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Mentzer RM Jr, Jahania MS, Lasley RD. Myocardial Protection.
In: Cohn LH, Edmunds LH Jr, eds. Cardiac Surgery in the Adult. New York: McGraw-Hill, 2003:413438.

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Chapter 14

Myocardial Protection

Robert M. Mentzer, Jr./ M. Salik Jahania/ Robert D. Lasley

HISTORY
ISCHEMIA/REPERFUSION INJURY
NONCARDIOPLEGIC TECHNIQUES
????Intermittent Cross-clamping with Fibrillation
SYSTEMIC HYPOTHERMIA AND ELECTIVE FIBRILLATORY ARREST
CARDIOPLEGIC TECHNIQUES
????Cold Crystalloid Cardioplegia
????????OPERATIVE PROCEDURE
????????RESULTS
????Cold Blood Cardioplegia
????Warm Blood Cardioplegia
????Tepid Blood Cardioplegia
????Methods of Delivery
PROTECTION STRATEGIES UNDER INVESTIGATION
????Ischemic Preconditioning
????Adenosine
????Sodium/Hydrogen Exchange Inhibition
????Nitric Oxide
MYOCARDIAL PROTECTION DURING BEATING HEART SURGERY
SUMMARY
REFERENCES

?? INTRODUCTION
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The term "myocardial protection" refers to strategies and methodologies used either to attenuate or to prevent postischemic myocardial dysfunction that occurs during and after heart surgery. Postischemic myocardial dysfunction is attributable, in part, to a phenomenon known as ischemia/reperfusion-induced injury. Clinically, it is manifest by low cardiac output and hypotension, and may be subdivided into two subgroups: reversible injury and irreversible injury. The two are typically differentiated by the presence of ECG abnormalities, elevations in the levels of specific plasma enzymes or proteins such as creatine kinase and troponin I or T, and/or the presence of regional or global echocardiographic wall motion abnormalities. With respect to coronary artery bypass surgery alone, 10% of patients may experience myocardial infarction, severe ventricular dysfunction, heart failure, and/or death, despite advances in surgical technique. The impact from these complications both on families and on society is enormous. From an economic standpoint alone, such dysfunctions consume an estimated additional $2 billion in U.S. health care resources each year.1 The purpose of this chapter is to review the history of myocardial protection, to update the reader regarding the current protective techniques, to examine the mechanisms underlying ischemia/reperfusion injury, and to discuss several new strategies currently under investigation.


?? HISTORY
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Over the past 50 years, many therapeutic strategies have been developed to protect the heart during surgery (Table 14-1). This concept of shielding the heart from perioperative insult originated in 1950 with the review article by Bigelow et al in which hypothermia was reported "as a form of anesthetic" that could be used to expand the scope of surgery.2 It was proposed that hypothermia could be used as "a technique that might permit surgeons to operate on the bloodless heart without recourse to extracorporeal pumps and perhaps allotransplantation of organs."2 Five years later, Melrose et al reported another way to reliably stop and restart the heart by injecting potassium citrate into the root of the aorta at both normal and reduced body temperatures.3 Soon thereafter, the clinical application of potassium citrate arrest was adopted by many centers. Interest in using the "Melrose technique" waned, however, with subsequent reports that potassium citrate arrest was associated with myocardial injury and necrosis. Within a short time, many cardiac surgeons shifted from using potassium-induced arrest to normothermic cardiac ischemia (normothermic heart surgery performed with the aorta occluded while the patient was on cardiopulmonary bypass), intermittent aortic occlusion, or coronary artery perfusion. Experimental and clinical evidence showed, however, that normothermic cardiac ischemia was associated with metabolic acidosis, hypotension, and low cardiac output.46


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TABLE 14-1 Therapeutic innovations for myocardial protection

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As a consequence, there was a renewed interest in discovering ways to arrest the heart. Bretschneider published the principle of arresting the heart with a low-sodium, calcium-free solution.7 It was Hearse and colleagues, however, who studied the various components of cardioplegic solutions, which led to the development and use of St. Thomas solution.8 The components of this crystalloid solution were based on Ringer's solution with its normal concentrations of sodium and calcium, with the addition of potassium chloride (16 mmol/L) and 16 mmol/L of magnesium chloride to arrest the heart instantly. The latter component was shown by Hearse to provide an additional cardioprotective benefit. In 1975, Braimbridge et al introduced this crystalloid solution into clinical practice at St. Thomas Hospital.9

Gay and Ebert showed experimentally that lower concentrations of potassium chloride could achieve the same degree of chemical arrest and myocardial protection afforded by the Melrose solution without the associated myocardial necrosis reported earlier.1013 Shortly thereafter, Roe et al reported an operative mortality of 5.4% for patients who underwent cardiac surgery with potassium-induced arrest as the primary form of myocardial protection.14 In 1977, Tyers et al reported that potassium cardioplegia provided satisfactory protection in over 100 consecutive cardiac patients.15

By the 1980s, the use of normothermic aortic occlusion had been replaced for the most part with the use of cardioplegia to protect the heart during cardiac surgery. The major controversy at the time (and one that persists today) was not whether cardioplegic solutions should be used, but what the ideal components of those solutions were. The chief variants consisted of: (1) the Bretschneider solution, consisting primarily of sodium, magnesium, and procaine; (2) the St. Thomas solution, consisting of potassium, magnesium, and procaine added to Ringer's solution; and (3) potassium-enriched solutions, containing no magnesium or procaine (Table 14-2). Coincident to this controversy, another variant of cardioplegia was introduced, that of using potassium-enriched blood cardioplegia.16,17 The theory was that blood would be a superior delivery vehicle based on its oxygenating and buffering capacity. Ironically, Melrose et al initially used blood as the vehicle to deliver high concentrations of potassium citrate more than 20 years earlier.


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TABLE 14-2 Components of various cardioplegic solutions

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While hypothermia and potassium infusions remain the cornerstone of myocardial protection during on-pump heart surgery, there are many other cardioprotective techniques and methodologies available.3,18 While many of these techniques have been reported to confer superior protection and improve patient outcomes, the ideal cardioprotective technique, solution, and/or method of administration has yet to be found. Fortunately, the majority of cardioprotective strategies now available do allow patients to undergo conventional and complex heart operations with an operative mortality rate ranging from less than 2% to 4%.


?? ISCHEMIA/REPERFUSION INJURY
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While the etiology of postischemic myocardial dysfunction after cardiac surgery is multifactorial, three basic types of injury occur during heart surgery: myocardial stunning, apoptosis, and myocardial infarction. Myocardial stunning is an injury that may last for only a few hours or persist for several days despite the restoration of normal blood flow. Cells that have been reversibly injured (stunned) exhibit no sign of ultrastructural damage. Apoptosis is "suicidal" programmed cell death, characterized by retention of an intact cell membrane, cell shrinkage, chromatin condensation, and phagocytosis without inflammation.1921 There is increasing evidence that apoptotic death of cardiomyocytes caused by ischemia/reperfusion contributes significantly to the development of infarction as well as the loss of cells surrounding the infarct area. A large fraction of dying cells may exhibit features of both apoptosis and necrosis, i.e., both nuclear condensation and plasma membrane damage. Ultimately, however, after more prolonged ischemia, the heart begins to sustain irreversible injury in the form of infarction and necrosis. This is manifest as membrane destruction, cell swelling, DNA degradation, cytolysis, and the induction of an inflammatory response.

While the consequences of inadequate myocardial protection are usually apparent in the immediate postoperative period, the full impact may not be fully appreciated for months. Klatte et al reported that patients with increased peak creatine kinase-myocardial band (CK-MB) enzyme ratios after CABG surgery exhibited a greater 6-month mortality.22 Specifically, the 6-month mortality rates for patients with peak CK-MB ratios of =" border="0">5 to =" border="0">10 to =" border="0">20 upper limits of normal were 3.4%, 5.8%, 7.8%, and 20.2%, respectively. Conversely, the cumulative 6-month survival was inversely related to the peak CK-MB ratio. These observations support the concept that myocardial injury occurring as a result of inadequate myocardial protection intraoperatively is associated with subsequent death.

In order to appreciate the strategies that have evolved to protect the myocardium during heart surgery it is important to understand the mechanisms implicated in the etiology of the various types of myocardial ischemia/reperfusion injury. Significant evidence now exists that the primary mediators of reversible and irreversible myocardial ischemia/reperfusion injury include intracellular Ca2+ overload during ischemia and reperfusion, and oxidative stress induced by reactive oxygen species (ROS) generated at the onset of reperfusion (Fig. 14-1).2325 The molecule nitric oxide (NO) can also interact with ROS to generate various reactive nitrogen species that appear capable of both contributing to and reducing injury.26,27 In addition, metabolic alterations occurring during ischemia can contribute directly and indirectly to Ca2+ overload and ROS formation. For example, decreased cytosolic phosphorylation potential ([ATP] / ([ADP] x [Pi]) results in less free energy from ATP hydrolysis than is necessary to drive the energy-dependent pumps (SR Ca2+ -ATPase, the sarcolemmal Ca2+ -ATPase) that maintain intracellular calcium homeostasis.28 Restoration of intracellular pH at the onset of reperfusion via Na+ -H+ exchange contributes to intracellular Ca2+ overload via reversed Na+ -Ca2+ exchange.29,30



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FIGURE 14-1 Intracellular mechanisms regulate cardiomyocyte Ca2+ homeostasis and reactive oxygen species formation, the two primary mediators of myocyte ischemia/reperfusion injury. During ischemia, intracellular Ca2+ increases via the inability of energy-dependent Ca2+ pumps in the sarcolemma and sarcoplasmic reticulum (SR) to maintain normal low resting cytosolic Ca2+ concentrations. Activation of various G proteincoupled receptors (R, alpha- and beta-adrenergic, angiotensin, endothelin, etc.) initiates signaling mechanisms (stimulatory G proteins [Gs] and PLC [phospholipase C]) and also increases Ca2+ . The generation of inositol triphosphate (IP3) from this latter pathway increases Ca2+ release from intracellular stores, including SR. Diacylglycerol (DAG) formation via the PLC pathway leads to the activation of Ca2+ dependent and independent isoforms of protein kinase C (PKC). PKC phosphorylation of various proteins and enzymes further modulates Ca2+ concentration, metabolism, and contractile protein Ca2+ sensitivity. The sodium-hydrogen exchanger (NHE) exchanges intracellular H+ for extracellular Na+, and the resulting increase in intracellular Na+ may result in reverse Na+ -Ca2+ exchange via the sodium-calcium exchanger (NaCa). During reperfusion, the generation of reactive oxygen species (superoxide anion [O2-], hydroxyl anion [OH-], and hydrogen peroxide [H2O2]) oxidizes various proteins (Ca2+ pumps, SR Ca2+ release channels, contractile proteins, etc.) that contribute to both reversible and irreversible injury. Superoxide may combine with nitric oxide (NO) generated from sarcolemmal NOS and possibly mitochondrial NOS to form peroxynitrite (ONOO-) and other reactive nitrogen species to modulate ischemia/reperfusion injury. Early reperfusion is also associated with increased NHE activity, further exacerbating Ca2+ overload via reverse Na+ -Ca2+ exchange. Preischemic activation of some myocyte inhibitory G protein (Gi)coupled receptors, such as adenosine A1 and opioid receptors, reduces these deleterious effects of ischemia/reperfusion. It has also been proposed that PKC may phosphorylate an ATP-dependent K+ channel in the mitochondrial membrane and/or membrane-bound nitric oxide synthase (NOS) that protects the myocyte against ischemia/reperfusion injury.

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The metabolic changes that occur during ischemia also reduce the endogenous antioxidant defense systems of cardiac myocytes. The first line of defense against mitochondrial ROS formation and its deleterious effects is the GSH (reduced glutathione)/GSSG (oxidized glutathione) system, which is directly linked to the NADPH/NADP+ ratio via the enzyme glutathione reductase. The depletion of glutathione levels increases ROS formation, oxidative stress, and [Ca2+]i.3134 Since NADPH is not formed during ischemia, the normal metabolic mechanism for regenerating the reduced glutathione does not function. Thus, the formation of ROS during reperfusion occurs at a time when the myocyte's endogenous defense mechanisms are depressed. The NADPH/NADP+ ratio is a primary determinant of the redox state of the cell, and there is evidence that redox state plays a key role in determining the bioactivity and redox state of NO.27,35,36 In addition, there are several reports that, in the absence of normal levels of its cofactors, nitric oxide synthase (NOS) itself can generate superoxide anion.37,38 Although systolic calcium [Ca2+]i may return to normal levels early in reperfused stunned myocardium, the transient increases in intracellular [Ca2+]i can activate Ca2+ -dependent protein kinase (PKC); proteases, such as calpain; and endonucleases.3941 Calpain activation and subsequent action on contractile proteins has been implicated in the reduction in myofilament Ca2+ sensitivity observed in stunned myocardium.42,43

Similarly, there is significant evidence that ROS are involved in mediating myocardial stunning. Various spin trap agents and chemical probes have demonstrated the rapid release of ROS into the vascular space during reperfusion after brief ischemia in vivo.4447 It is also now recognized that mitochondria are a primary source of intracellular ROS in cardiac myocytes.48,49 Scavengers of ROS and antioxidants attenuate myocardial stunning in vitro and in vivo, and these interventions are effective when administered prior to or at the onset of reperfusion.23,50,51 It has been shown that ROS can attack thiol residues of numerous proteins such as the SR Ca2+ -ATPase, the ryanodine receptor, and contractile proteins.5254 This may explain why myofibrils isolated from in vivo reperfused stunned, but not ischemic, myocardium exhibit reduced Ca2+ sensitivity.55

More prolonged ischemia, which produces irreversible injury, is associated with more severe intracellular Ca2+ overload and further depletion of endogenous antioxidants, conditions which both contribute to and are exacerbated during reperfusion by the production of ROS. The production of ROS during reperfusion appears to contribute to Ca2+ overload, as exposure of normal myocytes to exogenous ROS is associated with increased L-type Ca2+ channel current and increased [Ca2+]i.34,5657 Conversely, increases in [Ca2+]i during ischemia/reperfusion may adversely affect mitochondrial function, leading to further ROS production.5859 Mitochondria can buffer small increases in intracellular Ca2+ via the Ca-uniporter, a process that is energetically favorable due to the [Ca2+] gradient and the mitochondrial membrane potential. During reperfusion, the increase in cytosolic Ca2+ enhances mitochondrial Ca2+ uptake. Since excess cytosolic Ca2+ has been associated with the loss of myocyte viability, mitochondrial Ca2+ buffering is initially cardioprotective.60 However, continued mitochondrial Ca2+ buffering in the face of decreased antioxidant reserves and excess ROS formation sets up a cycle which may ultimately lead to the total collapse of mitochondrial membrane potential and cell death.58 The synergistic interactions between Ca2+ overload and ROS formation during conditions of decreased antioxidant reserves may also provide an explanation of why ROS scavengers are not very effective at reducing irreversible injury when administered at reperfusion.61,62

Historically, myocardial ischemia/reperfusion injury has been characterized as either reversible or irreversible (based on staining techniques, enzyme release, and histology). There is now increasing evidence that this injury represents a transition from reversible to irreversible injury, and that it occurs as a continuum and not as an all-or-none phenomenon. For example, apoptosis occurs prior to severe depletion in ATP and loss of membrane integrity, but ultimately leads to cell death.20,21 The phenomenon of apoptosis appears to commence during reperfusion with the formation of intracellular ROS and/or intracellular calcium overload (Fig. 14-2).6365 This process is initiated by the translocation of the proapoptotic proteins Bad and Bax from the cytosol to the mitochondrial membrane. Heterodimerization of Bad or Bax with the anti-apoptotic Bcl-2 or Bcl-xl can lead to the release of the mitochondrially localized cytochrome c into the cytosol.6668 Formation of a cytosolic complex consisting of cytochrome c, apoptosis activating factor-1 (APAF-1), and caspase-9 leads to activation of caspase 3 and the cleavage of poly (ADP)-ribosylating (PARP) protein. Activation of PARP is the final step in apoptosis, leading to DNA fragmentation.69 As described above, the increased intracellular ROS and/or intracellular calcium overload collapse the mitochondrial membrane potential, leading to mitochondrial permeability transition pore (MPTP) opening, which if not reversed can result in the loss of mitochondrial proteins, such as cytochrome c.58



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FIGURE 14-2 Proposed mechanisms of cardiomyocyte apoptosis following ischemia/reperfusion injury. Intracellular Ca2+ overload during ischemia and reperfusion and reactive oxygen species (ROS) formation during reperfusion are thought to be the primary mediators of the intrinsic pathway of apoptosis. The mechanisms of Ca2+ overload and ROS formation are described in detail in the text. Ischemia/reperfusion-associated effects on metabolism and decreased levels of the endogenous antioxidant glutathione lead to excess electron leak from the mitochondrial electron transport chain generating mitochondrial ROS. The mitochondrial Ca2+ uniporter can buffer increases in cytosolic Ca2+, but increased mitochondrial Ca2+ can induce excess ROS formation. Likewise, ROS formation can induce intracellular Ca2+ overload. Through mechanisms that are not well defined, two families of closely related proteins (Bcl-2 and Bax) modulate the cell's response to apoptotic stimuli. Bcl-2 is an antiapoptotic protein that appears to be capable of inhibiting cytochrome c release either directly or by forming a complex with and inhibiting the proapoptotic family of proteins (Bax). Bax is thought to translocate from the cytosol to the mitochondrial membrane during the apoptotic process. Two early events in apoptosis are the externalization of phosphatidylserine (PS) residues in the sarcolemma and the release of cytochrome c from the mitochondria. The significance of PS externalization is not clear; however, its occurrence can be detected with fluorescently tagged annexin-5, thus permitting the detection of the early stages of apoptosis. Cytochrome c released from the mitochondria complexes with an apoptotic protease-activating factor-1 (Apaf-1) and procaspase 9. In the presence of near normal ATP levels, procaspase 9 is cleaved into the active caspase 9 with the resulting activation of the cytosolic protease caspase 3, often referred to as the executioner caspase. Caspase 3 protease activity leads to irreversible damage to cell morphology and DNA fragmentation and laddering.

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The physiological relevance of apoptosis during myocardial ischemia/reperfusion has yet to be determined. This is due to the fact that the majority of reports on apoptosis in this setting have been based on measurements of DNA fragmentation and laddering, the final steps in apoptotic cell death. Once DNA is fragmented, the cell's ability to synthesize new proteins to repair itself is severely compromised, and these cells, even if they survive a first ischemic episode, may die at an accelerated rate during subsequent stress or ischemia. However, studies conducted in other tissues and in isolated cells (including cardiomyocytes) indicate that the apoptotic program can be detected much earlier than these late stages. One of the earliest signs of apoptosis is the translocation of phosphatidylserine from the inner face of the plasma membrane to the cell surface, a process that can be detected by annexin V, which has a strong affinity for phosphatidylserine.70,71 Apoptosis in cardiac myocytes can be demonstrated with (FITC)-conjugated annexin V staining of the plasma membrane much earlier than DNA fragmentation (via the TUNEL assay and DNA laddering).7275 There are also reports that this early stage of apoptosis does not irreversibly commit cells to programmed cell death in noncardiac tissue, and that a significant proportion of myocytes, when submitted to simulated ischemia/reperfusion, exhibit signs of early apoptosis (positive annexin-FITC staining, intact membrane cell death, decreased cell width, and increased mitochondrial [Ca2+]).7577

Thus, it appears that ischemia/reperfusion injury (myocardial stunning, apoptosis, infarction) is manifest in a variety of interrelated ways. For example, apoptosis may proceed to necrosis when mitochondria are no longer able to withstand the intracellular Ca2+ overload and oxidative stress induced by ROS, and when oxidative phosphorylation is unable to keep pace with energy demands. Due to the resulting decrease in the myocardial phosphorylation potential, energy-dependent ion pumps cannot maintain normal ion gradients. This results in cell swelling and, ultimately, loss of membrane integrity. These disturbances can be further exacerbated by the influx of macrophages and leukocytes, complement activation, and endothelial plugging by platelets and neutrophils. If cell death in ischemic/reperfused myocardium progresses from apoptosis to necrosis, and if early apoptosis is indeed reversible, then one therapeutic approach for the treatment or prevention of ischemia/reperfusion injury would be to target the early events in apoptosis. Regardless of which stage is being addressed, current cardioprotection strategies are designed to reduce cellular and subcellular ROS formation and oxidative stress, to enhance the heart's endogenous antioxidant defense mechanisms, and to prevent intracellular Ca2+ overload.


?? NONCARDIOPLEGIC TECHNIQUES
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Intermittent Cross-clamping with Fibrillation

One of the earliest forms of cardioprotection, still used at some centers today, is known as intermittent aortic crossclamping with fibrillation and moderate hypothermic perfusion (30?C to 32?C). Using this approach, coronary artery bypass surgery can be performed on the unarrested heart with ascending aorta cannulation and generally a two-stage single venous cannula. This technique allows the surgeon to operate in a relatively quiet field (during ventricular fibrillation) and to avoid the consequences of profound metabolic changes that occur with more prolonged periods of ischemia. The duration of fibrillation is determined by how long it takes to complete the distal anastomoses. After completion of the last distal graft, the heart can be defibrillated and the proximal aortic-based graft anastomoses performed on the beating heart, using an aortic partial occlusion clamp.

As a result of increasing pressures to reduce costs, and yet maintain acceptable levels of myocardial protection, there has been a renewed interest in this approach. There are, in fact, a number of reports that indicate that satisfactory protection can be conferred using this technique. In 1992, Bonchek et al reported a large clinical series in which the advantages and safety of using this technique were meticulously analyzed.78 In this study, the authors reviewed the outcomes of the first 3000 patients at their institution who underwent primary coronary bypass surgery utilizing the intermittent aortic cross-clamping technique. Preoperative risk factors (age, gender, left ventricular dysfunction, preoperative intra-aortic balloon pumping, and urgency of operation) and operative deaths were analyzed as well. In this series, 29% of the patients were more than 70 years of age; 27% were females; 9.7% had an ejection fraction less than 0.30; 13% had a myocardial infarction less than one week preoperatively; and 31% had preinfarction angina in the hospital. Only 26% underwent purely elective operations. Using the noncardioplegic cardioprotective technique, the authors reported an elective operative mortality of rate 0.5%, an urgent mortality rate of 1.7%, and an emergency rate of 2.3%. Postoperatively, inotropic support was needed in only 6.6% of the patients and only 1% required intra-aortic balloon pumping. It is important to note, however, that this was a retrospective, single-center institutional experience. The findings would have been more enlightening if the analysis had included a similarly matched group of patients at the same institution in which cardioplegic arrest had been employed. Nevertheless, the findings do suggest that noncardioplegic strategies can provide a satisfactory means of myocardial protection even in high-risk patients.

As a result, a minority of surgeons continue to use this technique.7981 In 2002, Raco et al82 reported the results of 800 consecutive coronary artery bypass grafting operations performed by a single surgeon using aortic cross-clamping in both elective and nonelective procedures. The patients were divided into three cohorts: (1) elective, (2) urgent, and (3) emergent. The mean age, number of distal grafts, and mortality in the elective group were 61.5 years, 3.2, and 0.6%, respectively. For the urgent group, they were 63.1 years, 3.2, and 3.1 %. In the emergent group they were 63.8 years, 2.9, and 5.6%. These findings support the contention that intermittent aortic cross-clamping is a safe technique both in elective and nonelective patients when performed by an experienced surgeon.


?? SYSTEMIC HYPOTHERMIA AND ELECTIVE FIBRILLATORY ARREST
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Although infrequently used, this technique appears to be a safe approach to protecting the heart during coronary artery bypass surgery. In 1984, Atkins et al reported a low incidence of perioperative infarction and a low hospital mortality rate in 500 consecutive patients using this technique.83 With this method, systemic hypothermia (28?C), elective fibrillatory arrest, and maintenance of systemic perfusion pressure between 80 mm Hg and 100 mm Hg are the key elements. Upon fibrillatory arrest, the local vessel can be isolated and myocardial revascularization performed. The limitations of this technique include: (1) the surgical field may be obscured by blood during revascularization; (2) ventricular fibrillation is associated with increased muscular tone, which can limit the surgeon's ability to position the heart for optimal exposure; and (3) it is generally not applicable for intracardiac procedures.


?? CARDIOPLEGIC TECHNIQUES
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Cardioplegic solutions contain a variety of chemical agents that are designed to arrest the heart rapidly in diastole, create a quiescent operating field, and provide reliable protection against ischemia/reperfusion injury. In general, there are two types of cardioplegic solutions: crystalloid cardioplegia and blood cardioplegia. These solutions are administered most frequently under hypothermic conditions.

Cold Crystalloid Cardioplegia

There are basically two types of crystalloid cardioplegic solutions: the intracellular type and the extracellular type. The intracellular types are characterized by absent or low concentrations of sodium and calcium. The extracellular types contain relatively higher concentrations of sodium, calcium, and magnesium. Both groups avoid concentrations of potassium greater than 40 mmol/L, contain bicarbonate for buffering, and are osmotically balanced. In both types the concentration of potassium used ranges between 10 mmol/L and 40 mmol/L (for potassium 1 mmol/L = 1 mEq/L). Examples of some of the various crystalloid cardioplegic solutions used are shown in Table 14-2.

OPERATIVE PROCEDURE

While the degree of core cooling varies from center to center, patients undergoing cardiac surgery are placed on cardiopulmonary bypass and often cooled to between 33?C and 28?C. To initiate immediate chemical arrest, the solution is infused after cross-clamping the aorta through a cardioplegic catheter inserted into the aorta proximal to the cross-clamp. The catheter may or may not be accompanied by a separate vent cannula. The cold hyperkalemic crystalloid solution is then infused (antegrade) at a volume that generally does not exceed 1000 mL. One or more infusions of 300 to 500 mL of the cardioplegic solution may be administered if there is evidence of electrical heart activity resumption, or if a prolonged ischemic time is anticipated. If myocardial revascularization is being performed, the aortic cross-clamp can be removed after completing the distal anastomoses, and the heart reperfused while the proximal anatomoses are completed, using a partial occlusion clamp. Alternatively, the proximal grafts can be performed after the distal grafts have been completed with the cross-clamp still in place (the single-clamp technique). Another approach is to perform the proximal aortic grafts first, then cross-clamp the aorta and infuse the cardioplegic solution. When valve repair or replacement is being performed, the crystalloid cardioplegia can be administered directly into the coronary arteries via cannulation of the coronary ostia. Crystalloid cardioplegia can also be administered retrograde via a coronary sinus catheter, with or without a self-inflating silicone cuff.

RESULTS

Numerous studies have been performed to determine the efficacy of using cold crystalloid cardioplegic solutions to protect the heart during cardiac surgery. While there is considerable controversy regarding the "ideal" solution and its components, there is evidence that in those centers in which crystalloid cardioplegia is used almost exclusively, excellent myocardial protection can be achieved. In many reports the perioperative myocardial infarction rate is less than 4%, and the operative mortality rate is less than 2%.

Cold Blood Cardioplegia

Cold blood cardioplegia, widely employed throughout the world, is the cardioplegic technique most commonly used in the United States today. Although there are a variety of formulations, it is usually prepared by combining autologous blood obtained from the extracorporeal circuit while the patient is on cardiopulmonary bypass with a crystalloid solution consisting of citrate-phosphate-dextrose (CPD), tris-hydroxymethyl-aminomethane (tham) or bicarbonate (buffers), and potassium chloride. The CPD is used to lower the ionic calcium, the buffer is used to maintain an alkaline pH of approximately 7.8, and the final concentration of potassium is used to arrest the heart (approximately 30 mmol/L).

Prior to administering blood cardioplegia, the temperature of the solution is usually lowered with a heat exchanging coil to between 12?C and 4?C. The ratio of blood to crystalloid varies among centers, with the most common ratios being 8:1, 4:1, and 2:1. This in turn affects the final hematocrit of the blood cardioplegia infused. For example, if the hematocrit of the autologous blood obtained from the extracorporeal circuit is 30, these ratios would result in a blood cardioplegia with a hematocrit of approximately 27, 24, and 20, respectively.

The use of undiluted blood cardioplegia or "miniplegia" (using a minimum amount of crystalloid additives) has also been reported to be effective. In an acute ischemia/ reperfusion canine preparation, Velez and colleagues tested the hypothesis that an all-blood cardioplegia (66:1 blood to crystalloid ratio) would provide superior protection compared to a 4:1 blood cardioplegia delivered in a continuous retrograde fashion.84 They found very little difference between the animal groups with respect to infarct size or postischemic recovery of function. This is consistent with the findings by Rousou et al years earlier that it is the level of hypothermia that is important in blood cardioplegia, not necessarily the hematocrit.85

The rationales for using blood as a vehicle for hypothermic potassium-induced cardiac arrest include:

  1. It can provide an oxygenated environment.
  2. It can provide a method for intermittent reoxygenation of the heart during arrest.
  3. It can limit hemodilution when large volumes of cardioplegia are used.
  4. It has an excellent buffering capacity.
  5. It has excellent osmotic properties.
  6. The electrolyte composition and pH are physiologic.
  7. It contains a number of endogenous antioxidants and free radical scavengers.
  8. It can be less complex than other solutions to prepare.

With respect to efficacy, there are numerous preclinical studies as well as nonrandomized and randomized clinical trials that demonstrate that cold blood cardioplegia is an effective way to provide excellent myocardial protection. While many of these same studies have also suggested that cold blood cardioplegia is superior to cold crystalloid cardioplegia, it is important to note that other investigators have shown crystalloid cardioplegia to be just as cardioprotective as well as cost-effective, if not more so, and that crystalloid cardioplegia more reliably ensures a quiet, bloodless operative field. Unfortunately, many of the clinical trials that have compared the efficacy of blood and crystalloid cardioplegia have been single-center studies, involved a limited number of patients, focused on a specific subset of patients, and/or omitted details of the clinical management of the two techniques.

Warm Blood Cardioplegia

The concept of using warm (normothermic) blood cardioplegia as a cardioprotective strategy in humans dates back to the 1980s. In 1982, Rosenkranz et al reported that warm induction with normothermic blood cardioplegia, with a multidose cold blood cardioplegia maintenance of arrest, resulted in better recovery of function in canines than a similar protocol using cold blood induction.86 In 1986, Teoh et al reported an experimental study demonstrating that a terminal infusion of warm blood cardioplegia before removing the cross-clamp (a "hot shot") accelerated myocardial metabolic recovery.87 This was followed by reports in 1991 by Lichtenstein et al that normothermic blood cardioplegia in humans is an effective cardioprotective approach.88 They compared the results of 121 consecutive patients who received antegrade normothermic blood cardioplegia during myocardial revascularization operations with a historical group of 133 patients who received antegrade hypothermic blood cardioplegia. The operative mortality in the warm cardioplegic group was 0.9% compared to 2.2% for the historical controls. At about the same time, Salerno et al reported a series of 113 consecutive patients in which continuous warm blood cardioplegia was administered via the coronary sinus.89 In this series, 96% had spontaneous return of rhythm upon reperfusion, 7% needed transient intra-aortic balloon pump circulatory support, 6% had evidence of a perioperative myocardial infarction, and 3% did not recover. A control cohort was not provided for comparison.

Despite these encouraging reports, there are still concerns with this approach. For example, for any given patient it is not known just how long the warm heart can tolerate an ischemic event, which may occur when the infusion is interrupted, flow rates are reduced due to an obscured surgical field, or a maldistribution of the cardioplegic solution occurs. Another concern is the report by Martin et al which suggested that the use of warm cardioplegia is associated with increased incidence of neurological deficits.90 In their prospective, randomized study (conducted on more than 1000 patients), the efficacy of warm blood cardioplegia and cold oxygenated crystalloid cardioplegia was analyzed. While operative mortalities were similar between the warm blood group and the cold oxygenated crystalloid cardioplegia cohort (1.0% vs.1.6%, respectively), the incidence of permanent neurologic deficits was threefold greater in the warm blood group (3.1% vs.1.0%). Thus, it appears that warm blood cardioplegia offers no distinct advantage over cold blood or cold crystalloid cardioplegia, and it may be less than ideal if its delivery is interrupted for any reason.

Tepid Blood Cardioplegia

Both cold blood (4?C to10?C) and warm blood cardioplegic solutions (37?C) have temperature-related advantages and disadvantages. As a consequence, a number of studies were performed in the 1990s to determine the optimal temperature.

Hayashida et al were one of the first groups to study specifically the efficacy of tepid (29oC) blood cardioplegia.91 In this study, 72 patients undergoing coronary artery bypass grafting were randomized to receive cold (8oC) antegrade or retrograde, tepid (29oC) antegrade or retrograde, or warm (37oC) antegrade or retrograde blood cardioplegia. While protection was adequate for all three, the tepid antegrade cardioplegia was the most effective in reducing anaerobic lactate acid release during the arrest period. These authors reported similar findings when the tepid solution was delivered continuously retrograde and intermittently antegrade.92 Since then, other studies have also demonstrated that tepid blood cardioplegia is safe and effective. The majority of these studies, however, have been single-center studies and/or conducted in a relatively small cohort of patients. Whether tepid cardioplegia confers better protection over other current methodologies remains to be determined.

Methods of Delivery

In addition to a variety of solutions and temperatures, there are also many different ways of administering the solutions (Fig. 14-3). As one might expect with so many options, the optimal delivery method of a cardioplegic solution also remains controversial. These include: intermittent antegrade, antegrade via the graft, continuous antegrade, continuous retrograde, intermittent retrograde, antegrade followed by retrograde, and simultaneous antegrade and retrograde infusions. While all methods are generally good, comparisons are difficult because there are numerous confounding factors such as the: (1) composition of the solution, (2) temperature of the solution, (3) duration of the infusion, (4) infusion pressure, (5) type and complexity of the operation, (6) need for surgical exposure, and (7) expected versus actual cross-clamp time. One method that is being used more frequently is the retrograde technique. This approach originated with a concept developed by Pratt in 1898, who suggested that oxygenated blood could be supplied to the ischemic heart via the coronary venous system.93 Sixty years later, Lillehei et al used retrograde coronary sinus perfusion to protect the heart during aortic valve surgery.94 Today, it is an accepted method for delivering a cardioplegic solution and is used frequently as an adjunct to antegrade cardioplegia.



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FIGURE 14-3 Methods and delivery of cardioplegic solutions.

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In favor of the retrograde approach is the theoretical advantage of ensuring a more homogeneous distribution of the cardioplegic solution to regions of the heart that are poorly collateralized. It is also effective in: (1) the setting of aortic regurgitation and valve surgery; (2) reducing the risk of embolization from saphenous vein grafts that could occur during antegrade perfusion during reoperative coronary artery surgery; and (3) delivering cardioplegia in a continuous manner.

Despite these advantages, retrograde cardioplegia is not without its limitations. Numerous experimental and clinical studies have shown that cardioplegia administered via the coronary sinus can result in a poor distribution of the solution to the right ventricle. This may be related to the variable venous anatomy of the heart. Because the anterior region of the right ventricle is not drained by the coronary sinus, and it is not uncommon for the heart to have a number of coronary sinus anomalies, these factors may result in the heterogeneous distribution of cardioplegic solutions and thus limit myocardial protection.

As a consequence, a technique for simultaneously delivering cardioplegia both antegrade and retrograde is available. The feasibility and safety of this approach was reported in 1984 by Ihnken et al.95 In a more recent study, Cohen et al used sonicated albumen and transesophageal echocardiography intraoperatively to assess the effects of delivering a cardioplegic solution antegrade and retrograde simultaneously.96 Compared to the antegrade or retrograde routes, the best and most consistent perfusion of the anterior left and right ventricles was achieved using the simultaneous technique. These investigators also reported that antegrade infusion resulted in superior perfusion of the left ventricle when compared to retrograde delivery alone, and that right ventricular perfusion was inconsistent with both antegrade and retrograde delivery. Thus, it remains to be determined in which setting the simultaneous use of both methods is most appropriate.

With respect to intermittent cardioplegic infusions versus continuous infusions, the major advantage of the former is the ability to achieve and to sustain a dry quiescent operative field. While a continuous infusion, especially if it is oxygenated, has the theoretical advantage of minimizing ischemia, from a practical aspect it is unlikely that this can be achieved reliably. There is also the theoretical potential for an excessive infusion of cardioplegic solution.


?? PROTECTION STRATEGIES UNDER INVESTIGATION
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Ischemic Preconditioning

Ischemic preconditioning is an adaptive biological phenomenon in which the heart (and numerous other tissues) becomes more tolerant to a period of prolonged ischemia if first exposed to a prior episode of brief ischemia and reperfusion. This adaptation to ischemia was first described by Murry et al, and is referred to as classic or early phase preconditioning.97 This increased tolerance to ischemia is associated with a reduction in infarct size, apoptosis, and reperfusion-associated arrhythmias.98102 It has been demonstrated in every animal species studied and appears to persist as long as 1 to 2 hours after the ischemic preconditioning stimulus.103,104 It becomes ineffective when the sustained ischemic insult exceeds 3 hours.105 This suggests that the protection is conferred only when prolonged ischemia is followed by timely reperfusion.106

Further characterization of this phenomenon has also revealed a second phase of protection that requires 24 hours to appear and is sustained for up to 72 hours. This has been referred to as the second window of protection (SWOP), late phase preconditioning, or delayed preconditioning. Unlike classical preconditioning, which protects only against infarction, the late phase protects against both infarction and myocardial stunning.99,107 This observation, coupled with a longer interval of protection, has resulted in major investigative efforts to elucidate the intracellular mechanism(s) that underlie both phenomena. The assumption is that a better understanding of these mechanism(s) could lead to the development of potent new therapeutic modalities that are more effective in treating or preventing the deleterious consequences of ischemia/reperfusion injury.

To elucidate the trigger(s) and mediator(s) of this powerful endogenous defense mechanism, numerous in vivo and in vitro studies have been performed. One of the earliest hypotheses was that the adenosine receptor (the A1 receptor) was the primary mediator of this phenomenon (Fig. 14-4).104,108 Subsequent studies have shown, however, that in addition to adenosine, there are multiple guanine nucleotide binding (G) protein coupled receptors that, once activated, can mimic protection against infarction (e.g., bradykinin, endothelin, alpha1-adrenergic, muscarinic, angiotensin II, and delta-opioid receptors).104,108 Transient infusion of exogenous agents that mimic ischemic preconditioning is referred to as pharmacologic preconditioning. Exactly which of these receptors is the most important in mediating endogenous preconditioning is unknown, since there appear to be species differences and redundant pathways. Regardless, it is now thought that these triggers of ischemic preconditioning result in alterations in certain enzymes, such as tyrosine kinases, protein kinase C (PKC), and mitogen-activated protein kinases or c-jun-N-terminal kinases, which in turn confer protection against irreversible injury prior to the onset of prolonged ischemia.104,108 While the actual effector(s) of the protection has yet to be determined, one hypothesis is that the activation of these signaling pathways ultimately leads to the activation of an ATP-sensitive K+ (KATP) channel in the mitochondria.104,108



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FIGURE 14-4 Proposed mechanisms of late-phase preconditioning. Important upstream triggers of adaptation include adenosine, nitric oxide (NO), and reactive oxygen species (ROS). It is also possible that other ligands binding with 7 transmembrane domain receptors such as bradykinin, opioids, and noradrenaline may act as triggers of delayed preconditioning, but there is only indirect evidence for this at present. Mitochondrial KATP channel (MT-KATP) opening may initiate delayed protection, speculatively through the generation of superoxide anion. The rapid release of cytokines may also trigger delayed protection. The activation of a complex kinase signaling cascade is proposed, and experimental evidence suggests the activation of protein kinase C-{epsilon} and ? isoforms (PKC); tyrosine phosphorylation by protein tyrosine kinases (pTK) and MAP kinases (MKK); and involvement of some MAP kinase familes. Activation of NF-KB occurs rapidly and may be a key transciptional mechanism, although other transcription factors are activated also by preconditioning. De novo protein synthesis is proposed to be central to the mechanism of delayed preconditioning, but it is likely that posttranslational modifications of constituitively expressed proteins may also be involved. The identities of distal mediators and the effectors of protection are not established. Possible candidates include members of the heat shock protein family (HSPs), inducible nitric oxide synthase (iNOS), and manganese-dependent superoxide dismutase (Mn-SOD). The regulation of proteins associated with mt-KATP function may be involved. Cyclo-oxygenase-2 (COX-2) may be upregulated in delayed preconditioning. The final common pathway leading to cell survival during index ischemia is unknown, but it is highly likely that several distal proteins cooperate to confer protection. (Reproduced with permission from Springer-Verlag from Baxter GF, Ferdinandy P: Delayed preconditioning of myocardium: current perspectives. Basic Res Cardiol 2001; 96:338.)

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While early-phase preconditioning shares many of the same signaling mechanisms with late-phase preconditioning, the most obvious difference between the two is the apparent requirement for protein synthesis in the latter. Both late-phase ischemic and pharmacologic preconditioning have been shown to be associated with the upregulation of various proteins, such as heat shock proteins, inducible NOS (iNOS), cyclo-oxygenase 2, and manganese superoxide dismutase.109111

To date, the evidence that ischemic preconditioning exists in the human heart is circumstantial. A number of investigators have reported that patients experiencing angina prior to a myocardial infarct have better in-hospital prognoses and a reduced incidence of cardiogenic shock, fewer and less severe episodes of congestive heart failure, and smaller infarcts as assessed by cardiac enzyme release.112115 There are also follow-up studies that suggest patients who have had preinfarction angina prior to an infarct have better long-term survival rates.116118 There are also a myriad of reports that patients who undergo percutaneous transluminal coronary angioplasty (PTCA) have an enhanced tolerance to ischemia after the first balloon inflation, providing that the first balloon inflation exceeds 60 to 90 seconds.106 Chest pain severity, regional wall motion abnormalities, ST-segment elevation, QT dispersion, lactate production, and CK-MB release have all been reported to be attenuated in this setting as well.119124

In patients undergoing PTCA, a preconditioning-like effect has also been mimicked by the administration of a variety of pharmacologic agents that are known to induce preconditioning in animal studies. For example, the administration of adenosine prior to PTCA has been reported to attenuate myocardial ischemic indices during the first balloon inflation. Conversely, the administration of bamiphylline or aminophylline (nonselective adenosine receptor antagonists) reportedly abolishes markers of myocardial ischemia during the second balloon inflation.125 Tomai et al reported that the administration of oral glibenclamide (a KATP channel blocker) before angioplasty abolishes the reduction in ischemic indices observed after subsequent balloon inflations.126 This finding is consistent with the observation that nicorandil (a potassium channel opener) administration is associated with a reduction in ECG evidence of ischemia during PTCA.127 Opioid and bradykinin receptor activation have also been implicated in mediating the myocardial protection induced by the first balloon inflation.128 Thus, there are many observational studies that support the hypothesis that myocardial protection conferred by ischemic preconditioning and its possible mediators in animal studies is translatable to humans. It is important to note, however, that classic or early ischemic preconditioning observed in animals is associated with a reduction in infarct size, not stunning, and that many of the clinical studies are either retrospective in nature or have used surrogate markers of injury as end points.

With respect to cardiac surgery, one of the first studies indicating that preconditioning may exist in humans was conducted by Yellon et al in 1993.129 In this study, patients undergoing cardiac surgery were subjected to a protocol that involved 2 cycles of 3 minutes of global ischemia. Cross-clamping the aorta intermittently and pacing the heart at 90 beats per minute were used to induce ischemia. This was followed by 2 minutes of reperfusion before a 10-minute period of global ischemia and ventricular fibrillation. Myocardial biopsies were obtained during a 10-minute period of global ischemia, and ATP tissue content was measured. The results showed that the ATP levels in the biopsies obtained from patients subjected to the preconditioning-like protocol were higher. However, since ATP content is not a marker of necrosis, a follow-up study was performed and troponin T serum levels were used. In this study the investigators reported that the release of this marker of necrosis was also less in patients subjected to the preconditioning protocol.130

Whether the phenomenon of ischemic preconditioning plays any role in conferring protection when the intermittent cross-clamp technique is used perioperatively is unknown. It is important to note, however, that Teoh et al reported that ischemic preconditioning might confer additional myocardial protection beyond that provided by intermittent cross-clamp fibrillation in patients undergoing coronary artery bypass surgery.131 The fact that other human studies have not shown additional protection when ischemic preconditioning was added to a myocardial protection protocol makes the use of this procedure less likely.132,133 For the immediate future, the most promising strategy for developing new methods to protect the heart against ischemia/reperfusion injury lies with the elucidation of the intracellular events underlying the phenomenon of late-phase ischemic preconditioning (Fig. 14-4).

Adenosine

There is considerable experimental evidence that the preischemic administration of the nucleoside adenosine retards the rate of ischemia-induced ATP depletion, prolongs the time to onset of ischemic contracture, attenuates myocardial stunning, enhances postischemic myocardial energetics, and reduces infarct size.134 As mentioned earlier, there is also evidence that adenosine may play a role in mediating the infarct sizelimiting effects of ischemic preconditioning.104,135 A transient infusion of adenosine or certain adenosine receptor agonists prior to ischemia is associated with infarct size reduction similar to that of ischemic preconditioning.135137 There are, however, conflicting reports regarding the ability of adenosine receptor antagonists to block ischemic preconditioning.138,139 When analyzing these studies, it is important to recognize a small but important difference between adenosine preconditioning and adenosine pretreatment. The former involves a brief infusion of adenosine that is terminated prior to the onset of ischemia, whereas the latter involves the continuous infusion of adenosine until the onset of ischemia. The significance of this difference lies with the observation that adenosine pretreatment, not adenosine preconditioning, attenuates myocardial stunning.140,141 Myocardial stunning is the most common form of injury in patients after heart surgery. The beneficial effects of adenosine infusions prior to ischemia appear to be due to the direct effects of adenosine on the cardiac myocyte since: (1) adenosine must be infused at a dose that reaches the interstitial fluid (ISF) space that surrounds the cardiac myocyte; and (2) adenosine reduction of ischemic and hypoxic injury can be demonstrated in isolated myocyte preparations.142144

Although the cardioprotective effects of adenosine have been recognized for some time, there are still many questions regarding its mechanism of action. Genetic, biochemical, and pharmacologic studies indicate that there are at least four distinct sarcolemmal adenosine receptor subtypes, designated A1, A2a, A2b, and A3(Table 14-3), that couple to a variety of guanine nucleotide binding (G) proteins, Go, Gi {alpha} 2, Gi {alpha} 3, Gq, and Gs, depending upon the receptor subtype and tissue studied. Currently, there is direct evidence that two, possibly three, of these receptors are expressed in the adult heart. Radioligand binding studies have documented the presence of A1 and A2a adenosine receptors in mammalian myocardium, and numerous studies since have reported the physiological roles of these receptors.145 The results of recent studies suggest that adenosine A2b receptors may be expressed in the coronary vasculature.146,147 Although there are some reports of A3 receptor mRNA expression in cardiac tissue, presently there is no definitive evidence for the expression of this receptor in the normal mammalian heart.148,149


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TABLE 14-3 Cardiac adenosine receptor subtypes

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In normal myocardium, activation of A1 receptors on atrial myocytes exerts negative chronotropic, dromotropic, and inotropic effects via modulation of K+ and Ca2+ channel conductances. Activation of the same receptor exerts few, if any, direct effects in ventricular myocardium. However, A1 receptor activation significantly blunts the metabolic and contractile effects of beta-adrenergic receptor stimulation.150 Adenosine cardioprotection appears to be another physiologically relevant indirect effect of A1 receptor activation. The administration of various A1 receptor agonists prior to ischemia in multiple species and preparations, including human papillary muscle strips and atrial myocytes, has also been shown to mimic the cardioprotective effects of adenosine.134,151 Likewise, there are reports that the beneficial effects of adenosine pretreatment are blocked by the A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX).134 Like the A1 antiadrenergic effect, the A1 receptor cardioprotective effect is blocked by pertussis toxin pretreatment, which ADP ribosylates and blocks Gi protein activation.152 Preischemic adenosine A1 receptor activation appears more effective in modulating the deleterious effects of ischemia, since A1 receptor agonist infusion during reperfusion appears to exert little or no beneficial effect afterwards.153

With respect to the A2a receptor, recent studies indicate that selective adenosine A2a receptor activation can be beneficial to the reperfused myocardium. Reperfusion infusions of the relatively selective A2a agonist 2-[4-[(2-carboxyethyl)phenyl]-ethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680, at doses that exert minimal effects on systemic blood pressure and heart rate, have been shown to decrease infarct size in intact canine and porcine myocardium.154157 There is now evidence that adenosine A2a receptor activation during reperfusion in stunned porcine myocardium is associated with a flow-independent increase in myocardial contractility (assessed by load-insensitive measurements of preload recruitable stroke work index). This effect appears to occur only in the stunned myocardium. This is in contrast to earlier studies that showed adenosine infusion upon reperfusion had little or no effect on infarct size. These negative findings were probably due to the rapid rate of degradation of adenosine in blood and the use of high-dose adenosine which may have masked or offset any potential beneficial effects of selective A2a receptor activation. In addition to vascular smooth muscle and endothelial cells, adenosine A2a receptors have been shown to be expressed in porcine, human, and rat ventricular myocytes.158,159 This suggests that the beneficial effect of A2a agonists may be due to a direct effect on the cardiac myocytes.

More recent studies have suggested that the A3 receptor plays an important role in conferring protection.160,161 This hypothesis has been supported by studies with novel adenosine agonists, such as N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (IBMECA) and 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IBMECA). These agents exhibit selectivity for cloned A3 receptors.162164 However, there have been no reports verifying the expression of A3 receptors on mammalian adult cardiomyocytes. Moreover, the effects of these agents can be blocked by A1 and A2a receptor antagonists.165167 It is also important to note that A3 receptor agonists have been reported to simulate apoptosis in several cell types, including neonatal rat ventricular myocytes, an undesirable effect if applicable to adult myocytes.168,169

As noted earlier, the specific subcellular events and signaling pathways that are involved after the adenosine receptor has been activated have yet to be clearly defined. There is no known direct signaling mechanism activated by adenosine in normal ventricular myocardium. The mechanisms that have received the most interest and study are the stimulation of protein kinase C (PKC) isoforms and/or ATP-dependent K+ (KATP) channels.104,135 It has been proposed that A1 receptor activation results in the cytosol-to-membrane translocation of one or more PKC isoforms, which phosphorylate the KATP channels, leading to increased channel activity.104 Another possibility is that the A1 receptor can activate a protein tyrosine kinase and p38 mitogen activated protein (MAP) kinase, processes that occur distal to PKC.170 The evidence that supports this hypothesis is based on pharmacologic studies designed to explore the effects of specific agents on infarct size. However, other studies indicate that adenosine A1 receptor activation blunts the PKC-dependent negative inotropic effects in intact myocardium and isolated myocytes, and may, in fact, activate a serine-threonine protein phosphatase.171174 With respect to the KATP channel mechanism, there is evidence that adenosine activates sarcolemmal KATP channels in neonatal rat ventricular myocytes (although there is no direct evidence that this occurs in adult myocytes).175 This latter observation and recent results with the agent diazoxide (a mitochondrial KATP channel activator) have led to the hypothesis that adenosine A1 cardioprotection is mediated via the activation of mitochondrial KATP channels.104 It is possible that the A1 receptor activates a PKC isoform that translocates to mitochondria to phosphorylate the KATP channel, and (in a yet-to-be-determined manner) confers myocardial protection.

Another possibility is that activation of the A1 receptor results in an attenuation of oxidative stress. Both adenosine and A1 receptor agonists have been shown to attenuate the deleterious contractile and metabolic effects of hydrogen peroxide.176,177 They have also been reported to decrease lipid peroxidation and increase the activity of superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase in various cell types.178 This suggests that adenosine may play an important role in counteracting the deleterious effects of the formation of reactive oxygen species (a major component of ischemia/reperfusion injury) and in this manner attenuate stunning, apoptosis, and myocardial infarction.

Adenosine A1 receptor agonists, in addition to their acute cardioprotective effects, have been shown to induce a second window of protection (SWOP) or delayed preconditioning. The injection of various species with the A1 receptor agonist 2-chloro-N6-cyclopentyladenosine (CCPA) has been shown to reduce myocardial infarct size 24 hours later, an effect that has been shown to persist for 72 hours in the rabbit.179185 This is similar to the second window of protection induced by a transient ischemic episode. If A1receptormediated delayed and ischemia-induced late-phase preconditioning share some of the same signaling pathways, this would implicate a trigger role for the upregulation of inducible nitric oxide synthase (iNOS).186 Although 24-hour CCPA protection is associated with the upregulation of iNOS in murine myocardium, a loss of protection in iNOS knockout mice has not been a consistent finding, and NOS inhibitors do not block the A1 SWOP in rabbit myocardium.181,183 Thus much work needs to be done to fully understand the mechanism underlying A1 delayed preconditioning and ischemia-induced late-phase preconditioning. A better understanding of both phenomena could lead to new therapies designed to protect the heart during cardiac surgery.

Although there have been fewer studies on adenosine A2a receptormediated attenuation of myocardial reperfusion injury, there appears to be more definitive evidence regarding its mechanism of action. Adenosine- and A2a agonistinduced reduction in infarct size are associated with decreased neutrophil infiltration and adherence to coronary endothelium.155,156,187 Although this effect could simply be the result, rather than the cause, of myocardial protection, it is known that adenosine A2a receptors are expressed on neutrophils, and that their activation leads to decreased superoxide radical production.188 As described earlier, adenosine A2a receptors are also expressed on coronary endothelial cells, and there is evidence that their activation may result in nitric oxide (NO) release.189 The platelet antiaggregatory effects of adenosine may also play a role in A2a-mediated reduction of reperfusion injury.

With respect to clinical studies, Lee et al pretreated 7 patients undergoing coronary artery bypass surgery with adenosine and compared their postoperative course to a similar group of untreated patients.190 Adenosine was infused incrementally before the initiation of cardiopulmonary bypass at a rate of 50 ?g/kg/min every minute until a dose of 350 ?g/kg/min was reached. The total duration of the adenosine infusion lasted for 10 minutes, or until the patient developed systemic arterial pressures less than 70 mm Hg, at which time the infusion was discontinued. Five minutes after completion of the adenosine or the saline control infusion, patients were placed on cardiopulmonary bypass and underwent coronary artery bypass grafting. Cold blood cardioplegia was used to facilitate the arrest. The investigators reported that adenosine pretreatment was associated with improved postoperative myocardial function. Major limitations of this study were the small number of patients studied and limited number of parameters used to assess ventricular function. In contrast, Fremes et al reported the results of an open label, nonrandomized adenosine study in which no effect was observed.191 In this study, the patients also underwent coronary artery bypass surgery. Antegrade warm blood cardioplegia was used, with adenosine added to the initial 1-L dose and the final 500-mL dose of cardioplegia. The adenosine concentrations studied were 15, 20, and 25 ?mol/L. These investigators found that adenosine could be safely added as a supplement to cardioplegic solutions, but the agent had no effect on myocardial function at the doses studied.

A similar lack of efficacy in humans was reported by Cohen et al in a phase II double-blind, placebo-controlled trial performed in patients also undergoing coronary artery bypass surgery.192 Patients were treated with placebo (saline) or warm blood cardioplegia supplemented with 15 ?M, 50 ?M, or 100 ?M adenosine. These investigators also reported that the adenosine additive had no effect on survival, on the incidence of myocardial infarction (as determined by CK-MB levels), or on the incidence of low cardiac output syndrome. A major limitation of this study was the use of low concentrations of adenosine in the setting of warm blood cardioplegia. The nucleoside is rapidly metabolized to inosine and hypoxanthine, and the half-life in blood is measured in seconds.

In contrast, Mentzer et al reported a beneficial effect in an open label, single-center study in which the safety, tolerance, and efficacy of high doses of adenosine were assessed.193 Like the previous studies, adenosine was added to cold blood cardioplegia in patients undergoing coronary artery bypass surgery. In this study, 61 patients were randomized to receive standard cold blood cardioplegia or cold blood cardioplegia containing 1 of 5 adenosine doses (100 ?M, 500 ?M, 1 mM, 2 mM, and 2 mM with a preischemic infusion of 140 ?g/kg/ min). Invasive and noninvasive studies of myocardial function were obtained at 1, 2, 4, 8, 16, and 24 hours postbypass. This included the recording of inotropic utilization rates for the postoperative treatment of low cardiac output. Blood samples were collected before and after the first, second, and last dose of cardioplegia, as well as at 1 hour and 24 hours after cessation of cardiopulmonary bypass, for the measurement of nucleoside levels. These investigators found that high-dose adenosine treatment was associated with a 249-fold increase in the plasma adenosine concentration and a 69-fold increase in the combined levels of adenosine and its degradation products, inosine and hypoxanthine. The high-dose adenosine and associated high plasma levels of adenosine were associated with a reduction in postbypass inotropic drug utilization and improved regional wall motion and global function measured by transthoracic echocardiography.

Using a similar protocol, Mentzer et al examined the effects of high-dose adenosine treatment in 253 patients randomized to one of three treatment arms.194 This was a double-blind, placebo-controlled multicenter trial. The three cohorts consisted of those patients who were administered intraoperative cold blood cardioplegia, those administered cold blood cardioplegia containing 500 ?M adenosine, and those receiving cold blood cardioplegia containing 2 mM adenosine. Patients receiving the adenosine cardioplegia were also given an infusion of adenosine (200 ?g/kg/min) 10 minutes before and 15 minutes after removal of the aortic cross-clamp. Invasive and noninvasive measurements of ventricular performance were obtained before, during, and after surgery. The results of this study revealed a trend toward a decrease in high-dose inotropic agent utilization rates and a lower incidence of myocardial infarction. A composite outcome analysis showed that patients who received the high-dose adenosine were less likely to experience one of five adverse events: high-dose dopamine use, epinephrine use, insertion of an intra-aortic balloon pump, myocardial infarction, or death. A major limitation of this study was the failure to demonstrate a reduction in dopamine use or overall inotropic use, the two primary end points of the study. Another factor was the relatively low adverse event rates of myocardial infarction and death, namely 5.1% and 3.6%.

In summary, there is preclinical and clinical evidence that adenosine is a cardioprotective agent. Its clinical use, however, is somewhat limited since large doses are associated with marked hypotension. Although this can be easily managed while the patient is on cardiopulmonary bypass, it would be preferable to use a more selective A1 receptor agonist that would confer protection without peripheral vasodilation. The administration of such an agent prior to surgery (much in the same way that late-phase preconditioning has a salutary effect in limiting myocardial stunning, apoptosis, and infarction 24 hours later) could result in a reduction in the current rates of postoperative stunning and infarction, and represent a significant advance in the field of myocardial protection.

Sodium/Hydrogen Exchange Inhibition

The sodium hydrogen exchangers (NHEs) are a family of membrane proteins that are involved in the transport of hydrogen ions in exchange for sodium ions. The driving force behind the exchange is the transmembrane Na+ gradient, the Ca2+ gradient, and the membrane potential.195,196 The gradient is regulated by the intracellular pH through interaction of H+, with a sensor site on the exchanger protein (Fig. 14-5). To date, seven NHEs have been identified and are designated as NHE-1 through NHE-7.197200 In contrast to the NHE-1 and NHE-6 isoforms, which are ubiquitously distributed, the NHE-2 and NHE-5 isoforms have a much more limited expression. All isoforms except NHE-6 and NHE-7, which are located intracellularly, are localized primarily in the sarcolemmal membrane. In the mammalian heart the NHE-1 is the predominant isoform, although NHE-6 has been identified in the heart as well.197199 While the exact role the exchanger plays in the normal excitation-contraction coupling process has yet to be determined, there is increasing evidence that these proteins perform an important role in many pathophysiological conditions. They have been implicated in the etiology of arrhythmias, stunning, apoptosis, necrosis associated with acute myocardial ischemia/reperfusion injury, and postinfarction ventricular remodeling and heart failure.201,202



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FIGURE 14-5 Potential mechanism through which Na+/H+ exchanger (NHE) inhibition preserves intracellular ion homeostasis and thereby myocardial integrity and function after ischemia and reperfusion. (A) Under basal conditions, NHE is relatively quiescent, the Na+/K+ ATPase (Na+ pump) utilizes ATP to extrude Na+, and the bidirectional Na+ /Ca2+ exchanger (NCX) works predominantly in forward (Ca2+ efflux) mode. (B) During ischemia, NHE becomes activated in response to intracellular acidosis and possibly by other NHE-stimulatory factors. The resulting influx of Na+, occurring in the presence of ischemia-induced attenuation of Na+ pump activity, causes the intracellular accumulation of Na+ . Such a rise in the intracellular Na+ concentration during ischemia alters the reversal potential of the NCX in a manner that inhibits its operation in forward mode but favors its operation in reverse (Ca2+ influx) mode, thus producing intracellular Ca2+ accumulation (Ca2+ overload) during both ischemia and subsequent reperfusion. (C) NHE inhibitors are likely to afford a cardioprotective effect during ischemia and reperfusion by inhibiting this sequence at an early stage, through the limitation of Na+ influx during ischemia. Note that the illustration has been simplified for clarity, and that mechanisms other than NHE activity are also likely to contribute to the intracellular accumulation of Na+ and consequently Ca2+ during ischemia and reperfusion. (Reproduced with permission from the American College of Cardiology Foundation, from Avkiran M, Marber MS: Na+/H+ exchange inhibitors for cardioprotective therapy: progress, problems and prospects. J Am Coll Cardiol 2002; 39:749.)

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One of the primary mechanisms of injury that all these conditions have in common is the deleterious effect of an excess accumulation of intracellular calcium.203 Normally, sodium-hydrogen (Na+ /H+) exchange plays an important role in regulating cardiac myocyte physiology. The influx of extracellular Na+ via its concentration gradient is coupled to the efflux of H+, helping to maintain the intracellular pH. The Na+/Ca2+ exchanger uses the normal Na+ gradient to extrude Ca2+ in order to maintain normal intracellular Ca2+ homeostasis. However, during ischemia intracellular Na+ accumulates due to decreased activity of the Na+ /K+ ATPase, and increased production of H+ due to anaerobic glycolysis. During the initial phase of reperfusion, the Na+ /H+ exchanger is accelerated in an attempt to restore intracellular pH. This results in even more sodium and ultimately more Ca2+ accumulating intracellularly. As a consequence of increased intracellular sodium, the Na+ /Ca2+ exchanger operates in the reverse direction, resulting in a marked increase in the intracellular Ca2+ concentration (Ca2+ overload).

This Ca2+ overload can result in the activation of various enzyme systems and signaling pathways that over time can lead to cell contracture, membrane rupture, gap junction dysfunction, and cell death.204 As a consequence of this deleterious process, numerous preclinical studies have been performed to determine whether interruption of the process that leads to calcium overload can prevent ischemia/reperfusion injury. This has involved the use of agents with NHE inhibitor properties such as amiloride and its 5-amino-substituted derivatives, and the more novel benzoylguanidine derivatives: cariporide (HOE-642), eniporide (EMD-96785), and zoniporide (CP-597, 396).197,202,205,206

One of the first studies to suggest that inhibition of the Na+ -H+ exchange mechanism might exert a beneficial effect on postischemic recovery of ventricular function was reported by Karmazyn in 1988.207 Using the isolated rat heart subjected to low-flow ischemia, this investigator reported that the compound amiloride improved contractile recovery and decreased creatine kinase and 6-ketoprostaglandin F1 {alpha} release. In addition to NHE inhibition, however, the compound was known to exhibit numerous other pharmacologic properties. Using the more selective NHI inhibitor HOE-694, du Toit and Opie208 found that this agent, given either prior to global ischemia or at the onset of reperfusion, attenuated myocardial stunning in the ejecting isolated perfused rat heart. Amiloride, at a 100-fold higher dose, was also protective, although only when administered prior to ischemia. When given at the time of reperfusion, both agents were reported to reduce the incidence of reperfusion arrhythmias. Likewise, Sack et al reported that HOE-694 given prior to ischemia and throughout reperfusion improved regional ventricular function and decreased the incidence of arrhythmias following brief coronary occlusion in the intact pig.209 Using a chronic porcine preparation, Klein et al demonstrated that pigs treated with HOE-694 10 minutes prior to 45 minutes of regional ischemia showed a significant decrease in infarct size and an improved regional systolic shortening after 24 hours of reperfusion.210 When the drug was administered 10 minutes prior to reperfusion, less infarct size reduction and no improvement in function were observed. This is consistent with the report by Rohmann et al, who found that HOE-694 pretreatment reduced infarct size to a much greater extent when administered prior to reperfusion.211 Over time, the beneficial effects of NHE inhibition have been corroborated by numerous other investigators in a variety of experimental animals and under numerous conditions (Fig. 14-6).212216 Thus, it is not surprising that there has been considerable interest in exploring the use of these agents in the clinical setting.



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FIGURE 14-6 Effects on infarct size of intracoronary infusion of the Na+/H+ exchanger (NHE) inhibitor cariporide during various periods, in pig hearts subjected to 60 min of regional low-flow ischemia and 24 h of reperfusion. Infarct size was measured at the end of 24 h of reperfusion by both histochemical and histologic methods. The top panel illustrates the experimental protocol, with the vertically hatched bars indicating the periods of cariporide infusion and the arrows showing the coronary sinus cariporide concentrations (in ?mol/L) after 30 min of ischemia, immediately before reperfusion and immediately after reperfusion, in the various study groups. Note that a minimum concentration of approximately 1 ?mol/L cariporide is required for effective inhibition of sarcolemmal NHE activity in cardiac ventricular myocytes. As shown, infarct size was significantly limited by the intracoronary infusion of cariporide during the first 30 min of ischemia or throughout the entire 60 min of ischemia plus the first 10 min of reperfusion. In contrast, infusion of cariporide during the last 15 min of ischemia plus the first 10 min of reperfusion provided no benefit, even though the coronary sinus cariporide concentrations at the end of ischemia and the beginning of reperfusion were sufficient to inhibit NHE activity. Thus, NHE activity during early ischemia, rather than that during late ischemia and early reperfusion, appears to be the principal determinant of the extent of myocardial infarction. I = ischemia; R = reperfusion, *p <.05 versus control. the figure is based on data from klein et al href="#R210">210 (Reproduced with permission from the American College of Cardiology Foundation, from Avkiran M, Marber MS: Na+/H+ exchange inhibitors for cardioprotective therapy: progress, problems and prospects. J Am Coll Cardiol 2002; 39:750.)

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One of the first trials to examine the efficacy of using an NHE inhibitor was reported by Rupprecht et al.217 In this study, the NHE inhibitor cariporide was administered to patients within 6 hours after the onset of symptoms of an anterior myocardial infarction. One hundred patients were randomized to receive placebo or cariporide prior to reperfusion therapy. Cardiac enzymes were determined in blood samples obtained at predetermined times after reperfusion. Left ventricular function was ascertained by contrast ventriculography before treatment and 3 weeks after treatment. In the cariporide group of patients, the ejection fraction was greater, the resolution of regional left ventricular wall motion abnormalities tended to occur earlier, and the cumulative release of CK-MB was less. These findings supported the contention that NHE inhibition is beneficial to patients at risk of sustaining a reperfusion injury.

In a subsequent study performed by Zeymer et al, the salutary effects of NHE inhibitor therapy were not confirmed.218 In the ESCAMI (Evaluation of the Safety and Cardioprotective Effects of Eniporide in Acute Myocardial Infarction) trial, 433 patients with either an anterior or inferior myocardial infarction were studied. Patients were randomized to receive placebo or three different doses (low, intermediate, or high) of eniporide. Treatment was initiated in the form of a 10-minute infusion prior to the start of coronary angioplasty or within 15 minutes after the start of thrombolytic therapy. The initial findings indicated a reduction in cumulative cardiac enzyme release of approximately 15% and 30% with the low- and intermediate-dose eniporide therapies, respectively. There was, however, no observable effect at the highest dose studied. Consequently, 978 additional patients were recruited to the placebo, low-dose, and intermediate-dose eniporide groups. The results of this second phase (the extended study), however, failed to substantiate the positive findings of the first stage of the trial; i.e., there was no significant reduction in cardiac enzyme release by eniporide.218 This led to the conclusion that NHE inhibition was not effective in attenuating reperfusion injury.

Curiously, both of these studies were undertaken with the assumption that the beneficial effect of NHE inhibition would be realized during late ischemia or early reperfusion, despite considerable preclinical data that indicated that the salutary effect was realized during early ischemia. Despite these observations, a third clinical trial (GUARDIAN) was initiated to determine effective dosing and clinical efficacy in the heterogeneous group of patients experiencing variable degrees of ischemia.203 Specifically, 11,590 patients with unstable angina and nonST elevation myocardial infarction who were undergoing high-risk percutaneous and surgical revascularization were randomized to receive a placebo, and one of three doses of cariporide for the period of risk. The results of this study also indicate that NHE inhibition failed to reduce the incidence of myocardial infarction and death. In a subgroup analysis, however, there did appear to be a salutary effect among patients undergoing coronary artery bypass surgery. In this cohort, there was a 25% risk reduction for myocardial infarction or death for 6 months after surgery. Thus, while the GUARDIAN trial failed to show a clinical benefit of NHE inhibition in all patients studied, the findings suggested that NHE inhibition was beneficial when administered to patients undergoing cardiac surgery. Whether NHE inhibition is an effective form of protection against myocardial stunning, apoptosis, infarction, ventricular remodeling, and heart failure in humans during heart surgery awaits further study.

Nitric Oxide

There is increasing evidence that the signaling molecule nitric oxide (NO) plays an important role in modulating the heart's tolerance to ischemia. However, the combination of the short half-life of NO, the multiple redox states in which it can exist, the subcellular compartmentalization of NOS isoforms, and the multiple targets of its actions has hindered the determination of the specific role this molecule plays in modulating ischemia/reperfusion injury. This explains, in part, why NO and related reactive nitrogen species have been reported to both exacerbate injury and exert a cardioprotective effect.219 For the most part, however, studies performed using in vivo preparations indicate that the administration of NO donors reduces infarct size.219 There are also reports that the infusion of NOS inhibitors during reperfusion exacerbates ischemia/reperfusion injury.220,221 These reports are consistent with evidence that NO modulates coronary blood flow, and that the molecule can reduce neutrophil adherence to endothelium and inhibit platelet aggregation. There is also evidence that NO produced during reperfusion may scavenge the oxygen free radical superoxide (O2-), an effect observed in oxidant stressed cells exposed to constant low levels (12 ? M) of NO.222,223

Additional support for the beneficial effects of NO is derived from studies of late-phase ischemic preconditioning. Several reports (in multiple species) indicate that delayed protection against infarction induced by ischemic and pharmacologic preconditioning is blocked by various NOS inhibitors.224229 Conversely, delayed preconditioning can be induced by NO donors.226,230 There are also reports of increased iNOS upregulation following delayed ischemic and pharmacologic preconditioning, and that this protection is ablated in iNOS knockout mice.181,185,186,231,232 Late-phase preconditioning against myocardial stunning has also been reported to be induced by nitroglycerin and the NO donor SNAP.233 If NO does play a role in mediating late-phase preconditioning, it is still not known whether this protection is a direct result of increased NO production during prolonged ischemia/reperfusion or whether a transient increase in NO production during the preconditioning period triggers the upregulation of one or more endogenous protective proteins.

Although acute ischemic preconditioning has cardioprotective effects similar to delayed preconditioning and may have some common signal transduction mechanisms, there is much less evidence supporting NO as a trigger or mediator of this form of preconditioning. While there are reports that transient infusions of NO donors mimic acute ischemic preconditioning, there are also reports in multiple species that NOS inhibitors do not block early-phase conditioning.233238 Moreover, as in the studies on late-phase preconditioning, there have been very few, if any, studies in which NO levels have actually been measured.

With respect to clinical studies, there is only a modest amount of evidence that the NO mechanism can be manipulated to confer myocardial protection in humans. Leesar et al reported that the infusion of nitroglycerin (an NO donor) 24 hours prior to angioplasty mimicked the protection associated with the first balloon inflation in the control group in terms of ST-segment shifts, regional wall motion abnormalities, and chest pain scores.239 Zhang and Galinanes studied the effects of arginine (a precursor to NO) on human myocardial specimens obtained from right atrial appendages of patients undergoing elective coronary artery bypass graft surgery.240 The tissues were subjected to 120 minutes of simulated ischemia followed by 120 minutes of reoxygenation. Ischemic injury was assessed by measuring the leakage of lactate dehydrogenase into the incubation medium and the capacity of the tissue to reduce MTT [3-(4-dimethylthiazol-yl)-2,5-diphenyltetrazolium bromide] to a formazan product. In this study, L-arginine (a precursor of NO) decreased LDH leakage but had no effect on MTT reduction or oxygen consumption. However, the effect of arginine was not reversed by L-NAME (an NO synthase inhibitor), nor was it mimicked by S-nitroso-N-acetylpenicillamine (an NO donor). These data suggested that L-arginine was effective, but provided only a modest degree of protection against simulated ischemia/reperfusion injury in human myocardial atria tissue.

In a more relevant setting, Carrier et al reported that an L-arginine-enriched blood cardioplegia solution provided superior protection in 200 patients undergoing coronary artery bypass surgery.241 This was a prospective, randomized, double-blind clinical trial that compared standard blood cardioplegia to an L-arginine-enriched solution. The results indicated that the use of the arginine-supplemented blood cardioplegic solution was associated with a reduction in the release of a biochemical marker (troponin T) of myocardial damage.

All of these studies are limited, however, because the demonstration of efficacy was dependent on the measurement of surrogate markers of injury and sample sizes were small. Additional preclinical studies and clinical trials are needed to demonstrate whether NO plays an important role in protecting the ischemic heart. As noted by Wang and colleagues, the potential cytoprotective role of iNOS-derived NO needs to be reconciled with the known detrimental actions associated with iNOS in the setting of septic shock, organ rejection, inflammation, autoimmune diseases, and ischemic brain injury.186


?? MYOCARDIAL PROTECTION DURING BEATING HEART SURGERY
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As many as 10% to 15% of the patients in the United States who undergo coronary artery bypass surgery have the operation performed on the beating heart. The acceptance of this technique is due, in part, to the development and availability of better shunt appliances and mechanical stabilization devices, as well as the demonstration of satisfactory outcomes.242 The infarction rate among patients undergoing beating heart coronary artery surgery (OPCAB) has been reported to range between 0% and 5%, while on-pump surgery ranges between 2% and 6%.243245 If sensitive myocardial marker proteins are used to detect myocardial necrosis, it is possible that cell necrosis may be less in OPCAB patients. The fact that any necrosis occurs suggests that even OPCAB patients may be susceptible to ischemia/reperfusion injury. For the most part, however, little is known about the indications for and long-term outcomes of beating heart surgery. Likewise, there is a paucity of data regarding the extent to which the myocardium is reversibly or irreversibly damaged after OPCAB surgery. This is due, in part, to the fact that the primary focus among beating heart advocates has been the demonstration of feasibility, safety, and cost-effectiveness.

In an effort to elucidate the impact of regional myocardial ischemia/reperfusion injury on regional myocardial function and metabolism during OPCAB surgery, Dapunt et al studied the effects of performing a left internal mammary artery to left anterior coronary artery (LAD) anastomosis on the beating heart in an in vivo juvenile porcine heart preparation.246 The anastomosis was performed during either a 15-minute LAD occlusion or 15-minute intraLAD shunt insertion to maintain blood supply to the myocardium distal to the anastomosis. Functional and biochemical data were obtained at baseline, 15 minutes after LAD occlusion or shunt insertion, and after 30 minutes of reperfusion. In both groups, the regional left ventricular wall motion score index (WMSI) (using epimyocardial echocardiography) was similar at 15 minutes after occlusion or shunt insertion. After 30 minutes of reperfusion, however, both global and regional WMSI were markedly better in the shunt insertion group compared to the occlusion group. Myocardial adenine nucleotide glycogen contents were significantly lower in the occlusion group, while HSP 70 expression was greater. The major limitations of this study include the following: (1) only healthy pig hearts were used, (2) only one anastomosis was performed, (3) the investigators did not assess the impact of the surgery on cardiac enzyme release, and (4) the duration of reperfusion was limited. While the salutary effect of shunting is noteworthy, equally important were the findings that the hearts in the nonshunted animals were susceptible to ischemia/reperfusion injury. This suggests that patients with multivessel disease and poor ventricular function who undergo OPCABG might benefit from additional forms of myocardial protection.

In an effort to clarify the incidence of perioperative myocardial injury as determined by myocardial marker protein release, Bonatti et al studied 15 consecutive patients undergoing beating heart surgery.247 A stabilizing device was used along with the short-acting beta blocker esmolol to reduce heart rate and myocardial contractility. Target vessels were snared using silicone tubearmed prolene sutures placed around a bolster of epicardial and myocardial tissue. Prior to performing each anastomosis, the vessel was occluded for 5 minutes and reperfused for another 5 minutes. Intraoperative and postoperative assessments included echocardiography, ECG monitoring, and the measurement of creatine kinase and cardiac troponin I. The results of this study revealed that 9 of 14 surviving patients showed signs of transient myocardial ischemia on ECG, echocardiography, or both. This was observed only intraoperatively during target vessel occlusion in 3 patients, and postoperatively in 3 patients. Three other patients exhibited reversible signs of ischemia both intraoperatively and postoperatively. The Tn I level was elevated in 4 of the 9 patients showing ischemia, and the CK-MB mass concentration was elevated in 5. There was one death related to severe myocardial ischemia with infarction confirmed on autopsy. The overall conclusion was that despite the use of an agent to reduce heart rate and contractility, subclinical myocardial injury is a common event in coronary artery surgery on the beating heart.

In contrast, Koh et al reported their findings in a prospective observational study that compared 18 patients undergoing coronary grafting on the beating heart with 8 patients undergoing grafting with cardiopulmonary bypass.248 In the beating heart surgery cohort, pharmacologic agents were not used to slow the heart rate. These investigators found that the intraoperative and serial venous cardiac troponin T concentrations were lower in the beating heart surgery group. Neither group required inotropic support or pacing, and none of the patients sustained a perioperative myocardial infarct as determined by ECG. The conclusion of these investigators was that their results provided strong evidence that a lower degree of myocardial injury is associated with beating heart coronary surgery. Again, it is important to note that some degree of myocyte injury was observed regardless of whether the surgery was performed on or off pump.

To determine if the reversible injury that occurs during OPCAB surgery can be reduced using pharmacologic agents, Hendrikx et al studied the phenomenon in an in vivo, stunning only, sheep preparation.249 Animals were randomized to one of four groups. Group I received no treatment, Group II was administered the NHE inhibitor cariporide, Group III was given aprotinin, and Group IV was treated with a combination of cariporide and aprotinin. Each animal was then subjected to 20 minutes of regional ischemia by temporarily occluding the first lateral branch of the circumflex artery. This was followed by 1 hour of reperfusion. These investigators found that compared to the control group, both cariporide and aprotinin treatments, and especially the combination of the two, were associated with a marked attenuation of stunning. Thus, there is some preclinical evidence that patients undergoing beating heart surgery could benefit from pharmacologic pretreatment with selected cardioprotective agents.


?? SUMMARY
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While considerable progress has been made in the field of myocardial protection, the ideal solution, technique, or delivery method has yet to be identified. This is due, in part, to our increasing awareness of the complexity of ischemia/reperfusion injury, and the recognition that the definition of ideal protection is no longer limited to the time that the patient is in the operating room. As long as the incidence of myocardial stunning ranges from 20% to 80%, postischemic ventricular dysfunction from 3% to 7%, severe dysfunction in high-risk patients from 15% to 20%, and non-Q-wave infarction and Q-wave infarction from 5% to 7%, there is clearly a need to develop new therapeutic strategies to protect the heart during heart surgery. This need is even greater when one considers that long-term survival after heart operations is determined, in part, by adequate myocardial protection during the operation itself.


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