arterial thrombolysis via catheter navigation through the posterior communicating artery to the site of MCA arterial occlusion. No attempt was made to transverse the occluded ICA.

RESULTS

Near complete restoration of flow was achieved in one patient and minimal vessel reopening was observed in the other patient. Both patients had good outcomes.

CONCLUSION

Intra-arterial thrombolysis via Circle of Willis collaterals such as the posterior communicating artery for the treatment of acute thrombotic occlusion of the cervical internal carotid artery associated with embolic occlusion of the middle cerebral artery is a therapeutic option. This treatment option avoids the potential complications of navigating through an occluded proximal internal carotid artery and may expedite reopening of the MCA.

In vivo flow territory mapping of major brain feeding arteries

The ability to visualize the perfusion territories of major feeding arteries to the brain is important for many clinical applications. Since the work of Duret in 1874 on vascularization of the brain, many textbooks and atlases have shown schematic drawings of the supply areas of the major cerebral arteries. Recent postmortem studies demonstrated that the variability of the cerebral vascular territories is significantly greater than previously assumed. The aim of the present study was to investigate in vivo, the variability of flow territories of major brain feeding arteries. Flow territory mapping of the anterior (internal carotid arteries) and posterior (basilar artery) circulation was performed in 115 (58 +/- 9 years of age) subjects with selective arterial spin labeling MRI. Flow territory maps for the entire population indicated significant variation in flow territories. However, when the subjects are further categorized into groups with a complete circle of Willis, with a missing A1 segment and with a unilateral or bilateral fetal-type posterior cerebral artery, the results showed considerably lower variation within groups. It is therefore concluded that, the variation observed from the entire population is mainly caused by anatomical variants of the circle of Willis. To relate focal brain lesions to underlying flow territories in individual cases, knowledge of the anatomy of the circle of Willis is essential.

Microanatomy of the subependymal arteries of the lateral ventricle

BACKGROUND

Scarce information about the anatomy of the subependymal arteries (SEAs) is present in the scientific literature.

METHODS

Twenty cerebral hemispheres with injected arteries were microdissected, and the magnetic resonance imaging scans of 100 patients with lacunar infarcts were examined.

RESULTS

The SEAs were found to range in diameter from 40 to 490 microm (mean, 149 microm) and in number between 3 and 12 (average, 5.2). Of these, numbers from 1 to 3 originated from the anterior choroidal artery (AChA), between 1 and 10 from the lateral posterior choroidal artery (LPChA), 1 from the medial posterior choroidal artery (MPChA), and 1 from the internal carotid artery. The SEAs most often arose from the choroidal branches (90%) and less frequently from the thalamic (30%), caudate (35%), or thalamocaudate twigs (20%). The SEAs of the AChA supplied the walls of the temporal horn (100%), the occipital horn (85%), and the atrium (35%). Those of the LPChA perfused the walls of the occipital horn (15%), the atrium (65%), the body of the ventricle (100%), and partially the frontal horn. The SEAs of the MPChA partially nourished the body and the frontal horn (10%). The SEAs may also occasionally supply the caudate nucleus (20%) and the stria terminalis. The anastomoses involving the SEAs were absent. In spite of this, ischemia in the territory of a single SEA was noticed in only 1% of our patients.

CONCLUSIONS

The SEAs are tiny vessels that supply the walls of the lateral ventricle, as well as the caudate nucleus and the stria terminalis occasionally. The obtained anatomic data can have important neurosurgical implications in intraventricular operations.

Non-invasive visualization of collateral blood flow patterns of the circle of Willis by dynamic MR angiography

The circle of Willis plays an important role in the distribution of blood flow in the brain. To obtain dynamic information of the blood flow through the circle of Willis, a dynamic MRA technique based on arterial spin labeling (ASL) is introduced as a non-invasive technique. When the ASL labeling slab is restricted to a single artery, it is possible to visualize selectively the flow distribution of that specific artery. However, because of the decay of the label and the presence of noise it is difficult to extract functional information from these images. In the present study we propose three visualization and post-processing methods for the interpretation of these images. Firstly, the passage of labeled blood was corrected for decay of the label and hereafter shown as a movie. Secondly, by calculating the time of arrival at every location in the arteries of the circle of Willis, a 2D image was reconstructed summarizing the information of the movie. Finally, quantitative flow values were obtained by relating the arterial input function to the passage of labeled blood through a region of interest encompassing the vessel under investigation. Experiments in a circle of Willis phantom showed a high linear relation between measured flow and true flow, although the measured values were 10-15% lower than the true flow values. Measurements in healthy volunteers showed the potential to quantify the flow in all major arteries of the circle of Willis.

Investigations of flow and pressure distributions in physical model of the circle of Willis

The paper presents the results of experiments concerning flow in the model of cerebral supplying arteries and the circle of Willis (CW). Vascular phantom was prepared on the basis of anatomical specimens. The most typical artery shapes and dimensions were considered. Pressure distribution in six characteristic points is provided, and so are the average flow rates in the anterior, middle and posterior section of the brain. Tests were run in the conditions replicating the physiological state (i.e. when the supplying arteries were fully patent) and in pathological conditions, in which the internal carotid and vertebral arteries were occluded on one or both sides. Thus obtained results were compared with the results of computer simulations based on linear and non-linear flow models. To estimate the non-linear resistance of vascular segment two phenomenological formulae were proposed. High degree of correlation between the values obtained from experiments and those registered in non-linear computer model proves usefulness of proposed formulae. It verifies the hypothesis that non-linearity of flow characteristics of the vessel segments to a great extent is caused by their tortuousity and small length in relation to diameter. Non-linear effects are particularly pronounced in conditions of pathological occlusion of supplying vessels.

Effect of carotid endarterectomy on primary collateral blood flow in patients with severe carotid artery lesions

BACKGROUND AND PURPOSE

In patients with severe obstruction of the internal carotid artery (ICA), it is recognized that the preoperative failure to visualize collaterals of the circle of Willis increases the risk of hemispheric ischemia before, during, and after carotid endarterectomy (CEA). The purpose of the present study was to assess the effect of CEA on the anatomy and function of the circle of Willis.

METHODS

Time-of-flight and phase-contrast MR angiography were used to study changes in vessel diameter and collateral flow of the circle of Willis in 48 patients with 70% to 99% ICA stenosis before and after CEA.

RESULTS

In patients with unilateral ICA stenosis, all preoperative vessel diameters on both sides of the circle of Willis were larger than in control subjects. All demonstrated a significant diameter decrease to reach normal values after CEA. Furthermore, preoperative collateral flow patterns normalized after CEA (P=0.03). In patients with stenosis and contralateral ICA occlusion, CEA resulted in a significant increase in the prevalence of collateral flow via the anterior communicating artery (33% to 83%, P<0.01) and a significant increase in diameter of both A1 segments (P<0.05) in patients in whom collateral flow developed after CEA.

CONCLUSIONS

CEA reduces the caliber of compensatory collateral channels to normal levels by MR angiography measurements in the presence of severe unilateral stenosis; when the opposite side is occluded and the stenosis is removed ipsilaterally, a greater amount of compensatory collateral circulation can be measured on both the occluded side and the fully opened side.

Arteriogenesis in hypoperfused rat brain

Experimental and clinical studies have provided evidence for spontaneous and therapeutically induced arteriogenesis after occlusion of major peripheral or cardiac vessels. Such evidence is lacking for the cerebrovascular system. In halothane-anesthetized rats, different degrees of brain hypoperfusion were induced by one- to four-vessel occlusion, that is, one or both common carotid arteries in combination with or without bilateral vertebral artery occlusion. The flow decline was monitored by laser Doppler flowmetry, the residual hemodynamic reserve by testing flow reactivity to ventilation with 6% CO(2) and arteriogenesis by intravascular latex infusion and immunohistochemistry of vascular proliferation and monocyte adhesion. The optimum condition for induction of arteriogenesis was three-vessel (one carotid plus both vertebral arteries) occlusion, which led to reduction of blood flow to about 50% and complete suppression of CO(2) reactivity, but no histologic injury. One week after three-vessel occlusion, the ipsilateral posterior cerebral artery significantly enlarged by 39%, and after 3 weeks by 72%, paralleled by the partial return of CO(2) reactivity and the appearance of immunohistochemical markers of arteriogenesis. Three-vessel occlusion is a reliable model for the induction of arteriogenesis in the adult brain and is a new approach for exploring the potentials of arteriogenesis for the prevention of progressing brain ischemia.

The circle of Willis in healthy older persons

BACKGROUND

The current indication for carotid surgery is based upon the percentage of stenosis of the internal carotid artery. This approach shows many limitations, one of which is to presume the anatomical completeness of the circle of Willis in all patients who are evaluated. On the contrary, there is increasing evidence of a great natural variability in the configuration of the circle of Willis.

METHODS

The aim of the present paper was to investigate, by magnetic resonance angiography, the variability of the circle of Willis in a cohort of 118 healthy older persons.

RESULTS

The circle of Willis showed an entirely complete configuration in 47% of the subjects, a complete configuration of its anterior part in 90% of the subjects, and a complete configuration of its posterior part in 48.5% of the subjects.

CONCLUSIONS

These findings confirm the great variability of the circle of Willis even in healthy older persons and suggest that, in indicating carotid surgery, the configuration of the circle of Willis should also be taken into account.

Changes in middle cerebral artery blood flow after carotid endarterectomy as monitored by transcranial Doppler

OBJECTIVE

By using transcranial Doppler (TCD) it is possible to measure blood flow velocities within the circle of Willis. In this study, TCD was performed before and after carotid endarterectomy (CEA) with the aim to describe cerebral hemodynamics after normalization of the carotid artery blood flow.

METHODS

Thirty CEA patients were consecutively entered into the TCD study, whereas 15 patients were referred for postoperative TCD for various clinical reasons. All 45 patients were investigated by using TCD: first preoperatively, then during the first few days after CEA before discharge from the hospital, and finally 3 to 12 months later. In addition, all patients underwent duplex investigation of the internal carotid artery the day before surgery and 3 months postoperatively. For the analysis, the patients were divided into two groups, one with (S-group), suspected postoperative neurologic complications/symptoms and another one without (C-group). Six patients were assigned to the S-group and 37 to the C-group, the latter including two patients who underwent bilateral CEAs.

RESULTS

In the whole study group,a significant postoperative increase in systolic flow velocity was recorded bilaterally in the middle cerebral artery (MCA) as measured some days after surgery. The patients in the S-group showed high blood flow velocities mainly in the MCA on the ipsilateral side. A contralateral flow velocity increase did not occur in patients with very severe contralateral stenosis or occlusion (n = 9) if the late follow-up investigation was chosen as a reference value. Twenty patients in the C-group formed a subgroup with high blood pressure and/or headache postoperatively (CB-group) The other 19 patients were referred to as the CA-group. The CB- and S-groups showed more pronounced vessel disease in internal carotid artery on the contralateral side combined with lower collateral capacity in the circle of Willis compared to the CA-group. In the S-group the mean +/- standard deviation peak systolic velocity in ipsilateral MCA increased from a preoperative value of 0.71 +/- 0.22 m/sec to 2.23 +/- 0.72 m/sec (P <.005). In the CB-group, we observed a bilateral MCA blood flow velocity increase from 0.72 +/- 0.18 to 1.35 +/- 0.56 m/sec (P <.0001) on the ipsilateral side and from 0.82 +/- 0.37 to 1.28 +/- 0.66 m/sec (P < 0.001) on the contralateral side. In the CA-group, we observed minor bilateral blood flow velocity increases in the MCA, from 0.79 +/- 0.25 m/sec to 1.03 +/- 0.33 m/sec on the ipsilateral (P <.001) and from 0.70 +/- 0.17 m/sec to 0.93 +/- 0.26 m/sec on the contralateral side (P <.005). At the follow-up 3 to 12 months after surgery, the MCA flow velocities had returned to normal.

CONCLUSIONS

Soon after surgery, blood flow velocity increases often bilaterally in the MCA. However a contralateral flow velocity increase did not occur in patients with very severe contralateral stenosis or occlusion if the late follow-up investigation was chosen as a reference value. The clinical significance of bilateral flow velocity increases is uncertain, but very high blood flow velocities might be a signal for cerebrovascular hyperperfusion. In those patients, increased postoperative surveillance is recommended.

Computational models of blood flow in the circle of Willis

A two-dimensional, steady state model of the circle of Willis has been developed. To simulate the peripheral resistance of the cerebrovascular tree, blocks of porous media were used. Their effective resistance was kept constant, disregarding the effects of arterial auto-regulation. The model was then used to simulate different common abnormalities of the circle of Willis while a range of varying boundary conditions was imposed to the right internal carotid artery (ICA). The total flux was tabulated and compared favourably with both clinical measurements and other models of the circle of Willis. Relevant fluid dynamics effects were also observed and analysed. The present model demonstrates that the use of CFD can produce physiological results if the appropriate boundary conditions are used. We can provide clinicians with a priority list of the severity of the flux reduction for the considered abnormalities for different degrees of stenosis of the right ICA. From this study it is apparent that the redistribution of blood via the circle of Willis is mainly driven by changes in the vascular resistance of the brain rather than in the local arterial geometry. The use of valid peripheral resistances allows for a more realistic model of the circle of Willis but also highlights the need for more accurate means to estimate the vascular resistance of a patient.

[Clinical importance of assessment of collateral capacity in the circle of Willis. Benefit of a mathematical blood flow model for the every-day practice of vascular surgery]

Collateral capacity of the Willisian arteries is of clinical importance during and after carotid endarterectomies.

AIMS

Assessment of cerebral hemodynamics using a flow circulation model based on a mathematical formula.

PATIENTS AND METHODS

Four patients suffering from ischemic stroke in moribund stage were investigated using transcranial color-coded duplex sonography. By compressing the common carotid arteries, the function of the Willisian collaterals was assessed. After the death of the patients, the circles were removed, the diameters and lengths of the arterial segments were measured. The data were analysed with the mentioned circulation model.

RESULTS

The diameters of non-functioning collateral arteries were 0.4 mm, while that of the functional ones were 0.7 and 0.8 mm, respectively. In the two cases where the anterior communicating arteries did not function, a near-critical hemodynamical status was found in the end-arteries. This was especially true if the mean arterial blood pressure was 70 mmHg. The most critical hemodynamical status developed in case 4, where internal carotid occlusion on one side, a contralateral severe carotid stenosis and a non-functioning anterior communicating artery were observed.

CONCLUSIONS

A special flow circulation model based on mathematical formula enables the calculation of the cerebral blood flow in the different arterial segments of the circle of Willis. Further studies are needed to clarify whether the method can be used for preoperative modeling of the cross-clamping phase of carotid endarterectomy.

A model of the cerebral and cerebrospinal fluid circulations to examine asymmetry in cerebrovascular reactivity

The authors examined the steal phenomenon using a new mathematical model of cerebral blood flow and the cerebrospinal fluid circulation. In this model, the two hemispheres are connected through the circle of Willis by an anterior communicating artery (ACoA) of varying size. The right hemisphere has no cerebrovascular reactivity and the left is normally reactive. The authors studied the asymmetry of hemispheric blood flow in response to simulated changes in arterial blood pressure and carbon dioxide concentration. The hemispheric blood flow was dependent on the local regulatory capacity but not on the size of the ACoA. Flow through the ACoA and carotid artery was strongly dependent on the size of the communicating artery. A global interhemispheric "steal effect" was demonstrated to be unlikely to occur in subjects with nonstenosed carotid arteries. Vasoreactive effects on intracranial pressure had a major influence on the circulation in both hemispheres, provoking additional changes in blood flow on the nonregulating side. A method for the quantification of the crosscirculatory capacity has been proposed.

Omental transplantation for Alzheimer's disease

The acetylcholine, dopamine, serotonin and other neurotransmitters may be reduced in the subcommissural regions even in the early stages of Alzheimer's disease(AD), due to hypoperfusion of the anterior perforating and anterior choroidal arteries. This hypothesis was confirmed after the transplant of omental tissue on the optic chiasma, carotid crotch and anterior perforated space in a woman with moderate AD. Neurological improvement was better in the first week after the surgery than in the following months.

Evaluation of posterior cerebral artery blood flow with transcranial Doppler sonography: value and risk of common carotid artery compression

PURPOSE

Investigations of the posterior cerebral arteries (PCA) by transcranial Doppler sonography (TCD) may be less reliable than investigations of the anterior part of the circle of Willis. Nevertheless, a true PCA may be identified by manual compression of the proximal common carotid artery (CCA) during TCD. Therefore, we used CCA compression in clinically indicated TCD studies and assessed retrospectively its risks and prospectively its benefits for PCA evaluations.

METHODS

Using the transtemporal approach, we prospectively assessed flow velocities in posteriorly located blood vessels in 180 consecutive patients before and during CCA compression. The complications of CCA compression were retrospectively reviewed in all 3,383 clinical TCD investigations performed over an 8-year period.

RESULTS

Decreased flow velocities during ipsilateral CCA compression occurred in 17% of patients. A PCA-like vessel with perfusion from the carotid artery or PCA supply from the carotid circulation was unmasked. Mixed distal PCA support by the posterior communicating artery and proximal PCA could not be shown by TCD. Transient cerebral symptoms occurred in less than 0.4% of the 3,383 retrospectively reviewed TCD investigations; no other adverse effects were seen.

CONCLUSIONS

TCD without CCA compression may lead to false identification of the PCA. Since transient cerebral symptoms during CCA compression are rare, CCA compression can be used when a clinical TCD investigation of intracranial collateral blood flow compensation is indicated or when the identification of a cerebral artery is uncertain.

A nonlinear biomathematical model for the study of intracranial aneurysms

The formation and rupture of aneurysms is a significant medical problem, but is not clearly understood. Most intracranial aneurysm are located in the circle of Willis. We consider a nonlinear mathematical model that simulates the blood flow inside the aneurysm, one of the relevant factors in the evolution of an aneurysm. Different techniques from nonlinear analysis are used in order to obtain, from the model, several consequences that would help to understand some medical aspects of aneurysms of the circle of Willis.

A comparison of cerebral hemodynamic parameters between transient monocular blindness patients, transient ischemic attack patients and control subjects

PURPOSE

To assess whether patients with transient monocular blindness (TMB) and patients with hemispheric transient ischemic attacks (hTIA) differ from each other with respect to cerebral hemodynamic parameters.

METHODS

Seventeen TMB patients and 23 hTIA patients with a moderate to severe stenosis or an occlusion of the internal carotid artery (ICA) underwent magnetic resonance (MR) angiography, (1)H MR spectroscopy and transcranial Doppler sonography. Thirty-one control subjects were investigated to obtain reference values for the MR investigations. Quantitative flow was measured in the ICAs, the basilar artery and the middle cerebral arteries (MCA). Metabolic changes in the MCA territory were studied by assessing N-acetyl-aspartate (NAA)/choline ratios and prevalences of lactate. The prevalence of collateral flow was assessed in the circle of Willis and the ophthalmic arteries. The vasomotor reactivity was studied by measuring the CO(2) reactivity of the MCA territories.

RESULTS

Quantitative flow in the cerebropetal arteries and the MCAs did not differ between TMB patients and hTIA patients. Also patterns of collateral flow, prevalence of lactate and CO(2) reactivity were similar. The mean ipsilateral NAA/choline ratio was lower in hTIA patients compared with TMB patients (p < 0.01), and was predominantly correlated with symptomatology (p < 0.01), i.e. whether patients had TMB or hTIA, and not with ipsilateral MCA flow (p = 0.2) or ipsilateral CO(2) reactivity (p = 0.7).

CONCLUSION

The results of this study indicate that there are no cerebral hemodynamic differences between TMB patients and hTIA patients. It is therefore unlikely that hemodynamic factors account for differences in clinical characteristics between the two patient groups.

Changes in selective brain cooling across the behavioral states of the ultradian wake-sleep cycle

In cats, the behavioral state-dependent negative correlation of the pontine-hypothalamic temperature difference, an indicator of selective brain cooling, with the hypothalamic-ear pinna temperature difference, which is an indicator of heat loss from the heat exchangers of the head, is suppressed after bilateral common carotid ligature. Behavioral state-dependent selective brain cooling may underlie a thermal feedback mechanism differentiating the relative influences of hypothalamic and extra-hypothalamic thermoreceptors on the thermoregulatory system during quiet wakefulness and NREM sleep.

Inconsistent MRI findings in acute pre-Willisian brain ischemia in rabbit: a useful model?

BACKGROUND

We have investigated a model of "preterminal ischemia" in order A) to assess the MRI findings and the repeatability of the anatomical changes as showed by MRI, and B) to compare the data so obtained with the humans.

METHODS

Twenty rabbits were used. Under general anesthesia, the occlusion of 1) the two common carotid arteries (from 2 hours up to 24 hours) was performed in 8 cases, and of 2) the epi-aortic vessels at the aortic arch (from 2 hours up to 4 hours) was performed in 4 cases in general anesthesia.

RESULTS

In the vessels occlusion group MRI study showed inconsistent, inconstant, predominantly ill defined, linear and/or spotlike hyperintense deep cerebral alterations, variable in location, unilateral and bilateral in 8 cases. Although in carotid occlusion the damage was already evident within the first two hours, aortic arch occlusion was unable to produce global brain lesional pattern even up to 4 hours. MRI study of the sham operated animals was negative.

CONCLUSIONS

The selective vulnerability of mesial deep cerebral structures to hypoxia, different in location from the human's, and the more effective pre-Willisian compensation, suggests to look for different animals when dealing with preterminal ischemic models comparable to the humans' as studied with MRI.

Multimodality monitoring during passive tilt and Valsalva maneuver under hypercapnia

In occlusive cerebrovascular disease cerebral blood flow (CBF) autoregulation can be impaired and constant CBF during fluctuations in blood pressure (BP) cannot be guaranteed. Therefore, an assessment of cerebral autoregulation should consider not only responsiveness to CO2 or Diamox. Passive tilting (PT) and Valsalva maneuver (VM) are established tests for cardiovascular autoregulatory function by provoking BP changes. To develop a comprehensive test for vasomotor reactivity with a potential increase of sensitivity and specificity, the authors combined these maneuvers. Blood pressure, corrected to represent arterial pressure at the level of the circle of Willis, middle cerebral artery Doppler frequencies (DF), heart rate (HR) and endtidal partial pressure of CO2 (PtCO2) were measured continuously and noninvasively in 81 healthy subjects (19-74 years). Passive tilt and Valsalva maneuver were performed under normocapnia (mean, 39 + 4 mmHg CO2) and under hypercapnia (mean, 51 + 5 mm Hg CO2). Resting BP, HR, and DF increased significantly under hypercapnia. Under normocapnia and hypercapnia, PT induced only minor, nonsignificant changes in mean BP at the level of the circle of Willis compared to baseline (normocapnia: + 2 + 15 mm Hg; hypercapnia: -3 +/- 13 mm Hg). This corresponded with a nonsignificant decrease of the mean of DF (normocapnia: -4 +/- 11%; hypercapnia -6 +/- 12%). Orthostasis reduced pulsatility of BP by a predominantly diastolic increase of BP without significant changes in pulsatility of DF. Valsalva maneuver, with its characteristic rapid changes of BP due to elevated intrathoracic pressure, showed no significant BP differences in changes to baseline between normocapnic and hypercapnic conditions. Under both conditions the decrease in BP in phase II was accompanied by significantly increased pulsatility index ratio (PIDF/PIBP). Valsalva maneuver and PT as established tests in autonomic control of circulation provoked not only changes in time-mean of BP but also in pulsatility of BP. The significant increase in pulsatility ratio and decrease of the DF/BP ratio during normocapnia and hypercapnia indicated preserved CBF autoregulation within a wide range of CO2 partial pressures. Hypercapnia did not significantly influence the autoregulatory indices during VM and PT. Physiologically submaximally dilated cerebral arterioles can guarantee unchanged dynamics of cerebral autoregulation. Combined BP and MCA-DF assessment under hypercapnia enables investigating the effect of rapid changes of blood pressure on CO2-induced predilated cerebral arterioles. Assuming no interference of hypercapnia-induced vasodilation, VM, with its rapid, distinct changes in BP, seems especially to be adequate provocation for CBF autoregulation. This combined vasomotor reactivity might provide a more sensitive diagnostic tool to detect impaired cerebral autoregulation very early.

Occlusion of the MCA by an intraluminal filament may cause disturbances in the hippocampal blood flow due to anomalies of circle of Willis and filament thickness

We examined blood flow changes and histology in the hippocampus induced by occlusion of the middle cerebral artery (MCA) by a filament in Swiss albino and SV-129 mice (n=67) and in Wistar rats (n=64). Filling cerebral arteries with carbon black revealed that one or both posterior communicating arteries were hypoplastic in 50% of Swiss mice. Ischemic changes were detected in the ipsilateral hippocampus with 2,3,5-triphenyl tetrazolium chloride or hematoxylin and eosin staining when these mice were subjected to 2-h MCA occlusion and 22-h reperfusion. No such abnormalities were found in SV-129 mice and Wistar rats (except one). The hippocampal blood flow dropped to 60+/-2.3% of the baseline in mice with a normal circle of Willis but to 37+/-4.2% in those with an incomplete circle when the MCA was occluded with a 6/0 nylon filament. When an 8/0 filament was used, no flow change in mice with a normal circle but a decrease to 60+/-2% in those with an incomplete circle was observed. A flow drop to 63+/-4% was also seen in Wistar rats when a 3/0 filament used. These data demonstrate that occlusion of the MCA by a thick filament may cause flow reduction in the hippocampus, which may be severe enough to lead to infarction if the circle of Willis is anomalous.

Hemodynamic effect of cerebral vasospasm in humans: a modeling study

A mathematical model of cerebral hemodynamics during vasospasm is presented. The model divides arterial hemodynamics into two cerebral territories: with and without spasm. It also includes collateral circulation between the two territories, cerebral venous hemodynamics, cerebrospinal fluid circulation, intracranial pressure (ICP) and the craniospinal storage capacity. Moreover, the pial artery circulation in both territories is affected by cerebral blood flow (CBF) autoregulation mechanisms. In this work, a numerical value to model parameters was given assuming that vasospasm affects only a single middle cerebral artery (MCA). In a first stage, the model is used to simulate some clinical results reported in the literature, concerning the patterns of MCA velocity, CBF and pressure losses during vasospasm. The agreement with clinical data turns out fairly good. In a second stage, a sensitivity analysis on some model parameters is performed (severity of caliber reduction, longitudinal extension of the spasm, autoregulation gain, ICP, resistance of the collateral circulation, and mean systemic arterial pressure) to clarify their influence on hemodynamics in the spastic territory. The results suggest that the clinical impact of vasospasm depends on several concomitant factors, which should be simultaneously taken into account to reach a proper diagnosis. In particular, while a negative correlation between MCA velocity and cross sectional area can be found until CBF is well preserved, a positive correlation may occur when CBF starts to decrease significantly. This might induce false-negative results if vasospasm is assessed merely through velocity measurements performed by the transcranial Doppler technique.

Cavernous sinus syndrome during balloon test occlusion of the cervical internal carotid artery. Report of two cases

The authors report the occurrence of ipsilateral transient cavernous sinus syndrome during balloon test occlusion (BTO) of the cervical internal carotid artery (ICA) and discuss the involved pathomechanisms. The authors reviewed their series of 129 BTOs of the ICA performed between 1989 and 1996. Two patients developed facial paresthesias and transient palsies of the third through sixth cranial nerves during test occlusion of the cervical ICA. The tests were performed prior to planned permanent carotid artery occlusion for the treatment of a neck sarcoma in one patient and a giant cavernous carotid artery aneurysm in the other. The patients' symptoms resolved with deflation of the balloon. When the balloon was subsequently inflated above the inferior cavernous sinus artery (ICSA), one of the patients complained of mild facial discomfort. There was no contralateral weakness or mental status change during test occlusion in either patient. Angiography demonstrated good filling of the ipsilateral intracranial circulation via collateral vessels of the circle of Willis. In these two cases, the cranial nerves in the cavernous sinus were likely supplied by the ICA via the meningohypophyseal trunk and the ICSA. In each case, there was excellent blood supply to the ipsilateral cerebral hemisphere; however, there was probably inadequate retrograde filling of the cranial nerve collateral vessels located where the meningohypophyseal trunk and ICSA originated. These cases emphasize the importance of a patent external carotid artery-ICA connection for successful cervical carotid artery occlusion. Neurological examination during BTO was critical to interpret the clinical manifestations caused by the hemodynamic changes.

Flow velocity of central retinal artery and retrobulbar vessels during cardiovascular operations

OBJECTIVE

Both blood flow monitoring and pressure monitoring are necessary to avoid inadequate cerebral perfusion during cardiovascular operations. Inasmuch as transcranial Doppler ultrasonography does not provide a consistently good signal, especially during cardiopulmonary bypass, we examined the blood flow through the central retinal artery, which has proved to reflect an obstruction of the carotid artery.

METHOD

Twenty-eight consecutive cases were examined with a 5 or 7.5 MHz conventional echocardiographic probe. Correlation between the maximal velocity at the central retinal artery and systolic blood pressure was examined. The blood flow of the central retinal artery and retrobulbar vessels was examined during selective or retrograde cerebral perfusion or intraaortic balloon pumping.

RESULTS

Blood flow could be clearly visualized but disappeared below a certain pressure in every case. With data from 478 measuring points, systolic blood pressure correlated with maximal velocity (r = 0.6902, p < 0.0001). The blood pressure-axis intercept, known as "critical closing pressure," was 35.8 +/- 14.8 mm Hg, varying among individuals and bilateral eyes. Pulsatility index increased after cardiopulmonary bypass (1.095 +/- 0.245 to 1.525 +/- 0.268, p < 0.0001). Patency of the circle of Willis was confirmed by the blood flow during anastomosis of the ipsilateral artery. During retrograde cerebral perfusion, blood flow was detectable at the retrobulbar vessels. During intraaortic balloon pumping, the central retinal artery flow was augmented on inflation of the balloon.

CONCLUSION

Orbital vessel monitoring provides the critical closing pressure of the central retinal artery and confirms patency of the circle of Willis. The eye can be "an acoustic window" into intracranial blood flow during cardiovascular surgery.

Extended Willis circle model to explain clinical observations in periorbital arterial flow

A fluid-dynamic model of the circle of Willis and its periorbital links with the external carotid arteries has been established and tested. It is based on anatomic data and takes Doppler measurements as flow input conditions. The model explains, on fluid-dynamic grounds, the clinical observations of periorbital reverse flow and arrival pulse time delay. It also obtains the velocity and pressure pulse at any point of the studied area. This allows the comparison between the normal or healthy condition and the flow distribution when an internal carotid is externally or pathologically occluded. Several combinations of the communicating artery sizes are explored to obtain the reduced cerebral flow. The combination of the communicating diameters can lead to insufficient irrigation which can be hydrodynamically assessed. No other physiological response is included, and the results must be considered as a minimum assured. These results show the need for a common evaluation of the alternative paths and explain some paradoxes found in literature.

[Impedance of cerebrovascular system simulated by a hemodynamic model of brain circulation]

A hemodynamics model of cerebral Willis circulation is proposed. The model consists of the compensation of blood flow as well as the compliance, resistance and inductance of arteries. The impedances of carotid and vertebral arteries are obtained by solving the govern equations. The theoretical impedances correspond well with the experimental results. The simulating results demonstrate that the alteration of the arterial dynamic parameters influences not only the ipsilateral impedance but also the contralateral arterial system. These conclusions suggest that the carotid or vertebral impedance is not suitable for describing the behaviour of its own arterial system, because the impedance is also influenced by the other arterial system and blood pressure.