Dissociation of vasoreactivity to acetazolamide and hypercapnia. Comparative study in patients with chronic occlusive major cerebral artery disease

Cortical oxygen extraction fraction using quantitative BOLD MRI and cerebral blood flow during vasodilation

Introduction: We aimed to demonstrate non-invasive measurements of regional oxygen extraction fraction (OEF) from quantitative BOLD MRI modeling at baseline and after pharmacological vasodilation. We hypothesized that OEF decreases in response to vasodilation with acetazolamide (ACZ) in healthy conditions, reflecting compensation in regions with increased cerebral blood flow (CBF), while cerebral metabolic rate of oxygen (CMRO2) remained unchanged. We also aimed to assess the relationship between OEF and perfusion in the default mode network (DMN) regions that have shown associations with vascular risk factors and cerebrovascular reactivity in different neurological conditions. Material and methods: Eight healthy subjects (47 ± 13 years, 6 female) were scanned on a 3 T scanner with a 32-channel head coil before and after administration of 15 mg/kg ACZ as a pharmacological vasodilator. The MR imaging acquisition protocols included: 1) A Gradient Echo Slice Excitation Profile Imaging Asymmetric Spin Echo scan to quantify OEF, deoxygenated blood volume, and reversible transverse relaxation rate (R2 ') and 2) a multi-post labeling delay arterial spin labeling scan to measure CBF. To assess changes in each parameter due to vasodilation, two-way t-tests were performed for all pairs (baseline versus vasodilation) in the DMN brain regions with Bonferroni correction for multiple comparisons. The relationships between CBF versus OEF and CBF versus R2' were analyzed and compared across DMN regions using linear, mixed-effect models. Results: During vasodilation, CBF significantly increased in the medial frontal cortex (P = 0.004 ), posterior cingulate gyrus (pCG) (P = 0.004 ), precuneus cortex (PCun) (P = 0.004 ), and occipital pole (P = 0.001 ). Concurrently, a significant decrease in OEF was observed only in the pCG (8.8%, P = 0.003 ) and PCun (8.7 % , P = 0.001 ). CMRO2 showed a trend of increased values after vasodilation, but these differences were not significant after correction for multiple comparisons. Although R2' showed a slightly decreasing trend, no statistically significant changes were found in any regions in response to ACZ. The CBF response to ACZ exhibited a stronger negative correlation with OEF (β = - 0.104 ± 0.027 ; t = - 3.852 , P < 0.001 ), than with R2' (β = - 0.016 ± 0.006 ; t = - 2.692 , P = 0.008 ). Conclusion: Quantitative BOLD modeling can reliably measure OEF across multiple physiological conditions and captures vascular changes with higher sensitivity than R2' values. The inverse correlation between OEF and CBF across regions in DMN, suggests that these two measurements, in response to ACZ vasodilation, are reliable indicators of tissue health in this healthy cohort.

Hemodynamics and oxygen extraction in chronic large artery steno-occlusive disease: Clinical applications for predicting stroke risk

Depending on the adequacy of collateral sources of blood flow, arterial stenosis or occlusion may lead to reduced perfusion pressure and ultimately reduced blood flow in the distal territory supplied by that vessel. There are two well-defined compensatory mechanisms to reduced pressure or flow - autoregulatory vasodilation and increased oxygen extraction fraction. Other changes, such as metabolic downregulation, are likely. The positive identification of autoregulatory vasodilation and increased oxygen extraction fraction in humans is an established risk factor for future ischemic stroke in some disease states such as atherosclerotic carotid stenosis and occlusion. The mechanisms by which ischemic stroke may occur are not clear, and may include an increased vulnerability to embolic events. The use of hemodynamic assessment to identify patients with occlusive vasculopathy at an increased risk for stroke is very appealing for several different patient populations, such as those with symptomatic intracranial atherosclerotic disease, moyamoya phenomenon, complete internal carotid artery occlusion, and asymptomatic cervical carotid artery stenosis. While there is very good data for stroke risk prediction in some of these groups, no intervention based on these tools has been proven effective yet. In this manuscript, we will review these topics above and identify areas for future research.

Acetazolamide-augmented dynamic BOLD (aczBOLD) imaging for assessing cerebrovascular reactivity in chronic steno-occlusive disease of the anterior circulation: An initial experience

The purpose of this study was to measure cerebrovascular reactivity (CVR) in chronic steno-occlusive disease using a novel approach that couples BOLD imaging with acetazolamide (ACZ) vasoreactivity (aczBOLD), to evaluate dynamic effects of ACZ on BOLD and to establish the relationship between aczBOLD and dynamic susceptibility contrast (DSC) perfusion MRI. Eighteen patients with unilateral chronic steno-occlusive disease of the anterior circulation underwent a 20-min aczBOLD imaging protocol, with ACZ infusion starting at 5 min of scan initiation. AczBOLD reactivity was calculated on a voxel-by-voxel basis to generate CVR maps for subsequent quantitative analyses. Reduced CVR was observed in the diseased vs. the normal hemisphere both by qualitative and quantitative assessment (gray matter (GM): 4.13% ± 1.16% vs. 4.90% ± 0.98%, P = 0.002; white matter (WM): 2.83% ± 1.23% vs. 3.50% ± 0.94%, P = 0.005). In all cases BOLD signal began increasing immediately following ACZ infusion, approaching a plateau at ~ 8.5 min after infusion, with the tissue volume of reduced augmentation increasing progressively with time, peaking at 2.60 min (time range above 95% of the maximum value: 0–4.43 min) for the GM and 1.80 min (time range above 95% of the maximum value: 1.40–3.53 min) for the WM. In the diseased hemisphere, aczBOLD CVR significantly correlated with baseline DSC time-to-maximum of the residue function (T_max) (P = 0.008 for the WM) and normalized cerebral blood flow (P = 0.003 for the GM, and P = 0.001 for the WM). AczBOLD provides a novel, safe, easily implementable approach to CVR measurement in the routine clinical environments. Further studies can establish quantitative thresholds from aczBOLD towards identification of patients at heightened risk of recurrent ischemia and cognitive decline.

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A method coupling BOLD with ACZ challenge (aczBOLD) for CVR assessment was proposed.

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Compromised CVR was detected in patients with cerebrovascular disease.

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Dynamic effects of ACZ on BOLD were characterized.

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CVR correlated with baseline dynamic susceptibility contrast perfusion MRI.

Is Carotid Stenosis in Women a Gender-Related Condition?

BACKGROUND

We set out to study, through ultrasound examinations, the carotid bifurcation in men and women with/without carotid stenosis to look for anatomical and electrophysiologic differences. We evaluated other variables to look for differences that might explain the dissimilar behavior of this disease in the two sexes and the presence and impact of risk factors.

METHODS

We examined 974 subjects aged 25 to 88 years (478 men and 496 women) in whom we considered heart rate, smoking status, and the presence of hypertension, diabetes, hypercholesterolemia, and hypertriglyceridemia. Ultrasound examination of the neck vessels included measurement of intimal medial thickness (IMT), vessel diameter, and outflow area/inflow area ratio. We established plaque location, echogenicity and echostructure, and the percentage of stenosis owing to plaque and measured systolic velocity, flow direction, and the depth of detection of these parameters. We used the apnea and hyperpnea test to assess cerebrovascular reactivity.

RESULTS

Hypertension and hypercholesterolemia were the most frequent risk factors. Women had a higher heart rate, whereas men had significantly greater IMT. The presence of atheromatous plaque was significantly correlated with age in both sexes, with men having a higher prevalence of carotid plaques. The sexes differed significantly with regard to plaque location, echogenicity, echostructure, and intracranial circulation. Women had a slightly higher blood flow velocity in the intracranial arteries. Risk factors affected plaque formation and extent more in men than in women.

CONCLUSIONS

These findings suggest that carotid stenosis is a gender-related trait.

Cerebrovascular reserve may be a more accurate predictor of stroke than degree of ICA or MCA stenosis

Background

It is currently unclear whether the degree of stenosis or the cerebrovascular reserve (CVR) is a better predictor of ischemic stroke.

Material/Methods

In this study, CVR was measured by perfusion computed tomography with inhalation of 5% CO₂ in 37 symptomatic patients with internal carotid artery (ICA) or middle cerebral artery (MCA) stenosis or occlusion. Patients were divided into groups according to the degree of stenosis: ≥70% stenosis (stenosis group 1) or <70% stenosis (stenosis group 2); and according to CVR: ≥10% CVR (CVR group 1) or <10% CVR (CVR group 2). All patients were given medical treatment.

Results

During a mean follow-up period of 56.9 months (range 24–73 months), recurrent ipsilateral ischemic stroke occurred in 7 patients. Ischemic stroke occurred in 0 of 19 patients in CVR group 1 (annual risk 0%), 7 of 18 patients in CVR group 2 (annual risk 7.7%), 3 of 18 patients in stenosis group 1 (annual risk 3.3%), and 4 of 19 patients in stenosis group 2 (annual risk 4.7%). Comparisons using Pearson’s chi-square test showed a significant difference in the rate of ischemic stroke between CVR group 1 and CVR group 2 (odds ratio 1.700; 95% confidence interval 1.142–2.530; P=0.003), but no significant difference between stenosis group 1 and stenosis group 2 (P=0.691).

Conclusions

Cerebrovascular reserve may be a more accurate predictor of stroke than degree of ICA or MCA stenosis.

Chinese consensus statement on the evaluation and intervention of collateral circulation for ischemic stroke

Background

Collateral circulation is becoming more significant in the individual management strategy of ischemic stroke, there are more data updated recently.

Aim

To make the further acknowledgment of the evaluation and how to improving collateral flow, for better treatment selection.

Method

A panel of experts on stroke providing related statement based on review the results from most up‐to‐date clinical research.

Results

DSA is the gold standard in evaluating all levels of collaterals. CTA can be used for evaluating leptomeningeal collaterals, MRA for CoW, TCD or TCCS can be used as screening tool for primary evaluation. The treatment modalities include direct interventions, such as Extracranial–Intracranial bypass, and indirect interventions, as External counterpulsation and pressor therapy. The consideration of methodology to augment and improve can be considered on an individual basis.

Discussion

In this consensus, we interpret the definition, neuroimaging evaluation, intervention and potential strategy on collaterals in the future.

Conclusion

Assessment of collateral circulation is crucial for selecting therapeutic options, predicting infarction volume and making prognosis after ischemic stroke. Data is still needed to provide therapeutic evidence for many new developed technologies. Until more evidence is available, the clinical significance of applying the new technologies is unclear and perhaps limited.

Evaluation of cerebrovascular reserve using xenon-enhanced CT scanning in patients with symptomatic middle cerebral artery stenosis

Cerebrovascular reserve (CVR) is an important prognostic factor in patients with major cerebral arterial steno-occlusive disease. However, few studies have examined CVR in symptomatic intracranial stenosis without ipsilateral extracranial internal carotid artery stenosis. This study sought to evaluate CVR in patients with symptomatic middle cerebral artery (MCA) stenosis using xenon-enhanced computed tomography (Xe/CT) with acetazolamide (ACZ) challenge. Twelve patients with symptomatic MCA stenosis were recruited. All patients were examined by Xe/CT to quantitatively measure resting cerebral blood flow (CBF) and received ACZ challenge to evaluate CVR. For resting CBF, no significant differences were found between the sides in four regions of interest. After the ACZ challenge test, the CVR was significantly different between hemispheres (ipsilateral versus contralateral CVR: 12.9 ± 24.3% versus 28.0 ± 16.8%, respectively; p=0.005) and in the MCA territory (ipsilateral versus contralateral CVR: 8.7 ± 24.7% versus 29.3 ± 24%, respectively; p=0.003). However, no significant differences in CVR were detected between cortical comparisons and white matter comparisons from the two sides. Thus, ACZ-challenge Xe/CT is useful for the measurement of CBF and CVR in these patients. Impaired CVR is an important characteristic of patients with symptomatic MCA stenosis.

Assessing Cerebrovascular Reactivity in Carotid Steno-Occlusive Disease Using MRI BOLD and ASL Techniques

Impaired cerebrovascular reactivity (CVR), a predictive factor of imminent stroke, has been shown to be associated with carotid steno-occlusive disease. Magnetic resonance imaging (MRI) techniques, such as blood oxygenation level-dependent (BOLD) and arterial spin labeling (ASL), have emerged as promising noninvasive tools to evaluate altered CVR with whole-brain coverage, when combined with a vasoactive stimulus, such as respiratory task or injection of acetazolamide. Under normal cerebrovascular conditions, CVR has been shown to be globally and homogenously distributed between hemispheres, but with differences among cerebral regions. Such differences can be explained by anatomical specificities and different biochemical mechanisms responsible for vascular regulation. In patients with carotid steno-occlusive disease, studies have shown that MRI techniques can detect impaired CVR in brain tissue supplied by the affected artery. Moreover, resulting CVR estimations have been well correlated to those obtained with more established techniques, indicating that BOLD and ASL are robust and reliable methods to assess CVR in patients with cerebrovascular diseases. Therefore, the present paper aims to review recent studies which use BOLD and ASL to evaluate CVR, in healthy individuals and in patients with carotid steno-occlusive disease, providing a source of information regarding the obtained results and the methodological difficulties.

PET in Cerebrovascular Disease

Investigation of the interplay between the cerebral circulation and brain cellular function is fundamental to understanding both the pathophysiology and treatment of stroke. Currently, PET is the only technique that provides accurate, quantitative in vivo regional measurements of both cerebral circulation and cellular metabolism in human subjects. We review normal human cerebral blood flow and metabolism and human PET studies of ischemic stroke, carotid artery disease, vascular dementia, intracerebral hemorrhage and aneurysmal subarachnoid hemorrhage and discuss how these studies have added to our understanding of the pathophysiology of human cerebrovascular disease.

Are the local blood oxygen level-dependent (BOLD) signals caused by neural stimulation response dependent on global BOLD signals induced by hypercapnia in the functional MR imaging experiment? Experiments of long-duration hypercapnia and multilevel carbon dioxide concentration

BACKGROUND AND PURPOSE

The relationship between the local blood oxygen level-dependent (BOLD) signals caused by neural stimulation (fBOLD) and the global BOLD signals induced by hypercapnia (hBOLD) has not been fully investigated. In this study, we examine whether fBOLD is modulated by hBOLD signals, by means of experiments using a relatively wide range of inhaled carbon dioxide (CO(2)) for a long duration of 5 minutes.

MATERIALS AND METHODS

Ten healthy volunteers were recruited, each undergoing 6 separate experiments by inhaling gas mixtures with different fractions of CO(2) (room air, 3%-7%). Each experiment contained 3 phases, prehypercapnic, hypercapnic, and posthypercapnic, during which boxcar visual stimulus was given. The local fBOLD signals were measured from areas showing activation patterns highly correlated with the visual stimulus paradigm, whereas the global hBOLD signals were measured from areas showing no visual activations. Percentage changes in fBOLD during transient-state hypercapnia and steady-state hypercapnia were both investigated in response to varying degrees of hypercapnic perturbations.

RESULTS

The hBOLD signals increased with increase of inhaled CO(2) fractions. The duration for the hBOLD signals to reach steady state prolonged with increase of inhaled CO(2) fractions. Normalized fBOLD ratio was inversely related to the inhaled CO(2) during steady-state hypercapnia but showed positive association with hBOLD during transient-state hypercapnia.

CONCLUSION

Our study concludes that the steady-state fBOLD signal intensity is dependent on and inversely related to the hBOLD signals. Previous reports documenting independent and additive relationships between hBOLD and fBOLD may likely be due to transient-state observations.

Evaluation of vasomotor reactivity by transcranial Doppler sonography in patients with acute stroke who have symptomatic intracranial and extracranial stenosis

OBJECTIVE

In patients with large artery disease, determining the cerebral hemodynamic state and following its alterations may be a good marker for predicting long-term outcome. The aim of our study was to compare the changes in vasomotor reactivity (VMR) of patients with symptomatic intracranial and extracranial artery stenosis. We also examined whether VMR after stroke influences the long-term prognosis for these patients.

METHODS

Forty-one patients were included in the study. To determine the cerebral hemodynamic state, transcranial Doppler ultrasound examinations and acetazolamide tests were performed after acute stroke and repeated after 6 months. We compared the VMR on admission and at 6 months, together with changes in VMR, of the patients with symptomatic intracranial and extracranial artery stenosis. By calculating the Barthel index at 6 months, we examined whether VMR had an effect on an improvement in their quality of life.

RESULTS

We observed a significantly higher initial VMR of the ipsilateral hemisphere in patients with intracranial stenosis (22.4 +/- 9.1 versus 13.4 +/- 12.8; P = .013). At 6 months, the VMR obtained from the ipsilateral hemisphere was better in patients with extracranial stenosis than in the patients with intracranial stenosis (P = .01). The ipsilateral VMR measured on admission showed a positive correlation with the Barthel index at 6 months (P = .007; r = 0.434).

CONCLUSIONS

Our study showed that VMR in patients with acute stroke who have extracranial and intracranial artery stenosis measured by using a transcranial Doppler examination may have value in predicting long-term outcome.

Perfusion Imaging of Cerebrovascular Reserve

Cerebrovascular reserve reflects the capacity of the brain to maintain adequate blood flow in the face of decreased perfusion pressure. Perfusion imaging, combined with a physiologic or pharmacologic challenge, is a direct method of measuring cerebrovascular reserve. The authors discuss the strengths and drawbacks of each of the methods of cerebrovascular reserve assessment. They review the applications of cerebrovascular reserve testing, particularly in the assessment of stroke risk in the setting of chronic stenosis or occlusion of vessels in the head and neck.

Optic nerve oxygenation

The oxygen tension of the optic nerve is regulated by the intraocular pressure and systemic blood pressure, the resistance in the blood vessels and oxygen consumption of the tissue. The oxygen tension is autoregulated and moderate changes in intraocular pressure or blood pressure do not affect the optic nerve oxygen tension. If the intraocular pressure is increased above 40 mmHg or the ocular perfusion pressure decreased below 50 mmHg the autoregulation is overwhelmed and the optic nerve becomes hypoxic. A disturbance in oxidative metabolism in the cytochromes of the optic nerve can be seen at similar levels of perfusion pressure. The levels of perfusion pressure that lead to optic nerve hypoxia in the laboratory correspond remarkably well to the levels that increase the risk of glaucomatous optic nerve atrophy in human glaucoma patients. The risk for progressive optic nerve atrophy in human glaucoma patients is six times higher at a perfusion pressure of 30 mmHg, which corresponds to a level where the optic nerve is hypoxic in experimental animals, as compared to perfusion pressure levels above 50 mmHg where the optic nerve is normoxic. Medical intervention can affect optic nerve oxygen tension. Lowering the intraocular pressure tends to increase the optic nerve oxygen tension, even though this effect may be masked by the autoregulation when the optic nerve oxygen tension and perfusion pressure is in the normal range. Carbonic anhydrase inhibitors increase the optic nerve oxygen tension through a mechanism of vasodilatation and lowering of the intraocular pressure. Carbonic anhydrase inhibition reduces the removal of CO2 from the tissue and the CO2 accumulation induces vasodilatation resulting in increased blood flow and improved oxygen supply. This effect is inhibited by the cyclo-oxygenase inhibitor, indomethacin, which indicates that prostaglandin metabolism plays a role. Laboratory studies suggest that carbonic anhydrase inhibitors might be useful for medical treatment of optic nerve and retinal ischemia, potentially in diseases such as glaucoma and diabetic retinopathy. However, clinical trials and needed to test this hypotheses.

Comparison of regional vasomotor responses to acetazolamide and CO2 in rabbit cerebrum and cerebellum, measured by a hydrogen clearance method

AIM

Many investigators have proved the usefulness of acetazolamide provocation and the carbon dioxide test for assessment of the local cerebrovascular reactivity by measurement of the regional cerebral blood flow in patients with occlusive cerebrovascular disease. Data originating from a comparison of these two different vasomotor stimuli as concerns the differences in sensitivity to them in various parts of the central nervous system are scarce. Our aim was to compare the cerebral blood flow responses to hypercapnic and acetazolamide stimuli in different brain regions.

METHODS

The cerebral blood flow was measured in the cerebrum (cortex and caudate nucleus) and cerebellum (cortex), as measured by a hydrogen clearance method in anaesthetized, artificially ventilated rabbits.

RESULTS

In normocapnia, the cerebral blood flow values in the cerebrum and the cerebellum differed significantly. The cerebral blood flow responses to both vasodilatory stimuli were to be significantly higher in the cerebrum than in the cerebellum, but the relative increases, i.e. the mean relative reactivities, were similar in the different regions measured.

CONCLUSION

The regional dissimilarity might explain to some extent the different sensitivities of the various brain areas to sudden blood pressure changes (infarction or haemorrhage). The results further suggest that heterogeneity in cerebrovascular reactivity should be considered in the assessment of vasoreactivity in patients with occlusive cerebrovascular disease. Since the comparison of the carbon dioxide and acetazolamide-induced cerebrovascular reactivities revealed a strong linear relationship, it was concluded that acetazolamide provocation is equivalent to the carbon dioxide test in the evaluation of cerebrovascular reactivity.

The cerebrovascular dilatation effects of olprinone, a phosphodiesterase III inhibitor, in comparison with acetazolamide—a pliot study

To examine the effects of olprinone, a phosphodiesterase III inhibitor, on cerebral blood flow (CBF), we compared the effects of olprinone on CBF to that of acetazolamide. Using technetium-99m-ethyl cysteinate dimer (99mTc-ECD) brain SPECT, we measured regional CBF (rCBF) at 33 sites, including 16 right and left pairs of non-infarct cerebral cortexes, in seven stroke patients (66.0+/-3.2 years) in a resting state and 15 min after the administration of acetazolamide. Within 1 week, rCBF at each site was measured 15 min after the initiation of olprinone infusion. Resting rCBF showed a significant negative correlation with the change in rCBF (DeltaCBF) during olprinone infusion (r = -0.43, P=0.013), but no significant correlation was seen following acetazolamide administration. The difference in rCBF between the right and left cortex increased more following acetazolamide administration (14.1+/-10.9 ml/(min 100 g)) than during olprinone infusion (5.4+/-4.8 ml/(min 100 g), P=0.013). The rCBF at four regions of interest (ROI) with low-resting CBF (< 49 ml/(min 100 g)) further decreased following the administration of acetazolamide. The vasodilatory effects of olprinone are dependent on resting CBF instead of on the intracerebral steal phenomenon that occurs with acetazolamide.

Olprinone: a phosphodiesterase III inhibitor with positive inotropic and vasodilator effects

Olprinone is a newly developed phosphodiesterase III inhibitor characterized by several properties. First, olprinone has positive inotropic and vasodilator actions and improves myocardial mechanical efficiency. Second, olprinone augments cerebral blood flow by a direct vasodilatory effect on cerebral arteries. The cerebrovascular reactivity to olprinone is marked in patients with impaired cerebral circulation. Third, olprinone selectively improves carotid artery distensibility, which may be attributable to differences in the arterial structural components or the reactivity of smooth muscle cells to olprinone. Fourth, olprinone improves inadequate redistribution of brain perfusion and may prevent cerebral metabolic abnormalities in heart failure.

Cerebrovascular reactivity in patients under long-term acetazolamide treatment

BACKGROUND AND PURPOSE

The acetazolamide (AZA) test is a well-accepted method for measuring the vascular reactivity of the cerebral arteries. In order to investigate the nature of this reactivity after long-term daily AZA treatment, the cerebral blood velocity (CBV) was measured using transcranial Doppler in patients under continuous AZA treatment after a single AZA 1 g intravenous (IV) dose.

METHODS

Thirteen patients (eight women, five men) on long-term daily AZA (750 mg/day, mean treatment duration 68 +/- 12+ months) were included in the study. The CBV of the middle cerebral artery (MCA) and the basilar artery (BA), including the values of peak velocity, mean velocity and pulsatility index (PI) were measured. The examination was performed twice - with the initial IV administration of AZA and 20 min later. The results were compared with those of 10 age matched volunteers.

RESULTS

A consistent significant increase of CBV in the right and left MCA (P < 0.001 for both arteries) was found in all study participants. A highly significant decrease of peak CBV in the BA (P < 0.001) was found in the post-AZA velocities of the patient's group. In the control group, a consistent significant increase in all post-AZA tests was demonstrated (P < 0.001).

CONCLUSIONS

A mild elevation of blood velocity in the MCAs concomitant with a highly significant decrease of velocity in the BA was present in all examined patients. These patterns of CBV changes indicate the presence of a 'steal phenomenon' from the posterior to the anterior circulation and stress the necessity for caution when evaluating the indications for performance of the AZA test in patients under continuous AZA therapy.

Effects of acetazolamide on cerebral blood flow, blood volume, and oxygen metabolism: a positron emission tomography study with healthy volunteers

To evaluate changes in cerebral hemodynamics and metabolism induced by acetazolamide in healthy subjects, positron emission tomography studies for measurement of cerebral perfusion and oxygen consumption were performed. Sixteen healthy volunteers underwent positron emission tomography studies with 15O-gas and water before and after intravenous administration of acetazolamide. Dynamic positron emission tomography data were acquired after bolus injection of H2[15O] and bolus inhalation of 15O2. Cerebral blood flow, metabolic rate of oxygen, and arterial-to-capillary blood volume images were calculated using the three-weighted integral method. The images of cerebral blood volume were calculated using the bolus inhalation technique of C[15O]. The scans for cerebral blood flow and volume and metabolic rate of oxygen after acetazolamide challenge were performed at 10, 20, and 30 minutes after drug injection. The parametric images obtained under the two conditions at baseline and after acetazolamide administration were compared. The global and regional values for cerebral blood flow and volume and arterial-to-capillary blood volume increased significantly after acetazolamide administration compared with the baseline condition, whereas no difference in metabolic rate of oxygen was observed. Acetazolamide-induced increases in both blood flow and volume in the normal brain occurred as a vasodilatory reaction of functioning vessels. The increase in arterial-to-capillary blood volume made the major contribution to the cerebral blood volume increase, indicating that the raise in cerebral blood flow during the acetazolamide challenge is closely related to arterial-to-capillary vasomotor responsiveness.

Mapping of cerebrovascular reactivity using BOLD magnetic resonance imaging

Blood oxygen level-dependent (BOLD) contrast MRI is a simple non-invasive method of estimating "perfusion," and combined with a vasodilatory stimulus, may allow estimation of cerebral vascular reserve. We compared BOLD carbon dioxide (CO2) reactivity in the middle cerebral artery (MCA) perfusion territory to MCA flow velocity reactivity determined using transcranial Doppler ultrasound (TCD) in 16 patients with unilateral carotid artery stenosis or occlusion. Both BOLD and TCD reactivities were calculated from measurements acquired when the subjects were breathing air, and again when breathing a 6% CO2/air mixture, and were normalized by dividing by the difference in end tidal (ET) CO2. There was a significant correlation between interhemispheric MCA reactivity difference (contralateral-ipsilateral to the stenosis or occlusion) determined by BOLD MRI and TCD (r = 0.75, p < 0.001). In contrast, treating each hemisphere individually, there was no correlation between the absolute BOLD and TCD MCA CO2 reactivities (r = 0.08, p = 0.670). This appeared to be due to a variable BOLD signal change in the non-stenosed hemisphere between subjects, with little change in the normal hemisphere of a few subjects. In one patient, focal regions of reduced reactivity were seen in non-infarcted regions of the stenosed hemisphere, in the borderzones between arterial territories. BOLD reactivity maps provide information on the whole MCA territory reactivity, and may identify small regions of impaired reactivity which are not detected using TCD. However, BOLD reactivity maps only appear to provide semi-quantitative rather than quantitative data.