Hemodynamic significance of internal carotid artery disease

Incomplete circle of Willis as a risk factor for intraoperative ischemic events during carotid endarterectomies performed under regional anesthesia - A prospective case-series

Background

The role of the willisian collaterals during carotid endarterectomies (CEAs) is a debated issue. The aim of the present work was to test whether an incomplete or non-functional circle of Willis (CoW) is a risk factor for ischemic events during CEA.

Patients and methods

CEAs were performed under local anesthesia. Patients were considered symptomatic (SY) if neurological signs appeared after the cross-clamping phase. In SY patients shunt insertion was performed. CoW on CT angiograms (CTa) were analyzed offline and categorized as non-functional (missing or hypoplastic collaterals) or functional collaterals by three neuroradiologists. Near-infrared spectroscopy (NIRS) was performed throughout the procedure.

Results

Based on CTa, 67 incomplete circles were found, 54 were asymptomatic (ASY) and 13 were SY. No complete CoW was found among the SY patients. Significant differences could be detected between incomplete and complete circles between ASY and SY groups (Chi-square: 6.08; p = 0.013). The anterior communicating artery was missing or hypoplastic in 5/13 SY cases. There were no cases of the non-functional anterior communicating arteries in the ASY group (Chi-square: 32.9; p = 10^-8). A missing or non-functional bilateral posterior communicating artery was observed in 9/13 SY and in 9/81 ASY patients (Chi-square: 24.4; p = 10^-7). NIRS had a sensitivity of 76.9% and a specificity of 74.5% in detecting neurological symptoms.

Conclusions

Collateral ability of the CoW may be a risk factor for ischemic events during CEAs. Further studies should delineate whether the preoperative assessment of collateral capacity may be useful in decision-making about shunt use during CEA.

Resting-state BOLD functional connectivity depends on the heterogeneity of capillary transit times in the human brain A combined lesion and simulation study about the influence of blood flow response timing

Functional connectivity (FC) derived from blood oxygenation level dependent (BOLD) functional magnetic resonance imaging at rest (rs-fMRI), is commonly interpreted as indicator of neuronal connectivity. In a number of brain disorders, however, metabolic, vascular, and hemodynamic impairments can be expected to alter BOLD-FC independently from neuronal activity. By means of a neurovascular coupling (NVC) model of BOLD-FC, we recently demonstrated that aberrant timing of cerebral blood flow (CBF) responses may influence BOLD-FC. In the current work, we support and extend this finding by empirically linking BOLD-FC with capillary transit time heterogeneity (CTH), which we consider as an indicator of delayed and broadened CBF responses. We assessed 28 asymptomatic patients with unilateral high-grade internal carotid artery stenosis (ICAS) as a hemodynamic lesion model with largely preserved neurocognitive functioning and 27 age-matched healthy controls. For each participant, we obtained rs-fMRI, arterial spin labeling, and dynamic susceptibility contrast MRI to study the dependence of left-right homotopic BOLD-FC on local perfusion parameters. Additionally, we investigated the dependency of BOLD-FC on CBF response timing by detailed simulations. Homotopic BOLD-FC was negatively associated with increasing CTH differences between homotopic brain areas. This relation was more pronounced in asymptomatic ICAS patients even after controlling for baseline CBF and relative cerebral blood volume influences. These findings match simulation results that predict an influence of delayed and broadened CBF responses on BOLD-FC. Results demonstrate that increasing CTH differences between homotopic brain areas lead to BOLD-FC reductions. Simulations suggest that CTH increases correspond to broadened and delayed CBF responses to fluctuations in ongoing neuronal activity.

A narrative review of the pathophysiology of ischemic stroke in carotid plaques: a distinction versus a compromise between hemodynamic and embolic mechanism

Atherosclerotic carotid artery stenosis causes about 10–20% of all ischemic strokes through two main mechanisms: hemodynamic impairment in case of significant stenosis and thromboembolism from an atherosclerotic plaque regardless of the degree of stenosis. The latter is the most frequent mechanism and appear to result from embolization from a vulnerable atherosclerotic plaque or acute occlusion of the carotid artery and propagation of thrombus distally. Downstream infarcts may occur in a territory of major cerebral artery or at the most distal areas between two territories of major cerebral arteries, the so-called watershed (WS), or border zone area. Although WS infarcts, especially deep WS infarct, were historically thought to be due to hemodynamic compromise, the role of microembolism has also been documented, both mechanisms may act synergistically to promote WS infarcts. Routine and more advanced imaging techniques may provide information on the underlying mechanism involved in ipsilateral ischemic stroke. A better understanding of ischemic stroke pathogenesis in carotid stenosis may limit the use of routine non-selective shunt, whose benefit-risk balance is debated, to patients with hemodynamic impairment.

After reviewing existing evidence underpinning the contribution of the two mechanisms in downstream ischemic stroke and the various imaging techniques available to investigate them, we will focus on the pathogenesis of WS infarcts that remains debated.

Hemodynamic impairments within individual watershed areas in asymptomatic carotid artery stenosis by multimodal MRI

Improved understanding of complex hemodynamic impairments in asymptomatic internal carotid artery stenosis (ICAS) is crucial to better assess stroke risks. Multimodal MRI is ideal for measuring brain hemodynamics and has the potential to improve diagnostics and treatment selections. We applied MRI-based perfusion and oxygenation-sensitive imaging in ICAS with the hypothesis that the sensitivity to hemodynamic impairments will improve within individual watershed areas (iWSA). We studied cerebral blood flow (CBF), cerebrovascular reactivity (CVR), relative cerebral blood volume (rCBV), relative oxygen extraction fraction (rOEF), oxygen extraction capacity (OEC) and capillary transit-time heterogeneity (CTH) in 29 patients with asymptomatic, unilateral ICAS (age 70.3 ± 7.0 y) and 30 age-matched healthy controls. In ICAS, we found significant impairments of CBF, CVR, rCBV, OEC, and CTH (strongest lateralization ΔCVR = -24%), but not of rOEF. Although the spatial overlap of compromised hemodynamic parameters within each patient varied in a complex manner, most pronounced changes of CBF, CVR and rCBV were detected within iWSAs (strongest effect ΔCVR = +117%). At the same time, CTH impairments were iWSA independent, indicating widespread dysfunction of capillary-level oxygen diffusivity. In summary, complementary MRI-based perfusion and oxygenation parameters offer deeper perspectives on complex microvascular impairments in individual patients. Furthermore, knowledge about iWSAs improves the sensitivity to hemodynamic impairments.

Assessment of the Cerebral Hemodynamic Benefits of Carotid Artery Stenting for Patients with Preoperative Hemodynamic Impairment Using Cerebral Single Photon Emission Computed Tomography (SPECT) and Carbon Dioxide Inhalation

BACKGROUND The aim of this study was to evaluate the effects of carotid artery angioplasty and carotid artery stenting (CAS) on cerebral blood flow (CBF) and cerebrovascular reactivity (CVR) in patients with preoperative cerebrovascular hemodynamic impairment. MATERIAL AND METHODS Seventeen patients with unilateral severe internal carotid artery (ICA) stenosis and ipsilateral CVR impairment underwent CAS. CBF and CVR were measured by single photon emission computed tomography (SPECT) with inhalation of carbon dioxide (CO2) one week before and three months after CAS. Sixty-eight ROIs in the middle cerebral artery (MCA) territory were analyzed in 17 patients. RESULTS Before CAS, CVR was impaired in all ROIs. CBF was impaired in 16 ROIs (23.5%). The percentage of ROIs with impaired CBF was significantly increased in patients with ≥90% carotid artery stenosis (p=0.047) without collateral flow through the circle of Willis (p=0.005). CAS significantly increased CVR in ROIs with a normal preoperative CBF and impaired CVR, indicating mild hemodynamic impairment (0.9±6.7% vs. 4.9±8.6%) (p=0.014). CAS significantly increased CBF in ROIs with preoperative impaired CBF and impaired CVR, indicating severe hemodynamic impairment (79.1±7.5% vs. 86.7±10.0%) (p<0.001). Following CAS, ROIs with normal CBF and impaired CVR had a significantly increased percentage of improved CVR (p=0.047); ROIs with impaired CBF and impaired CVR had a significantly increased percentage of improved CBF (p=0.027). CONCLUSIONS The severity of preoperative hemodynamic impairment, which is related to the degree of carotid artery stenosis and cerebral collateral flow, may influence hemodynamic benefits by CAS.

Slow gait speed - an indicator of lower cerebral vasoreactivity in type 2 diabetes mellitus

OBJECTIVE

Gait speed is an important predictor of health that is negatively affected by aging and type 2 diabetes. Diabetes has been linked to reduced vasoreactivity, i.e., the capacity to regulate cerebral blood flow in response to CO2 challenges. This study aimed to determine the relationship between cerebral vasoreactivity and gait speed in older adults with and without diabetes.

RESEARCH DESIGN AND METHODS

We studied 61 adults with diabetes (65 ± 8 years) and 67 without diabetes (67 ± 9 years) but with similar distribution of cardiovascular risk factors. Preferred gait speed was calculated from a 75 m walk. Global and regional perfusion, vasoreactivity and vasodilation reserve were measured using 3-D continuous arterial spin labeling MRI at 3 Tesla during normo-, hyper- and hypocapnia and normalized for end-tidal CO2.

RESULTS

Diabetic participants had slower gait speed as compared to non-diabetic participants (1.05 ± 0.15 m/s vs. 1.14 ± 0.14 m/s, p < 0.001). Lower global vasoreactivity (r (2) adj = 0.13, p = 0.007), or lower global vasodilation reserve (r (2) adj = 0.33, p < 0.001), was associated with slower walking in the diabetic group independently of age, BMI and hematocrit concentration. For every 1 mL/100 g/min/mmHg less vasodilation reserve, for example, gait speed was 0.05 m/s slower. Similar relationships between vasodilation reserve and gait speed were also observed regionally within the cerebellum, frontal, temporal, parietal, and occipital lobes (r (2) adj = 0.27-0.33, p < 0.0001). In contrast, vasoreactivity outcomes were not associated with walking speed in non-diabetic participants, despite similar vasoreactivity ranges across groups.

CONCLUSION

In the diabetic group only, lower global vasoreactivity was associated with slower walking speed. Slower walking in older diabetic adults may thus hallmark reduced vasomotor reserve and thus the inability to increase perfusion in response to greater metabolic demands during walking.

Change in cerebral perfusion after carotid angioplasty with stenting is related to cerebral vasoreactivity: a study using dynamic susceptibility-weighted contrast-enhanced MR imaging and functional MR imaging with a breath-holding paradigm

BACKGROUND AND PURPOSE

Carotid angioplasty with stent placement (CAS) is an optional treatment for significant carotid stenosis. Cerebral vasoreactivity (CVR), representing the reserve capacity of cerebral perfusion, usually decreases in patients with severe carotid stenosis. This study aimed to investigate the relationship between the baseline CVR assessed by functional MR imaging (fMRI) and the changes in cerebral blood flow (CBF) after CAS.

MATERIALS AND METHODS

Fourteen patients with at least 70% unilateral carotid stenosis underwent CAS. Baseline CVR was evaluated by fMRI a under breath-holding paradigm. CBF was assessed by dynamic susceptibility-weighted contrast-enhanced MR imaging before and 3-5 days after CAS. The lateral index (LI) was defined as (n - L) / (n + L), where n and L represent the number of activated voxels in fMRI on the normal and lesion hemispheres, respectively.

RESULTS

No subject had clinical evidence of hyperperfusion syndrome. The LI represented baseline CVR. Patients were divided into normal (LI < 0, n = 6) and impaired (LI > 0, n = 8) CVR groups. The CBF on the normal and lesion sides was calculated separately. CBF increment on the lesion side after CAS was significantly higher in the impaired CVR group than that in the normal CVR group (P = .035). There was a significantly positive correlation between CVR impairment and the CBF increment (P = .026).

CONCLUSIONS

fMRI could be a reproducible tool in evaluating CVR. After CAS, early CBF changes on the lesion side are more prominent in patients with impaired CVR. Baseline CVR might predict early CBF increase after CAS.

Endovascular treatment of carotid stump syndrome

We present the case of a carotid stump syndrome in a 72-year-old woman with a 3-day history of recurrent transient ischemic attacks. Computed tomographic angiography showed the occlusion of the ipsilateral internal carotid artery, and the presence of an internal stump with ophthalmic reverse flow, confirming the suspect of a stump syndrome. The patient underwent stent-graft exclusion of the carotid stump; she was last seen 12 months after the procedure when she remained totally asymptomatic.

Changes in Cerebral Perfusion after Revascularization of Symptomatic Carotid Artery Stenosis: CT Measurement

PURPOSE

To prospectively evaluate changes in brain perfusion computed tomographic (CT) parameters after revascularization of unilateral symptomatic carotid artery stenosis and to determine whether pretreatment perfusion CT parameters can be used to predict changes in cerebral hemodynamics after treatment.

MATERIALS AND METHODS

This study was medical ethics committee approved, and written informed consent was obtained from all patients. Thirty-six patients (23 men, 13 women; mean age, 67 years) with unilateral symptomatic carotid artery stenosis underwent multi-detector row perfusion CT before and after revascularization. Mean transit time (MTT), cerebral blood volume (CBV), and cerebral blood flow (CBF) were calculated, and relative values based on the comparison between symptomatic and asymptomatic hemispheres-specifically, relative CBV, relative CBF, and difference in MTT-were derived. The absolute and relative perfusion values before treatment were assessed and compared with posttreatment values. These analyses were performed for the group as a whole by using the t test and after subdividing patients into three tertiles according to the difference in MTT by using the Wilcoxon signed rank test.

RESULTS

Among the absolute perfusion values, only the MTT in the symptomatic hemisphere improved significantly after treatment (P < .01). All relative values (difference in MTT, relative CBV, and relative CBF) changed significantly after treatment (P < .05). When the patients were subdivided into three tertiles according to difference in MTT, no significant change in any relative perfusion value could be demonstrated in the lowest tertile, only the difference in MTT improved significantly (P = .004) in the middle tertile, and all relative perfusion values changed significantly (P = .002) in the highest tertile.

CONCLUSION

Compared with relative CT perfusion values based on interhemispheric comparison, absolute perfusion CT values are less suited for demonstrating changes in cerebral perfusion after revascularization in patients with unilateral symptomatic carotid artery stenosis.

Endovascular Treatment of the Carotid Stump Syndrome

In patients with an occluded internal carotid artery, the carotid stump syndrome is a potential source of microemboli that pass through the ipsilateral external carotid artery and the ophthalmic artery to the territory of the middle cerebral artery. Thus, the syndrome is associated with carotid territory symptoms although the internal carotid artery is occluded. Surgical exclusion of the internal carotid artery associated with endarterectomy of the external carotid artery has been described as the gold standard of treatment by many authors. This report is the second case, to our knowledge, of endovascular treatment of the carotid stump syndrome with the use of a stent-graft.

Management of the Near Total Internal Carotid Artery Occlusion

OBJECTIVES

The risk of stroke in patients with near total internal carotid artery (ICA) occlusion is perceived to be high as stroke risk increases with severity of the stenosis. The management of this entity has not been addressed specifically in the existing randomised trials and thus it remains controversial.

METHODS

Systematic review of the relevant literature.

RESULTS

The management of patients with near total ICA occlusion remains controversial: some favour intervention whereas others have condemned it as dangerous or of no benefit. A prospective multicentre randomised trial regarding intervention versus best medical treatment for patients with symptomatic near total ICA occlusion seems difficult because of the large number of patients required to power the study. Nevertheless, it appears hard to decline surgery based on the current evidence.

CONCLUSIONS

Because of the current controversy over the best management of the near total ICA occlusion, prospective observational studies are needed to demonstrate its prevalence in the symptomatic and asymptomatic population and any associated excess stroke risk. Based on the current evidence, surgery is the treatment of choice in most centres but its validity over best medical treatment remains untested.

Angiographically defined collateral circulation and risk of stroke in patients with severe carotid artery stenosis. North American Symptomatic Carotid Endarterectomy Trial (NASCET) Group

BACKGROUND AND PURPOSE

Blood supply through collateral pathways improves regional cerebral blood flow and may protect against ischemic events. The effect of collaterals on the risk of stroke and transient ischemic attack (TIA), in the presence of angiographic severe internal carotid artery (ICA) stenosis, was assessed.

METHODS

Angiographic collateral filling through anterior communicating and posterior communicating arteries and retrograde filling through ophthalmic arteries were determined in all patients at entry into the North American Symptomatic Carotid Endarterectomy Trial. Kaplan-Meier event-free survival analyses were performed on 339 medically treated and 342 surgically treated patients.

RESULTS

The presence of collaterals supplying the symptomatic ICA increased with severity of stenosis. Two-year risk of hemispheric stroke in medically treated patients with severe ICA stenosis was reduced in the presence of collaterals: 27.8% to 11.3% (P=0.005). Similar reductions were observed for hemispheric TIA (36.1% versus 19.1%; P=0.008) and disabling or fatal strokes (13.3% versus 6.3%; P=0.11). For surgically treated patients, the perioperative risk of hemispheric stroke was 1.1% in the presence of collaterals versus 4. 9% when absent. The 2-year stroke risks for surgical patients with and without collaterals were 5.9% versus 8.4%, respectively. Neither comparison in the surgical group was statistically significant. The observed reductions were independent of the degree of ICA stenosis and other vascular risk factors.

CONCLUSIONS

Collaterals are associated with a lower risk of hemispheric stroke and TIA, both long term and perioperatively. Angiographic identification of collaterals assists in identifying patients with severe ICA stenosis at lower risk of stroke and TIA.

Vasomotor response to CO2 and L-Arginine in patients with severe internal carotid artery stenosis; pre- and post-surgical evaluation with transcranial Doppler

BACKGROUND AND AIM

Carotid artery disease may cause both thromboembolism and cerebral blood flow disturbances, particularly in subjects with impaired hemodynamic compensatory mechanisms. The aim of this study was to evaluate by transcranial Doppler (TCD) the hemodynamic changes induced by CO2 and L-Arginine stimulation in a selected population with severe unilateral carotid stenosis (70-80%), before and after carotid endarterectomy, in order to determine the effect of surgery in the vascular hemodynamics of these patients.

METHODS

We studied 20 subjects (mean age 66.4 years) consecutively admitted to our institute with ischemia and unilateral severe internal carotid artery stenosis (70-80%) detected by Color Doppler. All patients underwent arterial digital subtraction angiography to confirm the ultrasonographic evaluation. TCD was performed bilaterally; blood flow velocity was monitored during CO2 and L-Arginine stimulation both in basal conditions and three months after surgery.

RESULTS

After endarterectomy, mean velocity increased in response to both stimuli with a trend toward statistical significance. A significantly lower reactivity to L-Arginine on the stenotic side was found in the pre-operative phase: this asymmetrical reactivity was no longer observable after carotid endarterectomy.

CONCLUSIONS

We found a statistically significant difference in L-Arginine reactivity in the stenotic side of patients with severe unilateral internal carotid stenosis. This is probably related to an alteration of the endothelium function due to the carotid pathology, since the abnormalities disappeared three months after endarterectomy.

Middle cerebral artery blood velocity, embolisation, and neurological outcome during carotid endarterectomy: a prospective comparison of the Javid and the Pruitt-Inahara shunts

OBJECTIVES

To investigate the in vivo haemodynamic performance and neurological outcome of two types of carotid shunt.

DESIGN

Randomised single surgeon study of consecutive symptomatic patients.

SETTING

163 consecutive patients undergoing carotid endarterectomy for symptomatic carotid disease were randomised to the Javid or Pruitt shunt.

CHIEF OUTCOME MEASURES

Middle cerebral artery velocity (MCAV), preoperatively, during clamping, during shunting and post-restoration of flow, embolic episodes, neurological outcome.

MAIN RESULTS

The MCAV preoperatively, at carotid clamping, and postoperatively was the same for both groups (p > 0.15). During shunting the MCAV was significantly lower in the Pruitt group, p < 0.005, 59% of the Javid and 34% of the Pruitt shunts maintained MCAV at preoperative levels p < 0.005, chi 2 = 8.92. The Javid shunt produced significantly more emboli (73% of cases) at declamping than the Pruitt (41%), p < 0.0002, chi 2 = 14.7. Four Javid patients and one Pruitt had disabling thromboembolic strokes; overall thromboembolic stroke rate 3.7%. The difference in stroke rates was not statistically significant (p = 0.14).

CONCLUSIONS

The Pruitt shunt was unable to maintain preoperative MCAV in 66% of cases, the Javid shunt had a higher incidence of emboli on declamping. These factors may lead to an increased risk of stroke; however, the numbers required for statistical confirmation would be large.

Measurement of carotid bifurcation pressure gradients using the Bernoulli principle

Current randomized prospective studies suggest that the degree of carotid stenosis is a critical element in deciding whether surgical or medical treatment is appropriate. Of potential interest is the actual pressure drop caused by the blockage, but no direct non-invasive means of quantifying the hemodynamic consequences of carotid artery stenoses currently exists. The present prospective study examined whether preoperative pulsed-Doppler duplex ultrasonographic velocity (v) measurements could be used to predict pressure gradients (delta P) caused by carotid artery stenoses, and whether such measurements could be used to predict angiographic percent diameter reduction. Preoperative Doppler velocity and intraoperative direct pressure measurements were obtained, and per cent diameter angiographic stenosis measured in 76 consecutive patients who underwent 77 elective carotid endarterectomies. Using the Bernoulli principle (delta P = 4v(2), pressure gradients across the stenoses were calculated. The predicted delta P, as well as absolute velocities and internal carotid artery/common carotid velocity ratios were compared with the actual delta P measured intraoperatively and with preoperative angiography and oculopneumoplethysmography (OPG) results. An end-diastolic velocity of > or = 1 m/s and an end-diastolic internal carotid artery/common carotid artery velocity ratio of > or = 10 predicted a 50% diameter angiographic stenosis with 100% specificity. Although statistical significance was reached, preoperative pressure gradients derived from the Bernoulli equation could not predict actual individual intraoperative pressure gradients with enough accuracy to allow decision making on an individual basis. Velocity measurements were as specific and more sensitive than OPG results. Delta P as predicted by the Bernoulli equation is not sufficiently accurate at the carotid bifurcation to be useful for clinical decision making on an individual basis. However, end-diastolic velocities alone as well as internal carotid artery/ common carotid artery velocity ratios are highly specific in the prediction of clinically significant carotid stenoses. An end-diastolic velocity of > or = 1 m/s accurately identifies a 50% or greater diameter stenosis, and thus may in some cases be sufficient for operation.

Effects of anterior communicating artery diameter on cerebral hemodynamics in internal carotid artery disease. A model study

BACKGROUND

Collateral circulatory pathways are considered the primary determinant of cerebral hemodynamics in patients with obstructive lesions of the internal carotid arteries (ICaAs). However, the hemodynamic effects of the diameter of the anterior communicating artery (ACoA) have never been assessed quantitatively in humans.

METHODS AND RESULTS

Two different mathematical models were used to simulate changes affecting blood pressures and flows in cerebral arteries as a function of ACoA diameter and ICaA stenoses or occlusions. Small changes in ACoA diameter were found to have marked hemodynamic effects when they occurred within the range of 0.4 to 1.6 mm, a situation observed in 80% of the cases. Outside this range, changes in ACoA diameter had no effect. Simulated pressure drops through a stenotic ICaA were consistent with those observed. They were found to depend on the degrees of the stenoses in both ICaAs and on ACoA diameter according to a simple equation. Pressure reserve in the middle and anterior cerebral arteries decreased to below the lower limit of autoregulation, despite a normal mean arterial blood pressure, when the arteries were distal to a unique 70% ICaA stenosis associated with a small-diameter ACoA or to a 50% ICaA stenosis associated with a contralateral ICaA occlusion and a large-diameter ACoA. Above these thresholds, the circle of Willis allowed for an almost complete global cerebral blood flow compensation that involved all the afferent and communicating vessels.

CONCLUSIONS

ACoA diameter strongly modulates the effects of ICaA lesions on cerebral hemodynamics. Some proposals for endarterectomy indications can be derived from our study.

Simple hyperaemia test as a screening method in the postoperative surveillance of infrainguinal in situ vein bypasses

OBJECTIVES

To develop a simple protocol for ultrasound Duplex surveillance of infrainguinal vein bypasses.

DESIGN

The value of three Doppler waveform parameters, obtained from a single point of the bypass, for identification of stenoses was studied in 91 in situ vein bypasses. Midgraft peak systolic velocity (PSV), pulsatility index (PI) and ratio of hyperaemic and resting time-average mean velocities (TAMV), (TAMV ratio = TAMVhyperaemia/TAMVrest) were correlated with the presence and severity of stenoses as assessed by conventional Duplex scanning and ankle-brachial index (ABI) measurements. The optimal value of the waveform parameters for discrimination between bypasses with and without evidence of stenoses was determined by receiver operating characteristics (ROC) analysis.

MAIN RESULTS

Complete Duplex scanning of the entire graft revealed an increase in the peak systolic velocity by a factor 2.5 indicative of significant stenoses in 24 (26%) patients. A PSV below 55 cm/s was a poor indicator of stenoses (sensitivity 46%, specificity 76%) and PI < or = 3.8 only allowed suboptimal discrimination between normal and stenotic bypasses (sensitivity 63%, specificity 75%). The hyperaemic response assessed by TAMV ratio proved the best parameter for identification of graft stenoses. A TAMV ratio of 2.0 or less correctly identified 21 of the 24 lesions (sensitivity 88%, specificity 75%) and none of the three bypasses with evidence of stenoses and TAMV ratios exceeding 2.0 failed during follow-up.

CONCLUSIONS

Single point waveform analysis of vein bypass velocity profile at rest and during reactive hyperaemia is a simple screening method providing diagnostic and prognostic information which may be of value in the postoperative surveillance of infrainguinal vein bypasses.

Imaging studies of cerebral hyperperfusion after carotid endarterectomy. Case report

A case is reported of severe unilateral hemispheric edema and localized hemorrhage associated with seizures following endarterectomy of an ipsilateral high-grade carotid stenosis. Imaging studies including angiography, computerized tomography (CT), magnetic resonance imaging/angiography, and xenon-CT, suggested postoperative ipsilateral cerebral hyperperfusion. Cerebral hyperperfusion syndromes caused by a probable failure of vascular autoregulation are rare but potentially serious complications after endarterectomy. The literature on this type of complication is briefly reviewed, and the role of various imaging modalities in identification of the syndrome and in guiding management decisions is emphasized.

SPECT imaging of stroke

Single-photon emission computed tomography (SPECT) is a helpful tool for the management of stroke patients. Brain perfusion SPECT can help differentiate an ischemic event from peri-ictal phenomena such as Todd's paresis. Initial data suggest that SPECT may be useful in prognosticating the likelihood of an early stroke after a transient ischemic attack and in distinguishing lacunar from cortical stroke. After an acute stroke, early SPECT depicts the area of ischemia with greater accuracy than either computed tomography or magnetic resonance imaging. When the perfusion defect is large, the likelihood of hemorrhagic complications or herniation increases. Reperfusion of an arterial territory after thrombolysis can be documented more conveniently with SPECT than with angiography. SPECT before and after the injection of acetazolamide has been used to assess the vascular reserve in patients with severe stenosis of the proximal vessels of the cerebrovascular tree. Combined with transcranial Doppler studies, SPECT is used to document ischemia after subarachnoid hemorrhage. It has also been used to assess the effect of arterial ligation intended to treat arteriovenous malformations or aneurysms on brain perfusion and to evaluate ischemia secondary to pressure from an intracranial hemorrhage.

Correlation of cerebral blood flow and MCA flow velocity measured in healthy volunteers during acetazolamide and CO2 stimulation

The assessment of the cerebrovascular reserve capacity (RC) has become a widely used tool in the management of cerebrovascular disease. Discrepancies become obvious, however, if results obtained with different methods are compared. Aim of the present study, therefore, was to compare blood velocity and cerebral perfusion data in the same group of healthy test persons. In 32 volunteers regional cerebral blood flow (rCBF) was measured with the 133Xe-inhalation method. F1 as grey matter flow and the initial slope index (ISI) were computed. Simultaneously flow velocity in the middle cerebral artery (VMCA) was assessed by transcranial Doppler sonography (TCD). Measurements were performed in the resting state, during inhalation of 7% CO2 and after 1 g acetazolamide. Baseline VMCA was 62.38 +/- 16.1 cm/s, 90.84 +/- 23.85 cm/s during hypercapnia and 84.91 +/- 24.54 cm/s after acetazolamide. There was no significant change of baseline or stimulated values with age. F1 rose from baseline 76.25 +/- 12.48 ml/100 g/min to 103.90 +/- 14.6 ml/100 g/min in hypercapnia and to 98.4 +/- 14.9 ml/100 g/min after acetazolamide. The baseline F1 values and the response to CO2 decreased with age (p = 0.01) whereas for the acetazolamide reaction an age dependency could not be proven. ISI baseline values (41.5 +/- 6.1 ml/100 g/min) as well as those found after CO2 or acetazolamide decreased significantly with age. In hypercapnia changes of F1 and ISI were not too well related with changes of VMCA (F1: r = 0.599; ISI: r = 0.473), but better during acetazolamide exposure (F1: r4 = 0.715; ISI: r = 0.522). The age dependency of resting and stimulated values has to be considered when assessing the reserve capacity. There is a correlation between changes of the perfusion and flow parameters in healthy individuals which, however, may be worse in cerebrovascular disease.

Seizures following carotid endarterectomy in patients with severely compromised cerebral circulation

OBJECTIVES

To determine the incidence of postoperative neurological complications following carotid endarterectomy in patients with severely compromised cerebral circulation.

DESIGN

Prospective open clinical study.

SETTING

Department of Vascular Surgery, University Hospital.

MATERIALS AND METHODS

We determined the incidence of postendarterectomy seizures related to haemodynamic impairment in terms of intraoperatively measured perfusion pressure in 151 patients undergoing 153 carotid endarterectomies.

MAIN RESULTS

Cerebral perfusion pressure index (ICA/CCA pressure ratio) was significantly reduced (25% or more) in 47% (55/118) of patients with 70-99% stenosis compared to 6% (2/35) of patients with 30-69% stenosis (p < 0.00005). Among the 57 haemodynamically compromised patients five developed seizures considered due to cerebral hyperperfusion five to seven days after surgery. The seizures were associated with headache in two, focal neurological deficits in four and hypertensive episodes in all cases. The symptoms remitted within 2 weeks and no patients suffered cerebral haemorrhage.

CONCLUSIONS

Seizures following correction of high grade stenoses causing severe pressure reduction may be more common than previously assumed. Patients at risk should be identified and postoperative blood pressure controlled meticulously.

[Is there still a place for routine deep hypocapnia in intracranial surgery?]

Deliberate hypocapnia during the anaesthetic management of the patient undergoing craniotomy has become an accepted standard of care. However there has been a resurgence of interest, in how hypocapnia should be applied in intra- and extra-operative settings. There are three possible therapeutic effects of hypocapnia, namely, (a) reduction of brain bulk through a reduction in cerebral blood volume, with a decrease cerebral blood flow; (b) developing an "inverse steal" by redistribution of blood from normal to ischaemic regions and (c) acting to offset cerebral acidosis by increasing pH in the extracellular space. In anaesthetic intraoperative practice, hypocapnia is used as a specific treatment of, or prophylaxis against, intracranial hypertension during induction of anaesthesia and the period before dural exposure. More commonly, hypocapnia is used for intraoperative brain relaxation (intracranial pressure = 0). Severe hypocapnia (< 20 mmHg) may result in cerebral production of lactate; however no studies have shown that a Paco2 in the range of 23-28 mmHg has deleterious effects. Recent studies in head-injured patients suggest that routine long-term hyperventilation, without an objective index of cerebral flow/metabolism coupling, may place the brain at risk for adverse outcome. The few data available for intraoperative management suggest that Paco2 figures of 30-35 mmHg result in acceptable operating conditions. Unless otherwise specifically indicated by surgical conditions or cerebral flow/metabolism coupling (e.g. jugular O2 saturation), routine application of profound (Paco2 < 28-30 mmHg) hyperventilation should probably be avoided and its use needs reevaluation.

Tomographic cerebral blood flow measurement during carotid surgery

OBJECTIVES

The aim of the study was to depict regional cerebral blood flow (rCBF) during carotid cross clamping using 99mTechnetium-hexamethylpropylene amine oxime (TcHMPAO). This tracer rapidly passes the blood-brain barrier and is retained for hours in the brain tissue. Injecting TcHMPAO during surgery and performing single photon emission computer tomography (SPECT) scanning shortly after the operation thereby pictures rCBF at the time of injection.

DESIGN

Ongoing prospective study.

SETTINGS

Departments of Vascular Surgery, Neurology and Anaesthesiology, University Hospital, Rigshospitalet, Copenhagen, Denmark.

MATERIAL

15 patients who during a period of 4 months underwent carotid endarterectomy.

CHIEF OUTCOME MEASURES

Prior to surgery rCBF was determined using 133Xe and SPECT. Intraoperatively stump pressure was measured and a bolus of TcHMPAO was injected for later SPECT measurement.

MAIN RESULTS

We found a significant correlation between stump pressure and enhancement of side-to-side asymmetry in rCBF due to carotid cross clamping. Pronounced variations were seen in which regions were deprived of perfusion during clamping.

CONCLUSION

TcHMPAO allows tomographic assessment of CBF during carotid surgery. This method may serve as a reference tool in future research on intraoperative cerebral haemodynamics.

Cerebral vasoreactivity in unilateral carotid artery disease. A comparison of blood flow velocity and regional cerebral blood flow measurements

BACKGROUND AND PURPOSE

Hemodynamic information obtained by assessing cerebral vasoreactivity is of clinical interest and may have prognostic significance in patients with occlusive carotid disease. The aim of this study was to compare the results of transcranial Doppler and regional cerebral blood flow studies when used to assess cerebral vasoreactivity.

METHODS

Blood flow velocities in both middle cerebral arteries and regional cerebral blood flow in their respective perfusion territories were compared in 52 patients with severe unilateral carotid stenosis or occlusion. The studies were first performed under basal conditions and repeated after the intravenous administration of 1 g acetazolamide.

RESULTS

Asymmetry (normal compared with pathological side) in middle cerebral artery blood velocity increase was significantly greater than the asymmetry in cerebral blood flow increase in the perfusion territories of the arteries. A significant correlation (r = .63, P < .0001) was found between asymmetry in percent velocity increase and asymmetry in absolute cerebral blood flow increase. The two methods agreed in their assessment of either a normal or a reduced vasoreactivity in 38 subjects and disagreed in 14. In six of the latter patients, who had no evidence of cerebral infarction, the asymmetry in velocity increase was abnormal, whereas asymmetry in flow increase was assessed as normal.

CONCLUSIONS

We found a good correlation between the asymmetry in regional cerebral blood flow increase in the middle cerebral artery perfusion territories and asymmetry in the velocity increase in the middle cerebral arteries after administration of acetazolamide. These results suggest that transcranial Doppler examination combined with the acetazolamide test may be used in clinical situations to assess cerebral vasoreactivity.

Prevalence of impaired cerebrovascular reserve in patients with symptomatic carotid artery disease

Cerebrovascular reserve (CVR) was studied in 104 consecutive patients with symptomatic carotid territory disease and ipsilateral internal carotid artery stenosis. Overall, 30 of 104 patients (29 per cent) had impaired CVR. The frequency of CVR impairment increased with the severity of internal carotid artery stenosis: impairment was present in none of 11 patients with stenosis of less than 50 per cent, four of 24 with stenosis of 50-69 per cent, 14 of 41 with stenosis of 70-89 per cent and 12 of 28 with stenosis of 90-99 per cent. Patients presenting with a stroke were significantly more likely to have impaired CVR than those with transient ischaemic attacks and/or amaurosis fugax (odds ratio 3.7 (95 per cent confidence interval (c.i.) 1.5-9.0)), as were those with a residual neurological deficit (odds ratio 4.3 (95 per cent c.i. 1.6-11.5)) and evidence of infarction from computed tomography (odds ratio 3.8 (95 per cent c.i. 1.6-9.4)).

Serial imaging of the carotid bifurcation and cerebrovascular reserve after carotid endarterectomy

A radioisotopic method of quantifying mean cerebral transit time was used to assess the immediate effects of carotid endarterectomy on cerebrovascular reserve (CVR) in 69 patients. In addition, serial postoperative data were acquired on CVR, clinical status and non-invasive imaging of the internal carotid arteries in 56 patients over a period of 6-48 (median 24) months. Twenty-one patients (30 per cent) had preoperative evidence of impaired CVR in the symptomatic hemisphere. Within 4 days of surgery, however, reserve had returned to normal in 17 of the 21 patients. During follow-up, four of the 56 patients developed recurrent stenosis (> 50 per cent) or occlusion of the artery operated on but only two of these had impairment of CVR and none was symptomatic. Three patients suffered recurrent transient ischaemic attacks (TIAs) but none had recurrent internal carotid artery disease or impaired CVR. One patient suffered a TIA in the territory of the non-operated artery during follow-up in association with disease progression and CVR impairment. However, the TIA preceded recognition of either of these changes. Twelve other patients had (or developed) stenosis (> 50 per cent) in the non-operated artery during follow-up but none was symptomatic or developed impairment of CVR. Although assessment of CVR provided useful information on the frequency of haemodynamic compromise before carotid endarterectomy and on the natural history of disease progression, neither serial assessment of reserve nor non-invasive imaging of the carotid bifurcation influenced clinical practice during follow-up.

Embolic infarcts of carotid origin differ in size and site with contralateral carotid patency

Carotid emboli cause small infarcts in the ipsilateral hemisphere in rats when the contralateral carotid artery is fully patent. With contralateral carotid occlusion, embolic infarcts, both large and small, occur with equal proportions in both hemispheres. To determine if emboli also cross to the territory of a stenotic carotid artery, we combined high grade (78-96%) stenosis of the left common carotid artery with photochemically-induced (laser irradiation 632 nm, 200 mW/cm2, 15 min; intravenous Photofrin II, 12.5 mg/kg) embolism from the right common carotid artery in 12 rats. Ninety-eight cerebral infarcts occurred in 9 experimental animals, with eight infarcts being large (> 2.5 mm). The mean proportions of infarcts and emboli on the left were 25% and 19%, respectively. These results suggest that contralateral carotid artery stenosis, like occlusion, will influence the site and size of embolic infarcts and that the "symptomatic" carotid artery cannot always be determined by the side of the cerebral infarct.

A comparison of transcranial Doppler and cerebral blood flow studies to assess cerebral vasoreactivity

BACKGROUND AND PURPOSE

The aim of this study was to determine the ability of transcranial Doppler ultrasonography when used to assess cerebral vasoreactivity. The results of this method were compared with regional cerebral blood flow measurements.

METHODS

Forty-three patients with symptoms suggesting cerebrovascular disease took part. Transcranial Doppler findings in the middle cerebral arteries were compared with regional cerebral blood flow in the corresponding perfusion territories before and after acetazolamide administration.

RESULTS

There was a significant positive correlation between the absolute increase in cerebral blood flow in milliliters per 100 g per minute and the percent increase in velocity (r = 0.63). The right-left, side-to-side difference of the acetazolamide response obtained by the two methods also showed a positive correlation (r = 0.80). Control limits obtained from healthy subjects were used for both the blood flow increase (absolute values and asymmetry in absolute values) and the velocity increase (percent increase and asymmetry in percent increase). The two methods then agreed in their evaluation of vasoreactivity in 74 (86%) of the 86 middle cerebral artery perfusion territories; 20 (23%) were assessed by both methods as having a reduced vasodilatory reserve. Eleven hemispheres with a slightly reduced regional cerebral blood flow response to acetazolamide were not detected by transcranial Doppler, whereas all territories with a marked reduction were identified by Doppler. Only one hemisphere with a normal cerebral blood flow increase after acetazolamide administration was assessed by Doppler as having reduced vasoreactivity.

CONCLUSIONS

Transcranial Doppler and the acetazolamide test may be used in clinical situations to assess cerebral vasoreactivity.

Transcranial Doppler monitoring during carotid endarterectomy

Transcranial Doppler monitoring of the middle cerebral artery blood flow velocity was used as an adjunct to routine methods of cerebral monitoring in a prospective study of 30 consecutive patients undergoing carotid endarterectomy to investigate whether transcranial Doppler monitoring provided information influencing operative technique. Application of carotid clamps caused a significant fall in middle cerebral artery velocity and there was a linear relationship between middle cerebral artery velocity and internal carotid artery stump pressure. Assuming a stump pressure of less than 50 mmHg to be an indication for shunting, this would correspond to a systolic middle cerebral artery velocity of less than 42 cm/s and a mean velocity of less than 30 cm/s. Transcranial Doppler monitoring immediately identified problems with shunt function and demonstrated a higher frequency of intraoperative embolization than had been anticipated, particularly after shunt insertion and final restoration of flow. With revision of operative technique this phenomenon is rarely encountered now. Two of the 30 patients exhibited a minor neurological deficit on recovery of consciousness, and transcranial Doppler monitoring was able to identify the probable underlying cause in both cases. Unnecessary and potentially hazardous re-exploration was avoided. In the absence of transcranial Doppler monitoring the neurological deficit in one of the patients might have been much worse.

Risk factors for intra-operative neurological deficit during carotid endarterectomy

Although embolism is considered to be the major cause of intra-operative neurological deficit (IOND) during carotid endarterectomy, the possibility that patients may vary in susceptibility to ischaemic damage following minor embolisation or falls in cerebral perfusion pressure is rarely considered. A prospective study was undertaken in 60 consecutive patients undergoing carotid endarterectomy to identify risk factors for IOND. The hypothesis was that patients with impaired cerebral vascular reserve (CVR) may be more vulnerable to intra-operative emboli or falls in cerebral perfusion pressure than normal subjects. Recent work using Positron Emission Tomography has indicated that the best index of CVR is the ratio of cerebral blood flow to cerebral blood volume, which is another expression for the reciprocal of mean cerebral transit time (MCTT). In this study, a new isotopic method of quantifying MCTT was used to identify patients with impaired CVR. Six patients (10%) recovered from anaesthesia with an IOND, only one of which was disabling. Significant risk factors for IOND were: (i) age over 65 years (Odds Ratio 13.0 (95%CI 1.4-121), p = 0.013); (ii) a residual neurological deficit prior to operation (Odds Ratio 7.0 (95%CE 1.1-43), p = 0.038); (iii) complex plaque morphology (Odds Ratio 6.4 (95%CI 1.06-34), p = 0.046); and (iv) the combination of impaired CVR and a CT scan infarct in the symptomatic hemisphere (Odds Ratio 9.8 (95%CI 1.5-62), p = 0.026). These observations suggest that certain risk factors for IOND can be identified preoperatively and this may enable a more discriminating approach to patient selection for operation and a more critical evaluation of operative technique and methods for intra-operative protection and monitoring in the future.

Cerebral perfusion pressure and CO2-reactivity in the evaluation of carotid obstructions

In order to evaluate the hemodynamic effect of a carotid artery obstruction, the CO2-reactivity was determined and the systolic carotid artery pressure was measured in 34 patients with a stenosis (20) and/or an occlusion (17) of the internal carotid artery. In 19 patients this was done bilaterally and in 15 unilaterally, a total of 53 observations. A decreased CO2-reactivity was found in 49% (26/53). With a carotid artery blood pressure of less than or equal to 90 mmHg the reactivity Index was diminished in 86% (12/14), with greater than 90 mmHg in 36% (14/39) of cases. It is assumed that in the first mentioned situation the autoregulation tends to be exhausted owing to a fall in perfusion pressure while in the last mentioned the explanation may be ischemic damage or shift of the lower limit of autoregulation towards a higher value as a result of (chronic) hypertension.

Vertebrobasilar insufficiency revealed by xenon-133 inhalation SPECT

A study of cerebral and cerebellar blood flow reactivity to acetazolamide by xenon-133-inhalation single photon emission computed tomography (133Xe SPECT) was carried out in a patient with bouts of transient basilar ischemia, whose neurological examination, computed tomographic scan, and auditory evoked potentials were normal. Though the patient was symptom-free at the time of the study, 133Xe SPECT demonstrated vertebrobasilar insufficiency by showing an impaired vasodilatory response in both the occipital lobes and the right cerebellar hemisphere. Three weeks later, the patient suffered an extensive stroke in these same areas. We therefore suggest that this method could be of great value in the assessment of vertebrobasilar insufficiency.