Metabolic disturbances during myocardial ischemia and reperfusion

Metabolic Dikturbances During Myocardial lschemk and Reperfusion

Roberto Ferrari, MD, PhD

Myocardial ischemia is defined as an imbalance between fractional uptake of oxygen and the rate of cellular oxidation in the heart. This condition may have seveml potential outcomes: (1) when ischemia is brief, a transient post-ischemic ventricular dysfunction occurs on reperfusion, a condition termed “stunned myocardium”; (2) when it is prolonged and severe, irreversible damage occurs, with no recovery in contmctile function upon reperfusion; (3) when ischemia is less severe, but still prolonged, the myocytes may remain viable but exhibit depressed contmctile function. Under this condition, named “hibernating myocardium,” the reperfusion is able to restore contractility. During these different

ischemic conditions many biochemical changes hap- pen; initially they represent a defensive and protective reaction against ischemic insults such as cellular acidosis and increase of inorganic phosphate levels that rapidly abolish the contmctile activity. But with the prolongation of &hernia or restomtion of the coronary flow, alterations in ions and ovemll Ca2+ homeostasis occur, together with an oxidative stress mediated by oxygen free mdicals, which are not adequately countemcted by the cellular antioxidant defenses. All these biochemical alterations lead to membrane damage, mitochondrial swelling, and irreversible deterioration of contmctile function.

(Am J Cardioll995; 76:17B-24B)

T he term “myocardial &hernia” describes a condition that exists when fractional uptake of oxygen in the heart is insufficient to

maintain the rate of cellular oxidation of that moment. This leads to extremely complex intracellular changes, which have been extensively studied in recent years.1-3

Clinically, there are several potential manifesta- tions and outcomes associated with ischemia and reperfusion.4,5 Ventricular dysfunction (either systolic or diastolic) of the ischemic zone is the most reliable clinical sign of ischemia, since electrocar- diographic changes and symptoms are often absent and biochemical abnormalities are difficult to de- tect. At least initially, ventricular dysfunction is reversible on reperfusion, although recovery of contraction may occur instantaneously or, more frequently, after a considerable delay, yielding the condition recently recognized as “stunned” myocardium.4,6

On the other hand, when ischemia is severe and prolonged, cell damage may occur. Reperfusion at this stage is associated with the release of intracellular enzymes, disruption of cell membranes, influx of calcium, persistent reduction of contractility, and eventual necrosis of at least a portion of the tissue.7 This entity has been called reperfusion

From the Cattedra di Cardiologia, Universita’ degli Studi di Brescia, and Centro di Fisiopatologia Cardiovascolare “S. Maugeri,” Fonda- zione Clinica del Lavoro, Gussago, Brescia, Italy.

Address for reprints: Roberto Ferrari, MD, Cattedra di Cardiolo- gia, Universita’ degli Studi di Brescia, c;o Spedali Civili, P.le Spedali Civili, 1, 25123 Brescia, Italy.

damage by those who believe that much of the injury is the consequence of events occurring at the moment of reperfusion rather than a result of changes occurring during the period of ischemia. However, the existence of reperfusion damage has been questioned, and it has been argued that, with the exception of induction of arrhythmias,8 it is difficult to be certain that reperfusion causes further injury.2 The existence of such an entity has clinical relevance because it would imply the possi- bility of improving recovery with specific interventions applied at the time of reperfusion.

At the moment, there is no simple answer to the question of what determines cell death and no recovery on reperfusion. Problems arise because: (1) ischemic damage is not homogeneous and many factors may combine to cause severe cell damage; (2) severity of biochemical changes and development of necrosis are usually associated (both processes being dependent on the duration of ischemia) and it is impossible to establish a causal relationship; and (3) the inevitability of necrosis can only be assessed by reperfusion of the ischemic myocardium. Restoration of flow might, however, result in numerous further negative consequences, thus directly influencing the degree of recovery.9

Two major lines of research have emerged from these studies, dealing with the role of mitochondria and calcium and with that of oxygen free radicals and oxidative damage in causing lack of recovery during reperfusion after prolonged ischemia.

In 1985, Rahimtoolal” described another possible outcome of myocardial ischemia.5 He demon-

A SYMPOSIUM: MANAGEMENT OF MYOCARDIAL ISCHEMIA 178

strated that late reperfusion (after months or even years) of an ischemic area showing ventricular wall-motion abnormalities might restore normal metabolism and function. He was the first to introduce the term “hibernating myocardium,” referring to ischemic myocardium in which the myocytes remain viable but in which contraction is chronically depressed.

In the present article, we review some of our data on metabolic changes occurring during ischemia followed by reperfusion obtained either experimentally or in patients with coronary artery disease undergoing intracoronary thrombolysis or aortocoronary bypass grafting.

METABOLISM DURING EARLY PHASES OF ISCHEMIA

Within seconds after the onset of ischemia, intracellular acidosis develops as a result of the shortage of oxygen at the mitochondrial level. With a variable degree of delay, a breakdown of high- energy phosphates takes place, creatine phosphate declining earlier than adenosine triphosphate (ATP). This results in a rapid increase in the intracellular concentration of inorganic phosphate. Both a decline of intracellular pH and an increase in inorganic phosphate are the likely biochemical factors that cause down-regulation of contraction of the ischemic zone, which reaches quiescence within seconds or minutes after the onset of ischemia.7J1 Induction of ischemic quiescence can be considered a protective mechanism, as it allows a drastic reduction of mitochondrial oxidation and, in turn, of the oxygen need of the myocytes. However, a decline of intracellular pH has been proven to be protective up to a certain level, which, in isolated preparation, is estimated to be around pH 6.9-6.7. Acidosis below this level has deleteri- ous effects.‘*

Another biochemical mechanism of protection of the ischemic myocyte is the development of anaerobic metabolism, leading to lactate production and release. This metabolic pathway allows reduction of cytosolic acidosis, continuation of anaerobic metabolism, and production of a small amount of ATP without oxygen utilization. Lactate production occurs, however, only in the early phase of ischemia, as it is under the control of cytosolic enzymes that, in turn, are inhibited by acidosis.

Thus, during the early phases of ischemia, several metabolic changes occur within the myocyte, which allows a drastic reduction of the oxygen demand at the expense of active contraction, and production, although limited, of ATP, which can

be important for maintenance of intracellular integ- rity.

From the clinical point of view, this condition corresponds to transient ischemia occurring, for example, during an attack of angina when acidosis, lactate production, and ventricular dysfunction can be clearly demonstrated. Usually, the ventricular dysfunction is reversible if the condition causing ischemia (such as an increase in heart rate, contractility, afterload, or reduced oxygen availability) ceases or if reperfusion occurs, as in the case of a release of coronary artery spasm. It may, however, persist over hours or even days in patients after an episode of ischemia followed by reperfusion, leading to the so-called condition of stunning, which is characterized by specific metabolic alterations.

METABOLISM DURING MYOCARDIAL STUNNING

Stunning is defined as a transient left ventricular dysfunction that persists after reperfusion, despite the absence of irreversible damage and the restoration of normal or near-normal coronary flow. This definition implies that: (1) stunning is a transient, fully reversible abnormality provided that sufficient time is allowed for recovery; (2) stunning is a mild, sublethal injury that must be kept apart from the irreversible damage occurring in myocardial infarction; and (3) stunned myocardium has a normal or near-normal coronary flow.

Thus, hallmark of this condition is the presence of a flow-function “mismatch,” with normal flow but abnormal function. This is in contrast to the other forms of reversible myocardial dysfunction, such as ischemia and hibernation, in which depressed flow and function are matched.

A clear definition is certainly desirable and important, but its application to the clinical condition is often problematic. Diagnosis of stunning in patients would require the demonstration of 2 major points: first, that the contractile abnormality is reversible with time and, second, that the dysfunc- tional myocardium has normal or near-normal coronary flow. This implies that it is possible to measure accurately regional myocardial function and blood flow in humans. The resolution of the available techniques (contrast ventriculography, radionuclide angiography, 2-dimensional echocar- diography, positron emission tomography) is, however, not comparable to that obtainable with so- nomicrometry and radioactive microspheres in experimental animals.

To fulfill the second point, before making a definitive diagnosis of stunning, the physician should

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allow sufficient time for the myocardium to re- cover. This suggests that myocardial stunning is generally a well-tolerated condition and that its diagnosis is important retrospectively for a better appreciation of the effects of reperfusion. It does not imply a decision-making process for clinical management of the patient, basically because the myocardium is already reperfused. It could be argued that in some high-risk situations, stunning can become dangerous. Therefore, stunning should be recognized and treated with positive inotropic interventions or agents increasing preload or de- creasing afterload.

Other interventions such as antioxidant or calcium antagonists can also be applied to reduce the extent of stunning in experimental settings, but, in general, they should be used before the onset of ischemia. Thus, they prevent stunning from occurring.

This suggests that alteration in calcium homeostasis and free radical formation might be involved in the genesis of stunning, the two mechanisms being not necessarily mutually exclusive.13J4 The ability of oxygen metabolites to depress myocardial function has been demonstrated.15 The results of studies that have measured free radicals in the stunned myocardium provide evidence that these toxic metabolites are produced in excess.13 Fur- ther, antioxidants have been used experimentally to reduce the extent of stunning. Nevertheless, their efficacy is still controversial and it seems that the timing of their administration on reperfusion is very important.*6J7 N-2-Mercaptopropionylglycine and desferrioxamine, when administered immedi- ately before reperfusion, markedly attenuate myocardial stunning. The same agents given 1 minute after reperfusion have little effect.23*24 This very short time window probably explains the controversial data. It should also be considered that after a short period of ischemia the natural defense mechanism of the myocyte against oxygen free radicals is still intact and should buffer the toxic effect of these metabolites. However, oxygen free radicals produced during reperfusion might cause minor subtle changes in sulfhydryl-dependent proteins responsible, for example, for calcium handling.

Oxygen free radicals increase cytosolic calcium levels by damaging the sarcoplasmic reticulum, thereby enhancing calcium release.18 Interestingly, an enhanced calcium transient has been demonstrated to occur in stunning,19 but the final mechanism for the rise in intracellular calcium during ischemia and reperfusion and the role of calcium

overload in models of myocardial stunning remain to be defined.

Calcium antagonists have been used to gain insights into the role of calcium homeostasis in the genesis of stunning. Unfortunately, the results are difficult to interpret.

When used before the onset of ischemia, these compounds reduce stunning.17 Under these conditions, however, they reduce the severity of ischemia, which probably remains the most effective way to reduce postischemic dysfunction. In some experiments, but not exclusively, therapy with calcium antagonists reduces stunning even when given only at the time of reperfusion.20,21 Under these conditions, they might cause favorable modifica- tions of afterload, preload, heart rate, and regional myocardial blood flow, all of which could change the systolic and diastolic properties of the myocardium. Interestingly, Opie** has recently proposed a 2-stage model of the changes in intracellular calcium occurring during stunning, which would explain both why calcium antagonists are effective when given at an early stage of ischemia-reperfusion and why l3 agonists are paradoxically also effective when given later to hearts already stunned. The theory is that cytosolic calcium rises during ischemia, but the oscillation decreases. On reperfusion, as ATP for uptake of calcium by the sarcoplasmic reticulum becomes available, there are excess calcium oscillations that might damage the contractile mechanism.** Thereafter, cytosolic calcium declines and calcium agonists should be beneficial.

METABOLISM DURING LATE PHASES OF ISCHEMIA

Once ischemia has occurred, it might persist for prolonged periods of time and cause cell death. Clinically, this situation corresponds to an acute myocardial infarction, although it is difficult to have a condition of regional no-flow ischemia, as some degree of collateral circulation always develops. However, collateral flow, in most cases, is not able to deliver enough oxygen to support the need for mitochondrial oxidation and thereby ischemia persists at the cellular level.

From the metabolic point of view, prolongation of ischemia results in further decrease in intracellular pH and in a progressive increase in resting pressure and myocardial stiffness. The early increase in lactate is followed by a decline, together with a further decrease in tissue content of ATP and creatine phosphate. This supports the view that after an initial stimulation, anaerobic glycolysis is inhibited by the more severe intracellular


acidosis. At this stage profound ionic changes occur, with a depletion of intracellular K+ and Mg2+ and an increase in Na+ and in cytosolic Ca2+. Interestingly, even after prolonged ischemia, total tissue calcium concentration is unchanged but mitochondrial calcium is increased, indicating an intracellular redistribution of the ion.23 Isolated mitochondrial function is then maintained, since only a slight reduction in the initial rate of ATP production is observed. 23 Despite this, reperfusion does not restore myocardial and mitochondrial function. On the contrary, it produces a further increase of stiffness and no recovery of contractility and of tissue ATP and creatine phosphate concen- trations. During reperfusion, there is a significant and sustained release of lactate, ions, and creatine phosphokinase, massive influx of calcium, and severe mitochondrial damage, suggesting that late reperfusion causes not only a washout of these substances but also an exacerbation of their release.7 These findings indicate that a lesion of the cell membrane has occurred, leading to a breakdown of the permeability barrier to ions such as Ca2+ and Mg2+ as well as to larger molecules, such as creatine phosphokinase, and that mitochondria used the restored oxygen for buffering cytosolic Ca2+ rather than for ATP production. For this reason, mitochondria are supposed to play a cen- tral role in reperfusion damage.

ROLE OF MITOCHONDRIA IN LATE ISCHEMIA-REPERFUSION DAMAGE

In the heart, mitochondria exert 2 roles essen- tial for cell survival: ATP synthesis and maintenance of Ca2+ homeostasis. These 2 processes are driven by the same energy source: the H+ electro- chemical gradient (AkH), which is generated by electron transport along the inner mitochondrial membrane.24 Under aerobic physiologic conditions, mitochondria do not contribute to the beat- to-beat regulation of cytosolic Ca2+, although a Ca2+ transient in the mitochondrial matrix has been described. A micromolar increase of mitochondrial Ca2+ concentration stimulates the Krebs cycle and reduced nicotinamide-adenine dinucleotide (NADH) redox potential and, therefore, ATP synthesis.

Under pathologic conditions, however, mitochondrial Ca2+ transport and overload might cause a series of vicious cycles leading to irreversible cell damage. 23 Mitochondrial Ca2+ accumulation causes profound alterations in the permeability of the inner membrane to solutes, leading to severe mitochondrial swelling. 25 In addition, Ca2+ trans-

port takes precedence over ATP synthesis and inhibits utilization of AkH for energy production.26 These processes are important for understanding the sequence of molecular events occurring during myocardial reperfusion following prolonged ischemia that lead to irreversible cell damage.

During ischemia, an alteration of intracellular Ca2+ homeostasis occurs, and mitochondria are able to buffer cytosolic Ca2+, suggesting that they retain the Ca2+ transporting capacity. Accordingly, once isolated, even after prolonged ischemia, the majority of the mitochondria are able to use oxygen for ATP phosphorylation. When isolated after reperfusion, mitochondria are structurally altered, contain large quantities of Ca2+, produce excess of oxygen free radicals, and exhibit less superoxide dismutase activity; their membrane pores are stimulated and the oxidative phosphorylation capacity is irreversibly disrupted. Most likely, reperfusion pro- vides oxygen to reactivate mitochondrial respiration but also causes a large production of oxygen free radicals (as the electron transport chain is in a reduced state) and a large influx of Ca2+ in the cytosol as a result of sarcolemmal damage. Mito- chondrial Ca2+ transport is, therefore, stimulated at maximal rates and, consequently, the equilib- rium between ATP synthesis and Ca2+ influx is shifted toward Ca2+ influx, with a loss of capacity for ATP production, thus giving rise to a vicious cycle.27

The next question relates to the cause of the early sarcolemma damage that occurs during reperfusion following ischemia under these conditions, when molecular oxygen is reintroduced to the ischemic myocardium. One of the most fashionable ideas regarding the cause of membrane damage with reperfusion is that oxygen is converted to oxygen free radicals, which are toxic and can promote further injury. This concept supports a general role of oxygen free radicals in membrane damage occurring during reperfusion.

ROLE OF OXYGEN FREE RADICALS A free radical is any compound that has 2 1

unpaired electron. Molecular oxygen possesses 2 unpaired electrons in the external orbit. If a single electron is accepted by the ground state, the product is a superoxide anion. In the myocardium, the reduction of oxygen to water proceeds by 2 pathways: (1) by cytochrome oxidase, which reduces 95% of oxygen to water by tetravalent reduction without the production of intermediates; and (2) by a series of defense mechanisms that include the enzymes superoxide dismutase, cata-

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lase, and glutathione peroxidase, plus other endog- enous antioxidants such as vitamin E, ascorbic acid, and cysteine. 28 Thus, the aerobic myocardium is able to handle and survive the continuous physiologic oxygen free radical production because of the existence of a delicate balance between cellular systems that generate the various oxidants and those that maintain the antioxidant defense mechanisms.

During ischemia, oxygen is slightly or entirely unavailable. This does not mean that oxygen free radicals cannot be formed. On the contrary, the metabolic alterations that occur during ischemia facilitate the formation of free radicals from residual molecular oxygen.’ Obviously, on reperfusion, the restored availability of oxygen greatly enhances the formation of these toxic metabolites. There are several possible sources of oxygen free radicals during ischemia and reperfusion, perhaps the most important being the mitochondria, whose electron transport chain is reduced during ischemia, allowing an increase of electron leakage that reacts with residual molecular oxygen.1,23~28

In the capillary endothelial cell, the enzyme xanthine dehydrogenase is converted during ischemia to the oxidase form and catalyzes the conver- sion of hypoxanthine and xanthine to uric acid, using oxygen as an electron acceptor. On reperfusion, the delivered oxygen can be reduced by this system, producing oxygen free radicals.

Neutrophils, when activated, generate several types of free radicals that are relevant to the defense against bacterial infection and inflamma- tory reactions, such as acute myocardial infarction. Thus, neutrophils recruited to the infarct site may damage the myocardium, producing oxygen free radicals.

Free radicals may also be generated within membranes in association with the arachidonic acid cascade and with auto-oxidation of catechol- amines. During ischemia, activation of phospholi- pases increases the release of arachidonate.

It has been demonstrated that prolonged ischemia causes alterations at the level of mitochondrial superoxide dismutase, its activity being reduced by 50%. 29 Under these conditions, readmission of molecular oxygen stimulates the production of oxygen free radicals above the neu- tralizing capacity of mitochondrial superoxide dismutase. Consequently, the entire glutathione system, which represents the second line of defense against oxygen toxicity, becomes highly stimulated. As a result, oxidative stress occurs, which is evident by excess production of oxidized glutathione with

consequent membrane damage and, possibly, lipo- peroxidation.

Clinically, these events are expected to occur during reperfusion after prolonged ischemia, and thus in the course of thrombolysis and of coronary artery bypass surgery.

Evidence of the occurrence of oxidative stress in humans is still very poor. There are 3 main reasons for this: (1) difficulties in following the molecular changes occurring during the early phases of reperfusion; (2) the impossibility of standardizing the onset, severity, and duration of ischemia and reperfusion; and (3) the lack of reliable indices capable of detecting the occurrence of an oxidative stress in humans.

We attempted to resolve this problem by measur- ing the arterial and coronary sinus difference of reduced and oxidized glutathione of patients with coronary artery disease subjected to different periods of global ischemia followed by reperfusion during coronary artery bypass grafting or in patients subjected to intracoronary thrombolysis.*,31-33

The results obtained after thrombolysis suggest that the likelihood of metabolic and functional recovery after reperfusion depends on the rapidity of recanalization. Early thrombolysis (within 1 hour) restored aerobic metabolism and myocardial contractility.32 There was no oxidative stress or membrane damage (estimated in terms of myocardial arteriovenous difference for creatine phosphokinase). When the ischemic period was more prolonged (3-4 hours), reperfusion led to oxidative stress, massive release of creatine phosphokinase from the myocardium, no recovery of aerobic metabolism, and a worsening of the regional left ventricular dysfunction.31*32

In the course of open heart surgery, reperfusion of patients after a short period of ischemia (<30 minutes) resulted in a small and transient release in the coronary sinus of oxidized glutathione and in a progressive improvement of hemodynamic param- eters reaching a stable state 4 hours after the surgery. In patients with a period of ischemia > 30 minutes, reperfusion induced a marked and sustained release of oxidized glutathione; the arteriocoronary sinus difference of oxidized glutathione was still negative after the end of cardiopulmonary bypass, and the rate of functional recovery was significantly delayed, reaching the values of the early reperfused patients only 12 hours after surgery. In addition, in these patients there was a negative correlation between the arteriocoronary sinus difference for oxidized glutathione and cardiac index measured 6 hours after surgery. These

A SYMPOSIUM: MANAGEMENT OF MYOCARDIAL ISCHEMIA 21B

data suggest that, depending on the severity of the ischemic period, oxidative stress occurs during reperfusion in patients with coronary artery disease who are subjected to heart surgery and that it may be linked to a delay in postoperative recovery of cardiac function.33

Thus, it seems that there is enough experimental and clinical evidence to support a role for oxygen free radicals in membrane damage occurring on reperfusion after prolonged ischemia. This will result in a massive entry of calcium into the cell, which further impairs the recovery of normal aerobic metabolism.

METABOLISM DURING HIBERNATION Recently, it has been recognized that prolonged

ischemia does not necessarily mean a progression of cellular damage and necrosis. On the contrary, it is possible that after acute ischemia, a residual coronary flow develops that is, in turn, able to maintain intracellular viability without restoring a normal ventricular function. This condition has been named “hibernating myocardium.”

From the clinical point of view, hibernation presents itself as chronic left ventricular regional dysfunction. This condition arises from prolonged myocardial hypoperfusion, in which myocytes remain viable but contraction is depressed. The dysfunction can be partially or completely restored to normal if the myocardial oxygen supply is favorably altered by improving blood flow with reperfusion or by reducing demand. On cardiac imaging, the dysfunction is reflected in an area of left ventricular wall that may be hypokinetic, akinetic, or dyskinetic.

This definition implies that hibernation is a chronic, reversible abnormality provided that coronary flow is restored (in the case of ischemic heart disease) or oxygen demand is reduced (in the case of chronic left ventricular overload); that it represents a viable myocardium, showing residual contractile and coronary flow reserve, which must be distinguished from the irreversibly damaged tissue present after myocardial infarction; and that there is a moderately reduced coronary flow. Thus, the hallmark of this condition is a “matching” reduction in both flow and function.

The diagnosis of hibernation is clinically relevant because it has therapeutic implications, par- ticularly in patients with coronary artery disease. In such patients, differentiation of viable from nonviable myocardium is important, since regional and global left ventricular function due to hibernation

will improve after revascularization. This is associated with improved survival.34

The molecular factors responsible for hibernation and for the chronically depressed contractile function have not yet been defined. The hypoth- esized mechanism of a down-regulated contractile performance matching the reduced energy supply is, in part, supported by experimental and clinical studies using positron emission tomography. As in acute ischemia, during prolonged underperfusion, metabolism is shifted predominantly to glucose, with recruitment of glycolysis. In humans, a mismatch of flow and glucose metabolism predicts recovery of mechanical function after revascularization. There are few data to support or disprove this theory of a perfect balance between reduced oxygen supply and contractile function, so that myocardial injury is prevented. 35 This is because there are neither adequate animal models for chronic persistent ischemia nor clinical studies showing whether hibernating myocardium is truly a chronic condition.

If this mechanism is the only one responsible for hibernation, this condition should occur in every patient with a reduction of coronary flow. Obvi- ously, this is not the case. In addition, in clinical practice, hibernation is often present in patients with a history of acute ischemia, such as infarction or prolonged angina pain. This complicates not only the distinction between viable and nonviable myocardium, but also the understanding of the pathophysiologic mechanism underlying hibernation.

Different hypotheses have been suggested for the factor responsible for down-regulation of contraction during hibernation. An early theory suggests that decrement in coronary perfusion pressure reduces sarcomere length because of distension in the adjacent coronary microvasculature. There- fore, the extent of the contraction would be reduced by the Frank Starling mechanism. Several lines of evidence argue against this suggestion. Another theory considers that a reduction of energy stores might cause down-regulation of contraction. 31P nuclear magnetic resonance studies in living animals suggest, however, that net ATP and creatine phosphate stores are not depleted unless coronary flow is reduced to very low levels. Fur- ther, it has been shown that in hibernating myocardium ATP levels are norma1.36

We suggest that changes in intracellular pH, inorganic phosphate, and myocardial nicotinamide- adenine dinucleotide (NAD)/NADH ratio are responsible for contractile and possibly metabolic

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down-regulation in short-term hibernation in the experimental setting. 37,38 To mimic the clinical events, we used a model of short acute &hernia followed by prolonged severe low-flow reperfusion (10% of initial coronary flow). Acute ischemia caused a drop of pH, an increase of inorganic phosphate, a reduction of NAD/NADH ratio, a 50% reduction in tissue content of ATP, and complete mechanical quiescence. Low-flow reperfusion was unable to restore normal pH, inorganic phosphate, NAD/NADH ratio, and contraction. On the contrary, it resulted in a prolonged down- regulation of myocardial contractility in the presence of aerobic metabolism, as demonstrated by the absence of lactate and creatine phosphokinase release, maintenance of mitochondrial function, and full recovery of tissue stores of ATP and creatine phosphate. Reperfusion after as much as 4-6 hours of ischemia caused a near-complete recovery of mechanical function. Most likely, the factors controlling contractile performance would then also be the factors regulating metabolic respiration. The primary regulators of respiration (par- tial oxygen tension, pH, inorganic phosphate, cytosolic adenine nucleotides, and oxidized NAD) may all be involved and their relative importance in regulating contraction and respiration is still a matter of discussion.

Doubts exist whether this new metabolic state (hibernation) should be considered a true ischemic condition. In strict terms, ischemia is a condition that exists when fractional uptake of oxygen is not sufficient to meet the rate of mitochondrial oxidation, which, in the heart, is largely determined by the mechanical or physical activity of the myocytes.39 It is likely that in hibernating myocardium, the residual flow is able to deliver enough oxygen to meet the reduced rate of mitochondrial oxidation. This concept will explain why the hibernating myocardium does not produce lactate (a typical marker of ischemia) and shows indirect signs of metabolic activity. It also explains the full recovery after reperfusion and the retention of a contractile reserve. Thus, hibernation is not a state of chronic ischemia; rather it represents a new metabolic state that is consequent to an ischemic condition and is not actually ischemic. In strictly molecular terms, hibernation represents a chronic hypoperfusion of akinetic but aerobic myocytes.

A difficult concept to conceive in clinical terms, the question is how these unknown mechanisms, either discussed herein or elsewhere, could provide an exact balance between energy demand and supply for months or years. This would probably

require subcellular adaptative changes to occur, which are at present under investigation using a molecular biologic approach to this fascinating problem.

CONCLUSION There are several potential outcomes of myocar-

dial 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 during 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.”

It is important clinically to recognize hibernation, as reperfusion of hibernating myocardium by angioplasty or heart surgery restores contraction and this correlates with long-term survival.

A third possible outcome after a short period of myocardial ischemia is a transient postischemic ventricular dysfunction, a situation termed “stunned myocardium.”

Acknowledgments: This work was supported by the National Research Council (C.N.R.)- targeted project “Prevention and Control of Dis- ease Factors,” no. 93.00656, PF 41/115.19070. I thank Roberta Bonetti for secretarial assistance in preparing the manuscript and Dr. Bill Dotson Smith for his critical reading of the manuscript.

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248 THE AMERICAN JOURNAL OF CARDIOLOGY VOLUME 76 AUGUST 24, 1995

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Metabolic disturbances during myocardial ischemia and reperfusion