Measurement of Cerebrovascular Reactivity as Blood Oxygen Level-Dependent Magnetic Resonance Imaging Signal Response to a Hypercapnic Stimulus in Mechanically Ventilated Patients.

BACKGROUND: Impaired cerebrovascular reactivity (CVR) is an important prognostic marker of stroke. Most measures of CVR lack (1) a reproducible vasoactive stimulus and (2) a high time and spatial resolution measure of cerebral blood flow (CBF), particularly for mechanically ventilated patients. The aim of our study was to investigate the feasibility of measuring CVR using sequential gas delivery circuit and gas blender for precise targeting of end-tidal PCO2 (PetCO2), and blood oxygen level-dependent magnetic resonance imaging (BOLD-MRI) signal as a surrogate of CBF, in mechanically ventilated patients. METHODS: Four patients with known moyamoya disease requiring preoperative CVR measurements under general anesthesia were studied. All patients had standard anesthesia induction and maintenance with intravenous propofol and rocuronium. Patients were intubated and manually ventilated with a self-inflating bag connected to a sequential breathing circuit. A computer-controlled gas blender supplied the gas mixture in proportions to attain target PetCO2. BOLD-MRI was performed at 3.0 Tesla magnet. Changes in signal per change in PetCO2 were calculated, and their magnitude color-coded and mapped onto the anatomic scan to form CVR maps. RESULTS: CVR studies were successfully performed on all patients, and the CVR values were lower in both gray and white matter bilaterally when compared with healthy volunteers. In addition, CVR maps in 3 patients showed intracerebral steal phenomenon in spite of having had cerebral revascularization procedures, indicating that they are still at risk of cerebral ischemia. CONCLUSIONS: BOLD-MRI CVR studies are feasible in mechanically ventilated patients anesthetized with propofol.

BOLD MRI and early impairment of cerebrovascular reserve after aneurysmal subarachnoid hemorrhage.

Currently no biological or radiological marker is available to identify patients at risk of delayed ischemic deficit (DIND) after aneurysmal subarachnoid hemorrhage (aSAH). We hypothesized whether MR-based quantitative assessment of cerebrovascular reserve (CVR) would detect early radiological markers of vasospasm and DIND. This manuscript describes our initial experience with this population. Five patients with aSAH underwent blood-oxygen level dependent-MRI (BOLD-MRI) with CO2 challenge for assessment of whole brain CVR. Patients were examined as soon as possible after aneurysm treatment. We obtained good quality anatomical and functional images without complications. Initial anatomical cerebrovascular imaging showed no vasospasm in all patients. Two patients had abnormal CVR-MRI tests and both developed DIND. Of the 3 others with normal CVR-MRI, one developed posterior circulation DIND. One patient with a normal CVR-MRI developed angiographic vasospasm but no DIND. Changes in CVR maps as early as 36 h after hemorrhage had good spatial correlation with delayed ischemia during short-term follow-up. Our series shows that MRI with CO2 challenge is feasible in this difficult population. Further developments might allow BOLD-MRI with CO2 challenge to identify patients at risk and provide anatomical correlation with future DIND, opening a new venue for prophylactic treatments. Further study is warranted in a larger patient cohort.

Reduced contralateral cerebrovascular reserve in patients with unilateral steno-occlusive disease.

UNLABELLED: The purpose of this study was to evaluate cerebrovascular reactivity (CVR) of major arterial vascular territories, particularly in the contralateral hemodynamically unaffected hemisphere, in patients with unilateral internal carotid artery (ICA) steno-occlusive disease compared to control subjects with risk factors for cerebrovascular disease. METHODS: In this retrospective observational study, twenty-seven patients with right-sided unilateral ICA steno-occlusive disease (age range, 25 to 91 years; 17 males) and twenty-one patients with left-sided unilateral ICA steno-occlusive disease (age range, 24 to 83 years; 14 males) and 41 control subjects were studied. CVR was quantitated as the change in blood oxygen level dependent (BOLD) MRI signal (as a surrogate of cerebral blood flow), in response to a consistently applied step change in the arterial partial pressure of carbon dioxide (PaCO2). The CVR of each major arterial vascular territory was assessed in the ipsilateral hemodynamically affected hemisphere and compared to the corresponding territory in the contralateral hemisphere. RESULTS: In patients, a significant reduction in CVR was observed in the ipsilateral anterior circulation compared to that of the corresponding territory on the contralateral side (0.027 ± 0.083 vs. 0.109 ± 0.066% BOLD change/​mm Hg, p < 0.0001) and to controls (0.195 ± 0.054% BOLD change/mm Hg, p < 0.0001). The CVR of the contralateral anterior circulation was reduced on average by 50% compared to controls (p < 0.0001). CONCLUSIONS: The implication of these findings is that unilateral carotid stenosis affects the vascular reserve of both sides of the brain compared to control subjects. This indicates that the collateral blood flow support from the contralateral to the ipsilateral hemisphere comes at a cost of reduced reserve capacity in the contralateral hemisphere. The findings suggest that there may be a reduction in functional hyperemia associated with neuronal activation, not only affecting the hemisphere ipsilateral to an occlusion, but also the hemisphere contralateral to an occlusion. It remains to be determined if 'stealing' from the 'rich' to support the 'poor' has clinical consequences over the long term.

CO2 blood oxygen level-dependent MR mapping of cerebrovascular reserve in a clinical population: safety, tolerability, and technical feasibility.

PURPOSE: To evaluate the safety, tolerability, and technical feasibility of mapping cerebrovascular reactivity (CVR) in a clinical population by using a precise prospectively targeted CO(2) stimulus and blood oxygen level-dependent (BOLD) magnetic resonance (MR) imaging. MATERIALS AND METHODS: A chart review was performed of all CVR studies from institutional review board-approved projects at a tertiary care hospital between January 1, 2006, and December 1, 2010. Informed consent was obtained. Records were searched for the incidence of adverse events and failed examinations. CVR maps were evaluated for diagnostic quality by two blinded observers and were categorized as good, diagnostic but suboptimal, or nondiagnostic. Outcomes were presented as raw data and descriptive statistics (means ± standard deviations). Intraclass correlation coefficient was used to determine interobserver variability. RESULTS: Four hundred thirty-four consecutive CVR examinations from 294 patients (51.8% female patients) were studied. Patient age ranged from 9 to 88 years (mean age, 45.9 years ± 20.6). Transient symptoms, such as shortness of breath, headache, and dizziness, were reported in 48 subjects (11.1% of studies) during hypercapnic phases only. There were no neurologic ischemic events, myocardial infarctions, or other major complications. The success rate in generating CVR maps was 83.9% (364 of 434). Of the 70 (16.1%) failed examinations, 25 (35.7%) were due to discomfort; eight (11.4%), to head motion; two (2.9%), to inability to cooperate; seven (10.0%), to technical difficulties with equipment; and 28 (40.0%), to unknown or unspecified conditions. Among the 364 remaining successful examinations, good quality CVR maps were obtained in 340 (93.4%); diagnostic but suboptimal, in 12 (3.3%); and nondiagnostic, in 12 (3.3%). CONCLUSION: CVR mapping by using a prospectively targeted CO(2) stimulus and BOLD MR imaging is safe, well tolerated, and technically feasible in a clinical patient population.

The interaction of carbon dioxide and hypoxia in the control of cerebral blood flow.

Both hypoxia and carbon dioxide increase cerebral blood flow (CBF), and their effective interaction is currently thought to be additive. Our objective was to test this hypothesis. Eight healthy subjects breathed a series of progressively hypoxic gases at three levels of carbon dioxide. Middle cerebral artery velocity, as an index of CBF; partial pressures of carbon dioxide and oxygen and concentration of oxygen in arterial blood; and mean arterial blood pressure were monitored. The product of middle cerebral artery velocity and arterial concentration of oxygen was used as an index of cerebral oxygen delivery. Two-way repeated measures analyses of variance (rmANOVA) found a significant interaction of carbon dioxide and hypoxia factors for both CBF and cerebral oxygen delivery. Regression models using sigmoidal dependence on carbon dioxide and a rectangular hyperbolic dependence on hypoxia were fitted to the data to illustrate this interaction. We concluded that carbon dioxide and hypoxia act synergistically in their control of CBF so that the delivery of oxygen to the brain is enhanced during hypoxic hypercapnia and, although reduced during normoxic hypocapnia, can be restored to normal levels with progressive hypoxia.

Impaired peri-nidal cerebrovascular reserve in seizure patients with brain arteriovenous malformations.

Epileptic seizures are a common presentation in patients with newly diagnosed brain arteriovenous malformations, but the pathophysiological mechanisms causing the seizures remain poorly understood. We used magnetic resonance imaging-based quantitative cerebrovascular reactivity mapping and conventional angiography to determine whether seizure-prone patients with brain arteriovenous malformations exhibit impaired cerebrovascular reserve or morphological angiographic features predictive of seizures. Twenty consecutive patients with untreated brain arteriovenous malformations were recruited (10 with and 10 without epileptic seizures) along with 12 age-matched healthy controls. Blood oxygen level-dependent MRI was performed while applying iso-oxic step changes in end-tidal partial pressure of CO(2) to obtain quantitative cerebrovascular reactivity measurements. The brain arteriovenous malformation morphology was evaluated by angiography, to determine to what extent limitations of arterial blood supply or the presence of restricted venous outflow and tissue congestion correlated with seizure susceptibility. Only patients with seizures exhibited impaired peri-nidal cerebrovascular reactivity by magnetic resonance imaging (0.11 ± 0.10 versus 0.25 ± 0.07, respectively; P < 0.001) and venous drainage patterns suggestive of tissue congestion on angiography. However, cerebrovascular reactivity changes were not of a magnitude suggestive of arterial steal, and were probably compatible with venous congestion in aetiology. Our findings demonstrate a strong association between impaired peri-nidal cerebrovascular reserve and epileptic seizure presentation in patients with brain arteriovenous malformation. The impaired cerebrovascular reserve may be associated with venous congestion. Quantitative measurements of cerebrovascular reactivity using blood oxygen level-dependent MRI appear to correlate with seizure susceptibility in patients with brain arteriovenous malformation.

Impact of extracranial-intracranial bypass on cerebrovascular reactivity and clinical outcome in patients with symptomatic moyamoya vasculopathy.

BACKGROUND AND PURPOSE: The purpose of this study was to evaluate in symptomatic moyamoya patients the effect of surgical revascularization on impaired cerebrovascular reactivity (CVR) and its relationship to clinical outcome. METHODS: Brain revascularization was performed using a direct superficial temporal artery to middle cerebral artery bypass or indirect encephalo-dural-arterial synangiosis. CVR was measured pre- and 3 months postoperatively using blood oxygen level-dependent MRI during iso-oxic hypercapnic changes in end-tidal carbon dioxide. Outcomes were assessed by MRI, clinical examination, and modified Rankin Scale scores. RESULTS: Fifty-five hemispheres were revascularized in 39 patients (superficial temporal artery to middle cerebral artery in 47, encephalo-dural-arterial synangiosis in 8). Surgery reversed CVR impairment in 52 hemispheres (94.5%) and in 36 of 39 patients (92.3%; Fisher exact test, P<0.001), and this was predictive of a patent extracranial-intracranial bypass. New, clinically silent perioperative hemorrhages, cortical foci of ischemia, or new white matter T2 hyperintensities were detected after 11 surgeries (20%), but no new lesions arose after 3 postoperative months. One patient had a clinical perioperative stroke (1.8%). In clinical follow-up, 37 of 39 patients (95%) had stable or improved modified Rankin Scale scores and 2 patients (5.1%) worsened. No patients with patent bypasses or CVR improvements exhibited new clinical symptoms, but failure of CVR improvement corresponded to a poorer long-term outcome (Fisher exact test, P<0.001). CONCLUSIONS: Cerebral revascularization surgery is a safe and effective treatment for reversing preoperative CVR defects and may prevent recurrence of preoperative symptoms. Moreover, CVR measurements may be useful in long-term follow-up and for predicting bypass patency.

Measurement of cerebrovascular reactivity in pediatric patients with cerebral vasculopathy using blood oxygen level-dependent MRI.

BACKGROUND AND PURPOSE: Cerebrovascular reactivity (CVR) is an indicator of cerebral hemodynamics. In adults with cerebrovascular disease, impaired CVR has been shown to be associated with an increased risk of stroke. In children, however, CVR studies are not common. This may be due to the difficulties and risks associated with current CVR study methodologies. We have previously described the application of precise control of end-tidal carbon dioxide partial pressure for CVR studies in adults. Our aim is to report initial observations of CVR studies that were performed as part of a larger observational study regarding investigations in pediatric patients with cerebral vascular disease. METHODS: Thirteen patients between the ages of 10 and 16 years (10 with a diagnosis of Moyamoya vasculopathy and 3 with confirmed, or suspected, intracranial vascular stenosis) underwent angiography, MRI, and functional blood oxygen level-dependent MRI mapping of CVR to hypercapnia. The results of the CVR study were then related to both the structural imaging and clinical status. RESULTS: Sixteen blood oxygen level-dependent MRI CVR studies were performed successfully in 13 consecutive patients. Twelve of the 13 patients with angiographic abnormalities also had CVR deficits in the corresponding downstream vascular territories. CVR deficits were also seen in 8 of 9 symptomatic patients and 2 of the asymptomatic patients. Notably, in patients with abnormalities on angiography, the reductions in CVR extended beyond the ischemic lesions identified with MR structural imaging into normal-appearing brain parenchyma. CONCLUSIONS: This is the first case series reporting blood oxygen level-dependent MRI CVR in children with cerebrovascular disease. CVR studies performed so far provide information regarding hemodynamic compromise, which complements traditional clinical assessment and structural imaging.

Surgical revascularization reverses cerebral cortical thinning in patients with severe cerebrovascular steno-occlusive disease.

BACKGROUND AND PURPOSE: Chronic deficiencies in regional blood flow lead to cerebral cortical thinning without evidence of gross tissue loss at the same time as potentially negatively impacting on neurological and cognitive performance. This is most pronounced in patients with severe occlusive cerebrovascular disease in whom affected brain areas exhibit "steal physiology," a paradoxical reduction of cerebral blood flow in response to a global vasodilatory stimulus intended to increase blood flow. We tested whether surgical brain revascularization that eliminates steal physiology can reverse cortical thinning. METHODS: We identified 29 patients from our database who had undergone brain revascularization with pre- and postoperative studies of cerebrovascular reactivity using blood oxygen(ation) level-dependent MRI and whose preoperative study exhibited steal physiology without MRI-evident structural abnormalities. Cortical thickness in regions corresponding to steal physiology, and where applicable corresponding areas in the normal hemisphere, were measured using Freesurfer software. RESULTS: At an average of 11 months after surgery, cortical thickness increased in every successfully revascularized hemisphere (n=30). Mean cortical thickness in the revascularized regions increased by 5.1% (from 2.40 ± 0.03 to 2.53 ± 0.03; P<0.0001). CONCLUSIONS: Successful regional revascularization and reversal of steal physiology is followed by restoration of cortical thickness.

Exaggerated increase in cardiac output during exercise in patients with peripheral high-flow arteriovenous malformations.

PURPOSE: To test the hypothesis that cardiac output (Q) in patients with arteriovenous malformations (AVMs) is normal at rest and increases disproportionately during exercise. MATERIALS AND METHODS: Q was measured in eight patients with large peripheral AVMs and in nine healthy subjects using a noninvasive carbon dioxide (CO2)-based differential Fick method. Subjects were tested while seated at rest and during mild exercise (repeated leg straightening while sitting). Oxygen consumption (VO2) was monitored as an index of the degree of exercise. RESULTS: Average resting Q was similar between AVM patients and healthy subjects (7.40 L/min ± 3.29 vs 6.13 L/min ± 0.94, respectively, P = .29). During exercise, AVM patients showed a smaller increment in VO2 (0.50 L/min ± 0.11 vs 0.78 L/min ± 0.26, P = .012) but with more apparent effort and shortness of breath compared with healthy subjects. The change in Q per unit change in VO2 (ΔQ/ΔVO2) was greater in AVM patients than in healthy subjects (16.00 L/min ± 6.50 vs 9.79 L/min ± 5.33, P < .045). CONCLUSIONS: Exercise intolerance in AVM patients may be due to an imbalance in ΔQ/ΔVO2 resulting from increased shunting through the AVM. Exercise provocation may increase the sensitivity of Q in the clinical evaluation of AVM patients.

Impaired cerebrovascular reactivity with steal phenomenon is associated with increased diffusion in white matter of patients with Moyamoya disease.

BACKGROUND AND PURPOSE: Reduced cerebrovascular reactivity (CVR) with steal phenomenon is an independent predictor for stroke and may indicate tissue exposed to episodic low-grade ischemia. The apparent diffusion coefficient (ADC) calculated using diffusion-weighted MRI is effective in characterizing focal brain ischemia and subtle structural changes in normal-appearing white matter (WM). We hypothesized that regions of steal phenomenon are associated with increased ADC in normal-appearing WM of patients with Moyamoya disease. METHODS: Twenty-two patients with unilateral CVR impairment secondary to Moyamoya disease and 12 healthy control subjects underwent diffusion-weighted MRI and functional MRI mapping of the cerebrovascular response to hypercapnia. Parametric maps of ADC and CVR were calculated, coregistered, and segmented using automated image processing methods. ADC of normal-appearing WM was compared between hemispheres, and between WM with negative CVR (ie, steal phenomenon) and WM with positive CVR. RESULTS: In patients, ADC of normal-appearing WM was elevated in the hemisphere ipsilateral to the CVR impairment compared with the contralateral hemisphere (P<0.005) and in WM with negative CVR compared with WM with positive CVR (P<0.001). WM in regions of steal phenomenon within the affected hemisphere had higher ADC than homologous contralateral WM (P<0.005). In control subjects, negative CVR in WM was not associated with elevated ADC. CONCLUSIONS: Regions of steal phenomenon are spatially correlated with elevated ADC in normal-appearing WM of patients with Moyamoya disease. This structural abnormality may reflect low-grade ischemic injury after exhaustion of the cerebrovascular reserve capacity.

Personalized Ventilation to Multiple Patients Using a Single Ventilator: Description and Proof of Concept.

OBJECTIVES: To design and test a ventilator circuit that can be used for ventilation of two or more patients with a single ventilator, while allowing individualization of tidal volume, fractional concentration of oxygen, and positive end-expiratory pressure to each patient, irrespective of the other patient's respiratory system mechanics. DESIGN: Description and proof of concept studies. SETTINGS: Respiratory therapy laboratory. SUBJECTS: Ventilation of mechanical test lungs. INTERVENTIONS: Following a previously advocated design, we used components readily available in our hospital to assemble two "bag-in-a-box" breathing circuits. Each patient circuit consisted of a flexible bag in a rigid container connected via one-way valve to a test lung, along with an inline positive end-expiratory pressure valve, connected to the ventilator's expiratory limb. Compressed gas fills the bags during "patient" exhalation. During inspiration, gas from the ventilator, in pressure control mode, enters the containers and displaces gas from the bags to the test lungs. We varied tidal volume, "respiratory system" compliance, and positive end-expiratory pressure in one lung and observed the effect on the tidal volume of the other. MEASUREMENTS AND MAIN RESULTS: We were able to obtain different tidal volume, dynamic driving pressure, and positive end-expiratory pressure in the two lungs under widely different compliances in both lungs. Complete obstruction, or disconnection at the circuit connection to one test lung, had minimal effect (< 5% on average) on the ventilation to the co-ventilated lung. CONCLUSIONS: A secondary circuit "bag-in-the-box" system enables individualized ventilation of two lungs overcoming many of the concerns of ventilating more than one patient with a single ventilator.

Mapping cerebrovascular reactivity using blood oxygen level-dependent MRI in Patients with arterial steno-occlusive disease: comparison with arterial spin labeling MRI.

BACKGROUND AND PURPOSE: Blood oxygen level-dependent MRI (BOLD MRI) of hypercapnia-induced changes in cerebral blood flow is an emerging technique for mapping cerebrovascular reactivity (CVR). BOLD MRI signal reflects cerebral blood flow, but also depends on cerebral blood volume, cerebral metabolic rate, arterial oxygenation, and hematocrit. The purpose of this study was to determine whether, in patients with stenoocclusive disease, the BOLD MRI signal response to hypercapnia is directly related to changes in cerebral blood flow. METHODS: Thirty-eight patients with stenoocclusive disease underwent mapping of CVR by both BOLD MRI and arterial spin labeling MRI. The latter technique was used as a reference standard for measurement of cerebral blood flow changes. RESULTS: Hemispheric CVR measured by BOLD MRI was significantly correlated with that measured by arterial spin labeling MRI for both gray matter (R=0.83, P<0.0001) and white matter (R=0.80, P<0.0001). Diagnostic accuracy (area under receiver operating characteristic curve) for BOLD MRI discrimination between normal and abnormal hemispheric CVR was 0.90 (95% CI=0.81 to 0.98; P<0.001) for gray matter and 0.82 (95% CI=0.70 to 0.94; P<0.001) for white matter. Regions of paradoxical CVR on BOLD MRI had a moderate predictive value (14 of 19 hemispheres) for spatially corresponding paradoxical CVR on arterial spin labeling MRI. Complete absence of paradoxical CVR on BOLD MRI had a high predictive value (31 of 31 hemispheres) for corresponding nonparadoxical CVR on arterial spin labeling MRI. CONCLUSIONS: Arterial spin labeling MRI confirms that, even in patients with stenoocclusive disease, the BOLD MRI signal response to hypercapnia predominantly reflects changes in cerebral blood flow.

Selective reduction of blood flow to white matter during hypercapnia corresponds with leukoaraiosis.

BACKGROUND AND PURPOSE: Age-related white matter disease (leukoaraiosis) clusters in bands in the centrum semiovale, about the occipital and frontal horns of the lateral ventricles, in the corpus callosum, and internal capsule. Cerebrovascular anatomy suggests that some of these locations represent border zones between arterial supply territories. We hypothesized that there are zones of reduced cerebrovascular reserve (susceptible to selective reductions in blood flow, ie, steal phenomenon) in the white matter of young, healthy subjects, the physiological correlate of these anatomically defined border zones. Furthermore, we hypothesized that these zones spatially correspond with the regions where the elderly develop leukoaraiosis. METHODS: Twenty-eight healthy volunteers underwent functional MR mapping of the cerebrovascular response to hypercapnia. We studied 18 subjects by blood oxygen level-dependent MRI and 10 subjects by arterial spin labeling MRI. We controlled both end-tidal pCO(2) and pO(2). All functional data was registered in Montreal Neurological Institute space and generated composite blood oxygen level-dependent MR and arterial spin labeling MR maps of cerebrovascular reserve. We compared these maps with frequency maps of leukoaraiosis published previously. RESULTS: Composite maps demonstrated significant (90% CI excluding the value zero) steal phenomenon in the white matter. This steal was induced by relatively small changes in end-tidal pCO(2). It occurred precisely in those locations where elderly patients develop leukoaraiosis. CONCLUSIONS: This steal phenomenon likely represents the physiological correlate of the previously anatomically defined internal border zones. Spatial concordance with white matter changes in the elderly raises the possibility that this steal phenomenon may have a pathogenetic role.

BOLD-MRI cerebrovascular reactivity findings in cocaine-induced cerebral vasculitis.

BACKGROUND: An 18-year-old woman presented to a regional stroke center with dysphasia and right hemiparesis 2 days after consuming alcohol and inhaling cannabis and -- for the first time -- cocaine. INVESTIGATIONS: Physical examination, blood tests for inflammatory markers, vasculitis and toxicology screen, echocardiography, electrocardiography, CT scanning, brain MRI, magnetic resonance angiography, magnetic resonance vessel wall imaging, catheter angiography, and correlation of blood oxygen level-dependent (BOLD)-MRI signal intensity with changes in end-tidal partial pressure of carbon dioxide. DIAGNOSIS: Cocaine-induced cerebral vasculitis. MANAGEMENT: No specific therapy was initiated. The patient's vital signs and neurological status were monitored during her admission. Follow-up medical imaging was performed after the patient's discharge from hospital.

Long-term changes in cerebrovascular reactivity following EC-IC bypass for intracranial steno-occlusive disease.

The purpose of this retrospective observational study is to investigate the long-term changes in cerebrovascular reactivity (CVR) as a measure of cerebral hemodynamics in patients with intracranial steno-occlusive disease (IC-SOD) after they have undergone an Extracranial-intracranial (EC-IC) bypass. Twenty-six patients suffering from IC-SOD were selected from our CVR database. Nineteen patients underwent unilateral and 7 underwent bilateral revascularization. CVR measurements were done using BOLD-MRI and precisely controlled CO2 and expressed as ΔBOLD (%)/Δ PETCO2 (mmHg). Trends in CVR over time were compared in both vascularized and non-vascularized hemispheres. Repeated measures analysis of variance with Greenhouse-Geisser correction was used to determine CVR changes within the grey matter MCA for longitudinal assessments. Overall, re-vascularized hemisphere showed a significant increase in CVR at the first follow-up, followed by a slight decrease at the second follow-up that significantly increased compared to the pre-bypass. However, the changes in the postoperative CVR were quite variable across the patients. Similar variability was seen in subsequent follow-ups, with a slight overall decline in the long term CVR as compared with first post-operative CVR. Our study demonstrates that EC-IC bypass has a beneficial long-term effect on cerebral hemodynamics and this effect varies between patients probably due to the variability in the underlying vascular pattern receiving the bypass. Hence, in the postoperative follow-up of patients routine functional imaging to monitor cerebral hemodynamics may be useful as the risk of stroke and cognitive decline remain present with impaired CVR.

Assessing the effect of unilateral cerebral revascularisation on the vascular reactivity of the non-intervened hemisphere: a retrospective observational study.

OBJECTIVES: Unilateral haemodynamically significant large-vessel intracranial stenosis may be associated with reduced blood-oxygen-level-dependent (BOLD) cerebrovascular reactivity (CVR), an indicator of autoregulatory reserve. Reduced CVR has been associated with ipsilateral cortical thinning and loss in cognitive function. These effects have been shown to be reversible following revascularisation. Our aim was to study the effects of unilateral revascularisation on CVR in the non-intervened hemisphere in bilateral steno-occlusive or Moyamoya disease. STUDY DESIGN: A retrospective observational study. SETTING: A routine follow-up assessment of CVR after a revascularisation procedure at a research teaching hospital in Toronto (Journal wants us to generalise). PARTICIPANTS: Thirteen patients with bilateral Moyamoya disease (age range 18 to 52 years; 3 males), seven patients with steno-occlusive disease (age range 18 to 78 years; six males) and 27 approximately age-matched normal control subjects (age range 19-71 years; 16 males) with no history or findings suggestive of any neurological or systemic disease. INTERVENTION: Participants underwent BOLD CVR MRI using computerised prospective targeting of CO2, before and after unilateral revascularisation (extracranial-intracranial bypass, carotid endarterectomy or encephaloduroarteriosynangiosis). Pre-revascularisation and post-revascularisation CVR was assessed in each major arterial vascular territory of both hemispheres. RESULTS: As expected, surgical revascularisation improved grey matter CVR in the middle cerebral artery (MCA) territory of the intervened hemisphere (0.010±0.023 to 0.143±0.010%BOLD/mm Hg, p<0.01). There was also a significant post-revascularisation improvement in grey matter CVR in the MCA territory of the non-intervened hemisphere (0.101±0.025 to 0.165±0.015%BOLD/mm Hg, p<0.01). CONCLUSIONS: Not only does CVR improve in the hemisphere ipsilateral to a flow restoration procedure, but it also improves in the non-intervened hemisphere. This highlights the potential of CVR mapping for staging and evaluating surgical interventions.

The effect of hypercarbia and hyperoxia on the total blood flow to the retina as assessed by magnetic resonance imaging.

PURPOSE: The feasibility of measuring total blood flow to the retina with Arterial Spin Labeling Magnetic Resonance Imaging (ASL-MRI) has been described previously. In the present study, the hypothesis was that the reactivity that the ASL-MRI detects at the human retina is dominated by the choroidal blood flow, and thus it may serve as a useful tool for quantitative assessment of the choroidal vascular reactivity. METHODS: Before imaging, the intraocular pressure (IOP) was measured in the study eye of nine clinically healthy subjects (four males) while the subject performed the ventilatory protocol subsequently imaged by the scanner. End-tidal CO2 partial pressure (P(ET)CO2) was increased to target 45 mm Hg, (baseline P(ET)CO2 = 40 mm Hg and baseline P(ET)O2 = 100 mm Hg). P(ET)O2 was then increased to target 300 and 500 mm Hg while keeping P(ET)CO2 constant at 45 mm Hg. A background-suppressed, pulsed-continuous ASL sequence was used for blood flow imaging. RESULTS: The measured total blood flow increased significantly from 1.55 ± 0.17 µL/mm²/min at the baseline to 1.96 ± 0.18 µL/mm²/min during hypercarbia. With increasing P(ET)O2, the measured blood flow did not change significantly relative to the hypercarbia condition but remained significantly elevated relative to the baseline. There were no significant changes in systolic, diastolic, or mean blood pressure, heart rate, or IOP during all four breathing conditions. CONCLUSIONS: The lack of change in the ASL signal under hyperoxic conditions is consistent with the hypothesis that this noninvasive assessment technique is predominantly weighted by choroidal blood flow. The results indicate that a CO2 provocation challenge, in combination with ASL-MRI, is a promising noninvasive approach for investigating choroidal vascular reactivity under normal and disease states.

Ophthalmic injuries from a TASER.

The TASER (TASER International) is an energy-conducting weapon, that is becoming more frequently used by law enforcement officials to subdue combative individuals. Though generally regarded as a safe alternative, the use of such weapons has been reported to cause serious injuries. We describe a case in which ocular injuries were sustained by impalement with a TASER dart. Emergency physicians should be aware of the potential for serious ophthalmic injuries from TASERs and how such injuries should be managed.