1. Stroke is the neurological evidence of a critical reduction of cerebral blood flow in a circumscribed part of the brain, resulting from the sudden or gradually progressing obstruction of a large brain artery. Treatment of stroke requires the solid understanding of stroke pathophysiology and involves a broad range of hemodynamic and molecular interventions. This review summarizes research that has been carried out in many laboratories over a long period of time, but the main focus will be on own experimental research. 2. The first chapter deals with the hemodynamics of focal ischemia with particular emphasis on the collateral circulation of the brain, the regulation of blood flow and the microcirculation. In the second chapter the penumbra concept of ischemia is discussed, providing a detailed list of the physiological, biochemical and structural viability thresholds of ischemia and examples of how these thresholds can be applied for imaging the penumbra. The third chapter summarizes the pathophysiology of infarct progression, focusing on the role of peri-infarct depolarisation, the multitude of putative molecular injury pathways, brain edema and inflammation. Finally, the fourth chapter provides an overview of currently discussed therapeutic approaches, notably the effect of mechanical or thrombolytic reperfusion, arteriogenesis, pharmacological neuroprotection, ischemic preconditioning and regeneration. 3. The main emphasis of the review is placed on the balanced differentiation between hemodynamic and molecular factors contributing to the manifestation of ischemic injury in order to provide a rational basis for future therapeutic interventions.

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Summary: Middle cerebral artery occlusion was per formed in rats while the animals were inside the nuclear magnetic resonance (NMR) tomograph, Successful occlu sion was confirmed by the collapse of amplitude on an electrocorticogram, The ultrafast NMR imaging tech nique UFLARE was used to measure the apparent diffu sion coefficient (ADC) immediately after the induction of cerebral ischemia, ADC values of normal cortex and cau date-putamen were 726 ± 22 x 10-6 mm2/s and 659 ± 17 x 10-6 mm 2 /s, respectively, Within minutes of occlu sion, a large territory with reduced ADC became visible in the ipsilateral hemisphere, Over the 2 h observation period, this area grew continuously, Quantitative analysis of the ADC reduction in this region showed a gradual ADC decrease from the periphery to the core, the lowest ADC value amounting to about 60% of controL Two hours after the onset of occlusion, the regional distribu tion of ATP and tissue pH were determined with biolu minescence and fluorescence techniques, respectively. There was a depletion of A TP in the core of the ischemic Diffusion-weighted nuclear magnetic resonance (NMR) imaging (DWI) has been shown to be sensi tive to cerebral ischemia during the early, acute

Brain injury after focal ischemia evolves along two basically different pathophysiologies, depending on the severity of the primary flow reduction and the dynamics of postischemic recirculation. In permanent and gradually reversed focal ischemia as after thromboembolic occlusion, primary core injury is irreversible but the expansion of the core into the penumbra can be alleviated by hemodynamic and molecular interventions. Such alleviation can only be achieved within 3 hours after the onset of ischemia because untreated core injury expands to near maximum size during this interval. In promptly reversed transient ischemia as after mechanical vascular occlusion, primary core injury may recover but a secondary delayed injury evolves after a free interval of as long as 6 to 12 hours. This injury can be alleviated throughout the free interval but the longer window is without clinical relevance because transient mechanical vascular occlusion is not a model of naturally occurring stroke. As this difference is widely ignored in stroke research, most clinical trials have been designed with a far too long therapeutic window, which explains their failure. Transient mechanical vascular occlusion models should, therefore, be eliminated from the repertoire of preclinical stroke research.

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Summary: In focal ischemia of rats, the volume of ischemic lesion correlates with the number of peri-infarct depolariza tions, To test the hypothesis that depolarizations accelerate in farct growth, we combined focal ischemia with externally evoked spreading depression (SD) waves, Ischemic brain in farcts were produced in halothane-anaesthetized rats by intra luminal thread occlusion of the middle cerebral artery (MCA), In one group of animals, repeated SDs were evoked at IS-min intervals by microinjections of potassium acetate into the fron tal cortex, In another group, the spread of the potassium-evoked depolarizations was prevented by application of the N-methyl D-aspartate (NMDA) receptor antagonist dizocilpine (MK-80 I), The volume of ischemic lesion was monitored for 2 h by diffusion-weighted imaging (DWI) and correlated with electro physiological recordings and biochemical imaging techniques, In untreated rats, each microinjection produced an SD wave and a stepwise rise of the volume and signal intensity of the In focal cerebral ischemia of rat infarct size correlates with the number of peri-infarct spreading depression (SD)-like depolarizations (Mies et aI., 1993; Chen et aI., 1993). These depolarizations are generated in the border zone of the ischemic lesion and spread into the peri infarct surrounding (Nedergaard and Hansen, 1993; Ne dergaard and Astrup, 1986). Glutamate antagonists such as dizocilpine (MK-801) or 2,3-dihydroxy-6-nitro-7-Received January I L 1996; final received May 3. 1996; accepted May 29, 1996.Address correspondence. and reprint requests to Prof. Dr. K.-A. Hossmann, Max-Planck-Institut fUr neurologische Forschung. Gleuelerstrasse 50, D-50931 Kiiln, FR Germany.Abbreviations used: ADC, apparent diffusion coefficient; ANOY A. analysis of variance; DC, direct current; DWl, diffusion-weighted im aging; MCA, middle carotid artery; MR, magnetic resonance; NBQX, 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo (F)-quinoxaline; NMDA, N methyl-D-aspartate; RF, radiofrequency; ROI, region of interest; SD, spreading depression; TE. spin echo. 1090DWI-visible cortical lesion, The volume of this lesion in creased between IS min and 2 h of MCA occlusion from 19 ± 15% to 66 ± 16% of ipsilateral cortex, In dizocilpine-treated animals, microinjections of potassium did not evoke SDs, nor did the volume and signal intensity of the DWI-visible cortical lesion change, At 15 min after MCA occlusion, the DWI visible lesion was larger than in untreated animals-43 ± 16% of the ipsilateral cortex; however, after 2 h, it increased only slightly further to 49 ± 21 %. Slower lesion growth in the ab sence of SDs was also reflected by the volume of ATP-depleted tissue, which, after 2 h of MCA occlusion, involved 26 ± 12% of the ipsilateral cortex in treated and 49 ± 9% in untreated animals (p < 0.(1). These observations support the hypothesis that peri-infarct depolarizations accelerate cerebral infarct growth.

The ischemic threshold of protein synthesis and energy state was determined 1, 6, and 12 h after middle cerebral artery (MCA) occlusion in rats. Local blood flow and amino acid incorporation were measured by double tracer autoradiography, and local ATP content by substrate-induced bioluminescence. The various images were evaluated at the striatal level in cerebral cortex by scanning with a microdensitometer with 75 microns resolution. Each 75 x 75 microns digitized image pixel was then converted into the appropriate units of either protein synthesis, ATP content, or blood flow. The ischemic threshold was defined as the flow rate at which 50% of pixels exhibited complete metabolic suppression. One hour after MCA occlusion, the threshold of protein synthesis was 55.3 +/- 12.0 ml 100 g-1 min-1 and that of energy failure was 18.5 +/- 9.8 ml 100 g-1 min-1. After 6 and 12 h of MCA occlusion, the threshold of protein synthesis did not change (52.0 +/- 9.6 and 56.0 +/- 6.5 ml 100 g-1 min-1, respectively) but the threshold of energy failure increased significantly at 12 h following MCA occlusion to 31.9 +/- 9.7 ml 100 g-1 min-1 (p less than 0.05 compared to 1 h ATP threshold value; all values are mean +/- SD). In focal cerebral ischemia, therefore, the threshold of energy failure gradually approached that of protein synthesis. Our results suggest that with increasing duration of ischemia, survival of brain tissue is determined by the high threshold of persisting inhibition of protein synthesis and not by the much lower one of acute energy failure.(ABSTRACT TRUNCATED AT 250 WORDS)

Summary: Following permanent occlusion of the left middle cerebral artery (MCA) in rats, electrophysiologi cal and hemodynamic characteristics of the periinfarct border zone were investigated in sham-operated (n = 6), untreated (n = 6), and MK-801-treated 0.0 mg/kg; n = 6) animals. For this purpose, direct current potential (DC), EEG, and blood flow (laser-Doppler flowmetry) were re corded from the cortex in the periphery of the MCA ter ritory. In sham-operated rats, a single negative cortical DC deflection was observed after electrocoagulation of the cortex, whereas in untreated MCA-occluded animals, three to eight transient DC det1ections were monitored during the initial 3 h of ischemia. The duration of these cortical DC shifts gradually increased from 1.2 ± 0.3 to 3.7 ± 2.7 min (mean ± SD; p < 0.05) during this time. In animals treated intraperitoneally with MK-801 (3.0 mgt According to the classic concept of the threshold relationship between the density of ischemia and neuronal injury, different flow thresholds have been identified for the suppression of membrane poten tials and the suppression of neuronal transmission (Astrup et aI., 1977). Tissue perfused at a flow rate between these thresholds is referred to as the isch emic penumbra because the neurons in this area are thought to be functionally inactive but still viable (Astrup et aI., 1981 ). Threshold determinations of brain metabolism have revealed that the flow rate