Message: #2633 - BPI Tech Brief #4 Date: 08 Mar 94 01:09:45 EST From: Mike Darwin <> Message-Subject: SCI.CRYONICS BPI TECH BRIEF #4 BPI TECH BRIEF #4 It is a core objective of BPI TECH BRIEFS to not only report significant in-house progress in both cryonics research and technology but also to educate the technologically sophisticated public about various aspects of the science which underlies cryonics. Other than cryobiology and the neurobiology of learning and memory no area of investigation impacts cryonics more than the pathophysiology of cerebral ischemia. The nature of ischemic lesions and the rapidity of their development is likely to be of critical importance to the workability of cryonics. Clearly, if a period of ischemic injury destroys those structures responsible for human mentation and memory then no technology of preservation or recovery (given our current understanding of physics) will be of any use to a patient so injured. Unfortunately, our understanding of the workings of memory and identity are still in their infancy. Our understanding of the nature of ischemic injury is more advanced, but still far from complete. The following article is adapted from the first Chapter of the HUMAN CRYOPRESERVATION LEVEL 1 TRANSPORT PROTOCOL, Seventh Edition (Biopreservation, 1994). THE PATHOPHYSIOLOGY OF CEREBRAL ISCHEMIA by Michael G. Darwin, President Biopreservation, Inc. In 1960 Kouwenhoven, Jude, and Knickerbocker reported use of closed-chest cardiopulmonary resuscitation (CC-CPR) in 20 patients with a 70% overall survival rate (1). In the decades that followed, an international program of enormous scope and cost was launched to implement CC-CPR at every level of emergency care, including the instruction of millions of laypersons in the technique. In the intervening three decades since CC-CPR was first introduced with the enthusiastic statement by Kouwenhoven, et al. that "Anyone, anywhere, can now initiate cardiac resuscitation procedures. All that is needed are two hands"(2). Many studies have been published documenting its ineffectiveness (i.e., survival rates under 20%) in maintaining cerebral viability in cases of cardiac arrest both in the hospital3,4 and in the field (5,6,7). Indeed, there is evidence that the survival rate of patients experiencing in-hospital cardiac arrest has declined since CC-CPR replaced open chest CPR (OC-CPR) in the 1960's(8). In the thirty years since its implementation there has never been a formal, organized assessment of the utility of this technique in terms of cost vs. benefit either financially or medically. In patients who survive following resuscitation with CC- CPR, the incidence of both transient and permanent neurological deficits and reduced quality of life are high (9,10,11,12). In recent years there has been a growing awareness of the inadequacy of CC-CPR, with a call by some to return to OC-CPR (13) and vigorous research by others to optimize CC-CPR to address the dismal survival rates and usually poor neurological outcome. Increasingly, public healthcare policy is coming to reflect the reality that neurologists, cardiologists and intensivists have long understood: "CC-CPR doesn't work.". This is reflected in the recent policy change by the American Red Cross, wherein bystanders to cardiac arrest patients are now urged to activate the Emergency Medical System (EMS) first and start CPR second, instead of the other way around. This change reflects a growing awareness that CC-CPR is largely ineffective and that a patient's best chance for recovery is early defibrillation and associated definitive therapy. This may seem an extreme statement, particularly to those who have not witnessed the all too common tableaux, played out in intensive care units around the world, of the brain dead or vegetative cardiac arrest victim consuming tens of thousands of dollars in medical resources. The staggering cost of CC-CPR in teaching, healthcare, and patient/family emotional and financial resources when weighed against the dubious benefit suggests that society might have been better served if the CC-CPR program had never been implemented. The conclusion seems inescapable that what CC-CPR is most effective at is producing individuals who either are brain dead, or in a persistent vegetative state. The problem with CC-CPR (or any in-field resuscitation technique) is cerebral ischemia. While mechanical or other device-oriented means of optimizing CC-CPR may well be developed, and the first-response use of defibrillators may become more commonplace, the fundamental problem of ischemic time before restoration of adequate circulation remains. For many of the 325,000 persons who will experience sudden cardiac death (SCD) in the coming year, there will be little or no possibility of rescue. Cardiac arrest will occur without warning, often in situations not conducive to activation of the EMS. However, for many of those patients, there will have been a warning that they are at increased risk of SCD. A prior myocardial infarct (MI), familial history of arrhythmic disease, or iatrogenic risk such as CABG or angioplasty, will often provide ample warning that SCD could occur. In MI alone the incidence of SCD within the first year following infarct is 14%14. The development of more sophisticated markers for SCD in post MI patients, such as increased R-R interval regularity, is also making it possible to identify with increasing accuracy those who are at risk of SCD15. What can be done to improve the disappointing overall success rate of CPR? Does increasing the ability to identify patients at risk for SCD offer the possibility of therapeutic interventions such as anti-arrhythmic drugs and implantable defibrillators? Is there some way to pre-medicate or pre-treat patients who are at risk to increase their chances of surviving an ischemic episode with intact mentation? A review of the literature in experimental cerebral resuscitation and the pathophysiology of cerebral ischemia (CI) suggests a number of approaches using both pre- and post- medication which may provide protection against cerebral ischemia for those at risk for SCD and which have acceptable costs and risk to-benefit ratios. While a wide range of post-insult interventions are currently being investigated in animal and clinical trials, relatively little attention has been paid to the possibility of pre-medication of the at-risk population combined with post- insult therapy. Additionally, despite almost universal agreement that CI is a multifactorial insult, there has been little or no research aimed at developing a multimodal method of managing the multiple insults and compromises to brain metabolism that are known to occur. Before suggestions are put forth for prevention and/or amelioration of ischemic injury it is desirable to review briefly the requirements for adequate cerebral perfusion and the basic mechanisms of cerebral ischemic injury as they are currently understood: Requirements For Adequate Cerebral Perfusion Normal cerebral blood flow (CBF) in man is typically in the range of 45-50 ml/min/100g between a mean arterial pressure (MAP) of 60 and 130 mmHg20. When CBF falls below 20 to 30 ml/min/100g, marked disturbances in brain metabolism begin to occur, such as water and electrolyte shifts and regional areas of the cerebral cortex experience failed perfusion21. At blood flow rates below 10 ml/min/100g, sudden depolarization of the neurons occurs with rapid loss of intracellular potassium to the extracellular space22. The Mean Arterial Pressure (MAP) necessary for cerebral viability following extended resuscitation efforts in dogs has been found to be above 40 mm Hg23. It has been speculated that a minimum MAP of 45 to 50 mm Hg is required to preserve cerebral viability in man24. Unfortunately, as is now well documented, conventional CC- CPR is generally incapable of consistently delivering MAPs much above 30 mm Hg in man25,26. A clinical evaluation of manual and mechanical CPR (using a pneumatically driven chest compressor and ventilator) demonstrated that only 3 of 15 acute cardiac arrest patients presenting for emergency room resuscitation had MAPs above 40 mm Hg27. It should be emphasized that these studies evaluated a highly selected patient population, where the underlying cause of cardiac arrest was primary cardiac failure without other organ system failure, dehydration, sepsis, or pulmonary hypoxia as an underlying cause. Quite often, the patient presenting for cryonic suspension suffers from a variety of pathologies which can be expected to further reduce the ability of closed chest CPR to deliver adequate MAP or adequate arterial blood oxygenation (pa02). Pneumonia, pulmonary and systemic edema, hemorrhage, sepsis, liver failure, space-occupying lesions of the lungs, and a host of other pathologies can all compromise gas exchange and reduce vascular tone and circulating blood volume. Even in the patient experiencing optimum machine-delivered CPR, lung compliance and blood gases tend to deteriorate rapidly during CPR, perhaps as a result of pulmonary edema secondary to high intrathoracic venous pressures28. As the foregoing analysis makes clear, many, if not most, cryonic suspension patients will suffer significant periods of cerebral anoxia, ischemia, or hypoperfusion before they receive more effective cardiopulmonary support such as OC-CPR29, extracorporeal circulation utilizing a membrane or bubble oxygenator30, or high impulse CPR31,32. Mechanisms of Ischemic Injury Early observations on the mechanisms of ischemic injury focused on relatively simple biochemical and physiological changes which were known to result from interruption of circulation. Examples of these changes are: loss of high-energy compounds16, acidosis due to anaerobic generation of lactate17, and no reflow due to swelling of astrocytes with compression of brain capillaries18. Subsequent research has shown the problem to be far more complex than was previously thought, involving the action and interaction of many factors19. Biochemical Events Within 20 seconds of interruption of blood flow to the mammalian brain under conditions of normothermia, the EEG disappears, probably as a result of the failure of high-energy metabolism. Within 5 minutes, high-energy phosphate levels have virtually disappeared (ATP depletion)33 and profound disturbances in cell electrolyte balance start to occur: potassium begins to leak rapidly from the intracellular compartment and sodium and calcium begin to enter the cells34. Sodium influx results in a marked increase in cellular water content, particularly in the astrocytes35. Calcium Normally, calcium is present in the extracellular milieu at a concentration 10,000 times greater than it is present intracellularly. This 10,000:1 differential is maintained by at least the following four mechanisms: 1) active extrusion of calcium from the cell by an ATP-driven membrane pump36, 2) exchange of calcium for sodium at the cell membrane driven by the intracellular to extracellular differential in the concentration of Na+ as a result of the cell membrane's Na+ -- K+ pump37, 3) sequestration of intracellular calcium in the endoplasmic reticulum by an ATP-driven process38, and 4) accumulation of intracellular calcium by oxidation-dependent calcium sequestration inside the mitochondria39. The loss of cellular high-energy compounds during ischemia causing the loss of the Na+ -- K+ gradient, virtually eliminates three of the four mechanisms of cellular calcium homeostasis. This, in turn, causes a massive and rapid influx of calcium into the cell40. Mitochondrial sequestration, the remaining mechanism, causes overloading of the mitochondria with calcium and diminished capacity for oxidative phosphorylation. Elevated intracellular Ca++ activates membrane phospholipases and protein kinases. A consequence of phospholipase activation is the production of free fatty acids (FFA's) including the potent prostaglandin inducer, arachidonic acid (AA). The degradation of the membrane by phospholipases almost certainly damages membrane integrity, further reducing the efficiency of calcium pumping and leading to further calcium overload and a failure to regulate intracellular calcium levels following the ischemic episode41. Additionally, FFAs almost certainly have other degradative effects on cell membranes42. The production of AA as a result of FFA release causes a biochemical cascade ending with the production of throxboxane and leukotrienes. Both these compounds are profound tissue irritants which can cause platelet aggregation, clotting, vasospasm, and edema42,43,44, with resultant further compromise to restoration of adequate cerebral perfusion upon restoration of blood flow. Free Radicals During ischemia, the hydrolysis of ATP via AMP leads to an accumulation of hypoxanthine45. Increased intracellular calcium enhances the conversion of xanthine dehydrogenase (XD) to xanthine oxidase (XO). Upon reperfusion and reintroduction of oxygen, XO may produce superoxide and xanthine from hypoxanthine and oxygen46,47. Even more damaging free radicals could conceivably be produced by the metal catalyzed Haber-Weiss reaction as follows48-51: O2- + H2O ----Fe3 ------> O2 + OH-+ OH- Iron, the transition metal needed to drive this reaction, is present in abundant quantities in bound form in living systems in the form of cytochromes, transferrin, hemoglobin and others. Anaerobic conditions have long been known to release such normally bound iron52,53,54. Indirect experimental confirmation of the role of free iron in generating free-radical injury has come from a number of studies which have confirmed the presence of free-radical breakdown products such as conjugated dienes55,56 and low molecular weight species of iron57. During reperfusion and re-oxygenation, significantly increased levels of several free-radical species that degrade cell and capillary membranes have been postulated: 1) O2-, OH-, and free lipid radicals (FLRs). O2- may be formed by the previously described actions of XO and/or by release from neutrophils which have been activated by leukotrienes (see discussion below of the role of leukocytes in ischemia- reperfusion injury). Re-oxygenation also restores ATP levels, and this may in turn allow active uptake of calcium by the mitochondria, resulting in massive calcium overload and destruction of the mitochondria58. Mitochondrial Dysfunction Calcium loading and free-radical generation are no doubt major contributors to the mitochondrial ultrastructural changes which are known to occur following cerebral ischemia59. In addition to the structural alterations observed, there are biochemical derangements such as a marked decrease in adenine nucleotide translocase and oxidative phosphorylation. There is also an accumulation of FFAs, long-chain acyl-CoA, and long-chain carnitines. Of these alterations, the accumulation of long-chain acyl-CoA is perhaps most significant, since intramitochondrial accumulation of long-chain acyl-CoA is known to be deleterious to many different mitochondrial enzyme systems60. Lactic Acidosis While it is clearly not the sole or even the major source of injury in ischemia, lactic acidosis does apparently contribute to the pathophysiology of ischemia64,65. It has been shown, for instance, that lactate levels above a threshold of 18 - 25 micromol/g result in currently irreversible neuronal injury66,67,68. Decrease in pH as a consequence of lactic acidosis has been shown to injure and inactivate mitochondria. Lactic acid degradation of NADH (which is needed for ATP synthesis) may also interfere with adequate recovery of ATP levels post ischemically69. Lactic acid can also increase iron decompartmentalization, thus increasing the amount of free- radical mediated injury70. Excitotoxins A rapidly growing body of evidence indicates that excitatory neurotransmitters, which are released during ischemia, play an important role in the etiology of neuronal ischemic injury71,72,73. Those areas of the brain which show the most "selective vulnerability" to ischemia, such as the neocortex and hippocampus, are richly endowed with excitatory AMPA (alpha- amino-hydroxy-5-methyl-4-isoxazole proprionic acid) and NMDA (N- methyl-d-aspartate receptors)74. Initially there was much optimism that blockade of the NMDA receptor would provide protection against delayed neuronal death following global cerebral ischemia75,76,77. The use of NMDA receptor blocking drugs has shown significant promise in ameliorating focal cerebral ischemic injury; a number of studies have demonstrated marked reduction in the severity of ischemic injury to focal areas (particularly the poorly perfused "penumbra" surrounding the no-flow area) as a result of treatment with glutamate-blocking drugs such a dextrorophan78 or the experimental anticonvulsant MK-80179. In vitro studies with cultured neurons have demonstrated that excitatory neurotransmitters cause neuronal injury and death even in the absence of hypoxic or ischemic injury80. In vivo studies have confirmed a massive release of glutamate and aspartate during both regional and global cerebral ischemia81. In regional or focal cerebral ischemic injury, the NMDA remains activated for a long period due to the prolonged interval of poor perfusion in the area at the edges of the infarct (the "penumbra"). However, in complete or global ischemia there is good resumption of blood flow following restoration of circulation with prompt uptake of glutamate and aspartate and resultant relatively rapid inactivation of the NMDA receptors82. Another factor limiting the role of the NMDA receptor in mediating injury in global cerebral ischemia may be the rapid and pronounced drop in pH which occurs in global as opposed to focal ischemia, since low pH is known to inactivate the NMDA receptor. These reasons are probably why NMDA receptor inhibitors have not proved effective in preventing global cerebral ischemic injury83,84. Recently, attention has turned to non-NMDA antagonists such as inhibitors of the kainate and AMPA receptors85. The mechanisms by which excitotoxins cause cell injury is not yet fully understood. It is known that they facilitate calcium entry into neurons86. However, these agents are neurotoxic even in cell culture where the medium is calcium free87. In the case of kainate and AMPA receptor activation, the likely mode of injury is sensitization of the CA1 pyramidal cells during ischemia such that when normal signaling is restored at the end of the ischemic insult, and normal intensity input from the Schaffer collaterals is resumed, lethal cell injury results, perhaps from abnormal calcium regulation in the CA1 cells or other metabolic derangements not yet understood. Neutrophil Activation Since the late 1960s, polymorphonuclear leukocytes (PMNLs) and monocytes/macrophages have been implicated as significant causes of pathology in cerebral ischemia. During the last decade there has been a veritable explosion of research documenting the role of PMNLs in reperfusion injury. Most of the initial work done in this area focused on PMNL-mediated reperfusion injury to the myocardium, establishing that PMNL activation and subsequent plugging and degranulation (resulting in release of oxidizing compounds) is responsible for the no-reflow phenomenon following myocardial ischemia88,89,90. In particular, the work of Engler has demonstrated that PMNL activation is responsible for plugging at least 27% of myocardial capillaries and is further responsible for the development of edema and arrhythmias upon reperfusion91. To what extent leukocyte plugging occurs in the brain following global cerebral ischemia remains controversial92. Anderson, et al. have examined the question of how rapidly leukocyte plugging occurs following cerebral ischemia using a bilateral carotid artery plus hypotension model in the dog. They noted no leukocyte plugging after 3 hours of reperfusion following a 40-minute ischemic episode93. However, it is clear from a growing body of work that neutrophils are a major mediator of ischemic injury in a variety of organ systems and that their acute activation is responsible for many of the effects of ischemia observed in the brain and other body tissues, including the loss of capillary integrity and the degradation of ultrastructure upon reperfusion94. When PMNLs are activated they generate large amounts of hydrogen peroxide. A large fraction of the hydrogen peroxide, aided by myeloperoxide (also released by activated PMNLs), reacts with the halides Cl-, Br-, or I- to produce their corresponding hypohalous acids (HOX)95. Because the concentration of Cl- is more than a thousand times greater than the other halides, the hydrogen peroxide-myeloperoxidase system probably generates Cl- most often in the form of HOCl. HOCl is more commonly known as household bleach and is capable of damaging a wide range of organic molecules including most of those that make up the structure of the cells and proteinaceous extracellular matrix96. As Klebanoff has pointed out, the amounts of HOCl generated by the neutrophil are awesome: 106 neutrophils can generate 2 x 107 mol of HOCl - enough to destroy 150 million E. Coli in a matter of milliseconds97. However, the direct destructive effects of HOCl are probably limited in vivo by a variety of mechanisms98. Most probably the hypohalous acids act to inflict the lion's share of injury by interacting with PMNL, collagenase, elastase, gelatinase, and other proteinases. As is shown in the diagram below, it is now believed that the oxidants released from the neutrophil create a halo of oxidized alpha-1-proteinase inhibitor that allows released elastase (and probably others of the 20 or so known neutrophil-secreted proteolytic enzymes99) to begin degrading the extracellular matrix, thus destroying capillary integrity and interfering with tissue metabolism and anabolism. ***ILLUSTRATION NOT INCLUDED Figure 1: The role of PMNL in mediating ischemic injury (from Weiss, S.J., New Eng J Med 1989;320:365-76). In complete circulatory arrest, it is clear that neutrophil activation with accompanying release of HOCl and activation of elastase is a key factor in initiating the systemic cascade of inflammation/immune response which terminates in delayed multisystem organ failure100. The extent to which this pathway is a factor in acute global cerebral ischemic injury in cardiac arrest is not yet clear. Hypoperfusion Following Reperfusion An apparently significant contributor to reperfusion injury is hypoperfusion after restoration of spontaneous circulation. The work of Hossman, et al101, and Sterz, et al102, has demonstrated the critical importance of providing adequate circulatory support following global cerebral ischemia. Loss of autonomic regulation, depressed myocardial function secondary to ischemic insult of the myocardium, and autonomic dysfunction all serve to depress MAP and cerebral perfusion following restoration of circulation. Both Hossman's and Sterz's work has demonstrated significant improvements in neurological outcome if circulation is supported both extracorporeally and/or with pressors during reperfusion. Histological Ultrastructural Change Ischemic changes in cell architecture begin almost as rapidly as ischemic changes in biochemistry. Within seconds of the onset of cerebral ischemia, brain interstitial space almost completely disappears. Loss of interstitial space is a consequence of cell swelling secondary to sodium influx and failure of membrane ionic regulation. There have been several studies of the ultrastructural alterations associated with prolonged global cerebral ischemia. Notable is the work of Kalimo et al in the cat103, as well as Karlsson and Schultz104, and Van Nimwegen, et al105 in the rat. These investigators describe the following changes in common in these animals' brain ultrastructure after varying periods of global cerebral ischemia (GCI): 1) Changes At 10 Minutes After 10 minutes of GCI, a significant number of cells (but not all) show clumping of nuclear chromatin and a modest increase in electron lucency (probably due to dilution of the cytosol by extracellular fluid). After 30 minutes, further changes include increased cytoplasmic swelling (particularly in the astrocytes), swelling and shape change of the mitochondria, and some loss of mitochondrial matrix density. Microtubules disappear and there is detachment of the ribosomes from the cisternae of the endoplasmic reticulum. There is also disassociation of the polyribosomes, and single ribosomes lose their compact structure with associated failure of protein synthesis. Of note is the stability of the lysosomes over this time course106. 2) Changes At 60 Minutes After 60 minutes of GCI, the above changes have become more pronounced with more conspicuous swelling of the ER cisternae. The mitochondria begin to show slight inner matrix swelling and occasional flocculent densities (probably due to accumulated calcium). 3) Changes At 120 Minutes After 120 minutes of GCI, the changes discussed above are more pronounced and a larger number of mitochondria exhibit the presence of flocculent densities evidencing calcium overload which is currently considered irreversible. Published electron micrographs reveal intact lysosomes and seem to confirm other studies which indicate that lysosomal rupture and subsequent catastrophic autolysis is not a feature of early (1 - 4 hours) ischemic injury107. From a cryonics (i.e., information-theoretic perspective), it is important to point out that throughout even a 120-minute- period of normothermic cerebral ischemia, the appearance of the plasma membrane layers, including synapses and myelin sheaths, is only altered modestly. Indeed, the first ultrastructural changes associated with what is currently considered lethal cell injury are to the mitochondria and ribosomes, and these do not usually appear until after 30 minutes of GCI. At least one study of post-mortem ultrastructural degradation has been conducted on a small number of human subjects108. The histological and ultrastructural changes experienced in patients with 25 to 85 minutes of GCI, and without extensive pre-mortem brain trauma or pre-mortem cerebral no- reflow of prolonged duration, closely parallel those observed in animal models of GCI: astrocytic edema, clumping of nuclear chromatin, disassociation of the polyribosomes, detachment of the ribosomes from the ER cisternae, and swelling of the mitochondria with the presence of flocculent densities. Stability of the lysosomes and conservation of the structure of the neuropil over this time-course are well documented. Opportunities For Intervention With the understanding of the mechanisms of the pathophysiology of cerebral ischemia having evolved to the point outlined above, many possible interventions suggest themselves. Indeed, the literature of cerebral resuscitation is a vast one and is growing rapidly with the release of papers exploring a variety of monomodal approaches to treating cerebral injury secondary to both global and regional ischemic insults. However, despite the widely held belief that cerebral ischemic injury is multifactorial in nature, there has been almost no work done examining multimodal methods of treatment. There is also almost a complete absence of studies which address the potential of pre-treatment in ameliorating cerebral ischemic injury, particularly pretreatment with nonproprietary agents such as antioxidant nutrients. This kind of approach is of particular importance to the cryonics community where a significant number of patients present for cryonic suspension in a slow failure mode that allows for active intervention. The approach to protecting cryonic suspension patients against cerebral ischemic injury outlined in this text is a multimodal approach which address the following known sources of cerebral ischemic injury: 1)Numerous studies have suggested a cerebroprotective effect for a variety of calcium channel blockers administered post- insult109,110,111. 2)Free radical damage: Free radicals have long been understood to be a major source of cerebral ischemic pathology. Similarly, there have been a number of studies which suggest that free radical associated ischemic injury can be reduced greatly or eliminated by pre- or post-insult treatment with nutritional antioxidants such as vitamin E112,113,114, selenium115, vitamin C116, and beta carotene117. Theoretical considerations also suggest other possible therapeutic agents such as those known to elevate neuronal (intracellular) glutathione levels for protection from cerebral ischemic injury118,119. 3)Phospholipase activation has been implicated as a significant source of injury in both cold and warm ischemia. The phospholipase inhibitor quinacrine has reduced cold ischemic injury in an organ preservation model120 as well as myocardial reperfusion injury121. Quinacrine may be effective in attenuating normothermic cerebral ischemic injury as well. 4)The importance of mitochondrial dysfunction in preventing recovery following global cerebral ischemia has been demonstrated in a recent study by Rosenthal, et al. They demonstrated the effectiveness of acetyl-l-carnitine in improving both neurological function and normalizing brain high energy metabolism in the dog following 10 minutes of normothermic cardiac arrest122. ***ILLUSTRATION NOT INCLUDED Figure 2: The Pathophysiology of Cerebral Ischemia. Schematic summary of the hypothesized mechanics of tissue injury in cerebral ischemia during both the circulatory arrest (left) and reperfusion (right) intervals. During normal conditions intracellular calcium (Ca++) levels are maintained at approximately 100 nM. Ca++ regulation is achieved by the plasma membrane Ca/Mg-ATPase and the ATP dependent uptake of Ca++ into the endoplasmic reticulum (ER) and mitochondria. The release of bound Ca++ from the ER store is believed to be triggered by inositol-1,4,5 triphosphate (IP3) and/or by free arachidonic acid (AA). Release of Ca++ bound in the mitochondria is not thought to occur until the ER stores are depleted. The initial response of many different cell types to stimulation--i.e., ligand- receptor interaction, hormone receptor binding, chemotactic peptide binding to polymorphonuclear leukocytes, or presynaptic or post-synaptic neurotransmitter binding , in an increase in Ca++ due to release of intracellular ER-bound Ca++, an influx of extracellular Ca++, or both. Changes in many intracellular enzyme activities, including phospholipases and protein kinases, the polymerization of g-actin to f-actin, and that of tubulin to microtubules, all occur at different "set points" of Ca++. Therefore much of the control of intracellular processes is related to the level of Ca++. During ischemia (left), in all cells (including neurons) the level of ATP decreases rapidly to near zero. This causes an increase in free calcium, even without an increase in IP3. The addition of 2-deoxyglucose to cells, which acts as an ATP sink, causes a rapid increase in Ca++. Increases in Ca++ activate phospholipase A2 (p.lase), which breaks down membrane phospholipids (PL) into free fatty acids (FFA), particularly AA. The AA causes increased activity of the cyclooxygenase pathway to produce prostaglandins (PG), including thromboxane (TX) A2, the lipoxygenase pathway to produce leukotrienes (LT), or both. Furthermore, during ischemia the hydrolysis of ATP via AMP leads to accumulation of hypoxanthine (HX). Increased Ca++ enhances the conversion of xanthine dehydrogenase (XD) to xanthine oxidase (XO), priming the neuron for the production of the oxygen free radical O2- intracellularly, once O2 is reintroduced. During reoxygenation (right), significantly increased levels of at least three free radical species (in oblique boxes) that result in direct and indirect damage to cell membranes and the extracellular matrix (and thus lead to edema and microcirculatory failure) may be formed: O2-, OHo, and free lipid radicals (FLR). O2- may be formed from two sources: 1) the previously described XO system and 2) activation of neutrophils in the microvasculature due to increased LT production by the neurons or simply by absent blood flow and consequent margination and diapedesis of neutrophils from the microvasculature. Increased O2- production leads to increased H2O2 production as a result of the intracellular action of SOD. H2O2 is controlled by intracellular catalase. Increased O2- production leads to increased OHo, due to the Fenton reaction (Fe++ +H2O2--->Fe+++.+OH+OHo) with iron liberated from ferritin, and the Haber-Weiss reaction (O2-+H2O2--->OH-+ OHo).. Each or all of these oxidants can result in lipid peroxidation and the production of LFRs. All free radicals can cause leaky membranes and currently irreversible cell damage. Furthermore, reoxygenation restores ATP via oxidative phosphorylation, which may result in massive uptake of Ca++ into mitochondria. Thus, increased Ca++ as a result of ischemia and reoxygenation, by itself, and by triggering free radical reactions, may well be the principal cause of neuronal necrosis during reperfusion. The above text and figure are reproduced with some changes from Safar, P., and Bircher, N.G., Cardiopulmonary Cerebral Resuscitation. 1988; W.B. Saunders Company, Ltd., London, UK. pp. 236-37.Figure 2. The Pathophysiology of Cerebral Ischemic Injury 5)Protection against the deleterious effects of excitotoxicity has been addressed in a number of ways, including the use of both NMDA and kainate receptor inhibiting drugs. As has been previously discussed, excitotoxicity is clearly a significant source of reperfusion injury and must be addressed in any multimodal therapeutic approach to cerebral ischemia. The best compound(s) to use to achieve this effect has not been determined by the author as of this writing. 6)As was previously noted, extracorporeal perfusion to support MAP, facilitate reperfusion through initial hypertension, insure adequacy of cerebral perfusion, and allow for induction of mild hypothermia have been shown to be beneficial in achieving a favorable outcome following 10 to 12 minute periods of global cerebral ischemia. 7)Inhibition of the inflammatory cascade and the adhesion and degranulation of polymorphonuclear lymphocytes by both drug treatment and by their removal via filtration have been shown to lessen reperfusion injury in the lungs and heart. As a consequence, they presumably lessen the likelihood of development of the post resuscitation syndrome, at least in extracerebral tissues123. Summary As the foregoing has hopefully made clear, neuronal ischemic changes occur rapidly with significant structural changes being observed over a time-course of minutes rather than hours. The signifance of these changes in terms of damage to identity- critical structures (i.e., those encoding memory and personality) is not currently known since we do not yet understand how memory is encoded, or more generally, which brain structures (gross or ultrastructural) are critical to mentation. As a consequence of our ignorance about what structures need to be preserved, it is the opinion of this author that a very conservative approach to suspension patient transport should be followed. In practice, what this means is that every reasonable effort should be made to minimize cerebral ischemic injury. Achieving a reasonable cost versus benefit tradeoff in actual practice will naturally be a matter of some debate. An attempt has been made in the development of this protocol to strike a reasonable balance between cost and complexity and anticipated benefit to the patient. A fairly conservative approach has been used in the application of new technologies without a proven track record of clinical success in cerebral resuscitation. The author has been active in the fields of cerebral resuscitation and cryonics long enough to have observed a number of "fads" and "hot new techniques" come and go. An attempt has been made here to apply only those research modalities which have shown promise in a number of researchers' hands, and whenever possible, to have in-house verification of the effectiveness of these modalities. References 1) American Heart Association and National Research Council, Standards for Cardiopulmonary Resuscitation (CPR) and emergency cardiac care (ECC). J Amer Med Assoc (Suppl.) 1974;227:833-68 2) Ibid. 3) Weale FE, Rothwell-Jackson RL. The efficacy of cardiac massage. 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