Mechanism of myocardial "stunning"

Among the numerous mechanisms proposed for myocardial stunning, three appear to be more plausible: 1) generation of oxygen radicals, 2) calcium overload, and 3) excitation-contraction uncoupling. First, the evidence for a pathogenetic role of oxygen-derived free radicals in myocardial stunning is overwhelming. In the setting of a single 15-minute coronary occlusion, mitigation of stunning by antioxidants has been reproducibly observed by several independent laboratories. Similar protection has been recently demonstrated in the conscious animal, that is, in the most physiological experimental preparation available. Furthermore, generation of free radicals in the stunned myocardium has been directly demonstrated by spin trapping techniques, and attenuation of free radical generation has been repeatedly shown to result in attenuation of contractile dysfunction. Numerous observations suggest that oxyradicals also contribute to stunning in other settings: after global ischemia in vitro, after global ischemia during cardioplegic arrest in vivo, and after multiple brief episodes of regional ischemia in vivo. Compelling evidence indicates that the critical free radical damage occurs in the initial moments of reflow, so that myocardial stunning can be viewed as a sublethal form of oxyradical-mediated "reperfusion injury." Second, there is also considerable evidence that a transient calcium overload during early reperfusion contributes to postischemic dysfunction in vitro; however, the importance of this mechanism in vivo remains to be defined. Third, inadequate release of calcium by the sarcoplasmic reticulum, with consequent excitation-contraction uncoupling, may occur after multiple brief episodes of regional ischemia, but its role in other forms of postischemic dysfunction has not been explored. It is probable that multiple mechanisms contribute to the pathogenesis of myocardial stunning. The three hypotheses outlined above are not mutually exclusive and in fact may represent different steps of the same pathophysiological cascade. Thus, generation of oxyradicals may cause sarcoplasmic reticulum dysfunction, and both of these processes may lead to calcium overload, which in turn could exacerbate the damage initiated by oxygen species. The concepts discussed in this review should provide not only a conceptual framework for further investigation of the pathophysiology of reversible ischemia-reperfusion injury but also a rationale for developing clinically applicable interventions designed to prevent postischemic ventricular dysfunction.     

Oxidative stress and cardiac disease

Reactive oxygen species (ROS) are formed at an accelerated rate in postischemic myocardium. Cardiac myocytes, endothelial cells, and infiltrating neutrophils contribute to this ROS production. Exposure of these cellular components of the myocardium to exogenous ROS can lead to cellular dysfunction and necrosis. While it remains uncertain whether ROS contribute to the pathogenesis of myocardial infarction, there is strong support for ROS as mediators of the reversible ventricular dysfunction (stunning) that often accompanies reperfusion of the ischemic myocardium. The therapeutic potential of free radical-directed drugs in cardiac disease has not been fully realized.     

Direct evidence that oxygen-derived free radicals contribute to postischemic myocardial dysfunction in the intact dog.

Electron paramagnetic resonance (EPR) spectroscopy was used to investigate whether (i) the free radicals produced in the "stunned" myocardium (myocardium with postischemic contractile dysfunction) are derived from O2, (ii) inhibition of radical reactions improves function, and (iii) i.v. spin traps are effective. Open-chest dogs undergoing a 15-min coronary occlusion received an i.v. infusion of the spin trap, alpha-phenyl N-tert-butylnitrone (PBN) (50 mg/kg). In group I (n = 6), EPR signals characteristic of radical adducts of PBN appeared in the coronary venous blood during ischemia and increased dramatically after reperfusion. In group II (n = 6), which received PBN and i.v. superoxide dismutase (SOD; 16,000 units/kg) plus catalase (12,000 units/kg), myocardial production of PBN adducts was undetectable during ischemia (delta = -100%, P less than 0.01 vs. group I) and markedly inhibited after reperfusion (delta = -86%, P less than 0.001). This effect was seen at all levels of ischemic zone flow but was relatively greater in the low-flow range. In group III (n = 8), the same dosages of SOD and catalase without PBN markedly enhanced contractile recovery (measured as systolic wall thickening) after reperfusion [P less than 0.01 at 3 hr vs. controls (group IV, n = 7)]. Systemic plasma activity of SOD and catalase averaged 127 +/- 24 and 123 +/- 82 units/ml, respectively, 2 min after reperfusion. PBN produced no apparent adverse effects and actually improved postischemic contractile recovery in group I (P less than 0.05 at 3 hr vs. controls). This study shows that (i) SOD and catalase are highly effective in blocking free radical reactions in vivo, (ii) the radicals generated in the "stunned" myocardium are derived from univalent reduction of O2, and (iii) inhibition of radical reactions improves functional recovery. The results provide direct, in vivo evidence to support the hypothesis that reactive oxygen metabolites play a causal role in the myocardial "stunning" seen after brief ischemia.     

Myocardial stunning: an experimental act with a large clinical audience

Myocardial stunning, a prolonged post-ischemic contractile dysfunction following single or multiple brief episodes of myocardial ischemia, was a fortuitous laboratory observation. Several years later it is obvious that myocardial stunning is a ubiquitous clinical finding and occurs whenever ischemia and reperfusion are present. Reperfusion, by restoring oxygen supply, which is beneficial and protective for the ischemic myocardium, induces at the same time a burst of oxygen free radicals complicated with intracellular Ca2+ overload, which is harmful for the contractile proteins. If it is a one-time episode, stunning is a benign condition which usually does not need treatment. If repetitive, it may lead first to chronic stunning and later to hibernating myocardium, characterised by permanent cellular injury and cell death through apoptosis with secondary reduction in myocardial perfusion. Patients with severe left ventricular dysfunction on the basis of hibernating myocardium carry an unfavourable prognosis when not revascularized. Further insight into molecular and genomic adaptation to ischemia and reperfusion will undoubtedly help improve our ability to fight ischemic heart disease.     

Demonstration of secondary free radicals and the role of calpain in functional changes associated with the myocardial ischemia-reperfusion sequence

The aim of this study was to investigate the role of secondary free radicals and calpain, a calcium-activated cysteine protease, in the development of reperfusion injury in the heart. The time course of radical generation was assessed directly by Electron Paramagnetic Resonance (EPR) and spin trapping with N-ter butyl-alpha-phenylnitrone (PBN), in isolated perfused rat heart subjected to 30 minutes of global ischemia and 30 minutes of reperfusion. The effect of leupeptin, a calpain inhibitor, was assessed on postischemic dysfunction. The antioxidant properties of leupeptin were also investigated by using allophycocyanin, a fluorescent protein sensitive to oxidative stress generated by the H2O2 + Cu++ system. Moreover, we measured the capacities of leupeptin to scavenge hydroxyl (.OH) and superoxide (O2-.) radicals using EPR technique. Our results show that myocardial reperfusion is associated with an increase of alkyl, alkoxyl free radicals release; the administration of catalase 5.10(5) UI/L significantly reduces this release, but didn't improve the postischemic contractile function of the heart. In our study leupeptin 50 microM possess, in vitro, antioxidant properties and scavenging abilities against .OH and O2-., in return leupeptin does not influence the cardiac functions during reperfusion period. In conclusion, our results confirm that myocardial reperfusion induces an important production of secondary free radicals associated with contractile dysfunction. The role of calpain in myocardial ischemia-reperfusion injury remains to be clarified 1) by assessing the activities of calpain and calpastain, its main endogenous inhibitor, during these periods, 2) by measuring the ability of leupeptin in inhibiting the calpain dependent proteolysis.     

Postischemic myocardial "stunning". Identification of major differences between the open-chest and the conscious dog and evaluation of the oxygen radical hypothesis in the conscious dog

Recent studies suggest that oxygen-derived free radicals contribute to the pathogenesis of postischemic myocardial dysfunction (myocardial "stunning"). This concept, however, is predicated exclusively on results obtained in open-chest preparations, which are subject to the confounding influence of many unphysiological conditions. The lack of supporting evidence in more physiological animal models represents a major persisting limitation of the oxy-radical hypothesis of myocardial stunning. The goal of this study was to address two fundamental (and related) questions: 1) Does the open-chest animal model alter the phenomenon of myocardial stunning? 2) If so, how valid are the concepts, derived from such a model, regarding the pathogenetic role of oxy-radicals? In part 1 of the study, myocardial stunning after a 15-minute coronary occlusion was compared in 30 pentobarbital-anesthetized open-chest dogs and in 19 conscious dogs. For any given level of collateral flow during occlusion, the recovery of systolic wall thickening after reperfusion was markedly less in open-chest animals. In an additional group of five open-chest dogs, a close inverse relation was noted between body temperature and postischemic wall thickening, indicating that the recovery of the stunned myocardium in acute experiments may vary markedly depending on how temperature is controlled. Because of these major differences between open-chest and conscious dogs, the oxy-radical hypothesis needs to be tested in the latter model. Thus, in part 2 of the study, conscious unsedated dogs undergoing a 15-minute coronary occlusion were randomized to an intravenous infusion of either saline (19 coronary occlusions) or superoxide dismutase (SOD) plus catalase (CAT) (21 coronary occlusions). Despite the fact that the plasma levels of SOD and CAT declined rapidly after reperfusion, postischemic wall thickening was significantly greater in treated compared with control dogs throughout the first 6 hours of reflow. Thus, a brief (60-minute) infusion of SOD and CAT produced a sustained improvement of recovery of contractility. The magnitude of this beneficial effect was a function of the severity of ischemia: the lower the collateral perfusion, the greater the improvement effected by the enzymes. The accelerated recovery produced by SOD and CAT was not followed by any deterioration of contractility, suggesting that postischemic dysfunction is not a teleologically "protective" phenomenon. In conclusion, the severity of myocardial stunning is greatly exaggerated by the unphysiological conditions present in the barbiturate-anesthetized open-chest dog.(ABSTRACT TRUNCATED AT 400 WORDS)     

Deleterious effects of oxygen radicals in ischemia/reperfusion. Resolved and unresolved issues

Oxygen free radicals are known to be generated during periods of ischemia followed by reperfusion. There is still some controversy, however, concerning the use of electron paramagnetic resonance spectroscopy to accurately detect and identify the free radical species that are formed. There is no doubt that oxygen radicals are deleterious to the myocardium; free radicals cause left ventricular dysfunction and structural damage to myocytes and endothelial cells in both in vitro and in vivo preparations. Potential sources of these cytotoxic oxygen species include the xanthine oxidase pathway, activated neutrophils, mitochondria, and arachidonate metabolism, yet the crucial source of free radicals in the setting of ischemia and reperfusion is unresolved. There is little doubt that oxygen radicals play a role in the phenomenon of stunned myocardium induced by brief periods of ischemia followed by reperfusion; numerous studies have consistently observed that pretreatment with free radical scavengers and antioxidants enhances contractile function of stunned, postischemic tissue. Whether oxygen free radical scavengers administered only during reperfusion enhance recovery of stunned myocardium in models of brief ischemia remains to be determined. In models of prolonged ischemia (2 hours) followed by reperfusion, we have not observed a beneficial effect of scavengers on stunned myocardium. The issue of whether oxygen free radical scavengers are capable of reducing so-called irreversible or lethal reperfusion injury remains, in our opinion, unresolved. Although some studies have observed that agents such as superoxide dismutase and catalase reduce infarct size in ischemia and reperfusion models, many others have reported negative results. Additional studies will be needed to resolve this ongoing controversy. Oxygen free radicals may also contribute to reperfusion-induced arrhythmias in rodent heart preparations; however, less data are available in other animal models. The concept of reperfusion injury should not be considered a deterrent to reperfusion for the treatment of acute myocardial infarcts in the clinical setting. Thrombolytic therapy reduces myocardial infarct size, enhances recovery of left ventricular function, and improves survival. Whether incremental beneficial effects on these parameters will be obtained when oxygen radical-scavenging agents are used as adjuvant therapy to thrombolysis in patients remains to be determined.     

The role of oxidants and free radicals in reperfusion injury

While timely reperfusion of acute ischemic myocardium is essential for myocardial salvage, reperfusion results in a unique form of myocardial damage. Functional alterations occur, including depressed contractile function and decreased coronary flow as well as altered vascular reactivity. Both myocardial stunning and infarction are seen. Over the last two decades, it has become increasingly clear that oxidant and oxygen radical formation is greatly increased in the post-ischemic heart and serves as a critical central mechanism of post-ischemic injury. This oxidant formation is generated through a series of interacting pathways in cardiac myocytes and endothelial cells and triggers subsequent leukocyte chemotaxis and inflammation. Nitric oxide (NO) production and NO levels are also greatly increased in ischemic and post-ischemic myocardium, and this occurs through NO synthase (NOS)-dependent NO formation and NOS-independent nitrite reduction. Recently, it has been shown that the pathways of oxygen radical and NO generation interact and can modulate each other. Under conditions of oxidant stress, NOS can switch from NO to oxygen radical generation. Under ischemic conditions, xanthine oxidase can reduce nitrite to generate NO. NO and peroxynitrite can inhibit pathways of oxygen radical generation, and, in turn, oxidants can inhibit NO synthesis from NOS. Ischemic preconditioning markedly decreases NO and oxidant generation, and this appears to be an important mechanism contributing to preconditioning-induced myocardial protection.     

Reperfusion injury and its pharmacologic modification

Reperfusion injury includes a spectrum of events, such as reperfusion arrhythmias, vascular damage and no-reflow, and myocardial functional stunning. The concept of reperfusion injury remains controversial with many proposed mechanisms when applied to humans, whereas in animal models, there are two main proposed mechanisms: calcium over-load and formation of oxygen free radicals. To prove that reperfusion injury is specifically caused by reperfusion would require evidence that an intervention given at the time of reperfusion can diminish or abolish the injury as in the case of arrhythmias, which are thought to be mediated by excess recycling of cytosolic calcium with delayed afterdepolarizations and ventricular automaticity. In the case of myocardial stunning, the phenomenon may be mediated, at least in part, by a burst of free radicals formed within the first minute of reperfusion and improved by free radical scavengers given at the time of reperfusion. The alternate hypothesis is that cytosolic calcium overload damages mechanisms for normal intracellular calcium regulation so that the stunned myocardium responds to agents that are thought to increase intracellular cytosolic calcium, such as beta-receptor agonists. A further component of reperfusion injury, under active investigation, is microvascular damage with alterations at the level of platelets, leukocytes, and endothelial integrity. From the therapeutic point of view, the divergent results of experimental interventions and the possibility that the abrupt onset of reperfusion in animals differs from the situation in humans with thrombolysis means that the best way currently available to limit reperfusion injury is by minimizing the ischemic period by early reperfusion and by optimizing the metabolic status of the ischemic myocardium at the end of the ischemic period.     

Stunning: Damaging or protective to the myocardium?

Summary There are several potential outcomes of myocardial ischemia. When ischemia is severe and prolonged, irreversible damage occurs and there is no recovery of contractile function. Interventions aimed at reducing mechanical activity and oxygen demand, either before ischemia or during reperfusion, have been shown to delay the onset of ischemic damage and to improve recovery on reperfusion.When myocardial ischemia is less severe but still prolonged, myocytes may remain viable but exhibit depressed contractile function. Under these conditions, reperfusion restores complete contractile performance. This type of ischemia, leading to a reversible, chronic left ventricular dysfunction, has been termed hibernating myocardium. Depression of mechanical activity is, actually, a protective mechanism whereby the hibernating cells reduce their oxygen demands in the setting of reduced oxygen supply.A third possible outcome after a short period of myocardial ischemia is a transient postischemic ventricular dysfunction, a situation termed stunned myocardium. As in the case of hibernating myocardium, the depressed contractile function occurring during stunning could be a protective mechanism, allowing the reperfused cells to gradually recover their metabolism and function.     

Oxygen free radicals and cardiac reperfusion abnormalities.

Oxygen free radicals are highly reactive compounds causing peroxidation of lipids and proteins and are thought to play an important role in the pathogenesis of reperfusion abnormalities including myocardial stunning, irreversible injury, and reperfusion arrhythmias. Free radical accumulation has been measured in ischemic and reperfused myocardium directly using techniques such as electron paramagnetic resonance spectroscopy and tissue chemiluminescence and indirectly using biochemical assays of lipid peroxidation products. Potential sources of free radicals during ischemia and reperfusion have been identified in myocytes, vascular endothelium, and leukocytes. In several different experimental models exogenous free radical-generating systems have been shown to produce alterations in cardiac function that resemble the various reperfusion abnormalities described above. Injury to processes involved in regulation of the intracellular Ca2+ concentration may be a common mechanism underlying both free radical-induced and reperfusion abnormalities. Direct effects of free radicals on each of the known Ca(2+)-regulating mechanisms of the cell as well as the contractile proteins and various ionic membrane currents have been described. Free radicals also inhibit critical enzymes in anaerobic and aerobic metabolic pathways, which may limit the metabolic reserve of reperfused myocardium and contribute to intracellular Ca2+ overload. Inhibiting free radical accumulation during myocardial ischemia/reperfusion with free radical scavengers and inhibitors has been demonstrated to reduce the severity of myocardial stunning, irreversible injury, and reperfusion arrhythmias in many, but not all, studies. This evidence strongly implicates free radical accumulation during myocardial ischemia/reperfusion as an important pathophysiological mechanism of reperfusion abnormalities, although many issues remain unresolved.     

Detection of oxygen-derived free radical generation in the canine postischemic heart during late phase of reperfusion

To define the relation between oxygen-derived free radical (oxy-radical) generation in the reperfused ischemic myocardium and the progression of myocardial damage, we measured oxy-radical generation in the ischemic myocardium and the propagating infarct size in a model of canine coronary occlusion (90 minutes) and reperfusion. We used electron paramagnetic resonance spin-trapping techniques (5,5-dimethyl-1-pyrroline N-oxide [DMPO]) to detect oxy-radicals in the rapidly frozen myocardial samples taken by needle biopsy. There was no detectable generation of DMPO adducts in the normal myocardium before or after reperfusion. In the reperfused ischemic myocardium, electron paramagnetic resonance signals of DMPO-OOH (superoxide anion) and DMPO-OH (hydroxyl radical) were detected, with peak concentrations at 1 hour after reperfusion for DMPO-OOH and at 3 hours after reperfusion for DMPO-OH, respectively. These DMPO adducts were also detected during the early phase (15 seconds) of reperfusion, but the concentrations of these signals were much less than those during the late phase of reperfusion. Treatment with human recombinant superoxide dismutase (2.5 mg/kg/hr) and catalase (2.5 mg/kg/hr) during the course of experiments abolished DMPO-OOH formation but had little effect on DMPO-OH formation. Infarct size (percent of risk area infarcted), quantified by a dual staining method with Evans blue dye and triphenyltetrazolium chloride, was 18.3 +/- 4.8% (mean +/- SEM) at 90 minutes of occlusion. After 5 hours of reperfusion, infarct size increased to 43.6 +/- 7.2%. These results indicate that a greater magnitude of oxy-radical generation was sustained in the ischemic myocardial tissue during the late phase (1-3 hours) of reperfusion, associated with the progression of myocardial infarction. The concurrent appearance of oxy-radicals and progressive infarction may support the view that a chain reaction of oxy-radicals contributes to the propagation of myocardial cell damage in the postischemic heart.     

Sensitization of reperfused myocardium to subsequent coronary flow reductions. An extension of the concept of myocardial stunning

The purpose of the present study was to evaluate the response of briefly ischemic and reperfused myocardium to subsequent moderate reductions of coronary arterial flow. In mongrel dogs, a carotid to left anterior descending coronary shunt was constricted to produce moderate coronary flow reductions (50-60% of control) and to thereby reduce regional systolic thickening (measured by echocardiography or sonomicrometry). First, we demonstrated an abnormal response of reperfused myocardium to subsequent flow reductions. We performed two episodes of coronary shunt stenosis, with an intervening 5-minute complete coronary shunt occlusion followed by 30 minutes of reperfusion. In a control group, the same two shunt stenoses were done, but no intervening shunt occlusion was performed. In the control dogs, repeated coronary shunt stenosis that produced equivalent perfusion reductions also produced equivalent declines in regional wall thickening. In contrast, in the intervention group (animals undergoing the intervening occlusion-reperfusion sequence between two shunt stenoses), the second coronary shunt stenosis produced an exaggerated decline in regional systolic thickening, even though the decline in myocardial perfusion was similar to the first stenosis. Second, we sought to demonstrate the mechanism of the exaggerated decline of the reperfused myocardium to subsequent moderate flow reductions. Again, two groups of animals were studied. Each group underwent two episodes of coronary shunt stenosis with an intervening sequence of 5 minutes of complete shunt occlusion and 30 minutes of reperfusion. In addition, one of the groups received an infusion of the oxygen free radical scavengers superoxide dismutase and catalase during the occlusion-reperfusion sequence. In the superoxide dismutase and catalase-treated animals, the decline in regional systolic function during the postreperfusion shunt stenosis was similar to the preocclusion stenosis. Thus, oxygen free radical scavengers blocked the exaggerated contraction decline in response to the postreperfusion flow reduction. We conclude that briefly ischemic and reperfused myocardium displays an exaggerated response to subsequent coronary arterial flow reductions and that this response is a subtle manifestation of postischemic ventricular dyskinesis, or "stunning." The mechanism is probably oxygen free radical toxicity.     

Reversible myocardial dysfunction: basics and evaluation

Large areas of non-functional but viable myocardium with reversible dysfunction are commonly seen in patients with acute myocardial infarction. Both reperfusion of acutely ischemic myocardium and chronic myocardial ischemia may produce a reversible forms of ventricular dysfunction. The two main conditions that lead to reversible myocardial dysfunction are stunned myocardium and hibernating myocardium. Myocardial stunning represents post-ischemic myocardial dysfunction that persists despite restoration of normal flow, with gradual return of contractile function. Hibernating myocardium is a state of persistently impaired myocardial function at rest due to reduced coronary blood flow owing to residual stenosis that can be restored toward normal by revascularization. The success of the revascularization procedures depends on the presence of amount of dysfunctional but viable myocardium. The basics and evaluation of reversible myocardial dysfunction are reviewed.     

Attenuation of dysfunction in the postischemic 'stunned' myocardium by dimethylthiourea

The mechanism for the prolonged contractile dysfunction observed in myocardium reperfused after reversible regional ischemia ("stunned" myocardium) is unclear. Recent studies suggest that myocardial stunning may be mediated by oxygen-derived free radicals, but the precise molecular species involved remain unknown. Thus we explored the role of the highly cytotoxic hydroxyl radical in regional postischemic dysfunction by using dimethylthiourea (DMTU), an effective and highly permeable hydroxyl radical scavenger. Open-chest dogs undergoing a 15 min occlusion of the left anterior descending coronary artery followed by 4 hr of reperfusion received either DMTU (0.5 g/kg iv over 45 min starting 30 min before occlusion, n = 14) or saline (n = 15). Control and treated dogs were comparable with respect to variables that may affect postischemic dysfunction, including heart rate, aortic pressure, left atrial pressure, arterial blood gases and hemoglobin concentration, size of the occluded bed (determined by postmortem perfusion), and collateral blood flow (determined by radioactive microspheres). Regional myocardial function was assessed by measuring wall thickening with an epicardial Doppler probe. The two groups exhibited comparable systolic thickening under baseline conditions and similar degrees of dyskinesis during ischemia. After reperfusion, however, wall thickening (expressed as percent of baseline) was considerably greater in treated as compared with control dogs: 53 +/- 9% (mean +/- SEM) vs 9 +/- 14% (p less than .03) at 1 hr, 55 +/- 9% vs 23 +/- 13% (p less than .05) at 2 hr, 60 +/- 9% vs 28 +/- 14% (p less than .05) at 3 hr, and 67 +/- 5% vs 36 +/- 13% (p less than .05) at 4 hr. Thus DMTU produced a significant and sustained improvement in recovery of contractile function. In concentrations greater than the plasma levels attained in vivo, DMTU did not scavenge either hydrogen peroxide or superoxide anion in vitro. These results suggest that the myocardial dysfunction occurring after a brief episode of regional ischemia is mediated in part by the hydroxyl radical.     

Pathobiology and Clinical Impact of Reperfusion Injury

Reperfusion injury refers to cellular death or dysfunction caused by restoration of blood flow to previously alchemic tissue. This should be differentiated from the normal reparative processes that follow an ischemic insult. Four types of reperfusion injury have been described in the literature: (1) lethal reperfusion injury, (2) nonlethal reperfusion injury, (myocardial stunning), (3) reperfusion arrhythmias, and (4) vascular injury (including the "no-reflow" phenomenon). There is continued debate whether reperfusion itself is capable of killing viable myocytes, which otherwise would have survived the ischemic insult. However, there is firm evidence for the existence of myocardial stunning following various ischemic syndromes, including reperfusion therapy for acute myocardial infarction, unstable angina pectoris, vasospastic angina, effort-induced ischemia, coronary artery bypass surgery, and cardiac transplantation. Reperfusion arrhythmia is more common after short ischemic episodes than after long ischemic periods. Thus, while reperfusion arrhythmias in the setting of acute myocardial infarction are relatively rare, reperfusion arrhythmias may be an important cause of sudden death. The "no-reflow" phenomenon has been described following reperfusion in patients with acute myocardial infarction. Three major components have been proposed as mediators of reperfusion injury: (1) oxygen free radicals, (2) the complement system, and (3) neutrophils. Numerous experimental studies have shown short-term benefit by blocking various stages of the postischemic inflammatory response. Oxygen free radicals scavengers, complement inhibition, leukocyte depletion, and the use of antibodies against various adhesion molecules have shown a reduction of infarct size in many ischemic/reperfusion experimental models. However, many of these agents failed to show a benefit in the clinical setting. Moreover, the long-term benefit of such intervention is still unknown.     

Reperfusion Injury: Basic Concepts and Protection Strategies

Prolonged ischemia such as that following myocardial infarction or occurring during long-term coronary bypass procedures causes serious damage to the myocardium. Early reperfusion is an absolute prerequisite for the survival of ischemic tissue. However, reperfusion has been referred to as the double edged sword because reperfusing ischemic myocardium carries with it a component of injury known as reperfusion injury. Reperfusion injury includes a number of events, such as reperfusion arrhythmias, myocardial infarction, stunning, vascular damage, and endothelial dysfunction. The underlying mechanism of reperfusion injury is not entirely known, but the existing evidence suggests that oxygen free radicals generated during the first few minutes of reflow lead to damage of cellular membranes, intracellular calcium overload, and uncoupling of excitation-contraction coupling. Although controversial, free radical scavengers, in general, are highly effective in the attenuation of reperfusion injury in animal models. Newer endogenous protection strategies, which include ischemic and heat shock preconditioning, are known to reduce reperfusion injury following ischemia.     

降钙素基因相关肽对顿抑心肌心功能的影响

建立大鼠心肌顿抑模型,观察降钙素基因相关肽（ＣＧＲＰ）对顿抑心肌心功能的影响。发现再灌注前给予ＣＧＲＰ,可显著减轻心肌顿抑的程度,加快缺血后心功能障碍的恢复。说明ＣＧＲＰ可部分预防心肌顿抑的发生     

Measurement of endothelial cell free radical generation: evidence for a central mechanism of free radical injury in postischemic tissues.

Oxygen free radicals have been demonstrated to be important mediators of postischemic reperfusion injury in a broad variety of tissues; however, the cellular source of free radical generation is still unknown. In this study, electron paramagnetic resonance measurements with the spin trap 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO) demonstrate that bovine endothelial cells subjected to anoxia and reoxygenation become potent generators of superoxide and hydroxyl free radicals. A prominent DMPO-OH signal aN = aH = 14.9 G is observed on reoxygenation after 45 min of anoxic incubation. Quantitative measurements of this free radical generation and the time course of radical generation are performed. Both superoxide dismutase and catalase totally abolish this radical signal, suggesting that O2 is sequentially reduced from O2-. to H2O2 to OH.. Addition of ethanol resulted in trapping of the ethoxy radical, further confirming the generation of OH.. Endothelial radical generation was shown to cause cell death, as evidenced by trypan blue uptake. Radical generation was partially inhibited and partially scavenged by the xanthine oxidase inhibitor allopurinol. Marked inhibition of radical generation was observed with the potent xanthine oxidase inhibitor oxypurinol. These studies demonstrate that endothelial cells subjected to anoxia and reoxygenation, conditions observed in ischemic and reperfused tissues, generate a burst of superoxide-derived hydroxyl free radicals that in turn cause cell injury and cell death. Most of this free radical generation appears to be from the enzyme xanthine oxidase. Thus, endothelial cell free radical generation may be a central mechanism of cellular injury in postischemic tissues.     

Ischemic preconditioning reduces infarct size in swine myocardium

We evaluated the hypothesis that stunning swine myocardium with brief ischemia reduces oxygen demand in the stunned region and increases tolerance of myocardium to longer periods of ischemia. Wall function was quantified with ultrasonic crystals aligned to measure wall thickening, and stunning was achieved with two cycles of left anterior descending coronary artery (LAD) occlusion (10 minutes) and reperfusion (30 minutes), after which the LAD was occluded for 60 minutes and reperfused for 90 minutes. Infarct size (as a percent of risk region) was then determined by incubating myocardium with para-nitro blue tetrazolium. Regional oxygen demand was measured as myocardial oxygen consumption before the 60-minute LAD occlusion in the stunned region; tracer microspheres were used to determine blood flow, and blood from the anterior interventricular vein and left atrium was used to calculate oxygen saturations. After the second reperfusion period, wall thickening in the stunned region was reduced to 1.4 +/- 2.4% compared with 36.7 +/- 2.5% (mean +/- SEM) before ischemia (p less than 0.001). Regional myocardial oxygen consumption after stunning (3.1 +/- 0.7 ml O2/min/100 g) was no different from regional myocardial oxygen consumption before stunning (3.7 +/- 0.6 ml O2/min/100 g). In the nine pigs "preconditioned" by stunning, infarct size was 10.4 +/- 6.3% of the risk region compared with 48.0 +/- 12.7% in the six control pigs subjected to 60 minutes of ischemia without prior stunning (p less than 0.005). The risk regions were similar (14.4 +/- 1.5% vs. 14.6 +/- 1.9% of the left ventricle, preconditioned vs. control pigs, respectively). We conclude that stunning swine myocardium with two cycles of a 10-minute LAD occlusion followed by reperfusion increases ischemic tolerance but that changes in regional demand in stunned myocardium do not predict the marked reduction in infarct size that follows a subsequent 60-minute period of ischemia.