Basilar and middle cerebral artery reserve: a comparative study using transcranial Doppler and breath-holding techniques

The reliability of a breath-hold protocol to determine cerebrovascular reactivity in adolescents

PURPOSE

Cerebrovascular reactivity (CVR) is impaired in adolescents with cardiovascular disease risk factors. A breath-hold test is a noninvasive method of assessing CVR, yet there are no reliability data of this outcome in youth. This study aimed to assess the reliability of a breath-hold protocol to measure CVR in adolescents.

METHODS

Twenty-one 13 to 15 year old adolescents visited the laboratory on two separate occasions, to assess the within-test, within-day and between-day reliability of a breath-hold protocol, consisting of three breath-hold attempts. CVR was defined as the relative increase from baseline in middle cerebral artery mean blood velocity following a maximal breath-hold of up to 30 seconds, quantified via transcranial Doppler ultrasonography.

RESULTS

Mean breath-hold duration and CVR were never significantly correlated (r < .31, P > .08). The within-test coefficient of variation for CVR was 15.2%, with no significant differences across breath-holds (P = .88), so the three breath-hold attempts were averaged for subsequent analyses. The within- and between-day coefficients of variation for CVR were 10.8% and 15.3%, respectively.

CONCLUSIONS

CVR assessed via a three breath-hold protocol can be reliably measured in adolescents, yielding similar within- and between-day reliability. Analyses revealed that breath-hold length and CVR were unrelated, indicating the commonly reported normalization of CVR to breath-hold duration (breath-hold index) may be unnecessary in youth.

Transcranial Doppler as a Predictor of Ischemic Events in Vertebral Artery Dissection

BACKGROUND AND PURPOSE

Transcranial Doppler (TCD) helps identify patients with carotid dissections at risk of ischemic events (IEs). There is paucity of data identifying independent predictors of IE in vertebral arterial dissection (VAD). We sought to investigate the clinical and ultrasound predictors of IE.

METHODS

Patients with VAD admitted between June 2017 and February 2020 were evaluated clinically and with TCD; sonographic curves, microembolic signals (MES), and the breath-holding index (BHI) test were applied. Covariates found on univariate screen (P < .25) were included in a multivariable linear regression to identify independent predictors of IEs.

RESULTS

Of 88 patients with 100 VAD, 75 (85.2%) were females with a mean age 37.9 ± 7.5 years. All patients received antiplatelet treatment. TCD monitoring lasted an average of 21 ± 2.1 minutes. TCD was abnormal in 23 cases (26.1%); 21 patients had abnormal sonographic curves in the vertebral/basilar arteries, while in 4 cases, MES were present and in 5 (4.5%), BHI was abnormal. None of the patients with a normal TCD had an IE. Six strokes occurred during follow up. On univariate analysis, male sex, diabetes, dyslipidemia, a previous myocardial infarct, migraine, time of consultation to the ER, bilateral VAD, MES, BHI abnormalities, post stenotic flow in the basilar artery (PFB), and basilar/vertebral velocities were significantly associated with the risk of IEs. In the multivariate analysis, only the presence of PFB was a significant predictor of IE (OR: 68.6, 95% CI 5-937, <.001).

CONCLUSIONS

TCD in VAD predicts patients at high risk of IE.

Cerebral Hemodynamic Changes to Transcranial Doppler in Asymptomatic Patients with Fabry's Disease

BACKGROUND

Patients with Fabry's disease (FD) may be asymptomatic or show a spectrum of clinical manifestations, including cerebrovascular disease, mainly affecting posterior circulation. Few and conflicting studies on cerebral blood flow (CBF) velocity by transcranial Doppler sonography (TCD) in asymptomatic FD (aFD) subjects have been published. Our study aims to assess TCD in aFD subjects to identify any preclinical CBF change.

METHODS

A total of 30 aFD subjects were consecutively recruited and compared to 28 healthy controls. Brain magnetic resonance imaging was normal in all participants. TCD was used to study blood flow velocity and indices of resistance of intracranial arteries from the middle cerebral artery (MCA), bilaterally, and from the basilar artery (BA). Cerebral vasomotor reactivity (CVR) was also evaluated from MCA.

RESULTS

No difference was found between groups for MCA parameters of CBF velocity and CVR. Compared to controls, a higher mean blood flow velocity and a lower resistance index from BA were observed in FD subjects. No correlation was found between any BA-derived TCD parameter and the level of lyso-globotriaosylceramide.

CONCLUSIONS

aFD subjects show evidence of altered CBF velocity in posterior circulation. Preclinical detection of neurovascular involvement in FD might allow appropriate management and prevention of future cerebrovascular complications and disability.

Cerebral Hemodynamic Changes to Transcranial Doppler in Asymptomatic Patients with Fabry's Disease

BACKGROUND

Patients with Fabry's disease (FD) may be asymptomatic or show a spectrum of clinical manifestations, including cerebrovascular disease, mainly affecting posterior circulation. Few and conflicting studies on cerebral blood flow (CBF) velocity by transcranial Doppler sonography (TCD) in asymptomatic FD (aFD) subjects have been published. Our study aims to assess TCD in aFD subjects to identify any preclinical CBF change.

METHODS

A total of 30 aFD subjects were consecutively recruited and compared to 28 healthy controls. Brain magnetic resonance imaging was normal in all participants. TCD was used to study blood flow velocity and indices of resistance of intracranial arteries from the middle cerebral artery (MCA), bilaterally, and from the basilar artery (BA). Cerebral vasomotor reactivity (CVR) was also evaluated from MCA.

RESULTS

No difference was found between groups for MCA parameters of CBF velocity and CVR. Compared to controls, a higher mean blood flow velocity and a lower resistance index from BA were observed in FD subjects. No correlation was found between any BA-derived TCD parameter and the level of lyso-globotriaosylceramide.

CONCLUSIONS

aFD subjects show evidence of altered CBF velocity in posterior circulation. Preclinical detection of neurovascular involvement in FD might allow appropriate management and prevention of future cerebrovascular complications and disability.

The effect of water immersion and acute hypercapnia on ventilatory sensitivity and cerebrovascular reactivity

The partial pressure of end tidal carbon dioxide (PETCO2 ), ventilatory sensitivity to CO2 , and cerebral perfusion are augmented during thermoneutral head out water immersion (HOWI). We tested the hypotheses that HOWI and acute hypercapnia augments minute ventilation, ventilatory sensitivity to CO2 , cerebral perfusion, and cerebrovascular reactivity to CO2 . Twelve subjects (age: 24 ± 3 years, BMI: 25.3 ± 2.9 kg/m2 , 6 women) participated in two experimental visits: a HOWI visit (HOWI) and a matched hypercapnia visit (Dry + CO2 ). A rebreathing test was conducted at baseline, 10, 30, 60 min, and post HOWI and Dry + CO2 . PETCO2 , minute ventilation, expired gases, blood pressure, heart rate, and middle cerebral artery blood velocity were recorded continuously. PETCO2 increased throughout HOWI (baseline: 42 ± 2 mmHg; maximum at 10 min: 44 ± 2 mmHg, P ≤ 0.013) and Dry + CO2 (baseline: 42 ± 2 mmHg; maximum at 10 min: 44 ± 2 mmHg, P ≤ 0.013) and was matched between conditions (condition main effect: P = 0.494). Minute ventilation was lower during HOWI versus Dry + CO2 (maximum difference at 60 min: 13.2 ± 1.9 vs. 16.2 ± 2.7 L/min, P < 0.001). Ventilatory sensitivity to CO2 and middle cerebral artery blood velocity were greater during HOWI versus Dry + CO2 (maximum difference at 10 min: 2.60 ± 1.09 vs. 2.20 ± 1.05 L/min/mmHg, P < 0.001, and 63 ± 18 vs. 53 ± 14 cm/sec, P < 0.001 respectively). Cerebrovascular reactivity to CO2 decreased throughout HOWI and Dry + CO2 and was not different between conditions (condition main effect: P = 0.777). These data indicate that acute hypercapnia, matched to what occurs during HOWI, augments minute ventilation but not ventilatory sensitivity to CO2 or middle cerebral artery blood velocity despite an attenuated cerebrovascular reactivity to CO2 .

Human cerebrovascular function in health and disease: insights from integrative approaches

Background

The marked increase in the size of the brain, and consequently, in neural processing capability, throughout human evolution is the basis of the higher cognitive function in humans. However, greater neural, and thus information processing capability, comes at a significant metabolic cost; despite its relatively small size, the modern human brain consumes almost a quarter of the glucose and oxygen supply in the human body.

Fortunately, several vascular mechanisms ensure sufficient delivery of glucose and oxygen to the active neural tissue (neurovascular coupling), prompt removal of neural metabolic by-products (cerebral vasoreactivity), and constant global blood supply despite daily variations in perfusion pressure (cerebral autoregulation). The aim of this review is to provide an integrated overview of the available data on these vascular mechanisms and their underlying physiology. We also briefly review modern experimental approaches to assess these mechanisms in humans, and further highlight the importance of these mechanisms for humans’ evolutionary success by providing examples of their healthy adaptations as well as pathophysiological alterations.

Conclusions

Data reviewed in this paper demonstrate the importance of the cerebrovascular function to support humans’ unique ability to form new and different interactions with each other and their surroundings. This highlights that there is much insight into the neural and cognitive functions that could be gleaned from interrogating the cerebrovascular function.

Evaluation of cerebrovascular reserve in patients undergoing carotid artery stenting and its usefulness in predicting significant hemodynamic changes during temporary carotid occlusion

We investigated the usefulness of cerebrovascular reserve (CVR) testing to predict severe hemodynamic changes during proximally protected carotid artery stenting. Of 90 patients referred, 63 eligible underwent complete evaluation of the extent of carotid artery disease and transcranial Doppler ultrasound (TCD) assessment of CVR by means of a breath-holding test and ophthalmic artery flow pattern evaluation. Periprocedural TCD monitoring of the ipsilateral middle cerebral artery flow was performed in 24 patients undergoing proximally protected procedure (requiring induction of flow arrest within internal carotid artery). Abnormal CVR was significantly less common in patients with unilateral compared to bilateral carotid artery disease (26.3 % vs. 76.9 %, p=0.02), while ophthalmic artery flow reversal was rare in patients with unilateral carotid artery disease (2.5 % vs. 42.9 %, p<0.01). During the induction of carotid flow arrest, the average mean flow velocity drop following external carotid artery occlusion was low (3.5 %, p=0.67) compared to the induction of complete flow arrest (32.8 %, p<0.01). Six patients had a total mean flow velocity drop >50 %, including 2 patients with normal pre-procedural CVR. Our results suggest that TCD evaluation of CVR is not a reliable predictor of hemodynamic changes induced during proximally protected carotid artery stenting in patients with unilateral carotid artery disease.

Cerebral hemodynamic changes at basilar artery in patients with obstructive sleep apnea syndrome. A case-control study

INTRODUCTION

Obstructive sleep apnea syndrome (OSAS) is characterized in polysomnography by recurrent airflow obstruction during sleep. The underlying pathogenic mechanisms of neuropsychological and cerebrovascular events in patients with OSAS have not been clarified unequivocally.

MATERIAL AND METHODS

Case-control study to evaluate the cerebral vasomotor reactivity assessed by breath-holding maneuver at basilar artery in patients with OSAS compared to control subjects.

RESULTS

The study included 76 patients with OSAS and 76 controls. Vascular risk factors (arterial hypertension, diabetes mellitus, hypercholesterolemia, smoking), age, gender, coronary, and peripheral arterial diseases were similar in both groups. Patients with OSAS had breath-holding test values (31.9 ± 13.35%) lower than controls (39.06 ± 13.16%), (P = 0.001). Patients with OSAS had higher systolic and diastolic blood pressure both basal and apnea and also a higher basal heart rate.

CONCLUSIONS

Altered cerebral hemodynamics together with increased blood pressure values in patients with OSAS may play a role in the association between this disease and the development of cerebrovascular events. This implies that this disease should be identified through guide symptoms such as snoring, sleep apnea, and daytime sleepiness in all patients who consult for these symptoms to reduce the number of cerebrovascular events.

Correction for blood pressure improves correlation between cerebrovascular reactivity assessed by breath holding and 6% CO(2) breathing

BACKGROUND

Changes in cerebral blood flow velocity to hypercapnia are associated with changes in systemic blood pressure (BP). These confounding BP-dependent changes in cerebral blood flow velocity cause misinterpretation of cerebrovascular reactivity (CVR) results. The objective of the study was to determine the relationship between CVR assessed by breath holding and 6% CO2 breathing after correcting for BP-dependent changes in cerebral blood flow velocity.

METHODS

In 33 patients of uncomplicated type 2 diabetes mellitus, CVR was assessed as percentage changes in cerebral blood flow velocity and cerebrovascular conductance index.

RESULTS

Percentage change in cerebral blood flow velocity during breath holding was positively correlated with that of during 6% CO2 breathing (r = .35; P = .0448). CVR during breath holding and 6% CO2 breathing were better correlated when expressed as percentage changes in cerebrovascular conductance index (r = .49; P = .0040). Similarly, breath-holding test results expressed as percentage changes in cerebral blood flow velocity correctly identified only 37.5% of the poor reactors to 6% CO2 breathing. However, when the breath-holding test results were expressed as percentage changes in cerebrovascular conductance index, 62.5% of the poor reactors to 6% CO2 breathing were correctly identified indicating a better agreement between the test results obtained by the 2 methods.

CONCLUSION

Cerebrovascular response to breath holding is better correlated with that of 6% CO2 breathing when changes in cerebral blood flow velocity were corrected for associated changes in BP.

Simultaneous vasomotor reactivity testing in the middle cerebral and basilar artery with suboccipital probe fixation device

BACKGROUND

We assess the feasibility of using the newly designed suboccipital probe fixation device (SPFD) as a convenient and reliable tool for simultaneous measurement of vasomotor reactivity (VMR) in the middle cerebral artery (MCA) and basilar artery (BA).

METHODS

We analyzed 30 healthy volunteers' VMR values by using both SPFD and conventional handheld method. The VMR values were measured as percentage increase of the mean flow velocity on transcranial Doppler (TCD) in response to hypercapnia induced by the rebreathing method. The VMR tests were performed three times: (1) for both MCAs, (2) for the index MCA (the better signal window) and the BA by using the SPFD, and (3) for the index MCA and the BA by using the handheld technique.

RESULTS

The VMR values of the right and left MCAs were similar (P > .05). Although the VMR values of the index MCA and the BA obtained by SPFD application and the handheld technique were similar (P > .05), the correlation coefficient of VMR values obtained by using the SPFD was higher (r= .827, P < .001 vs. r= .568, P= .001).

CONCLUSION

The SPFD is a convenient and reliable tool for the evaluation of relative VMR between the MCA and BA during TCD monitoring.

Integration of cerebrovascular CO2 reactivity and chemoreflex control of breathing: mechanisms of regulation, measurement, and interpretation

Cerebral blood flow (CBF) and its distribution are highly sensitive to changes in the partial pressure of arterial CO(2) (Pa(CO(2))). This physiological response, termed cerebrovascular CO(2) reactivity, is a vital homeostatic function that helps regulate and maintain central pH and, therefore, affects the respiratory central chemoreceptor stimulus. CBF increases with hypercapnia to wash out CO(2) from brain tissue, thereby attenuating the rise in central Pco(2), whereas hypocapnia causes cerebral vasoconstriction, which reduces CBF and attenuates the fall of brain tissue Pco(2). Cerebrovascular reactivity and ventilatory response to Pa(CO(2)) are therefore tightly linked, so that the regulation of CBF has an important role in stabilizing breathing during fluctuating levels of chemical stimuli. Indeed, recent reports indicate that cerebrovascular responsiveness to CO(2), primarily via its effects at the level of the central chemoreceptors, is an important determinant of eupneic and hypercapnic ventilatory responsiveness in otherwise healthy humans during wakefulness, sleep, and exercise and at high altitude. In particular, reductions in cerebrovascular responsiveness to CO(2) that provoke an increase in the gain of the chemoreflex control of breathing may underpin breathing instability during central sleep apnea in patients with congestive heart failure and on ascent to high altitude. In this review, we summarize the major factors that regulate CBF to emphasize the integrated mechanisms, in addition to Pa(CO(2)), that control CBF. We discuss in detail the assessment and interpretation of cerebrovascular reactivity to CO(2). Next, we provide a detailed update on the integration of the role of cerebrovascular CO(2) reactivity and CBF in regulation of chemoreflex control of breathing in health and disease. Finally, we describe the use of a newly developed steady-state modeling approach to examine the effects of changes in CBF on the chemoreflex control of breathing and suggest avenues for future research.

Endovascular therapy of symptomatic intracranial stenosis in patients with impaired regional cerebral blood flow or failure of medical therapy

BACKGROUND AND PURPOSE

Symptomatic intracranial stenoses have a high risk for a recurrent stroke if treated medically. Although angioplasty and stent placement are proposed treatment options, data on longer-term outcome are limited.

MATERIALS AND METHODS

We analyzed all endovascular procedures on symptomatic intracranial stenosis at our institution from January 1998 to December 2005. We retrospectively assigned patients to group A (symptoms despite antithrombotic therapy) or group B (impaired regional cerebral blood flow [rCBF]). Primary outcome events were periprocedural major complications or recurrent ischemic strokes in the territory of the treated artery. We used the Kaplan-Meier method to calculate survival probabilities.

RESULTS

The procedural technical success rate was 92% (35/38) with periprocedural major complications in 4 cases (10.5%; group A [8.3%, 2/24], group B [14.3%, 2/14]). Median (range) follow-up for the 33 patients with technically successful procedures was 21 (0-72) months. Recurrent ischemic strokes occurred in 15% (3/20) of patients in group A and 0% (0/13) of patients in group B. Overall, there were 21% (7/33) primary outcome events (group A [25%, 5/20], group B [15%, 2/13]). There was a nonsignificant trend for better longer-term survival free of a major complication or recurrent stroke in patients with impaired rCBF compared with patients who were refractory to medical therapy treatment (Kaplan-Meier estimate 0.85 [SE 0.10] vs 0.72 [SE 0.11] at 2 years, respectively).

CONCLUSION

Interventional treatment of symptomatic intracranial stenosis carries significant risk for complications and recurrent stroke in high-risk patients. The observation that patients with impaired rCBF may have greater longer-term benefit than medically refractory deserves further study.

Spect measurements of regional cerebral perfusion and carbondioxide reactivity: correlation with cerebral collaterals in internal carotid artery occlusive disease

BACKGROUND

The aim of the present study was to assess the regional variation in cerebral perfusion, vasomotor reactivity (VMR) and the role of cerebral collaterals in patients with symptomatic internal carotid artery (ICA).

METHODS

Seventeen functionally independent patients (60+/-9 years, mean+/-SD) with a unilateral symptomatic internal carotid artery occlusion and a <30% contralateral ICA stenosis were investigated. (99 m) Tc-hexamethyl propyleneamine oxime (HMPAO) single photon emission computed tomography (SPECT) was performed to study cerebral blood flow in rest and during a CO(2) challenge in the cerebellum, temporal lobe, occipital lobe, basal ganglia, frontal lobe and parietal lobe. Time of flight and phase contrast MRA were used to study collateral flow via circle of Willis.

RESULTS

In rest, cerebral perfusion on the side ipsilateral to the ICA occlusion was decreased compared with the contralateral side in the basal ganglia (p<0.05), frontal lobe (p<0.01) and parietal lobe (p<0.01). During a CO(2) challenge only the ipsilateral frontal lobe demonstrated a perfusion decrease compared with the contralateral frontal lobe (p<0.05). Furthermore, in patients without collateral flow via the anterior circle of Willis the perfusion of the ipsilateral frontal lobe was significantly decreased (p<0.01) during the CO(2) challenge and crossed cerebellar diaschisis with a decreased perfusion on the contralateral cerebellar hemisphere was detected (p<0.05). No cerebral blood flow (CBF) differences were found for present/absent collateral flow via the posterior communicating artery.

CONCLUSION

Regional assessment of cerebral perfusion and VMR with SPECT demonstrated the heterogeneity of cerebral hemodynamics and the importance of collateral flow via the anterior circle of Willis.

Cerebral Autoregulation and CO2 Reactivity in Anterior and Posterior Cerebral Circulation During Sevoflurane Anesthesia

The purpose of the study was to compare cerebral autoregulation (CA) and CO2 reactivity (CO2R) between the anterior and posterior circulation under sevoflurane anesthesia. We studied 9 adult ASA physical status I patients (22-47 yr) scheduled for elective orthopedic surgery. Blood flow velocity in the middle cerebral artery (Vmca) and in the basilar artery (Vba) were measured using transcranial Doppler ultrasonography. For CA testing, arterial blood pressure was increased using phenylephrine infusion. CA was quantified with the autoregulatory index (ARI). CO2R was investigated at PaCO2 of 30 +/- 2.8 mm Hg, 39.4 +/- 2.6 mm Hg, and 48.7 +/- 2.8 mm Hg. Linear regression analysis was used for CO2R. We found ARI was preserved in both arteries: ARImca (middle cerebral artery) = 0.72 +/- 0.2; ARIba (basilar artery) = 0.66 +/- 0.2; P = 0.5. With regard to CO2R, Vmca increased with slope of 1.7 cm/s/mm Hg PaCO2, Vba increased with slope of 1.5 cm/s/mm Hg PaCO2; P = 0.83. Absolute Vmca was higher compared with Vba; P < 0.05. We conclude that in healthy individuals under 0.5 MAC of sevoflurane and small-dose remifentanil: 1) mean flow velocities of BA are less than those of MCA; 2) autoregulation and CO2R are preserved in the basilar artery and are similar to those of MCA.

Cortical laminar necrosis related to prolonged focal status epilepticus

Cortical laminar necrosis (CLN) is radiologically defined as high intensity cortical lesions on T1 weighted MRI images following a gyral distribution. Histopathologically, CLN is characterised by pannecrosis of the cortex involving neurones, glial cells, and blood vessels. It has been reported to be associated with hypoxia, metabolic disturbances, drugs, and infections. We present two patients who developed CLN and permanent neurological deficits after prolonged and repeated focal status epilepticus. The possible mechanisms leading to CLN in these patients are discussed, together with the implications of prompt and aggressive treatment in similar cases.

Basilar and Middle Cerebral Artery Reactivity in Patients With Migraine

BACKGROUND

Migraine has been reported as a possible risk factor for ischemic stroke. The mechanisms underlying this association are unknown.

OBJECTIVES

To evaluate cerebrovascular reactivity to hypercapnia in the anterior and posterior circulation of patients with migraine, as reduced cerebrovascular reactivity is associated with a predisposition to stroke in various clinical conditions.

METHODS

Using transcranial Doppler ultrasonography, changes in flow velocity during apnea were measured in both middle cerebral arteries and in the basilar artery of 15 control subjects and 30 patients with migraine (15 with aura and 15 without aura) during an attack-free period. Cerebrovascular reactivity was evaluated using the breath-holding index, which is calculated by dividing the percent increase in mean flow velocity recorded during a breath-holding episode by its duration (in seconds) after a normal inspiration.

RESULTS

Vascular reactivity in the middle cerebral arteries was similar in patients and controls and significantly lower in the basilar artery of patients with migraine with aura compared with the other 2 groups (P <.0001).

CONCLUSIONS

These findings show that in patients with migraine with aura, there is an impairment in the adaptive cerebral hemodynamic mechanisms in the posterior circulation. This fact could have pathogenetic implications since the association between migraine and stroke frequently regards patients with migraine with aura, and cerebral infarcts occur more commonly in the vertebrobasilar district.

Cerebral vasomotor reactivity of the posterior circulation in patients with carotid occlusive disease

We evaluated the hemodynamic features of the posterior circulation in patients with severe carotid stenosis by assessing and comparing cerebral vasomotor reactivity (VMR) in the middle cerebral (MCA) and vertebral arteries (VA) by transcranial Doppler and the Diamox (1 g acetazolamide i.v.) test. Sixty symptomatic and 111 asymptomatic patients with unilateral severe (>70%) internal carotid artery stenosis were studied. The VMR was 19.2 +/- 18.9% for the MCA ipsilateral to the stenosis and 27.3 +/- 17.4% on the contralateral side (P < 0.0001) for all patients. It was 18.2 +/- 23.2% for the VA ipsilateral to the stenosis and 19.7 +/- 21% on the contralateral side (P = NS). The symptomatic patients' VMR of the MCA on the side of stenosis and the opposite side were 19.2 +/- 17.6 and 29 +/- 17.2%, respectively (P < 0.03). The VMR of the VA remained similar (15.1 +/- 21 and 21.6 +/- 6%, respectively, P = NS). The asymptomatic patients' VMR of the MCA on the side of the stenosis was also lower (19.2 +/- 19.7 vs. 26.5 +/- 17.5% on the opposite side, P < 0.001). In contrast, the VMR in the VA was similar (19.8 +/- 21.4 and 18.7 +/- 19.5%, respectively, P < 0.6, NS). Thus, the VMR of the posterior circulation remained similar regardless of carotid stenosis and a symptomatic/asymptomatic course of carotid occlusive disease, suggesting an independent cerebral vascular reserve capacity of the posterior circulation.