Regional cortical metabolism in focal ischemia

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1 J Neurosurg 52: , 1980 Regional cortical metabolism in focal ischemia ROnERT A. RATCHESON, M.D., AND JAMES A. FERRENDELLI, M.D. Department of Neurology and Neurological Surgery, and Department of Pharmacology, Washington University School of Medicine, St. Louis, Missouri ~" Regional cortical levels of organic phosphates and carbohydrates were measured in cat brains, enzymatically inactivated by the technique of "funnel freezing" 1 hour after occlusion of a middle cerebral artery (MCA). Significant metabolic alterations occurred in all hemispheres ipsilateral to the site of occlusion. However, there was marked interindividual variability, with changes ranging from only slight increases in lactate, pyruvate, and adenosine monophosphate (AMP) in small regions of cortex at one extreme, to profound depletion of high-energy phosphates, depression of glucose and pyruvate levels, and increased lactate, adenosine diphosphate (ADP) and AMP levels in much of the hemisphere of the most severely involved animals. In contrast, metabolic changes in the hemisphere contralateral to the site of occlusion were very few or nonexistent. In addition, in all ipsilateral hemispheres there were regions peripheral to the areas of greatest metabolic alteration where there was excessive elevation of glucose levels. The results demonstrate that occlusion of a major cerebral vessel does not produce metabolic changes that are consistent in their distribution or severity. However, the findings of this study probably depict some of the complicated metabolic events that occur clinically during thrombotic or embolic infarction of brain. KEY WORDS 9 regional cortical metabolism 9 cerebral ischemia 9 glucose organic phosphate compounds 9 carbohydrate substrates 9 partial ischemia 9 cortex metabolism 9 metabolites C EREBRAL ischemia occurs when blood carrying glucose and oxygen reaches brain tissue in insufficient amounts. The resultant injury produces biochemical alterations with associated changes in cerebral oxidative metabolism. 4,2~ Alterations of high-energy phosphate compounds and carbohydrate substrates have been described in studies employing complete interruption of the blood supply to the brain, 4,5,s,~l,la,14,2~ and in models in which a partial flow of blood to the brain has been maintained. 3,4,~6'ls'23,2v'3~ During complete cerebral ischemia, there is a shift to anaerobic glycolysis for energy production, with a resultant rapid depletion of glucose, phosphocreatine, and adenosine triphosphate (ATP), and increases in the levels of adenosine diphosphate (ADP) and adenosine monophosphate (AMP). Lactate is increased in an amount roughly proportional to the decreases of available glucose and pyruvate. 2~ Similar changes have been reported in studies of incomplete ischemia. However, utilizing various models of incomplete ischemia, observers have reported variable changes of some carbohydrate substrates, 27,s~ suggesting that the metabolic response is influenced by both the degree of ischemia and the duration of ischemic insult. This variability in metabolic response would have considerable import when considered in the light of clinical cerebral ischemia, which is frequently a partial failure of blood flow to a region of the central nervous system (CNS). Models of subtotal focal cerebral ischemia have been developed to minimize the effects of surgery by employing extradural transorbital occlusion of the middle cerebral artery (MCA). 15,sl,se Significant variability has been reported in neurological deficit ~2,3~ and alterations in blood flow, ~,g,~ in addition to pathologicaw '36'41 and metabolic changes),1~ These apparent inconsistencies may reflect the actual response to focal cerebral ischemia. Previous studies of intermediary metabolic changes occurring during subtotal focal cerebral ischemia 2e have not included regional anatomical analysis, and therefore could not provide information concerning the variability that occurs chiefly because of anatomical differences in collateral vascular supply. However, study of the variations in metabolic response occurring at the periphery of ischemic tissue, in which the supply of substrate is only partially diminished, may provide important information con- J. Neurosurg. / Volume 52 / June,

2 R. A. Ratcheson and J. A. Ferrendelli TABLE 1 Physiological parameters in experimental groups* MCA Occlusion Parameters Control 1 Minute 1 Hour Preocclusion Postocclusion temperature (~ MABP (mm Hg) t arterial blood ph _ pc02 (mm Hg) po2(mm Hg) 161 -t :4 *Values are mean SEM. There were eight cats in the control group, and five in the group with middle cerebral artery (MCA) occlusion. MABP = mean arterial blood pressure. cerning tissue that has not suffered a maximum ischemic insult and may not be irretrievably damaged. Study of the pattern and variability of regional cerebral metabolism in conditions of focal subtotal cerebral ischemia required a large-animal model. Of primary concern was the development of reliable methods of enzymatic inactivation that would allow the accurate determination of labile intermediary metabolites in multiple discrete cortical areas. In this report, we will describe a successful model that incorporated the principles used for the optimum measurement of cerebral metabolites in small animalsy,ss A similar model has been described by Welsh, et aly In the cat, the occlusion of a terminal cerebral vessel produces profound regional alterations in cerebral energy and carbohydrate metabolism, which reflect the variable degree of ischemia an entire brain encounters following thrombotic or embolic stroke. Experimental Model Materials and Methods Thirteen adult cats weighing 2 to 5 kg were used in these experiments. They were fasted overnight; anesthesia was induced with halothane, and a tracheostomy performed. The animals were artificially ventilated with 70% nitrous oxide and 30% oxygen, and paralyzed with gallamine triethiodide. Femoral blood vessels were cannulated for monitoring blood pressure and obtaining samples to determine arterial pco2, po~, and ph. End-tidal CO2 was monitored with an infrared CO~ analyzer.* Temperature was monitored with a rectal probe and maintained near 37~ with a heating pad. A large bilateral craniotomy was performed and an open cylinder, 45 mm in diameter, was attached to the remaining peripheral bone with dental cement. Anaerobic samples of arterial blood and cerebro- *Infrared CO2 analyzer manufactured by Beckman Instruments, Inc., Fullerton, California. spinal fluid (CSF), obtained via atlanto-occipital membrane puncture, were collected at various times and frozen with liquid nitrogen. In six control animals, a steady state of acid-base normality was achieved (changes of blood gases of less than 10% over a 15- minute period). Brains were then frozen by pouring liquid nitrogen into the cylinder. Care was taken to avoid freezing of the airways with liquid nitrogen by shielding the airway tubing with aluminum foil. Freezing was continued until blood pressure fell ( minutes), indicating freezing of brain-stem structures. The animals were then decapitated and the head immediately placed in a liquid nitrogen bath. The heads were stored at -80~ Brains were chiseled free in a cold room at -22~ placed in a cryostat (-22~ and anatomically dissected. Seven experimental animals underwent transorbital exposure of the left MCA by the method of O'Brien and Waltz) 1 In two of these animals the MCA was dissected free but not occluded, while in the other five animals the artery was coagulated and divided. One hour later, the brains of these animals were prepared as described above. Sampling of Brain Tissue In the cryostat, pial blood vessels were dissected from the cortical surface. Multiple sample cores, 1.7 mm in diameter and approximately 1.5 mm in depth, were taken from the cerebral cortex by means of a squared-off No. 13 hypodermic needle. As each sample was taken, its anatomic origin was recorded on a diagram of the cat brain. From individual control and sham-operated animals, 24 samples, each representative of a different cortical region, were removed from both hemispheres. In the five animals with MCA occlusion, approximately 20 samples were obtained from each right hemisphere and 30 from each left hemisphere. Biochemical Analysis Following dissection, the samples were weighed at -20 ~ to -30~ (sample weights ranged from 3.5 to 5 mg), and acid methanol-hc10, extracts were prepared. The ATP, ADP, AMP, phosphocreatine (P-creatine), glucose-6-phosphate (G-6-P), fructose diphosphate (FDP), pyruvate, and lactate in each sample were analyzed using the enzymatic fluorometric methods of Lowry and Passonneau} ~ Cyclic AMP was measured by radioimmunoassay, ss The energy state of the tissue was calculated in terms of the energy charge potential (ECP) of the adenine nucleotide pool, as described by Atkinson: ~ ECP = Chemicals used reagent grade.t ATP ADP ATP + ADP + AMP in these experiments were of 1"Enzymes and biochemicals were purchased from Boehringer Mannheim Biochemicals, Indianapolis, Indiana, or Sigma Chemical Co., St. Louis, Missouri. 756 J. Neurosurg. / Volume 52 / June, 1980

3 Regional cortical metabolism in focal ischemia Physiological Parameters Results Table 1 depicts the physiological parameters measured in control animals immediately before sacrifice, and in experimental animals just before and 1 hour after left MCA occlusion. Similar values were obtained for both groups, with the exception of a mild nonrespiratory acidosis seen in the experimental group. Before and after MCA occlusion, all animals included in this study maintained adequate arterial po2 and mean blood pressure, with little variability in pco~ and ph. Animals with MCA occlusion that failed to maintain a mean arterial blood pressure above 120 mm Hg were excluded from the study. Biochemical Levels in Controls Levels of organic phosphates and selected carbohydrate substrates were determined in multiple cortical regions of the right and left hemispheres of control and sham-operated animals. No significant differences were found between cortical regions and, therefore, the results obtained from all 192 samples were pooled (Table 2). The calculated energy charge potential compared favorably with that found in smaller animals, as did levels of organic phosphates, carbohydrate substrates, and the lactate/pyruvate ratio?~, 88 Biochemical Changes Following Left MCA Occlusion Blood and Cerebrospinal Fluid. In general, following occlusion of the left MCA, blood and CSF glucose levels were elevated, and lactate levels were diminished when compared with control and shamoperated animals. These changes were not, however, statistically significant (Table 3). Right Hemisphere. Cortical metabolite levels in the right hemisphere of animals with left MCA occlusion exhibited greater variability between cortical regions than that seen in controls (Table 2). For example, we found statistically significant changes in levels of single metabolites in individual regions of the cortex. However, with one exception (see below), none of these changes seemed to fit into any metabolic pattern TABLE 2 Levels of organic phosphates and carbohydrate substrates in cat brain after i hour of left middle cerebral artery occlusion* Metabolite Control Postocclusion (rt hemisphere) ATP _.05 ADP AMP cyclic AMP P-creatine glucose _.30 G-6-P FDP pyruvate lactate _-_ energy charge potential~ *Values are means SEM. ATP = adenosine triphosphate; ADP = adenosine diphosphate; AMP = adenosine monophosphate; P-creatine = phosphocreatine; G-6-P = glucose-6-phosphate; FDP = fructose diphosphate. Metabolite levels are expressed in mmol/kg wet weight of tissue. ECP = energy charge potential, for the calculation of which, see Biochemical Analysis in text. with enough consistency to allow assignment of physiological significance. The mean concentration of metabolite levels in the right hemisphere of MCAoccluded animals was compared with that in control animals and revealed slightly lower ATP, P-creatine, and lactate levels and higher glucose concentration. The elevated concentration of tissue glucose in this hemisphere can be explained by the elevated blood glucose level of these animals (Table 3). The lower values of ATP account for the decrease in calculated energy charge potential, which, along with the increased variability of metabolite levels, suggests that the right hemisphere is undergoing subtle metabolic changes. The right hemisphere of Brain C5 demonstrated the only focal changes involving the right hemispheres of experimental animals (Fig. 1). The right middle ectosylvian gyrus of this animal had decreased levels of ATP and P-creatine, and elevated tissue levels of lactate and AMP. The ADP level was elevated in the anterior part of the gyrus as was the cyclic AMP level. TABLE 3 Glucose and lactate levels in arterial blood and CSF in controls and after MCA occlusion* Glucose (mg/100 ml) Lactate (mg/100 ml) Group Arterial CSF Arterial Blood Blood CSF controls & sham-operated animals _ 0.11 animals with It MCA occlusion *CSF = cerebrospinal fluid; MCA -- middle cerebral artery. J. Neurosurg. / Volume 52 / June,

4 R. A. Ratcheson and J. A. Ferrendelli Increases of glucose were seen in areas peripheral to this region, the posterior one also having an elevated pyruvate level. No discrete changes of either FDP or G-6-P were observed. Left Hemisphere. Analysis of metabolites in the left cerebral hemispheres demonstrated regional changes in the cerebral cortex reflecting definite alterations of metabolism (Fig. 2). Changes in metabolite levels were usually marked, and substantial variability of metabolic response was observed. The variability encountered added complexity to the statistical analysis; therefore, levels of metabolites were considered abnormal only when they were two or more standard deviations from both the pooled control and pooled right hemispheric data. Limited Metabolic Changes. The brains of Cats C1 and C4 demonstrated the least severe metabolic changes. In Brain C1 there were no alterations in the levels of ATP and P-creatine, while in Brain C4 a small portion of the anterior ectosylvian gyrus had lowered levels of both. The area of decreased P-creatine extended well beyond the area of decreased ATP. In much of the area of decreased P-creatine no associated elevation of lactic acid was observed, although in this region the lactate/pyruvate ratio was increased. Noncontiguous regions of elevated lactic acid were seen in both brains. Elevated pyruvate levels occurred in some regions where lactate was elevated. Brain CI had a large area of elevated AMP, while no alteration was observed in Brain C4. In both brains ADP was unchanged. Both animals demonstrated noncontiguous peripheral areas in which the glucose content was elevated to highly significant levels when compared with the elevated levels present in the right hemisphere of the experimental animals. Changes in FDP, G-6-P, and cyclic nucleotides consisted of elevations occurring in small patches of the cortex that could not be correlated with the changes in other metabolites. Diffuse Metabolic Changes. The left cerebral hemispheres of Cats C2, C3, and C5 demonstrated changes indicative of severe regional energy depletion. All brains contained areas with marked depletion of ATP and P-creatine, associated with elevated levels of lactate. In all animals, the greatest change occurred in the anterior ectosylvian and anterior Sylvian gyri (also see Fig. 2, Brain C4). In Brain C2, decreases of ATP in these regions were approximately 55%, while P- creatine was decreased 60%. In the periphery of these regions, decreases were only about 20% for both substrates. The changes in P-creatine included an area of the cortex beyond the region with decreased ATP levels. In Brain C3, levels of ATP in the affected area fell 20% to 85%, with lesser changes occurring at the periphery. Changes in P-creatine occupied an identical portion of the hemisphere, again of lesser degree at the periphery, and decreases ranged from 35% to 100%. Two areas where there was almost complete depletion II-rrl ~ ATP I~1 ~ PCr I~ ~ ADP II t AMP ~ Glucose 177J ~ Glucose 1"17 f Lactate ~ Pyruvate I~1 ~ Pyruvate FIG. 1. Drawings depicting regional cortical alterations of selected organic phosphates and carbohydrate substrates in the right hemisphere of Cat C5, 1 hour after occlusion of the left middle cerebral artery. ATP = adenosine triphosphate; PCr = phosphocreatine; ADP = adenosine diphosphate; AMP = adenosine monophosphate. of ATP and P-creatine were observed, one in the region of the anterior Sylvian gyrus, and the other a noncontiguous region in the anterior suprasylvian gyrus with interspersed areas of less severe changes. In Brain C5, there was total depletion of ATP and P-creatine in the anterior portion of the hemisphere. In the posterior limit of the affected area, the ATP and P-creatine levels were depressed 60% to 85% or more, respectively. In all three brains, lactate levels showed marked elevation both in the regions of ATP and P-creatine depletion and in some adjacent regions. As in Brain C4, some of the regions in Brain C2 with decreased levels of P-creatine did not have elevated levels of lactate. The highest lactate levels in Brains C2 (range 2.9 to 14.2 mmol/kg) and C3 (range 2.1 to 36.4 mmol/kg) occupied the regions of greatest ATP and P-creatine change. However, in Brain C5, where the highest lactate levels were found (range 2.1 to 41.9 mmol/kg), the changes were relatively greater in the more peripheral regicns (30 to 40 mmol/kg) than in the 758 J. Neurosurg. / Volume 52 / June, 1980

5 Regional cortical metabolism in focal isehemia FIG. 2. Drawings depicting regional cortical alterations of selected organic phosphates and carbohydrate substrates in the left hemisphere of five experimental animals (Cats C1-C5) 1 hour after occlusion of the left middle cerebral artery. ATP = adenosine triphosphate; PCr = phosphocreatine; ADP = adenosine diphosphate; AMP = adenosine monophosphate. region of the anterior ectosylvian and anterior Sylvian gyri (23 to 28 mmol/kg). In this region of C5, the level of pyruvate was also decreased when compared with controls. Elevated levels of pyruvate were seen in the most markedly affected regions of Brains C2 and C3, and the periphery of Brain C5, regions that demonstrated the highest lactate content. The AMP levels were elevated in all areas with decreased P-creatine and ATP and in most areas of elevated lactate content. The ADP value was elevated in the region of ATP and P-creatine change in Brain C3 and in multiple noncontiguous areas of Brain C5. As observed in the brains of Cats CI and C4, Brains C2, C3, and C5 contained areas of increased tissue glucose greater than that found in their right hemispheres. Brain C2 contained a small area peripheral to the region of depletion of high-energy substrates. Both Brains C3 and C5 contained large areas peripheral to the regions of severe energy depletion and extending beyond the region of elevated lactate. These brains also contained areas in which the tissue glucose content was decreased. Interposed between the areas of decreased and elevated tissue glucose were regions containing intermediate levels of glucose that statistically were unchanged from control and right hemispheric levels. In Brain C5, the glucose content was almost completely depleted in the area with exhausted ATP and P-creatine, decreased pyruvate, and relatively decreased lactic acid. In this tissue, the level of G-6-P was also found to be decreased, while in large areas of Brain C3 and most of the posterior portion of the left hemisphere of Brain C5, levels of G-6-P were increased. In many areas of the affected brains, FDP was increased in a patchy distribution; however, in Brain C5, the level of FDP was decreased throughout both right and left hemispheres to about 55% of control values. Cyclic AMP was increased in small areas within the described regions of maximum energy depletion. In Brain C5, the level of cyclic AMP was also decreased in the region with decreased pyruvate, glucose, G-6-P, and relative decrease in lactate. J. Neurosurg. / Volume 52 / June,

6 TABLE 4 Metabolic perturbations in cerebral cortex during incomplete focal ischemia* Metabolic Alteration P-creatine, ATP, glucose, pyruvate! lactate (limited), AMP, ADP P-creatine, ATP! lactate, pyruvate, ADP, AMP P-creatine lactate, AMP! lactate P-creatine T glucose R. A. Ratcheson and J. A. Ferrendelli Proposed Etiology severe ischemia; anaerobic glycolysis; total lack of substrate delivery severe ischemia; anaerobic glycolysis; partial delivery of substrate moderate ischemia; anaerobic glycolysis; partial delivery of substrate mild ischemia; reduced aerobic glycolysis mild ischemia;? acidosis glycolysis and/or hyperemia; possible recovery phase *ATP = adenosine triphosphate; ADP = adenosine diphosphate; AMP = adenosine monophosphate; P-creatine = phosphocreatine. I Lowered;! = elevated. Discussion Using the method described in this study, we found that the metabolite levels observed in our control animals were very similar to those obtained in smallanimal brains enzymatically inactivated by rapid freezing or microwave irradiation. This fact indicates that regional cortical energy and carbohydrate metabolism can be accurately evaluated in the CNS of paralyzed, ventilated large animals after enzymatic inactivation by freezing with topically applied liquid nitrogen. It has been observed in the rat that topical application of liquid nitrogen through a funnel freezes the brain in layers from top to bottom. In that model, circulation and oxygenation are upheld until the deeper tissue is frozen, with the exception of the most superior cortical regions where perforating vessels are frozen prior to the lower cortical regions and metabolic changes secondary to ischemia are not observed? ~ This finding is thought to be due to either the short duration of ischemia in the deeper tissues before freezing or the protective effects of advancing hypothermia. In experiments on freezing the brain through the intact skulls of cats, Welsh, et al.,~~ found changes compatible with ischemia in the depths of cortical sulci and ascribed this to the interruption of surface penetrating vessels supplying these areas. Control levels of high-energy phosphates obtained in this study, where freezing was performed by the application of liquid nitrogen directly to exposed dura, failed to show a difference between surface and sulcal cortex. This is likely due to a faster rate of freezing when overlying bone is removed. Occlusion of the left MCA in the cat produced metabolic changes in brain with marked inter- and intra-individual variability. However, these changes represent a spectrum that includes alterations similar to those observed during complete ischemia 8''a~ and 760 changes seen with partial ischemia, a,4,23'27,a~ Table 4 lists the spectrum of metabolic changes that we observed, and their probable causes. The most profound metabolic changes appeared in the anterior ectosylvian and anterior Sylvian gyri, while metabolic changes of lesser severity were located in adjacent and in more distant regions. Previous studies have indicated that brains demonstrating depletion of energy reserves similar to those observed in the more severely affected regions of the brains of this study will proceed to irreversible ischemic damage. 16,17,28-25 Consistent with this, we (unpublished observations) and others 86," have found by histopathological examination of the cat brain after MCA occlusion that the anterior ectosylvian and anterior Sylvian gyri are infarcted most frequently. This is not the area reported to have the lowest cerebral blood flow following MCA occlusion; 9 however, Yamaguchi, et al., 41 found only inconsistent correlation between areas of decreased flow and infarction. While evidence of severe energy derangement was seen in a number of brains, in only one brain of this series (Brain C5) was there evidence of total or near total absence of substrate. In this brain, not only was a region observed in which ATP, P-creatine, and glucose were totally depleted, but, in addition, there were diminished 'levels of pyruvate and G-6-P and a relatively smaller lactate accumulation when compared to the markedly elevated levels in more posterior regions of this brain. The accumulation of lactate here was probably limited by the decreased regional availability of glucose. The more posterior areas of the left cerebral cortex of Brain C5 and regions of other brains had biochemical characteristics of severe, but incomplete, ischemia with continued availability of substrate, that is, reduced J. Neurosurg. / Volume 52 / June, 1980

7 Regional cortical metabolism in focal ischemia high-energy phosphate reserves and very high lactate levels. Despite the relatively greater substrate delivery offered these regions, the extreme acidosis produced by the high lactic acid levels may adversely influence the capacity of these tissues to undergo metabolic recovery. In this series of experiments, areas were found with low P-creatine levels not associated with an elevation of tissue lactate. Increased concentration of lactic acid would be expected to occur if the fall in P-creatine reflected utilization of high-energy phosphate during anaerobic glycolysis. It is possible that this phenomenon has occurred in brain regions of very mild ischemia in which there has been an accumulation of carbon dioxide. During moderate hypercapnia, lactic acid concentrations in tissue will fall at a time when intracellular ph has decreased, causing a shift in the creatine phosphokinase equilibrium and a lowering of tissue P-creatine levels without alteration of the energy state of the brain. TM While changes in individual levels of lactate and pyruvate were not statistically significant, they tended to be low, and lactate/pyruvate ratios in these areas were increased. It is presumed that in this situation the fall in ph and shift in lactate dehydrogenase equilibrium produces only a limited fall in lactate, despite a continued decline in pyruvate. An alternative explanation for the finding of low P-creatine and unelevated lactate would require that, in the involved regions of incomplete ischemia, anaerobic glycolysis is occurring, but sufficient blood flow exists to remove the excess lactic acid. Only in those brains (Brains C3 and C5) that exhibited the most severe metabolic insult, both in degree of substrate depletion and area involved, was there seen central depletion of glucose. However, in all of the brains there were areas of glucose elevation peripheral to the regions of maximum injury. Ginsberg, et al., ~~ observed peripheral areas in which there was increased uptake of l*c-2-deoxyglucose following MCA occlusion; they were using a similar model of focal, subtotal ischemia, and they concluded that these areas were undergoing enhanced anaerobic glycolysis. It is likely that the peripheral regions with increased glucose concentration observed in this study can be correlated with those areas showing increased uptake of ~*C-2-deoxyglucose observed by Ginsberg, et al. Our additional metabolic data fail to confirm Ginsberg's hypothesis of enhanced anaerobic glycolysis, due to a mild ischemic condition which compromised oxygen delivery to the tissue. We found neither changes in G-6-P and FDP suggestive of enhanced glycolysis nor consistent regional correlation with elevated levels of lactic acid indicative of a shift to anaerobic glycolysis. In fact, tissue glucose accumulation has also been observed as a consequence of decreased glycolytic flux. 6 In light of observations that hyperglycemic monkeys suffered more severe brain tissue changes after cardiac arrest than fasted animals, it is difficult to ascribe a protective mechanism to explain the elevated blood and brain J. Neurosurg. / Volume 52 / June, 1980 tissue glucose content. 2s,~9 The interpretation that the regions observed in this study correspond to regions of hyperemia with enhanced phosphorylase activity is limited by studies indicating that increased cerebral blood flow is not prominent shortly after MCA occlusion, 9,37 although both absolute and relative hyperemia have been reported),41 Another explanation for this finding is that the tissue in question may be undergoing a phase of recovery. It has been observed that after total compression ischemia during the period of recirculation, brain glucose concentration reaches supranormal values at a time when other metabolic changes have resolved. 1~ The results of this study reveal that metabolic changes occurring in and about an area of focal ischemia are heterogeneous and complex, and vary considerably from individual to individual. While these responses may be determined by gradations in blood flow and delivery of oxygen and substrate, it is possible that they are also influenced by alterations in metabolite removal and the metabolic activity of adjacent tissue. The static picture provided indicates that cortical areas of abnormal metabolism may be separated by areas with normal metabolic activity. Further studies may well indicate that these regions are undergoing marked metabolic alterations and only appear to be relatively normal due to the timing of sampling. It is also thought that after focal ischemia there is an immediate widespread area of metabolic depression that becomes smaller with time? e If this is the case, not only is the brain undergoing a variety of patterns of metabolic derangement, but different areas of the brain may simultaneously be undergoing deterioration and improvement. Present experimental models are unable to duplicate the gradual thrombotic occlusion of a cerebral vessel and the temporal pattern of collateral circulatory development that occurs in the human stroke patient. It is also apparent that present methods used to study the metabolic consequences of stroke, including those of this study, are unable to depict the dynamic alterations in cerebral metabolism that would allow an accurate appraisal of the state of the tissue in regard to its potential for survival. Although the metabolic changes encountered in the present study may demonstrate some events that occur in the clinical situation, until a method of temporal analysis is developed, the regional variability demonstrated by this model will limit its usefulness. Acknowledgments The authors wish to thank Ms. Eloys Young and Ms. Louise Bilezikjian for their expert technical assistance and Ms. Diane Kenar for preparation of the manuscript. References 1. 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9 Regional cortical metabolism in focal ischemia processes in the brain at normal and reduced temperatures and under anoxic and ischaemic conditions.] J Neuroehem 2: , 1958 (Ger) 39. Welsh FA, Durity F, Langfitt TW: The appearance of regional variations in metabolism at a critical level of diffuse cerebral oligemia. J Neuroehem 28:71-79, Welsh FA, O'Connor M J, Rieder W, et al: Regional changes in metabolism in hypoxia-ischemia. Adv Exp Med Biol 78: , Yamaguchi T, Waltz AG, Okazaki H: Hyperemia and ischemia in experimental cerebral infarction: correla- tion of histopathology and regional blood flow. Neurology 21: , 1971 This study was supported in part by USPHS Grant NS Address reprint requests to: Robert A. Ratcheson, M.D., Department of Neurology and Neurological Surgery, Washington University School of Medicine, Barnes Hospital Plaza, St. Louis, Missouri J. Neurosurg. / Volume 52 / June,