CBF and time thresholds for the formation of ischemic cerebral edema, and effect of reperfusion in baboons

Noncontrast conventional computed tomography in the evaluation of acute stroke

The advantages of CT scanning in the assessment of hyperacute stroke patients include convenience, accuracy, speed, and low cost. CT scanning is presently considered to be the standard of care for the detection of acute extra-axial and parenchymal hemorrhage, although newer MRI techniques are challenging this claim. The accuracy of CT scanning for stroke detection can be optimized by the use of appropriate scanning technique, as well as interactive image review at a picture archiving and communication system workstation using narrow window and level settings. More importantly, CT scanning has prognostic value for patients receiving thrombolytic treatment by helping to predict both responses to treatment and hemorrhage risk. Finally, as will be discussed in other articles in this issue, CT angiography can be performed in the same imaging session as noncontrast CT scanning without substantially increasing the time required for patient evaluation. This provides data regarding vascular patency and tissue perfusion, which are valuable in the triage of stroke patients.

Quantitative T(1rho) and magnetization transfer magnetic resonance imaging of acute cerebral ischemia in the rat

It has been previously shown that T1 in the rotating frame (T(1rho)) is a very sensitive and early marker of cerebral ischemia and that, interestingly, it can provide prognostic information about the degree of subsequent neuronal damage. In the present study the authors have quantified T(1rho) together with the rate and other variables of magnetization transfer (MT) associated with spin interactions between the bulk and semisolid macromolecular pools by means of Z spectroscopy, to examine the possible overlap of mechanisms affecting these magnetic resonance imaging contrasts. Substantial prolongation of cerebral T(1rho) was observed minutes after induction of ischemia, this change progressing in a time-dependent manner. Difference Z spectra (contralateral nonischemic minus ischemic brain tissue) showed a significant positive reminder in the time points from 0.5 to 3 hours after induction of ischemia, the polarity of this change reversing by 24 hours. Detailed analysis of the MT variables showed that the initial Z spectral changes were due to concerted increase in the maximal MT (+3%) and amount of MT (+4%). Interestingly, the MT rates derived either from the entire frequency range of Z spectra or the time constant for the first-order forward exchange (k(sat)) were unchanged at this time, these variables reducing only one day after induction of ischemia. The authors conclude that T(1rho) changes in the acute phase of ischemia coincide with both elevated maximal MT and amount of MT. These changes occur independent of the overall MT rate and in the absence of net water gain to the tissue, whereas in the consolidating infarction the decrease in the rate and amount of MT, as well as the extensive prolongation of T(1rho), are associated with water accumulation.

Ischaemic brain oedema

Ischaemic brain oedema appears to involve two distinct processes, the relative contribution and time course of which depend on the duration and severity of ischaemia, and the presence of reperfusion. The first process involves an increase in tissue Na+ and water content accompanying increased pinocytosis and Na+, K+ ATPase activity across the endothelium. This is apparent during the early phase of infarction and before any structural damage is evident. This phenomenon is augmented by reperfusion. A second process results from a more indiscriminate and delayed BBB breakdown that is associated with infarction of both the parenchyma and the vasculature itself. Although, tissue Na+ level still seems to be the major osmotic force for oedema formation at this second stage, the extravasation of serum proteases is an additional potentially deleterious factor. The relative importance of protease action is not yet clear, however, degradation of the extracellular matrix conceivably leads to further BBB disruption and softening of the tissue, setting the stage for the most pronounced forms of brain swelling. A number of factors mediate or modulate ischaemic oedema formation, however, most current information comes from experimental models, and clinical data on this microcosmic level is lacking. Clinically significant brain oedema develops in a delayed fashion after large hemispheric strokes and is a cause of substantial mortality. Neurological signs appear to be at least as good as direct ICP measurement and neuroimaging in detecting and gauging the secondary damage produced by stroke oedema. The neuroimaging characteristics of the stroke, specifically the early involvement of greater than half of the MCA territory, are, however, highly predictive of the development of severe oedema over the subsequent hours and days. None of the available medical therapies provide substantial relief from the oedema and raised ICP, or at best, they are temporizing in most cases. Hemicraniectomy appears most promising as a method of avoiding death from brain compression, but the optimum timing and manner of patient selection are currently being investigated. All approaches to massive ischaemic brain swelling are clouded by the potential for survival with poor functional outcome. It is possible to manage blood pressure, serum osmolarity by way of selective fluid administration, and a number of other systemic factors that exaggerate brain oedema. Broad guidelines for treatment of stroke oedema can therefore be given at this time.

Quantitative assessment of the balance between oxygen delivery and consumption in the rat brain after transient ischemia with T2 -BOLD magnetic resonance imaging

The balance between oxygen consumption and delivery in the rat brain after exposure to transient ischemia was quantitatively studied with single-spin echo T2-BOLD (blood oxygenation level-dependent) magnetic resonance imaging at 4.7 T. The rats were exposed to graded common carotid artery occlusions using a modification of the four-vessel model of Pulsinelli. T2, diffusion, and cerebral blood volume were quantified with magnetic resonance imaging, and CBF was measured with the hydrogen clearance method. A transient common carotid artery occlusion below the CBF value of approximately 20 mL x 100 g(-1) x min(-1) was needed to yield a T2 increase of 4.6 +/- 1.2 milliseconds (approximately 9% of cerebral T2) and 6.8 +/- 1.7 milliseconds (approximately 13% of cerebral T2) after 7 and 15 minutes of ischemia, respectively. Increases in CBF of 103 +/- 75% and in cerebral blood volume of 29 +/- 20% were detected in the reperfusion phase. These hemodynamic changes alone could account for only approximately one third of the T2 increase in luxury perfusion, suggesting that a substantial increase in blood oxygen saturation (resulting from reduced oxygen extraction by the brain) is needed to explain the magnetic resonance imaging observation.

Mechanisms of edema formation after intracerebral hemorrhage: effects of extravasated red blood cells on blood flow and blood-brain barrier integrity

BACKGROUND AND PURPOSE

Red blood cell (RBC) lysis contributes to brain edema formation after intracerebral hemorrhage (ICH), and RBC hemolysate (oxyhemoglobin) has been implicated to be a spasminogen in subarachnoid hemorrhage. Whether cerebral ischemia contributes to brain edema formation after ICH remains unclear, however. The aims of this study were to test whether extravasation of RBCs induces cerebral ischemia and/or blood-brain barrier disruption in a rat ICH model characterized by perihematomal brain edema.

METHODS

In this study, 87 pentobarbital-anesthetized Sprague-Dawley rats were used. In each animal, saline, packed RBCs, or lysed RBCs were injected into the right caudate nucleus. Sham injections served as controls. Regional cerebral blood flow, brain water and ion contents, blood-brain barrier integrity, and plasma volume were measured.

RESULTS

Intraparenchymal infusion of lysed RBCs caused severe brain edema by the first day but did not induce ischemic cerebral blood flows. In contrast, blood-brain barrier permeability increased during the first day after infusion of lysed RBCs (a 3-fold increase) and 3 days after infusion of packed RBCs (a 4-fold increase).

CONCLUSIONS

These results suggest that ischemia is not present at 24 or 72 hours after hematoma induction by injection of intact or lysed RBCs. RBC constituents that appear after delayed lysis, however, increase blood-brain barrier permeability, which contributes to edema formation.

rCBF in hemorrhagic, non-hemorrhagic and mixed contusions after severe head injury and its effect on perilesional cerebral blood flow

Intracerebral contusions can lead to regional ischemia caused by extensive release of excitotoxic aminoacids leading to increased cytotoxic brain edema and raised intracranial pressure. rCBF measurements might provide further information about the risk of ischemia within and around contusions. Therefore, the aim of the presented study was to compare the intra- and perilesional rCBF of hemorrhagic, non-hemorrhagic and mixed intracerebral contusions. In 44 patients, 60 stable Xenon-enhanced CT CBF-studies were performed (EtCO2 30 +/- 4 mmHg SD), initially 29 hours (39 studies) and subsequent 95 hours after injury (21 studies). All lesions were classified according to localization and lesion type using CT/MRI scans. The rCBF was calculated within and 1-cm adjacent to each lesion in CT-isodens brain. The rCBF within all contusions (n = 100) of 29 +/- 11 ml/100 g/min was significantly lower (p < 0.0001, Mann-Whitney U) compared to perilesional rCBF of 44 +/- 12 ml/100 g/min and intra/perilesional correlation was 0.4 (p < 0.0005). Hemorrhagic contusions showed an intra/perilesional rCBF of 31 +/- 11/44 +/- 13 ml/100 g/min (p < 0.005), non-hemorrhagic contusions 35 +/- 13/46 +/- 10 ml/100 g/min (p < 0.01). rCBF in mixed contusions (25 +/- 9/44 +/- 12 ml/100 g/min, p < 0.0001) was significantly lower compared to hemorrhagic and non-hemorrhagic contusions (p < 0.02). Intracontusional rCBF is significantly reduced to 29 +/- 11 ml/100 g/min but reduced below ischemic levels of 18 ml/100 g/min in only 16% of all contusions. Perilesional CBF in CT normal appearing brain closed to contusions is not critically reduced. Further differentiation of contusions demonstrates significantly lower rCBF in mixed contusions (defined by both hyper- and hypodense areas in the CT-scan) compared to hemorrhagic and non-hemorrhagic contusions. Mixed contusions may evolve from hemorrhagic contusions with secondary increased perilesional cytotoxic brain edema leading to reduced cerebral blood flow and altered brain metabolism. Therefore, the treatment of ICP might be individually modified by the measurement of intra- and pericontusional cerebral blood.

Rapid differential endogenous plasminogen activator expression after acute middle cerebral artery occlusion

BACKGROUND AND PURPOSE

During focal cerebral ischemia, the microvascular matrix (ECM), which participates in microvascular integrity, is degraded and lost when neurons are injured. Loss of microvascular basal lamina antigens coincides with rapid expression of select matrix metalloproteinases (MMPs). Plasminogen activators (PAs) may also play a role in ECM degradation by the generation of plasmin or by MMP activation.

METHODS

The endogenous expressions of tissue-type plasminogen activator (tPA), urokinase (uPA), and PA inhibitor-1 (PAI-1) were quantified in 10-microm frozen sections from ischemic and matched nonischemic basal ganglia and in the plasma of 34 male healthy nonhuman primates before and after middle cerebral artery occlusion (MCA:O).

RESULTS

Within the ischemic basal ganglia, tissue uPA activity and antigen increased significantly within 1 hour after MCA:O (2P<0.005). tPA activity transiently decreased 2 hours after MCA:O (2P=0.01) in concert with an increase in PAI-1 antigen (2P=0.001) but otherwise did not change. The transient decrease in free tPA antigen was marked by an increase in the tPA-PAI-1 complex (2P<0.001). No significant relations to neuronal injury or intracerebral hemorrhage were discerned.

CONCLUSIONS

The rapid increase in endogenous PA activity is mainly due to significant increases in uPA, but not tPA, within the ischemic basal ganglia after MCA:O. This increase and an increase in PAI-1 coincided with latent MMP-2 generation and microvascular ECM degeneration but not neuronal injury.

Interrelations of T(1) and diffusion of water in acute cerebral ischemia of the rat

Interrelation of T(1) and diffusion of water was studied in rat models of acute global and focal cerebral ischemia. Cortical T(1), as quantified with an inversion recovery method, increased by 4-7% within a few minutes of global ischemia at 4.7 and 9.4 T, but a significantly smaller change was detected at 1.5 T. The initial T(1) change occurred within seconds of cardiac arrest, much earlier than the extensive diffusion drop after 1-2 min. Thus, the initial increase in T(1) upon acute cerebral ischemia is directly caused by cessation of blood flow. In transient middle cerebral artery occlusion (MCAO), prolonged T(1) relaxation was detected within 10 min, with a subsequent increase during the course of ischemia. Spin density did not change during the first hour, showing that T(1) increase was not caused by net accumulation of water. Interestingly, partial recovery of T(1) upon release of MCAO, occurring independent of long-term tissue outcome, was observed only in concert with diffusion recovery.

Quantitative analysis of the loss of distinction between gray and white matter in comatose patients after cardiac arrest

BACKGROUND AND PURPOSE

Anecdotal reports suggest that a loss of distinction between gray (GM) and white matter (WM) as adjudged by CT scan predicts poor outcome in comatose patients after cardiac arrest. To address this, we quantitatively assessed GM and WM intensities at various brain levels in comatose patients after cardiac arrest.

METHODS

Patients for whom consultation was requested within 24 hours of a cardiac arrest were identified with the use of a computerized database that tracks neurological consultations at our institution. Twenty-five comatose patients were identified for whom complete medical records and CT scans were available for review. Twenty-five consecutive patients for whom a CT scan was interpreted as normal served as controls. Hounsfield units (HUs) were measured in small defined areas obtained from axial images at the levels of the basal ganglia, centrum semiovale, and high convexity area.

RESULTS

At each level tested, lower GM intensity and higher WM intensity were noted in comatose patients compared with normal controls. The GM/WM ratio was significantly lower among comatose patients compared with controls (P:<0.0001, rank sum test). There was essentially no overlap in GM/WM ratios between control and study patients. The difference was greatest at the basal ganglia level. We also observed a marginally significant difference in the GM/WM ratio at the basal ganglia level between those patients who died and those who survived cardiac arrest (P:=0. 035, 1-tailed t test). Using receiver operating characteristic curve analysis, we determined that a difference in GM/WM ratio of <1.18 at the basal ganglia level was 100% predictive of death. At the basal ganglia level, none of 12 patients below this threshold survived, whereas the survival rate was 46% among patients in whom the ratio was >1.18. The empirical risk of death was 21.67 for comatose patients with a value below threshold.

CONCLUSIONS

The ratio in HUs of GM to WM provides a reproducible measure of the distinction between gray and white matter. A lower GM/WM ratio is observed in comatose patients immediately after cardiac arrest. The basal ganglia level seems to be the most sensitive location on CT for measuring this relationship. Although a GM/WM ratio <1.18 at this level predicted death in this retrospective study, the difference in this study is not robust enough to recommend that management decisions be dictated by CT results. The results, however, do warrant consideration of a prospective study to determine the reliability of CT scanning in predicting outcome for comatose patients after cardiac arrest.

Vertigo: analysis by magnetic resonance imaging and angiography

The relationship of vascular disease of the vertebrobasilar artery system to isolated vertigo was examined by magnetic resonance imaging and angiography. Eighty-nine individuals complaining of vertigo were evaluated by standard otoneurologic investigations, and the data were correlated with the vascular patterns of the cervical region and posterior fossa. The age distribution extended from the fourth decade to the ninth decade; the peak occurrence was observed in the eighth. Approximately 85% of the group experienced numerous episodes of vestibular dysfunction from months to years before examination; the remaining segment was examined following the first episode due to severity or persistence of symptoms. The criteria for vascular abnormality proposed by the authors are based upon comparison with previous normal findings. Approximately 52% of the cohort demonstrated abnormal configurations or evidence of diminished flow within the vertebrobasilar artery system. Of this segment, a vertebral artery was most frequently abnormal, in 76%; the basilar artery was judged pathological in 32%, and combined disease of several arteries was evident in 20%.

Graded reduction of cerebral blood flow in rat as detected by the nuclear magnetic resonance relaxation time T2: a theoretical and experimental approach

The ability of transverse nuclear magnetic resonance relaxation time, T2, to reveal acutely reduced CBF was assessed using magnetic resonance imaging (MRI). Graded reduction of CBF was produced in rats using a modification of Pulsinelli's four-vessel occlusion model. The CBF in cerebral cortex was quantified using the hydrogen clearance method, and both T2 and the trace of the diffusion tensor (Dav = 1/3TraceD) in the adjacent cortical tissue were determined as a function of reduced CBF at 4.7 T. A previously published theory, interrelating cerebral hemodynamic parameters, hemoglobin, and oxygen metabolism with T2, was used to estimate the effects of reduced CBF on cerebral T2. The MRI data show that T2 reduces in a U-shape manner as a function of CBF, reaching a level that is 2.5 to 2.8 milliseconds (5% to 6%) below the control value at CBF, between 15% and 60% of normal. This reduction could be estimated by the theory using the literature values of cerebral blood volume, oxygen extraction ratio, and precapillary oxygen extraction during compromised CBF. Dav dropped with two apparent flow thresholds, so that a small 11% to 17% reduction occurred between CBF values of 16% to 45% of normal, followed by a precipitous collapse by more than 20% at CBF below 15% of normal. The current data show that T2 can be used as an indicator of acute hypoperfusion because of its ability to indicate blood oxygenation level-dependent phenomena on reduced CBF.

Acute stroke: improved nonenhanced CT detection--benefits of soft-copy interpretation by using variable window width and center level settings

PURPOSE

To assess the use of nonstandard, variable window width and level review settings in computed tomography (CT) without contrast material administration in the detection of acute stroke.

MATERIALS AND METHODS

Nonenhanced CT was performed in 21 patients with acute (< 6 hours) middle cerebral arterial stroke and nine control patients. Two blinded neuroradiologists rated all scans for presence of parenchymal hypoattenuation. Images were reviewed at a picture archiving and communication system (PACS) workstation, with standard, locally determined center level and window width settings of 20 and 80 HU and with variable soft-copy settings initially centered at a level of 32 HU with a width of 8 HU. Reviewers altered settings to accentuate gray and white matter contrast.

RESULTS

With standard viewing parameters, sensitivity and specificity for stroke detection were 57% and 100%. Sensitivity increased to 71% with variable window width and center level settings, without loss of specificity. Receiver operating characteristic analysis revealed a significant improvement in accuracy with nonstandard, soft-copy review settings (P = .03, one-tailed z test).

CONCLUSION

In nonehanced CT of the head, detection of ischemic brain parenchyma is facilitated by soft-copy review with variable window width and center level settings to accentuate the contrast between normal and edematous tissue.

Clinical application of perfusion and diffusion MR imaging in acute ischemic stroke

With the advances in new neuroimaging modalities, the role of imaging of acute ischemic stroke has broadened and progressed from making diagnoses to providing valuable information for patient management. The goal of thrombolytic therapy for acute ischemic stroke should be to salvage the ischemic tissue reversibility that can respond to recanalization and avoid reperfusion of the dead (nonviable) tissue. It is essential to have rapid diagnostic modalities that can distinguish viable ischemic tissue from irreversibly damaged tissue, because there is a risk of reperfusion injury such as hemorrhagic complications with early intervention. Although diffusion magnetic resonance (MR) imaging has been reported to have a high sensitivity and specificity for acute ischemia in acute stroke patients without early reperfusion therapy, the capability to differentiate reversible from irreversible ischemia by diffusion MR imaging has not been established. Perfusion MR imaging techniques provide direct information on parenchymal perfusion status (adequacy of the collateral circulation) and may have the potential for providing important information about tissue viability and/or reversibility for selecting appropriate patients for thrombolytic therapy.

Time course of lesion development in patients with acute stroke: serial diffusion- and hemodynamic-weighted magnetic resonance imaging

BACKGROUND AND PURPOSE

We sought to characterize the evolution of acute ischemic stroke by MRI and its relationship to patients' neurological outcome.

METHODS

Fourteen patients with acute ischemic stroke underwent MRI within 13 hours of symptom onset (mean, 7.4+/-3 hours) and underwent repeated imaging and concurrent neurological examination at 8, 24, 36, and 48 hours and 7 days and >42 days after first imaging.

RESULTS

Diffusion-weighted imaging (DWI) lesion volumes increased between the first and second scans in 10 of 14 patients; scans with maximum DWI lesion volume occurred at a mean of 70.4 hours. Initial DWI lesion volume correlated with the largest T2 lesion volume (r=0.97; P<0.001). Final lesion volume was smaller than maximum lesion volume in 12 of 14 patients. There was positive correlation between the follow-up National Institutes of Health Stroke Scale score and the initial DWI lesion volume (r=0.67; P=0. 01) and maximum T2 lesion volume (r=0.77; P<0.01) and negative correlation with initial mean apparent diffusion coefficient ratio (ADCr) (r=-0.64; P<0.05). The ADCr was 0.73 at initial imaging and fell between the initial and second scans in 10 of 14 patients. Mean ADCr did not rise above normal until 42 days after stroke onset (P<0. 001).

CONCLUSIONS

Serial MRI demonstrates the dynamic nature of progressive ischemic injury in acute stroke patients developing over hours to days, and it suggests that both primary and secondary pathophysiological processes can be valuable targets for neuroprotective interventions.

Interleukin-8 (IL-8) and monocyte chemotactic and activating factor (MCAF/MCP-1), chemokines essentially involved in inflammatory and immune reactions

Leukocyte infiltration is a hallmark of inflammation. Knowledge on molecular mechanisms of leukocyte infiltration has advanced rapidly due to the recent elucidation of structures and functions of adhesion molecules and chemokines. Since the discovery of interleukin-8 (IL-8), a prototype of CXC chemokines, in 1987 and monocyte chemotactic and activating factor/monocyte chemoattractant protein-1 (MCAF/MCP-1), a prototype of chemotactic cytokines (CC) chemokines, in 1989, more than 30 members of chemokines have been identified so far. Evidence is accumulating that these chemokines exert overlapping but distinct actions on specific types of leukocytes in vitro through interacting with their specific G-protein-coupled receptors with seven transmembrane domains. However, redundancy at receptor levels has frequently hindered the clarification on the precise physiological or pathophysiological roles of chemokines. Here, we describe the pathophysiological roles of IL-8 and MCAF/MCP-1 in several animal models of neutrophil- and macrophage-mediated inflammation, respectively, by focusing on our recent work using neutralizing antibodies to these chemokines. We discuss further potential roles of these chemokines in T-lymphocyte-mediated immune responses.

Temporary occlusion of the middle cerebral artery in intracranial aneurysm surgery: time limitation and advantage of brain protection

The risk of focal infarction secondary to the induced reversible arrest of local arterial flow during microsurgical dissection of middle cerebral artery (MCA) aneurysms was evaluated further to define the optimal approach to temporary arterial occlusion. To compare the effectiveness of potential brain-protection anesthetics, a group of patients treated with the intravenous agents propofol, etomidate, and pentobarbital, administered individually or in combination, was compared to a group treated with the inhalational agent isoflurane. Forty-nine consecutive MCA aneurysm surgeries involving the temporary clipping of the parent vessel were retrospectively reviewed. Thirty-eight patients received intravenous brain-protection (IVBP) anesthesia. Groups of patients with and without infarctions, and receiving and not receiving IVBP anesthesia, were compared based on the duration and nature of temporary arterial occlusion. Postoperative radiographic evidence of new infarction was used as the threshold for failure of occlusion tolerance. The overall infarction rate was 22.4% (11 of 49 patients), including 15.8% (six of 38 patients) in the IVBP group versus 45.5% (five of 11 patients) in the group that did not receive brain protection (NBP). In the NBP group, the mean duration of temporary occlusion was 3.9 +/- 2.2 minutes for patients without infarction versus 12.2 +/- 4.3 minutes for patients with focal infarction (p < 0.01). In contrast, the mean duration was 13.6 +/- 10.6 minutes for patients without infarction and 18.5 +/- 9.9 minutes for patients with infarction in the IVBP group. All patients (four of four) in the NBP group who underwent occlusion lasting 10 minutes or longer suffered an infarction versus five of 23 patients in the IVBP group (p < 0.0001). Patients with multiple aneurysms were found to be at increased risk of developing focal infarction, whereas those treated with intermittent temporary clip application were at decreased risk. It is concluded that patients in whom focal iatrogenic ischemia is induced during MCA aneurysm clip ligation have a significant advantage compared with those receiving isoflurane when they are given pentobarbital as the primary neuroprotective agent or when they receive propofol or etomidate titrated to achieve electroencephalographic burst suppression, particularly if more than 10 minutes of occlusion time is required. It is also concluded that 10 minutes is a general guideline for safe, temporary occlusion of the MCA. The use of intermittent temporary arterial occlusion and its use in patients with multiple aneurysms need further evaluation before specific recommendations can be made.

Temporary occlusion of the middle cerebral artery in intracranial aneurysm surgery: time limitation and advantage of brain protection

The risk of focal infarction secondary to the induced reversible arrest of local arterial flow during microsurgical dissection of middle cerebral artery (MCA) aneurysms was evaluated further to define the optimal approach to temporary arterial occlusion. To compare the effectiveness of brain-protection anesthetics, a group of patients treated with the intravenous agents, propofol, etomidate, and pentobarbital, administered individually or in combination, was compared to a group treated with the inhalational agent isoflurane.

Forty-nine consecutive MCA aneurysm surgeries involving the temporary clipping of the parent vessel were retrospectively reviewed. Thirty-eight patients received intravenous brain-protection (IVBP) anesthesia. Groups of patients with and without infarctions, and receiving and not receiving IVBP, were compared based on the duration and nature of temporary arterial occlusion. Postoperative radiographic evidence of new infarction was used as the threshold for failure of occlusion tolerance. The overall infarction rate was 22.4% (11 of 49 patients), including 15.8% (six of 38 patients) in the IVBP group versus 45.5% (five of 11 patients) in the isoflurane (ISO) group. In the ISO group, the mean duration of temporary occlusion was 3.9 ± 2.2 minutes for patients without infarction versus 12.2 ± 4.3 minutes for patients with focal infarction (p < 0.01). In contrast, the mean duration was 13.6 ± 10.6 minutes for patients without infarction and 18.5 ± 9.9 minutes for patients with infarction in the IVBP group. All patients in the ISO group who underwent occlusion lasting 10 minutes or longer suffered an infarction versus five of 23 patients in the IVBP group. Patients with multiple aneurysms were found to be at increased risk of developing focal infarction, whereas those treated with intermittent temporary clip application were at a decreased risk.

It is concluded that patients in whom focal iatrogenic ischemia is induced during MCA aneurysm clip ligation have a significant advantage compared with those receiving ISO when they are given pentobarbital as the primary neuroprotective agent or when they receive propofol or etomidate titrated to achieve electroencephalographic burst suppression, particularly if more than 10 minutes of occlusion time is required. It is also concluded that 10 minutes is a general guideline for safe, temporary occlusion of the MCA. The use of intermittent temporary arterial occlusion and patients with multiple aneurysms need further evaluation before specific recommendations can be made.

Repeat positron emission tomographic studies in transient middle cerebral artery occlusion in cats: residual perfusion and efficacy of postischemic reperfusion

The wider clinical acceptance of thrombolytic therapy for ischemic stroke has focused more attention on experimental models of reversible focal ischemia. Such models enable the study of the effect of ischemia of various durations and of reperfusion on the development of infarctions. We used high-resolution positron emission tomography (PET) to assess cerebral blood flow (CBF), cerebral metabolic rate of oxygen (CMRO2), oxygen extraction fraction (OEF), and cerebral metabolic rate of glucose (CMRglc) before, during, and up to 24 h after middle cerebral artery occlusion (MCAO) in cats. After determination of resting values, the MCA was occluded by a transorbital device. The MCA was reopened after 30 min in five, after 60 min in 11, and after 120 min in two cats. Whereas all cats survived 30-min MCAO, six died after 60-min and one after 120-min MCAO during 6-20 h of reperfusion. In those cats surviving the first day, infarct size was determined on serial histologic sections. The arterial occlusion immediately reduced CBF in the MCA territory to < 40% of control, while CMRO2 was less affected, causing an increase in OEF. Whereas in the cats surviving 24 h of reperfusion after 60- and 120-min MCAO, OEF remained elevated throughout the ischemic episode, the initial OEF increase had already disappeared during the later period of ischemia in those cats that died during the reperfusion period. After 30-min MCAO, the reperfusion period was characterized by a transient reactive hyperemia and fast normalization of CBF, CMRO2, and CMRglc, and no or only small infarcts in the deep nuclei were found in histology. After 60- and 120-min MCAO, the extent of hyperperfusion was related to the severity of ischemia, decreased CMRO2 and CMRglc persisted, and cortical/subcortical infarcts of varying sizes developed. A clear difference was found in the flow/metabolic pattern between surviving and dying cats: In cats dying during the observation period, extended postischemic hyperperfusion accompanied large defects in CMRO2 and CMRglc, large infarcts developed, and intracranial pressure increased fatally. In those surviving the day after MCAO, increased OEF persisted over the ischemic episode, postischemic hyperperfusion was less severe and shorter, and the perfusional and metabolic defects as well as the final infarcts were smaller. These results stress the importance of the severity of ischemia for the further course after reperfusion and help to explain the diverging outcome after thrombolysis, where a relation between the residual flow and the effectiveness of reperfusion was also observed.

Application of contrast agents in the evaluation of stroke: conventional MR and echo-planar MR imaging

The availability of new therapeutic interventions, including neuroprotective agents and endovascular thrombolysis, has given new hope to patients suffering an acute stroke. Early intervention remains a key factor in the effectiveness of these new and traditional treatments. More importantly, the capability to assess the viability and reversibility of the ischemic tissue became essential for better delineation and differentiation of infarcted versus ischemic tissue and patient management. Abnormal MR imaging (MRI) findings during acute stroke usually reflect the underlying pathophysiologic changes, which can be classified into three sequential stages: (a) hypoperfusion, (b) cellular dysfunction and (c) breakdown of the blood-brain barrier. The first stage is a kinetic phenomenon (not biologic) and, therefore, can be detected immediately. Contrast agents accentuate the abnormal flow kinetics and facilitate the early diagnosis of ischemia using either conventional MRI or newly developed echo-planar perfusion imaging (EPPI). The demonstration of abnormal arterial or parenchymal enhancement on conventional MRI during acute stroke provides the earliest sign of vascular occlusion/stenosis. EPPI, in contrast, provides information related to microcirculation (< 100 microns) and tissue reserve (cerebral blood volume) that cannot be obtained by conventional angiography and is directly related to the target end-organ. Further information obtained from both contrast MRI and EPPI may have a predictive value in the clinical outcome of acute stroke patients.

Induced hypertension during ischemia reduces infarct area after temporary middle cerebral artery occlusion in rats

BACKGROUND

Induced hypertension is one of the few interventions available to reverse acute ischemic neurologic deficit. The purpose of this study was to determine the safety and efficacy of hypertension during prolonged temporary focal ischemia.

METHODS

Anesthetized rats underwent 2 hours of endovascular middle cerebral artery occlusion. Angiotensin was given to increase mean arterial blood pressure to 40%-60% above baseline during ischemia. Neurologic deficit and infarct size in hypertensive rats were assessed at 72 hours.

RESULTS

Hypertensive rats had smaller infarcts than normotensive controls (mean, 12.40 +/- 3.71% versus 24.19 +/- 2.89; p < 0.05) without hemorrhage. Neuroscores were comparable.

CONCLUSION

Hypertension safely reduces infarction after 2 hours of focal ischemia in rats.

Lobar intracerebral hemorrhage model in pigs: rapid edema development in perihematomal white matter

BACKGROUND AND PURPOSE

The mechanisms underlying brain injury from intracerebral hemorrhage (ICH) are complex and poorly understood. To comprehensively examine pathophysiological and pathochemical alterations after ICH and to examine the effects of hematoma removal on these processes, we developed a physiologically controlled, reproducible, large-animal model of ICH in pigs (weight, 6 to 8 kg).

METHODS

We produced lobar hematomas by pressure- controlled infusions of 1.7 mL of autologous blood into the right frontal hemispheric white matter over 15 minutes. We froze brains in situ at 1, 3, 5, and 8 hours after hematoma induction and cut coronal sections of hematoma assessment, morphological brain examination, and immunohistochemical and water content determinations.

RESULTS

At 1 hour after blood infusion, "translucent" white matter areas were present directly adjacent to the hematoma. These markedly edematous regions had a greater than 10% increase in water content (>85%) compared with the contralateral white matter (73%), and this increased water content persisted through 8 hours. In addition, these areas were strongly immunoreactive for serum proteins. Intravascular Evans blue dye failed to penetrate into the brain tissue at all time points, demonstrating that this serum protein accumulation and edema development were not due to increased blood-brain barrier permeability.

CONCLUSIONS

Experimental lobar ICH in pigs models a prominent pathological feature of human ICH, ie, early perihematomal edema. Our findings suggest that serum proteins, originating from the hematoma, accumulate in adjacent white matter and result in rapid and prolonged edema after ICH. This interstitial edema likely corresponds to the low densities on CT scans and the hyperintensities on T2-weighted MR images that surround intracerebral hematomas acutely after human ICH.

Thresholds for cerebral ischemia after severe head injury: relationship with late CT findings and outcome

Cerebral ischemic insults in at least 30% of severely head injured patients at a very early stage following trauma and are associated with early death. To date, the threshold for ischemia of 18 mL/100g/min used in human head injury studies has been adopted from animal studies (by temporary occlusion of the middle cerebral artery). Since the traumatized brain becomes more susceptible to irreversible damage if accompanied by ischemia one may question whether the threshold for ischemic vulnerability is higher than 18 mL/100 g/min. Cerebral ischemia can cause atrophy. Therefore, the authors obtained computerized tomography (CT) scans in 33 comatose head-injured patients (Glasgow Coma Score of 8 or less) at least 3 months following injury and compared ventricle sizes (as a reflection of atrophy) with cerebral blood flow (CBF) obtained within 4 h (average 2.3 +/- 0.8 h) after injury. Ventricular measurements were performed in three fashions: the third ventricular size (cm), the bicaudate cerebral ventricular index (BCVI), and the hemispheric ventricular index (HCVI). No significant correlation was found between early CBF and any of the ventricule sizes. Applying a multiple correlation analysis with four independent parameters [CBF, CBF/time postinjury, CBF/(time postinjury)2, age], only age emerged as a significant indicator for predicting ventricle size (p < 0.001). We also compared CBF data, obtained within 4 h after trauma, from survivors at 3 months after injury (mean CBF of 32 mL/100 g/min) with CBF data from non-survivors (CBF 20 mL/100 g/min). The difference in CBF between survivors and nonsurvivors was significant at p < 0.001 (Wilcoxon rank-sum test). The proportion of patients with CBF less than or equal to 20 mL/100 g/min was 56% in the nonsurvivors and only 5% in survivors. The difference in the proportions was significant at p < 0.001 (chi-square test). We conclude that a measure of atrophy does not correlate with ultra-early CBF. However, based on the clear distinction between survivors and nonsurvivors, we suggest the threshold for ischemia after head injury be redefined as a CBF of 20 mL/100 g/min.

Graded hypotension and MCA occlusion duration: effect in transient focal ischemia

The first 2 h of middle cerebral artery occlusion (MCAO) are likely critical in determining the final outcome in ischemic stroke. To study this early postischemic period, male Wistar rats (n = 161) were subjected to right MCAO with closely spaced step variations in both duration of MCAO and blood pressure (BP), using the intraluminal suture technique. Quantitative neuropathology was performed at 25 coronal planes of the brain after 1-week survival. Atrophy was measured as the difference between the two hemispheres and was added to cortical and striatal necrosis to obtain total tissue loss. Damage consistently increased monotonically with increasing duration of occlusion only when infarct size was expressed as percentage of the contralateral hemisphere, but not when expressed as mm3, because of variable tissue size. The results showed that already at 1 week, the quantity of tissue loss due to resorption and transsynaptic effects approached the quantity of geographically traceable necrosis in cortex and striatum. Minimum brain damage (5%) occurred after 60 min at a BP of 80 mm Hg, with almost no cortical necrosis. Damage was extremely sensitive to hypotension and MCAO duration. At a BP of 40 mm Hg, 60 min of MCAO produced 25% damage, accelerating every 20 min during the 2-h period studied. At BP 80 mm Hg, 120 min of MCAO produced the same damage as only 80 min of MCAO at BP 60 mm Hg. At 60-, 80-, 100-, and 120-min duration of MCAO, infarct size was significantly reduced with increasing BP.(ABSTRACT TRUNCATED AT 250 WORDS)

Attenuation of postischemic brain hypoperfusion and reperfusion injury by the cyclooxygenase-lipoxygenase inhibitor BW755C

Arachidonic acid metabolites are believed to be important mediators of tissue injury during reperfusion after cerebral ischemia. To determine whether inhibiting the oxygen-dependent metabolism of arachidonic acid would reduce reperfusion injury, we administered the mixed cyclooxygenase-lipoxygenase inhibitor BW755C (3-amino-1-[m(trifluoromethyl)phenyl]-2-pyrazoline) near the time of reperfusion in a rat model of temporary focal ischemia. The duration of ischemia + reperfusion was 2 hours + 22 hours, 3 hours + 3 hours, or 3 hours + 21 hours. The effects of drug or saline treatment on infarct volume, blood-brain barrier permeability, and blood flow were determined. Cortical blood flow was monitored with laser Doppler flowmetry and blood-brain barrier permeability was evaluated by the Evans blue dye method. Infarct volume was determined in all groups by computerized image analysis of Nissl-stained sections. We found that BW755C treatment significantly attenuated delayed postischemic hypoperfusion in the 3 + 3 group (p < 0.05) and reduced the volume of Evans blue dye staining in the cortex (p < 0.01) and basal ganglia (p < 0.05). Hemispheric swelling was reduced in all treatment groups (p < 0.01), as was total infarct volume in the ischemic hemisphere (p < 0.05). These results support the hypothesis that arachidonic acid metabolites contribute to acute postischemic reperfusion injury and suggest that using a mixed cyclooxygenase-lipoxygenase inhibitor as an adjunct to thrombolytic or revascularization therapy could lengthen the ischemia time after which reperfusion is beneficial.

Focal ischemia due to traumatic contusions documented by stable xenon-CT and ultrastructural studies

A traumatic cerebral contusion causes a zone of perifocal neuronal necrosis, the cause of which is not known; the surgical management of these lesions remains controversial. To determine the pathophysiological mechanisms responsible for brain damage after contusions, the authors performed cerebral blood flow (CBF) mapping studies and related these to change in local cerebral blood volume (CBV) and ultrastructure. In 11 severely head injured patients with contusion, CBF and CBV were measured in pericontusional areas using stable xenon-computerized tomography (CT). These studies demonstrated a profound reduction in perilesional CBF (mean 17.5 +/- 4 ml/100 g/min), which was always accompanied by a zone of edema defined by CT density measurements. Mean CBV in these regions was 2.3 +/- 0.4 ml/100 g, a reduction to approximately one-half the value of 4.8 ml/100 g found in the nonedematous regions, and to approximately 35% of the value of 6.0 ml/100 g found in normal volunteers. Ultrastructural analysis of the pericontusional tissue, taken at surgery in four patients with high intracranial pressure showed glial swelling with narrowing of the microvascular lumina due to massive podocytic process swelling. Additionally, some suggestion of vascular occlusion due to erythrocyte and leukocyte stasis was seen. These data support the conclusion that microvascular compromise by compression and/or occlusion is a major event associated with profound perilesional hypoperfusion, which is a uniform finding within edematous pericontusional tissue.

CT and MRI diagnosis of cerebrovascular disease: going beyond the pixels

Cerebrovascular disease is one of the leading causes of morbidity and mortality in the United States, with an incidence of approximately 500,000 cases of stroke per year. Tremendous strides in the diagnosis of stroke have been made in recent years, although the main diagnostic tools available remain CT, MRI, and MR angiography. Because new treatment options are being developed, it has become imperative to analyze these anatomic studies more intensively and critically, with an eye toward eliciting vital pathophysiological information. This article reviews various imaging findings in that light and offers a perspective on the radiologist's role in the management of patients with these conditions.

Cerebral edema after temporary and permanent middle cerebral artery occlusion in the rat

BACKGROUND AND PURPOSE

The potential of thrombolytic agents to improve outcome after ischemic stroke could be negated if recanalization of an occluded artery exacerbates cerebral edema. We examined whether infarctions associated with reperfusion have more edema than those without reperfusion and whether the time course for the development of cerebral edema varied with and without reperfusion.

METHODS

Infarct volumes were measured 24 hours after permanent and 1, 2, and 3 hours of temporary right middle cerebral artery (MCA) occlusion in spontaneously hypertensive rats. Hemispheric volume, water, sodium, and potassium were measured 3, 6, 12, 24, 36, and 48 hours after permanent and 3 hours of temporary MCA occlusion and also determined 24 hours after permanent and 2 and 3 hours of temporary MCA occlusion.

RESULTS

Minimal tissue damage occurred after 1 hour of temporary ischemia. Infarct sizes were similar after permanent and 3 hours of temporary MCA occlusion and significantly greater than after 2 hours of temporary ischemia. Hemispheric volume, water, and sodium from the infarcted right hemisphere were significantly greater than those from the left hemisphere beginning 6 hours after MCA occlusion and continuing for 48 hours, with a peak at 24 hours. Right hemispheric water measured 24 hours after 2 hours of temporary ischemia was significantly less than after permanent or 3 hours of temporary ischemia.

CONCLUSIONS

This study demonstrates that cerebral edema after focal stroke is related to infarct size and is independent of reperfusion status. The results suggest that exacerbation of cerebral edema will not occur after thrombolytic treatment or spontaneous recanalization of occluded cerebral vessels.

[Neuro-anesthetic contribution to the prevention of complications caused by mechanical cerebral retraction: concept of a chemical brain retractor]

During most intracranial procedures, the microscope is used to allow the surgeon to work on structures which are deeply located in the brain. Under these circumstances, brain retraction is required for adequate exposure. It was rapidly suspected and later confirmed that brain retraction causes secondary brain damage. This is due not only to direct effect of the retractor on the cortical surface, but also because a pressure is generated under the retractor, on the brain tissue, which compromises local cerebral blood flow and local cerebral perfusion pressure, thus causing cerebral ischaemia. The need for retraction is increased if the lesion is located deeply and/or if the brain is tensed; thus the risk to generate ischaemic conditions is enhanced. These secondary surgical lesions are promoted and worsened by associated systemic conditions such as hypotension, hypoxaemia, hypercapnia. As an attempt to respond to the problem generated by surgical retraction, the "chemical brain retractor" concept is proposed. By compulsively rendering the brain as relaxed and compliant as possible, the chemical brain retractor should allow the surgeon to operate on without the use of a surgical brain retractor and, if such a retractor is still needed, to reduce the pressure under it. These goals are achieved with an osmotic agent like mannitol to improve brain compliance, and intravenous anaesthetic agents, moderate hypocarbia and a normal or elevated blood pressure, to minimize cerebral blood volume. In conjunction with the chemical brain retractor, two other manoeuvres should be used to enhance cerebral compliance: CSF drainage and moderate head up position during the procedure.

Mild intraischemic hypothermia reduces postischemic hyperperfusion, delayed postischemic hypoperfusion, blood-brain barrier disruption, brain edema, and neuronal damage volume after temporary focal cerebral ischemia in rats

Mild to moderate hypothermia (30-33 degrees C) reduces brain injury after brief (< 2-h) periods of focal ischemia, but its effectiveness in prolonged temporary ischemia is not fully understood. Thirty-two Sprague-Dawley rats anesthetized with 1.5% isoflurane underwent 3 h of middle cerebral artery occlusion under hypothermic (33 degrees C) or normothermic (37 degrees C) conditions followed by 3 or 21 h of reperfusion under normothermic conditions (n = 8/group). Laser-Doppler estimates of cortical blood flow showed that intraischemic hypothermia reduced both postischemic hyperperfusion (p < or = 0.01) and postischemic delayed hypoperfusion (p < or = 0.01). Hypothermia reduced the extent of blood-brain barrier (BBB) disruption as estimated from the extravasation of Evans blue dye at 6 h after the onset of ischemia (p < or = 0.01). Hypothermia also reduced the volume of both brain edema (p < or = 0.01) and neuronal damage (p < or = 0.01) as estimated from Nissl-stained slides at both 6 and 24 h after the onset of ischemia. These results demonstrate that mild intraischemic hypothermia reduces tissue injury after prolonged temporary ischemia, possibly by attenuating postischemic blood flow disturbances and by reducing vasogenic edema resulting from BBB disruption.

Diffusion-weighted magnetic resonance imaging of acute focal cerebral ischemia: comparison of signal intensity with changes in brain water and Na+,K(+)-ATPase activity

Diffusion-weighted magnetic resonance (MR) images from rats during acute cerebral ischemia induced by middle cerebral artery occlusion were analyzed for correspondence with changes in brain water, cation concentrations, and Na+,K(+)-ATPase activity measured in vitro after 30 or 60 min of ischemia. In the ischemic hemisphere, signal intensity was increased at 30 min (p < 0.05 vs contralateral hemisphere) and further increased at 60 min. Na+,K(+)-ATPase activity was 34% lower in ischemic cortex and 40% lower in ischemic basal ganglia after 30 min (p < 0.05), but water content and Na+ and K+ concentrations were not significantly different between hemispheres. After 60 min, water content and Na+ concentration were increased, and both Na+,K(+)-ATPase activity and K+ concentration were decreased in the ischemic hemisphere (p < 0.05). These findings are consistent with the hypothesis that the early onset of signal hyperintensity in diffusion-weighted MR images may reflect cellular edema associated with impaired membrane pump function. Early in vivo detection and localization of potentially reversible ischemic cerebral edema may have important research and clinical applications.

"Brain attack": the rationale for treating stroke as a medical emergency

Stroke is the third leading cause of death in the United States, behind only heart disease and cancer. With an estimated three million survivors of stroke in the United States, the cost to society, both directly in health care and indirectly in lost income, is staggering. Despite recent advances in basic and clinical neurosciences, which have the potential to improve the treatment of acute stroke, the general approach to the acute stroke patient remains one of therapeutic nihilism. Most basic science studies show that to be effective, acute intervention to reperfuse ischemic tissue must take place within the first several hours, as is the case with ischemic myocardium. In addition, most neuroprotective agents must also be administered within a short time frame to be effective at salvaging at-risk tissue. Recent studies have suggested that the outcome after intracerebral and subarachnoid hemorrhage is improved with early intervention. However, most stroke patients fail to present to medical attention within this short "window of opportunity." The public's knowledge about stroke is woefully inadequate. However, clinicians who deal with stroke can use the dramatic changes in the treatment of acute myocardial infarction over the last 2 decades as a guide for shaping changes in the management of acute stroke. Comprehensive educational efforts aimed at clinicians and the public at large have dramatically reduced the time from symptom onset to presentation and treatment for acute myocardial infarction, enabling treatment methods such as thrombolysis to be effective. The Decade of the Brain offers a unique opportunity to all concerned with the treatment of the patient with acute stroke to engage in a concerted effort to bring patients with a "brain attack" to specialized neurological attention within the same timeframe that the "heart attack" patient is handled. Such an effort is justified because, although at the present time there are few therapeutic interventions of "proven" value in the treatment of acute stroke, there is more than sufficient suggestive evidence that a number of approaches may be beneficial within the first few hours after the onset of the stroke.

A review of brain retraction and recommendations for minimizing intraoperative brain injury

Brain retraction is required for adequate exposure during many intracranial procedures. The incidence of contusion or infarction from overzealous brain retraction is probably 10% in cranial base procedures and 5% in intracranial aneurysm procedures. The literature on brain retraction injury is reviewed, with particular attention to the use of intermittent retraction. Intraoperative monitoring techniques--brain electrical activity, cerebral blood flow, and brain retraction pressure--are evaluated. Various intraoperative interventions--anesthetic agents, positioning, cerebrospinal fluid drainage, operative approaches involving bone resection or osteotomy, hyperventilation, induced hypotension, induced hypertension, mannitol, and nimodipine--are assessed with regard to their effects on brain retraction. Because brain retraction injury, like other forms of focal cerebral ischemia, is multifactorial in its origins, a multifaceted approach probably will be most advantageous in minimizing retraction injury. Recommendations for operative management of cases involving significant brain retraction are made. These recommendations optimize the following goals: anesthesia and metabolic depression, improvement in cerebral blood flow and calcium channel blockade, intraoperative monitoring, and operative exposure and retraction efficacy. Through a combination of judicious retraction, appropriate anesthetic and pharmacological management, and aggressive intraoperative monitoring, brain retraction should become a much less common source of morbidity in the future.

Vasodilators during cerebral aneurysm surgery

The objective of this review is to review the anaesthetic implications of vasoactive compounds particularly with regard to the cerebral circulation and their clinical importance for the practicing anaesthetist. Material was selected on the basis of validity and application to clinical practice and animal studies were selected only if human studies were lacking. Hypotensive drugs have been used to induce hypotension and in the treatment of intraoperative hypertension during cerebral aneurysm surgery. After subarachnoid haemorrhage, cerebral blood flow is reduced and cerebral vasoreactivity is disturbed which may lead to brain ischaemia. Also, cerebral arterial vasospasm decreases cerebral blood flow, and may lead to delayed ischaemic brain damage which is a major problem after subarachnoid haemorrhage. Recently, the use of induced hypotension has decreased although it is still useful in patients with intraoperative aneurysm rupture, giant cerebral aneurysm, fragile aneurysms and multiple cerebral aneurysms. In this review, a variety of vasodilating agents, prostaglandin E1, sodium nitroprusside, nitroglycerin, trimetaphan, adenosine, calcium antagonists, and inhalational anaesthetics, are discussed for their clinical usefulness. Sodium nitroprusside, nitroglycerin and isoflurane are the drugs of choice for induced hypotension. Prostaglandin E1, nicardipine and nitroglycerin have the advantage that they do not alter carbon dioxide reactivity. Local cerebral blood flow is increased with nitroglycerin, decreased with trimetaphan and unchanged with prostaglandin E1. Intraoperative hypertension is a dangerous complication occurring during cerebral aneurysm surgery, but its treatment in association with subarachnoid haemorrhage is complicated in cases of cerebral arterial vasospasm because fluctuations in cerebral blood flow may be exacerbated. Hypertension should be treated immediately to reduce the risk of rebleeding and intraoperative aneurysmal rupture and the choice of drugs is discussed. Although the use of induced hypotension has declined, the control of arterial blood pressure with vasoactive drugs to reduce the risk of intraoperative cerebral aneurysm rupture is a useful technique. Intraoperative hypertension should be treated immediately but the cerebral vascular effects of each vasodilator should be understood before their use as hypotensive agents.

Evaluation of critically perfused area in acute ischemic stroke for therapeutic reperfusion: a clinical PET study

To evaluate critically perfused areas in the acute ischemic brain, 9 patients were studied by positron emission tomography (PET) within 7-32 hours after the onset. The cerebral blood flow (CBF) and oxygen metabolic rate (CMRO2) were evaluated and compared with sequential change in CT findings. In all the regions developing subsequent necrosis on CT, CBF dropped below 17 ml/100 g/min. But in some of these lesions, CMRO2 remained above the minimum value for regions in which infarction did not develop, and the tissue density on CT obviously remained normal for several hours after PET scan. The mean CBF in these lesions (14.0 ml/100 g/min, range: 9.9-17.3 ml/100 g/min) was significantly higher than that in ischemic areas with low density on CT before or just after PET study (approximately 10 ml/100 g/min, range: 7.7-14.1 ml/100 g/min). These findings suggest that a part of the tissue with CBF between 10-17 ml/100 g/min is still viable at least 7 hours after the onset of ischemia, but becomes non-viable in a longer period of ischemia. These lesions should respond to effective treatment, including therapeutic reperfusion.

Changes in amino acid neurotransmitters and cerebral blood flow in the ischemic penumbral region following middle cerebral artery occlusion in the rat: correlation with histopathology

We simultaneously measured neurotransmitter amino acids by the microdialysis technique and cortical CBF by laser-Doppler flowmetry in the ischemic penumbral cortex of rats subjected to 2-h normothermic (36.5-37.5 degrees C) transient middle cerebral artery (MCA) clip-occlusion. Brains were perfusion-fixed 3 days later and infarct volume measured. CBF (% of preischemic values) fell to 32 +/- 2% (mean +/- SD) during ischemia and rose to 157 +/- 68% during recirculation. Extracellular glutamate levels increased from a baseline value of 7 +/- 3 microM to a peak value of 180 +/- 247 microM 20-30 min following onset of ischemia but subsequently returned to near baseline levels after 70 min of ischemia despite ongoing MCA occlusion. The threshold CBF for moderate glutamate release was 48%. Massive glutamate release was seen during the first 60 min of MCA occlusion in the two animals showing the largest infarcts and occurred at CBF values < or = 20% of control levels. Mean CBF during ischemia exhibited an inverse relationship with infarct volume, and the magnitude of glutamate release during ischemia was positively correlated with infarct volume. Extracellular gamma-aminobutyrate and glycine changes were similar to those of glutamate but showed no significant correlation with infarct volume. These results suggest that (a) accumulation of extracellular glutamate is an important determinant of injury in the setting of reversible MCA occlusion and (b) reuptake systems for neurotransmitter amino acids may be functional in the penumbra during transient focal ischemia.

Relationship between blood flow and blood-brain barrier permeability of sodium and albumin in focal ischaemia of rats: a triple tracer autoradiographic study

Local cerebral blood flow, the permeability of the blood-brain barrier to sodium and serum albumin, and the content of electrolytes were investigated in rats before and at 4 h and 24 h following permanent occlusion of the middle cerebral artery (MCA). Measurements were carried out by triple tracer autoradiography, using 131I-iodoantipyrin, 22NaCl and 125I-iodinated bovine serum albumin, respectively. Regional sodium and albumin transfer coefficients were calculated by multiple time point analysis, and correlated with the corresponding flow and tissue electrolyte values. In sham operated controls regional sodium and albumin transfer coefficients ranged between 2.16-2.30 x 10(-3) and 0.22-0.48 x 10(-3) ml/min per g, respectively. Four hours after MCA occlusion sodium and albumin transfer coefficients were unchanged although tissue sodium content was already increased. After 24 h the sodium-but not albumin-transfer coefficient increased 2-3 fold but the rise in tissue sodium content was slower than after 4 h. At both ischaemia times the unidirectional sodium influx was substantially higher than the actual changes of tissue sodium content. The development of stroke oedema is, therefore, not limited by the alterations of barrier permeability.

Relationship of cerebral blood flow disturbances with brain oedema formation

Brain oedema is an important factor which compromises maintenance of the cerebral blood flow. Conversely, primary blood flow disturbances are leading to brain oedema. The mechanisms underlying blood flow impairment by brain oedema are associated with an increased regional tissue pressure in proportion to the degree of water accumulation in the parenchyma. The release of vasoactive mediator compounds might be considered in addition. Primary disturbances of the cerebral blood flow, such as focal or global cerebral ischaemia are leading to an increased cerebral water content. A decrease of the cerebral blood flow to ca. 40% of normal or below has been found to result in the development of brain oedema. This flow threshold is in the neighbourhood of the ischaemic flow level causing irreversible tissue damage. Whereas in focal ischaemia oedema formation is a function of the severity of the flow decrease, it is a pathophysiological hallmark of early postischaemic recirculation in global cerebral ischaemia. Nevertheless, during complete interruption of cerebral blood flow translocation of interstitial fluid into the intracellular compartment occurs as manifestation of ischaemic cell swelling. Cell swelling under these conditions may, however, not necessarily indicate cell damage, but more likely a compensatory response attributable to the uptake of excitotoxic transmitters such as glutamate, and of K(+)-ions which are excessively released at the onset of ischaemia into the extracellular space. Purpose of the swelling process, thus, is clearance of extracellular fluid from this material to re-establish homeostasis.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of intermittent reperfusion during temporal focal ischemia

There is controversy regarding the role of intermittent reperfusion employed as a cerebroprotective measure when temporary arterial occlusion is necessary during repair of difficult aneurysms. The intraluminal suture middle cerebral artery (MCA) occlusion technique was used in 23 Wistar rats under barbiturate anesthesia to induce 60, 90, or 120 minutes of uninterrupted MCA occlusion. The total infarcted areas obtained were compared to those occurring in 27 animals subjected to identical cumulative ischemic periods but with 5 minutes of reperfusion after every 10-minute ischemic period. The mean total infarcted areas in the groups with 60-minute (1.8 +/- 0.89 sq mm), 90-minute (1.08 +/- 1.02 sq mm), and 120-minute (8.72 +/- 5.89 sq mm) intermittent reperfusion were significantly smaller than those occurring in the 60-minute (12.02 +/- 3.10 sq mm), 90-minute (11.54 +/- 2.68 sq mm), or 120-minute (30.43 +/- 6.51 sq mm) control groups, respectively (p < 0.05). Furthermore, there was no difference in the occurrence of blood-brain barrier breakdown, intraparenchymal hemorrhage, hemispheric edema, or seizures between control and intermittent reperfusion groups. The results support the hypothesis that intermittent reperfusion is beneficial if vessel occlusion is required during aneurysm repair.

A thermal clearance probe for continuous monitoring of cerebral blood flow

Thermal clearance has been used as a measure of tissue blood flow for over 60 years. We have developed a probe which can be used to monitor cerebral cortical thermal clearance in patients suffering from acute neurosurgical disorders. Despite its limitations of size, invasiveness, small sample volume, and difficult quantification it has proved a reliable method, showing a good correlation between measured thermal clearance and the onset of clinical signs of ischaemia.

Diffusion-weighted imaging studies of cerebral ischemia in gerbils. Potential relevance to energy failure

BACKGROUND AND PURPOSE

Diffusion-weighted magnetic resonance imaging has been shown to be particularly suited to the study of the acute phase of cerebral ischemia in animal models. The studies reported in this paper were undertaken to determine whether this technique is sensitive to the known ischemic thresholds for cerebral tissue energy failure and disturbance of membrane ion gradients.

METHODS

Diffusion-weighted images of the gerbil brain were acquired under two sets of experimental conditions: as a function of cerebral blood flow after controlled graded occlusion of the common carotid arteries (partial ischemia), as a function of time following complete bilateral carotid artery occlusion (severe global ischemia), and on deocclusion after 60 minutes of ischemia.

RESULTS

During partial cerebral ischemia, the diffusion-weighted images remained unchanged until the cerebral blood flow was reduced to 15-20 ml.100 g-1.min-1 and below, when image intensity increased as the cerebral blood flow was lowered further. This is similar to the critical flow threshold for maintenance of tissue high-energy metabolites and ion homeostasis. After the onset of severe global cerebral ischemia, diffusion-weighted image intensity increased gradually after a delay of approximately 2.5 minutes, consistent with complete loss of tissue adenosine triphosphate and with the time course of increase in extracellular potassium. This hyperintensity decreased on deocclusion following 60 minutes of ischemia.

CONCLUSIONS

The data suggest that diffusion-weighted imaging is sensitive to the disruption of tissue energy metabolism or a consequence of this disruption. This raises the possibility of imaging energy failure noninvasively. In humans, this could have potential in visualizing brain regions where energy metabolism is impaired, particularly during the acute phase following stroke.

Evaluation of experimental early acute cerebral ischemia before the development of edema: use of dynamic, contrast-enhanced and diffusion-weighted MR scanning

The ability of dynamic, contrast-enhanced, magnetic susceptibility-weighted scanning to delineate early experimental acute cerebral infarction was compared with that of heavily T2-weighted and diffusion-weighted spin echo scanning. Spontaneously hypertensive rats, which had undergone right middle cerebral artery occlusion, were studied from 15 min to 3 h post ligation on a 1.5-T clinical whole-body imager. In contrast to the diffusion- and T2-weighted spin echo scans, the dynamic, contrast-enhanced technique clearly and consistently delineated the nonperfused regions as early as 15 min post ligation.

[Adverse prognostic influence of diabetes mellitus and hyperglycemia on the clinical course of cerebral infarction]

In non-diabetic patients, the appearance of hyperglycemia in the acute phase of stroke is related to the extension of cellular injury, and hence to the physiologic stress response. In animal models of ischemic insult, the deleterious effects of hyperglycemia depend heavily on the production of lactic acid "via" activation of the glycolytic anaerobic pathway. The abnormal production of lactic acid and consequent tissular acidosis appear mainly in the early post-reperfusion period, or in states of marked but partial reduction of blood flow. A direct reduction of cerebral blood flow and, perhaps, the production of a hyperosmolar state may contribute to worsening of the ischemic injury. In diabetic patients, previous hemorrheologic and microcirculatory changes, and a greater susceptibility to infections may additionally reduce the chances of complete recovery after stroke.

The CBF threshold and dynamics for focal cerebral infarction in spontaneously hypertensive rats

Two strategies were used to estimate the blood flow threshold for focal cerebral infarction in spontaneously hypertensive rats (SHRs) subjected to permanent middle cerebral artery and common carotid artery occlusion (MCA/CCAO). The first compared the volume of cortical infarction (24 h after ischemia onset) to the volumes of ischemic cortex (image analysis of [14C]iodoantipyrine CBF autoradiographs) perfused below CBF values less than 50 (VIC50) and less than 25 ml 100 g-1 min-1 (VIC25) at serial intervals during the first 3 h of ischemia. The infarct process becomes irreversible within 3 h in this model. In the second, measurements of CBF at the border separating normal from infarcted cortex at 24 h after ischemia onset were used as an index of the threshold. During the first 3 h of ischemia, VIC50 increased slightly to reach a maximum size at 3 h that closely matched the 24 h infarct volume. VIC25, in contrast, consistently underestimated the infarct volume by a factor of 2-3. CBF at the 24 h infarct border averaged 50 ml 100 g-1 min -1. Taken together, the results indicate that the CBF threshold for infarction in SHRs approaches 50 ml 100 g-1 min-1 when ischemia persists for greater than or equal to 3 h. This threshold value is approximately three times higher than in primates. Since cortical neuronal density is also threefold greater in rats than in primates, the higher injury threshold in the rat may reflect a neuronal primacy in determining the brain's susceptibility to partial ischemia.

Stroke models: strengths and pitfalls

Recent editorials and reviews express disillusionment and sharp criticism with the contribution of animal experimental studies to stroke prevention and treatment. The basis for these comments appears to be a frustration with the absence of pharmacologic agents to effectively treat stroke patients despite considerable research efforts. In response to these nihilistic views, Zivin and Grotta (Stroke, 21 (1990) 981-983) have written a poignant rebuttle and Goldstein (Stroke, 21 (1990) 373-374) has outlined the considerable progress which has been achieved in experimental cerebral stroke research. It is our goal in the present discussion to highlight the strengths and to examine the potential pitfalls of stroke models, some of which may have contributed to the equivocal results obtained in testing pharmacologic agents. In addition, based on our own experience with a model of middle cerebral artery (MCA) occlusion in cats, we will describe the benefits of large versus small animal models for experimental stroke studies.