STUDIES OF CONTROLLED REPERFUSION AFTER ISCHEMIA

Controlling Reperfusion Injury With Controlled Reperfusion: Historical Perspectives and New Paradigms

Cardiac reperfusion injury is a well-established outcome following treatment of acute myocardial infarction and other types of ischemic heart conditions. Numerous cardioprotection protocols and therapies have been pursued with success in pre-clinical models. Unfortunately, there has been lack of successful large-scale clinical translation, perhaps in part due to the multiple pathways that reperfusion can contribute to cell death. The search continues for new cardioprotection protocols based on what has been learned from past results. One class of cardioprotection protocols that remain under active investigation is that of controlled reperfusion. This class consists of those approaches that modify, in a controlled manner, the content of the reperfusate or the mechanical properties of the reperfusate (e.g., pressure and flow). This review article first provides a basic overview of the primary pathways to cell death that have the potential to be addressed by various forms of controlled reperfusion, including no-reflow phenomenon, ion imbalances (particularly calcium overload), and oxidative stress. Descriptions of various controlled reperfusion approaches are described, along with summaries of both mechanistic and outcome-oriented studies at the pre-clinical and clinical phases. This review will constrain itself to approaches that modify endogenously-occurring blood components. These approaches include ischemic postconditioning, gentle reperfusion, controlled hypoxic reperfusion, controlled hyperoxic reperfusion, controlled acidotic reperfusion, and controlled ionic reperfusion. This review concludes with a discussion of the limitations of past approaches and how they point to potential directions of investigation for the future.

Modified Del Nido Cardioplegia and Its Evaluation in a Piglet Model

Del Nido cardioplegia (DN) is not available worldwide as the required solution is made from a commercial base (PlasmaLyte) that is not approved in all countries. We report our own modified DN solution and confirm its safety and effectiveness. Fourteen piglets were subjected to 90 minutes of global ischemia on cardiopulmonary bypass induced by original DN (n = 7) or NS (normal saline)-based DN (n = 7). Our DN solution begins with a base of NS (800 mL) and distilled water (200 mL), to which are added 15 mL KCl (2 mEq/mL), 17 mL NaHCO₃ (1 mEq/mL), 10 mL MgSO₄ (0.2 g/mL), 13 mL lidocaine 1%, and 13 mL mannitol 25%. LV function recovery was assessed in end-systolic elastance (EES) as systolic function and end-diastolic pressure-volume relationship (EDPVR) as diastolic function using a conductance catheter. Creatine kinase-MB (CK-MB) and mitochondrial score were also measured. Left ventricular (LV) contractility after ischemia (%EES ± SD) was not significantly different between the group induced by original DN (89.3 ± 20.6%) and the group induced by NS-based DN (99.3 ± 18.4%). LV compliance (%EDPVR ± SD) was likewise not significantly different between these groups (102.7 ± 28.2% vs 94.4 ± 22.8%, PL vs NS, respectively). CK-MB was equivalent between the groups. Mitochondrial scores were not significantly different between the groups, and this difference did not cause severe damage. NS-based DN preserves LV function recovery after prolonged global ischemia as effectively and as safely as original DN does. NS-based modified DN can be substituted for original DN.

Avoidance of Profound Hypothermia During Initial Reperfusion Improves the Functional Recovery of Hearts Donated After Circulatory Death

The resuscitation of hearts donated after circulatory death (DCD) is gaining widespread interest; however, the method of initial reperfusion (IR) that optimizes functional recovery has not been elucidated. We sought to determine the impact of IR temperature on the recovery of myocardial function during ex vivo heart perfusion (EVHP). Eighteen pigs were anesthetized, mechanical ventilation was discontinued, and cardiac arrest ensued. A 15-min standoff period was observed and then hearts were reperfused for 3 min at three different temperatures (5°C; N = 6, 25°C; N = 5, and 35°C; N = 7) with a normokalemic adenosine-lidocaine crystalloid cardioplegia. Hearts then underwent normothermic EVHP for 6 h during which time myocardial function was assessed in a working mode. We found that IR coronary blood flow differed among treatment groups (5°C = 483 ± 53, 25°C = 722 ± 60, 35°C = 906 ± 36 mL/min, p < 0.01). During subsequent EVHP, less myocardial injury (troponin I: 5°C = 91 ± 6, 25°C = 64 ± 16, 35°C = 57 ± 7 pg/mL/g, p = 0.04) and greater preservation of endothelial cell integrity (electron microscopy injury score: 5°C = 3.2 ± 0.5, 25°C = 1.8 ± 0.2, 35°C = 1.7 ± 0.3, p = 0.01) were evident in hearts initially reperfused at warmer temperatures. IR under profoundly hypothermic conditions impaired the recovery of myocardial function (cardiac index: 5°C = 3.9 ± 0.8, 25°C = 6.2 ± 0.4, 35°C = 6.5 ± 0.6 mL/minute/g, p = 0.03) during EVHP. We conclude that the avoidance of profound hypothermia during IR minimizes injury and improves the functional recovery of DCD hearts.

[Postconditioning in acute myocardial infarction: Primary angioplasty revisited?]

Early reperfusion of ischemic myocardium is the mean to improve prognosis of acute myocardial infarction. Nevertheless, reperfusion injury due to immediate acidosis correction and subsequent Ca(2+) overload results in formation of the mitochondrial permeability transition pore. The consequence is the death of viable myocardium due to onconecrosis and apoptosis. Mechanical (Stuttering reperfusion) or pharmacological postconditioning (cyclosporine A, adenosine…) is able to prevent reperfusion injury resulting in more myocardial salvage.

The use of controlled reperfusion strategies in cardiac surgery to minimize ischaemia/reperfusion damage

Ischaemia and reperfusion occur during almost every cardiac operation, and one of the key elements to achieve a successful operation is to counteract the detrimental effects of induced ischaemia and reperfusion during the operation. The cardiac surgeon is in a unique position to protect the heart before ischaemia is induced and to avoid further damage during the reperfusion period. The surgeon can alter the composition of the reperfusate and the conditions of reperfusion so that the ischaemia/reperfusion injury is minimal, even after very complex procedures that require long aortic cross-clamp periods. This in turn allows him to perform a near-perfect surgical repair of the underlying disease without the pressure of time. The vast knowledge gained in this field over the years has led to application in other organs, such as the limbs (acute limb ischaemia), lungs (lung transplantation), kidney and liver (kidney and liver transplantation), and more recently even for the brain [acute cerebral artery occlusion (stroke)] and the whole body (cardiopulmonary resuscitation). Further improvements in reperfusion strategies will allow salvage of tissue and even whole body after ischaemic periods thought previously to be irreversibly damaged.

Effects on Reperfusion Injury of Adding Diltiazem to Tepid Blood Cardioplegia

BACKGROUND

Although the present techniques of myocardial preservation for limiting ischemia/reperfusion injury in open heart operations yield excellent results for most patients, certain subgroups of patients with advanced coronary artery disease present a challenge in terms of intraoperative safety.

METHODS

In a prospective, randomized, controlled study, we assessed the myocardial protective effects of a total dose of 150 +/- 150 = 300 microg/kg diltiazem added to induction and terminal (reperfusion) doses of tepid blood cardioplegia. We determined the myocardial morphological (ultrastructural) and enzymatic (serum assays for the cardiospecific isoenzyme of creatine kinase [CK-MB]) changes and functional recovery (atrioventricular [AV]-node recovery time and postoperative need for inotropic support) in patients undergoing elective coronary artery bypass operations. The determinations were made with respect to values for control patients, who received the same cardioplegia but without the addition of diltiazem.

RESULTS

The mean isoenzyme CK-MB levels and semiquantitative ultrastructural score values of the diltiazem group were significantly less than those of the control group. Although AV-node recovery time was significantly prolonged (P < .05), this factor did not have major clinical impact.

CONCLUSIONS

We concluded that the addition of 150 +/- 150 microg/kg diltiazem to the induction and terminal doses of tepid cardioplegia enhanced myocardial protection in elective aortocoronary bypass surgery in high-risk patients and presented no significant additional operative risk.

Blood cardioplegic protection in profoundly damaged hearts: role of Na+-H+ exchange inhibition during pretreatment or during controlled reperfusion supplementation

BACKGROUND

Inhibition of the Na+/H+ exchanger before ischemia protects against ischemia-reperfusion injury, but use as pretreatment before blood cardioplegic protection or as a supplement to controlled blood cardioplegic reperfusion was not previously tested in jeopardized hearts.

METHODS

Control studies tested the safety of glutamate-aspartate-enriched blood cardioplegic solution in 4 Yorkshire-Duroc pigs undergoing 30 minutes of aortic clamping without prior unprotected ischemia. Twenty-four pigs underwent 30 minutes of unprotected normothermic global ischemia to create a jeopardized heart. Six of these hearts received normal blood reperfusion, and the other 18 jeopardized hearts underwent 30 more minutes of aortic clamping with cardioplegic protection. In 12 of these, the Na+/H+ exchanger inhibitor cariporide was used as intravenous pretreatment (n = 6) or added to the cardioplegic reperfusate (n = 6).

RESULTS

Complete functional, biochemical, and endothelial recovery occurred after 30 minutes of blood cardioplegic arrest without preceding unprotected ischemia. Thirty minutes of normothermic ischemia and normal blood reperfusion produced 33% mortality and severe left ventricular dysfunction in survivors (preload recruitable stroke work, 23% +/- 6% of baseline levels), with raised creatine kinase MB, conjugated dienes, endothelin-1, myeloperoxidase activity, and extensive myocardial edema. Blood cardioplegia was functionally protective, despite adding 30 more minutes of ischemia; there was no mortality, and left ventricular function improved (preload recruitable stroke work, 58% +/- 21%, p < 0.05 versus normal blood reperfusion), but adverse biochemical and endothelial variables did not change. In contrast, Na+/H+ exchanger inhibition as either pretreatment or added during cardioplegic reperfusion improved myocardial recovery (preload recruitable stroke work, 88% +/- 9% and 80% +/- 7%, respectively, p < 0.05 versus without cariporide) and comparably restored injury variables.

CONCLUSIONS

Na+/H+ exchanger blockage as either pretreatment or during blood cardioplegic reperfusion comparably delays functional, biochemical, and endothelial injury in jeopardized hearts.

Intracellular calcium increasing at the beginning of reperfusion assists the early recovery of myocardial contractility after diltiazem cardioplegia

OBJECTIVES

We investigated the ability of diltiazem to prevent myocardial injury by assessing heart function and intracellular calcium concentrations before and after ischemia-reperfusion.

METHOD

Isolated rat hearts underwent cardioplegia using the Langendorff perfusion model and were subjected to normothermic global ischemia for 60 minutes. The recovery rates for the heart function (heart rate, coronary flow, left ventricular systolic pressure) after reperfusion were monitored, and the intracellular Ca concentration was measured during ischemia and during the following reperfusion. Experimental groups were divided into three groups according to the diltiazem concentration used in the cardioplegic solution (potassium 20 mmol/l in Ringer's solution): (1) Group A: diltiazem 2.5 mg/l; (2) Group B: diltiazem 5 mg/l; and (3) Group C: no diltiazem.

RESULTS

Intracellular calcium concentration increased in all 3 groups during ischemia, but was significantly lower in Group B compared to either Group A or Group C. The heart function was significantly higher for Group A than for Group B or Group C. The hearts in Group B displayed markedly poor recovery in contractility and in heart rate.

CONCLUSIONS

Generally, a decrease in intracellular Ca concentration improves the heart function during ischemia and after reperfusion. However, this study showed that some increase in intracellular Ca at the beginning of reperfusion assisted the contractility of rat heart.

Pediatric myocardial protection: an overview

This article describes the experimental infrastructure and subsequent successful clinical application of a comprehensive bypass and cardioplegic strategy that limits intraoperative injury and improves postoperative outcomes in pediatric patients. The infant heart is at high risk of damage from poor protection because of preoperative hypertrophy, cyanosis, and ischemia. The background factors of vulnerability to damage caused by cyanosis and ischemia are discussed, together with studies of the infrastructure of strategies to use normoxia versus hyperoxia as bypass starts, white blood cell filtration, warm induction and reperfusion with substrate enhancements, multidose blood cardioplegia, and an integrated approach to allow ischemia only when vision is needed in pediatric surgeries. Data on cardioplegic management, including reducing calcium, increasing magnesium, and reducing perfusion pressure are shown, as used during this technique. These principles were applied to a consecutive series of 567 patients at the Heart Institute for Children and University of Illinois hospital over a 2-year period. Included also were 72 patients with hypoplastic left heart over a 4-year period with this myocardial management strategy. Application of these concepts may improve the safety of protection in infant hearts.

Superiority of magnesium cardioplegia in neonatal myocardial protection

BACKGROUND

We have shown that magnesium can offset the detrimental effects of normocalcemic cardioplegia in hypoxic neonatal hearts. It is not known, however, whether magnesium offers any additional benefit when used in conjunction with hypocalcemic cardioplegia.

METHODS

Twenty neonatal piglets underwent 60 minutes of ventilator hypoxia (FiO2 8% to 10%) followed by 20 minutes of normothermic ischemia on cardiopulmonary bypass (hypoxic-ischemic stress). They then underwent 70 minutes of multidose blood cardioplegic arrest. Five (Group 1), received a hypocalcemic (Ca+2 0.2 to 0.4 mM/L) cardiologic solution without magnesium, whereas in 10, magnesium was added at either a low dose (5 to 6 mEq/L, Group 2) or high dose (10 to 12 mEq/L, Group 3). In the last 5 (Group 4), magnesium (10 to 12 mEq/L) was added to a normocalcemic cardioplegic solution. Function was assessed using pressure volume loops and expressed as percentage of control.

RESULTS

Compared to hypocalcemia cardioplegic solution without magnesium (Group 1), both high- and low-dose magnesium enrichment (Groups 2 and 3) improved myocardial protection resulting in complete return of systolic (40% vs 101% vs 102%) (p < 0.001 vs Groups 2 and 3) and global myocardial function (39% vs 102% vs 101%) (p < 0.001 vs Groups 2 and 3), and reduced diastolic stiffness (267% vs 158% vs 154%) (p < 0.001 vs Groups 2 and 3). Conversely, even high-dose magnesium supplementation could not offset the detrimental effects of normocalcemic cardioplegia resulting in depressed systolic (End Systolic Elastance [EES] 41%+/-1%) (p < 0.001 vs Groups 2 and 3) and global myocardial function (40%+/-1%) (p < 0.001 vs Groups 2 and 3), and a marked rise in diastolic stiffness (258%+/-5%) (p < 0.001 vs Groups 2 and 3). Hypocalcemic magnesium cardioplegia has now been used successfully in 247 adult and pediatric patients.

CONCLUSIONS

Magnesium enrichment of hypocalcemic cardioplegic solutions improves myocardial protection resulting in complete functional preservation. However, magnesium cannot prevent the detrimental effects of normocalcemic cardioplegia when the heart is severely stressed. This study, therefore, strongly supports using both a hypocalcemic cardioplegic solution and magnesium supplementation as their benefits are additive.

A synergistic effect of extracellular hypocalcemic condition for hyperoxic reoxygenation injury in rat hepatocytes

BACKGROUND

Calcium accumulation of cells and mitochondria during reperfusion or reoxygenation has been implicated as a potential factor in cell injury as the result of mitochondrial damage. The objective of this study was to disclose whether or not low extracellular calcium ion concentration ([Ca2+]ex) in the medium at the time of reoxygenation might prevent calcium accumulation and attenuate hepatocytes injury after severe hypoxia.

METHODS

Isolated rat hepatocytes were incubated under a hyperoxic or hypoxic atmosphere for 60 min. During the ensuing 60-min hyperoxic reoxygenation, medium [Ca2+]ex was varied from 0.6 microM to 2.0 mM by altering total calcium and addition of chelators.

RESULTS

Incubation in low [Ca2+]ex reduced total cellular calcium and mitochondrial calcium in both the hyperoxic and hypoxic group. Under hyperoxic/hyperoxic incubation (control), hepatocytes were able to maintain potassium balance when [Ca2+]ex was >3.0 microM (pCa=5.5) and cellular viability (% lactate dehydrogenase release) at all levels of extracellular calcium. Under hypoxic/hyperoxic incubation (reoxygenation), however, loss of the ability to restore potassium balance as well as apparent increase in lactate dehydrogenase release were observed at severely low [Ca2+]ex (<30 microM; pCa=4.5). This low [Ca2+]ex-induced exacerbation of hepatocytes viability could not be generated under mild reoxygenation such as normoxia.

CONCLUSIONS

In normal isolated hepatocytes, very low [Ca2+]ex levels produce only very subtle changes in membrane permeability of isolated hepatocytes. After hypoxia, however, hypocalcemia acts synergistically with hyperoxic reoxygenation to produce more severe damage. These results suggested that [Ca2+]ex should be maintained on the physiological level to attenuate hepatocytes injury after severe hypoxia.

The scientific basis for hypocalcemic cardioplegia and reperfusion in cardiac surgery

The scientific rationale for avoiding the use of calcium-enriched cardioplegic solutions and calcium supplementation during cardioplegic induction and the early phase of reperfusion in open heart surgical procedures is reviewed. The role of the extracellular and intracellular free ionized calcium concentrations during ischemia and reperfusion is explored and the biochemical effects of ischemia on calcium fluxes, adenosine triphosphate levels, and mitochondrial function are discussed. The role of calcium in causing myocardial stunning and the biochemical basis of reperfusion injury are also addressed. Both prolonged ischemia and an increased concentration of Ca2+ during reperfusion have proved to be deleterious. I conclude on the basis of my review that there is no justification for the use of calcium chloride before and during the early phase of reperfusion and that hypocalcemic perfusion is an effective and easily controllable means of myocardial protection.

Continuous warm reperfusion during heart transplantation

From April 1991 to January 1993, 37 orthotopic heart transplantations were performed at our institution. Conventional preservation technique with cold crystalloid cardioplegia++ and topical hypothermia during storage and implantation was used in the first 15 cases (group A). After January 1992, for the next 22 patients (group B), we administered a first dose of hyperkalemic blood cardioplegia on arrival of the graft and thereafter instituted continuous warm reperfusion by infusion of oxygenated blood with added potassium. The groups were compared retrospectively, and significant differences were observed. In group B, the ischemic time was shortened by 31 minutes, the suture time lasted 12 minutes longer, sinus rhythm recovered spontaneously, the duration of inotropic support was reduced, postoperative arrythmias decreased, length of intensive care and hospital stays were reduced, there was less ischemic damage in the first endomyocardial biopsy sample, and right ventricular pressures a month after operation were lower. Continuous warm reperfusion during implantation of the donor heart is technically feasible and seems to provide enhanced myocardial preservation.

Resuscitative retrograde blood cardioplegia. Are amino acids or continuous warm techniques necessary?

This experiment was designed to determine the relative degree of cardiac functional recovery provided by various forms of resuscitative retrograde blood cardioplegia after global ischemic injury. Twenty-four dogs were subjected to 20 minutes of normothermic global myocardial ischemia followed by 60 minutes of cardioplegic arrest by one of three methods: group 1, standard cold blood cardioplegia with a cold terminal dose (n = 8); group 2, aspartate-glutamate-enhanced blood cardioplegia with warm induction and terminal enhancement (n = 8); and group 3, continuous warm blood cardioplegia (n = 8). Sonomicrometry was used to analyze left ventricular function for maximal elastance and preload recruitable stroke work area. Data were recorded at baseline and after 30 and 60 minutes of unloaded reperfusion. The results showed improved early recovery of preload recruitable stroke work area, but not of maximal elastance, after reperfusion of ischemic hearts with warm resuscitative blood cardioplegic solution enhanced with amino acids. The functional improvement provided by this technique was transient, however, and no significant differences were detectable among the groups after 60 minutes of unloaded reperfusion. Neither amino acid enhancement nor continuous warm cardioplegia offered a significant advantage in functional recovery over the standard method of cold blood cardioplegia reperfusion.

New surgical treatment for severe limb ischemia

Revascularization after prolonged complete limb ischemia may result in severe damage to skeletal muscle and systemic alterations (postischemic syndrome). Our previous experimental studies have shown that this injury can be reduced substantially by treating the jeopardized extremity by controlling the conditions of reperfusion and composition of the initial reperfusate. In the present study this concept of controlled limb reperfusion was applied in patients with prolonged severe limb ischemia. Controlled limb reperfusion was used in 14 patients after prolonged complete uni- or bilateral ischemia. The ischemic interval ranged from 5 to 21 h. Two patients were in cardiogenic shock, 11 had associated cardiac disease, and seven coexistent peripheral vascular disease. After systemic heparinization, standard thromboembolectomy was done using a Fogarty catheter. Cannulas were placed into the iliac, profunda, and superficial femoral arteries and were connected to a reperfusion set. Oxygenated blood was drawn from the iliac artery and mixed with an asanguineous solution (ratio 6:1). This controlled reperfusate was delivered into the profunda and superficial femoral arteries using a single rollerpump. The system allows control of the composition of the reperfusate (calcium, pH, osmolarity, glucose, substrate, pO2, free radical scavengers) and the conditions of reperfusion (pressure, flow, temperature). After 30 min of controlled limb reperfusion, the cannulas were removed and normal blood reperfusion started. All 12 patients who were stable hemodynamically before the operation survived the revascularization. Eleven patients, including one with acute aortic occlusion for several hours, were discharged with functional recovery of their extremities. Despite the severe ischemic insult, controlled limb reperfusion avoided amputation and profound systemic complications. Two patients who were in cardiogenic shock preoperatively died from progressive cardiac failure. We conclude that controlled arterioarterial limb reperfusion may reduce the local manifestations of the postischemic syndrome after prolonged periods of ischemia, may salvage limbs thought previously to be damaged irreversibly by prolonged ischemia, and can be done easily in the operating room.

Cardioplegia Solution

The mechanisms of the metabolic and ultrastructural changes that occur as a result of myocardial ischemia during cross-clamping of the aorta and the secondary injury that can occur during reperfusion after removal of the cross-clamp are important determinants of the composition and method of administration of cardioplegia solution (CS). Traditionally, basic principles of myocardial protection included hypothermia, potassium-induced diastolic cardiac arrest, buffering, membrane stabilization, and control of osmolarity and osmotic pressure. As the mechanisms of myocardial ischemia and reperfusion are further shown, methods of providing myocardial protection continue to expand. Current trends favor blood cardioplegia administration via a more complex approach to protect as much of the myocardium as possible; exogenous metabolic substrate enhancement in the presence of oxygen to allow aerobic metabolism to continue, with an end result of additional available myocardial energy; and warm (37°C) blood CS to optimize the metabolic rate for cellular repair. Thus, today the cardiothoracic surgeon can offer patients with energy-depleted hearts and those requiring more complex surgery improved myocardial protection with active myocardial resuscitation before and after the aortic cross-clamp is placed and removed. To this end, the pharmacist, ideally an operating room (OR) pharmacist, has a vital role in ensuring proper preparation, composition, storage, and quality control/assurance of CS. The basic principles of myocardial ischemic and reperfusion injury and how they relate to myocardial protection, CS composition, and methods of administration are described. By understanding these principles, in addition to surgeon and institution-specific factors, the pharmacist can build the foundation needed to maximize the role of pharmacy in ensuring optimal myocardial protection during open-heart surgery.

Postcardioplegia acute cardiac dysfunction and reperfusion injury

In cardiac surgery, an obligatory period of ischemia is imposed in order to provide a convenient operative field. Brief periods of ischemia produce systolic and diastolic abnormalities related to pathology occurring during ischemia per se (ischemic injury) or expressed after the onset of reperfusion (reperfusion injury). In the surgical setting, ischemia may be encountered preoperatively with preexisting coronary disease, hypotension, or ventricular fibrillation, between intermittent infusions of cardioplegia solutions, or as a result of maldistribution of cardioplegia solution. The potential for reperfusion injury exists not only at the time of cross-clamp removal, but also with each infusion of cardioplegia solution. Infusion of cardioplegic solution is, in fact, a form of reperfusion to previously ischemic myocardium. Ischemic injury and reperfusion injury are intimately linked in that the severity of ischemia sets the stage for and determines, in part, the extent of reperfusion injury. Mild-to-moderate systolic dysfunction, which may be called "postcardioplegia stunning," remains a significant complication after cardiac surgery. More significant postoperative functional depression may occur in hearts with severe preoperative dysfunction, and in operations requiring long cross-clamp times. In addition, the failure to adequately distribute cardioplegic solution to all areas of the myocardium because of coronary stenoses, high coronary resistance or inadequate delivery pressure-flow relations, contributes to postcardioplegia dysfunction. However, the cardioplegic solution itself may also contribute to postcardioplegic dysfunction by creating temporary ionic and metabolic abnormalities. In addition, systemic hypocalcemia or hyperkalemia resulting from using large doses of cardioplegic solution may temporarily aggravate postcardioplegic mechanical dysfunction. Current formulations and strategies for delivery of cardioplegia solutions are designed to address the various contributors to both ischemic and reperfusion injury that may impact on postoperative mechanical performance. Ischemic injury is avoided by reducing myocardial oxygen demand by engaging immediate arrest and cooling the heart to approximately 10 degrees centigrade, and intermittently infusing solution to reoxygenate the myocardium, maintain hypothermia, and wash out accumulated metabolites. Reperfusion injury may be avoided by infusing hyperosmotic solutions at moderate pressures, and by incorporating oxygen radical scavengers or inhibitors to reduce membrane lipid peroxidation, myocellular and microcirculatory (endothelium) damage.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in calcium fractions during cardiopulmonary bypass in infants and neonates

The changes in total blood calcium fractions were investigated in 20 infants and neonates undergoing open-heart surgery for correction of congenital heart defects. On initiation of cardiopulmonary bypass (CPB), total serum calcium fell from 2.29 ± 0.05 mmol/I to 1.8 ± 0.04 mmol/I and there was a profound fall in biologically active ionized calcium from 1.26 ± 0.02 mmol/I to 0.49 ± 0.04 mmol/l. Protein-bound calcium also fell from 1.03 ± 0.07 mmol/I to 0.09 ± 0.03 mmol/l. Complexed calcium increased from 0.08 ± 0.02 mmol/I to 1.1 ± 0.05 mmol/l. The large fall in ionized calcium and increase in complexed calcium could be accounted for by the corresponding large increases in serum citrate (162.2 ± 29.8 μmol/l to 1689.8 ± 163.1 μmol/l) and serum lactate (1.27 ± 0.18 mmol/I to 7.79 ± 0.72 mmol/l). All measured fractions of calcium returned towards their pre-operative levels by the end of bypass. However, 13 of the 20 patients had less than optimal levels of ionized calcium when being weaned from bypass despite the empirical use of intravenous calcium supplements. This study demonstrates that ionized calcium can be quickly and easily measured in the peri-operative period, that there can be extremely large changes in calcium fractions during CPB in infants and neonates, and that ionized calcium can be unexpectedly low in the critical period when weaning patients from bypass. Total calcium measurements gave an inaccurate estimate of the ionized calcium fraction.

Protean causes of myocardial stunning in infants and adults

Myocardial stunning can follow regional or global ischemia in the adult or immature heart. This report reviews some of our studies of the protean causes of stunning including energy and substrate depletion, inefficient oxygen utilization, calcium loading, acidosis, oxyradical damage, and summarizes studies and strategies to limit its occurrence. Data showing that reintroduction of molecular oxygen during reoxygenation of immature cyanotic hearts can cause stunning via a newly described biochemical pathway involving superoxide anion and nitric oxide are also included.

Pathophysiology and mediators of ischemia-reperfusion injury with special reference to cardiac surgery. A review

Although necessary for the ultimate tissue survival, reperfusion may paradoxically exacerbate the ischemic injury. Ischemia and reperfusion injury is intimately woven together. The relative role of reperfusion injury is not clarified and probably varies with the ischemic insult: Reperfusion is always preceded by ischemia, and some of the reperfusion-related events may represent a process continuing from the ischemic period; thus the proper designation should be ischemia-reperfusion injury. The reperfusion-related events are: arrhythmias, myocardial stunning with both systolic and diastolic dysfunction, and low reflow and microvascular stunning. Of pathogenetic importance are the mode and speed of reperfusion as well as the initiation of an intracoronary inflammatory reaction during reperfusion, including endothelium-leukocyte interaction, platelets, generation of oxygen free radical, generation and release of arachidonic acid metabolites, platelet activating factor, endothelium derived relaxing factor, endothelins, kinins, and histamine, complement activation, disturbances in calcium homeostasis, and disturbances in lipid and fatty acid metabolism.

Effect of Buckberg cardioplegia and peripheral cardiopulmonary bypass on infarct size in the closed chest dog

OBJECTIVES

To simulate a human catheterization laboratory setting of controlled reperfusion during myocardial infarction, regional infusion of commercially available Buckberg cardioplegic solution and peripheral vented bypass were administered in the closed chest dog.

BACKGROUND

Studies in open-chest dogs have demonstrated a significant reduction in infarct size and improvement in regional wall motion with a similar controlled reperfusion method using infusion of substrate-enriched (Buckberg) cardioplegic solution during cardiopulmonary bypass coupled with left ventricular venting.

METHODS

After 100 or 180 min of balloon occlusion of the proximal left anterior descending artery, controlled reperfusion was performed with cardioplegic infusion and vented bypass. Dogs matched for occlusion time underwent balloon deflation without bypass or cardioplegia (uncontrolled reperfusion groups). Microspheres were used to quantify coronary ischemia during balloon inflation. All four groups (n = 8 to 9 per group) were followed up at 1 week to determine regional wall motion and infarct size.

RESULTS

Qualitative echocardiographic analysis demonstrated no significant difference among groups in recovery of regional wall motion at 1 week; however, wall motion improved significantly in all groups between the ischemia and 1-week recovery periods. The histologic infarct size compared with the area at risk for dogs with uncontrolled versus controlled reperfusion, respectively, was 17.9 +/- 10.5% versus 31.9 +/- 8.3% (p < 0.05) for dogs with 100 min of occlusion and 40.1 +/- 11.7% versus 46.2 +/- 8.4% (p = NS) for dogs with 180 min of occlusion. A greater rate-pressure product in the dogs with controlled reperfusion after 100 min of occlusion (p < 0.05) may explain the larger infarct size observed for that group.

CONCLUSIONS

These results demonstrate that regional infusion of substrate-enriched cardioplegic solution in combination with peripheral vented bypass does not further reduce infarct size after prolonged ischemia in the closed chest dog (compared with uncontrolled reperfusion).

The effect of magnesium added to secondary cardioplegia on postischemic myocardial metabolism and contractile function--a 31P NMR spectroscopy and functional study in the isolated pig heart

This study investigated whether increasing the magnesium concentration during secondary cardioplegia improves postischemic myocardial recovery. Twenty-four isolated pig hearts were divided into four groups. All hearts were initially subjected to control perfusion with modified Krebs-Henseleit solution for 30 min, followed by a single infusion of St. Thomas' solution #2. The hearts were then maintained without perfusion at 12 degrees C for 4 h. Following this hypothermic preservation, the hearts in group I were reperfused with modified Krebs-Henseleit solution for 50 min, while hearts in group II and III were reperfused with a secondary cardioplegic solution containing 16 or 0 mmol/L magnesium, respectively, for 20 min followed by 30 min of perfusion with modified Krebs-Henseleit solution. In group IV, the hearts were initially reperfused with Krebs-Henseleit solution containing 16 mmol/L potassium for 20 min, followed by 30 min of reperfusion with modified Krebs-Henseleit solution. The changes in high-energy phosphates and intracellular pH were monitored throughout the experiments using 31P nuclear magnetic resonance (NMR) spectroscopy. Heart rate, left-ventricular systolic developed pressure, and rates of pressure increase and decrease were measured during control perfusion and reperfusion to calculate the percent contractile functional recovery. Needle biopsies for measurement of energy metabolites with high performance liquid chromatography were performed at the end of preservation and reperfusion to confirm the NMR measurements. All six hearts in group I showed significantly less recovery of contractile function during reperfusion when compared to the hearts in groups II, III, IV (p less than 0.05). There was no difference in either recovery of metabolism or mechanical function among the latter three groups of hearts. None of hearts in groups II, III, and IV showed ventricular fibrillation, which occurred in all six hearts of group I upon reperfusion. The results suggest that a short period of re-arrest perfusion following ischemia ("secondary cardioplegia") improves postischemic contractile functional recovery and prevents reperfusion-induced ventricular fibrillation. Increased magnesium concentration in the secondary cardioplegia did not provide additional benefit to the ischemic myocardium, possibly due to the low permeability of the sarcolemmal membrane to magnesium.

Successful transplantation of hearts harvested 30 minutes after death from exsanguination

The donor pool for heart transplants is severely limited. Unfortunately, many trauma patients who might be donors die of exsanguination before their organs can be used. We tested whether hearts "dead" for one half hour after exsanguination could be used as heart transplants in 8 lambs (mean weight, 8 kg). Four lambs were exsanguinated by severing the subclavian artery while simultaneously infusing intravenous saline solution to mimic resuscitation attempts. All animals died. Thirty minutes after hypotensive arrest and death, simulating the time needed to secure donation permission, the heart was harvested, perfused with 250 mL of cold cardioplegia containing 200,000 units of streptokinase to dissolve intravascular clots, and stored in iced saline solution for a mean of 1.5 hours while 4 recipient lambs were prepared for operation. After bypass and recipient heart excision, the "dead" donor heart was transplanted orthotopically. The heart was reperfused with low flow (25 mL/min), low pressure (30 mm Hg), low hematocrit (hematocrit, 0.08 to 0.12) blood supplemented with prostaglandin E1 and nifedipine for 15 minutes, followed by full flow rewarming for 45 minutes. All hearts resumed normal contractions. All animals were weaned from bypass without inotropes. Pressures a half hour after bypass were (in mm Hg): aorta, 80 +/- 10; pulmonary artery, 20 +/- 5; right atrium, 9 +/- 5; and left atrium, 9 +/- 2. We conclude that hearts "dead" for one half hour after exsanguination are capable of being reanimated and used successfully as donor organs. With further development, this method could potentially greatly expand the donor heart pool.

Adjunctive use of beta-adrenergic blockers, calcium antagonists and other therapies in coronary thrombolysis

The availability of thrombolytic agents for use in the treatment of acute myocardial infarction is an important step in the management of a common, often debilitating, and potentially lethal disorder. However, despite the proven benefits of coronary thrombolysis, the importance of adjunctive treatment modalities is being increasingly recognized. Beta-adrenergic blockers, calcium antagonists, nitrates, magnesium, and angiotensin-converting enzyme inhibitors each exert favorable cardiovascular properties that may offer additional benefits. Clinical trials combining thrombolytic and adjunctive pharmacologic agents offer hope for further advances in the treatment of acute myocardial infarction.

Controlled reperfusion during emergency coronary artery bypass surgery after angioplasty failure restores immediate segmental contractility

This study tests the hypothesis that careful control of the composition of the initial reperfusate and the conditions of the reperfusion during emergency CABG will restore immediate segmental contractility in the previously ischemia area despite ischemic intervals of greater than 2 hours. Between January 1987, and October 1990, 41 consecutive patients with acute coronary occlusion (90% due to PTCA failures) were reperfused during emergency myocardial revascularization according to one of two different protocols: in 25 patients the reperfusate was normal blood given at systemic pressure ("uncontrolled reperfusion"); in 16 patients the ischemic segment was reperfused during the first 20 minutes with a regional blood cardioplegic solution (substrate-enriched, hyperosmotic, hypocalcemic, alkalotic, diltiazem-containing) at 37 degrees C at a pressure of 50 mmHg. Thereafter, total bypass was prolonged for an additional 30 minutes before extracorporeal circulation was discontinued ("controlled reperfusion"). Assessment of regional contractility (echocardiography, radionuclide ventriculography), electrocardiographic evidence of myocardial infarction, release of CK and CK-MB enzymes, and hospital mortality were performed. Quantification of regional contractility was done with a scoring system from 0 (normokinesis) to 4 (dyskinesis). Data are expressed as mean +/- standard error of the mean. Both groups were well matched for age, sex, and the distribution of the occluded artery. In the controlled reperfusion group there was a higher incidence of previous infarctions (50% vs 30%), additional significant stenosis (1.1 +/- 0.2 vs 0.8 +/- 0.1), and cardiogenic shock (38% vs 20%) as compared to uncontrolled reperfusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Improved neonatal heart preservation with an intracellular cardioplegia and storage solution

The optimal conditions for preservation of the neonatal heart for transplantation remain uncertain. An isolated, working neonatal piglet heart model was used to compare a standard extracellular-like cardioplegic solution followed by storage at 4 degrees C in normal saline for 12 hr (n = 8) to cardioplegia and storage at 4 degrees C for 24 hr in an intracellular-like solution (n = 7). Seven of eight hearts in the 12-hr Extracellular Group failed to regain function, with a maximum stroke work index (SWI), developed at a left ventricular end-diastolic pressure (LVEDP) of 9 mm Hg of 0.91 +/- 0.30 x 10(3) erg/g (mean +/- standard error of the mean), 7.1% of nonpreserved control hearts. In contrast, all hearts arrested and stored for 24 hr in the intracellular solution regained function with a maximum SWI, again at a LVEDP of 9 mm Hg of 9.51 +/- 1.98 X 10(3) erg/g, 73.7% of control (P less than 0.05). Ultrastructural changes seen by electron microscopy paralleled the functional results. We conclude that an intracellular arrest and storage solution may be superior to conventional solutions for extended preservation of the neonatal heart.

Low Ca2+ reperfusion and enhanced susceptibility of the postischemic heart to the calcium paradox

This study was designed to define the effect of postischemic low Ca2+ perfusion on recovery of high-energy phosphates, intracellular pH, and contractile function in isolated rat hearts. Phosphorus-31 nuclear magnetic resonance spectroscopy was used to follow creatine phosphate, adenosine triphosphate, intracellular inorganic phosphate, and intracellular pH during control perfusion (15 minutes), total ischemia (30 minutes), and reperfusion (30 minutes). In Group I the perfusate [Ca2+] was 1.3 mmol/l throughout the experiment, whereas in Group II the perfusate [Ca2+] was reduced to 0.05 mmol/l during the first 10 minutes of reperfusion. Hearts from Group III were not made ischemic but were subjected to 10 minutes of low Ca2+ perfusion followed by 20 minutes of normal Ca2+ perfusion. During low Ca2+ reperfusion (Group II) recovery of high-energy phosphates and pH was significantly better than in controls (Group I). However, after reexposure to normal Ca2+, metabolic recovery was largely abolished, coronary flow was suddenly impaired, and contracture developed without any rhythmic contractions. These observations indicated the occurrence of a calcium paradox rather than postponed ischemia-reperfusion damage. On the other hand, normoxic hearts (Group III) tolerated temporary perfusion with 0.05 mmol/l Ca2+ very well with respect to left ventricular developed pressure, coronary flow, and metabolic parameters. In conclusion, postischemic low Ca2+ (0.05 mmol/l) perfusion may reduce reperfusion damage, but at the same time ischemia appears to enhance the susceptibility of the heart to the calcium paradox.

Controlled versus hyperemic flow during reperfusion of jeopardized ischemic myocardium

Controlled versus uncontrolled reperfusion of ischemic myocardium after experimental coronary artery occlusion was studied to determine the effect on regional ventricular wall motion and associated biochemical alterations. Fourteen pigs underwent coronary artery occlusion for 1 hour followed by 2 hours of reperfusion. In seven animals uncontrolled reperfusion was achieved by complete release of the arterial occlusion resulting in hyperemic flow. In seven other animals coronary flow during reperfusion was controlled at baseline levels eliminating hyperemic flow. Our results show that controlled reperfusion lessens end-diastolic wall thickness, reduces myocardial calcium deposition, increases the rate of mitochondrial oxidative phosphorylation, and preserves cellular high-energy phosphate stores in the ischemic-reperfused myocardium when compared to the uncontrolled reperfusion state. These data suggest that the magnitude of flow at an early stage of reperfusion is one of the important determinants in the outcome of ischemic myocardium.

Ischemia at the crossroads?

Understanding and controlling the consequences of myocardial ischemia requires us to acknowledge that we are dealing with a complex, dynamic, and highly variable process. The severity and progression of ischemic injury is not solely determined by the extent of oxygen deprivation, but by many other factors, including the accumulation of toxic metabolites. It may not be justified to assume that injury to the myocyte necessarily determines the survival of the organ; other components, such as the endothelium and the conducting system, may play a crucial role. Many factors can influence the severity and evolution of ischemic injury, perhaps the most important being the extent of residual (or collateral) flow to the affected tissue. If the ischemia is relatively mild, then the myocardium may survive for some long time, and drugs and other interventions may be used to further extend this period. However, reperfusion and the establishment of an adequate level of coronary flow is an absolute prerequisite for sustained tissue survival. The more severe the ischemia, the earlier must be the reperfusion. However, reperfusion of previously ischemic tissue is not without hazard, and it may precipitate potentially lethal events such as arrhythmias. Reperfusion may possibly result in the death of cells that were potentially viable in the moments before reflow was established, and there is good evidence that manipulation of reperfusion conditions may accelerate and possibly enhance recovery from ischemia. Much remains to be learned about myocardial ischemia and reperfusion, and in doing this we should perhaps put some of the older, yet well established, concepts behind us.

Effects of calcium channel blocker on responses of blood flow, function, arrhythmias, and extent of infarction following reperfusion in conscious baboons

Two groups of chronically instrumented, conscious baboons were studied. The effects of coronary artery occlusion for 3 hours and reperfusion for 1 week were examined on measurements of left ventricular function, ischemic-zone wall thickness, regional myocardial blood flow, arrhythmias, and extent of necrosis. The experimental group of animals (n = 7) was treated with the calcium channel blocker nisoldipine (0.1 microgram/kg/min) from 1 hour after coronary occlusion to 3 hours after coronary reperfusion. The control group (n = 6) received the vehicle (n = 4) or saline (n = 2). The effects of coronary artery occlusion and reperfusion on arterial pressure, left ventricular systolic pressure, heart rate, and left ventricular dP/dt were similar in both groups. Systolic wall thickening was reversed to paradoxical wall thinning during occlusion in both groups, and there was no recovery to systolic wall thickening over the 1-week period in either group. There were differences in regional blood flow; during coronary artery occlusion, nisoldipine increased blood flow significantly in the endocardium and epicardium of nonischemic and ischemic zones. There was a major difference in the number of arrhythmic beats per minute on reperfusion; during reperfusion, the number of arrhythmias rose markedly in the vehicle-treated group but actually fell in the nisoldipine-treated group. The size of areas at risk, infarcts, infarcts related to the area at risk, and amount of total creatine kinase (CK) and MB-CK appearing in blood were not significantly different in the two groups. Thus, in the conscious baboon, nisoldipine administered 1 hour after coronary artery occlusion exerted a marked effect in diminishing reperfusion-induced arrhythmias and improved blood flow to the ischemic zone during occlusion but did not salvage ischemic tissue.