Reperfusion injury after temporary coronary occlusion

Ischaemic conditioning and targeting reperfusion injury: a 30 year voyage of discovery

To commemorate the auspicious occasion of the 30th anniversary of IPC, leading pioneers in the field of cardioprotection gathered in Barcelona in May 2016 to review and discuss the history of IPC, its evolution to IPost and RIC, myocardial reperfusion injury as a therapeutic target, and future targets and strategies for cardioprotection. This article provides an overview of the major topics discussed at this special meeting and underscores the huge importance and impact, the discovery of IPC has made in the field of cardiovascular research.

Roles of endothelial nitric oxide synthase (eNOS) and mitochondrial permeability transition pore (MPTP) in epoxyeicosatrienoic acid (EET)-induced cardioprotection against infarction in intact rat hearts

We previously demonstrated that 11,12 and 14,15-epoxeicosatrienoic acids (EETs) produce cardioprotection against ischemia-reperfusion injury in dogs and rats. Several signaling mechanisms have been implicated in the cardioprotective actions of the EETs; however, their mechanisms remain largely elusive. Since nitric oxide (NO) plays a significant role in cardioprotection and EETs have been demonstrated to induce NO production in various tissues, we hypothesized that NO is involved in mediating the EET actions in cardioprotection. To test this hypothesis, we used an in vivo rat model of infarction in which intact rat hearts were subjected to 30-min occlusion of the left coronary artery and 2-hr reperfusion. 11,12-EET or 14,15-EET (2.5mg/kg) administered 10min prior to the occlusion reduced infarct size, expressed as a percentage of the AAR (IS/AAR), from 63.9±0.8% (control) to 45.3±1.2% and 45.5±1.7%, respectively. A nonselective nitric oxide synthase (NOS) inhibitor, L-NAME (1.0mg/kg) or a selective endothelial NOS inhibitor, L-NIO (0.30mg/kg) alone did not affect IS/AAR but they completely abolished the cardioprotective effects of the EETs. On the other hand, a selective neuronal NOS inhibitor, nNOS I (0.03mg/kg) and a selective inducible NOS inhibitor, 1400W (0.10mg/kg) did not affect IS/AAR or block the cardioprotective effects of the EETs. Administration of 11,12-EET (2.5mg/kg) to the rats also transiently increased the plasma NO concentration. 14,15-EET (10μM) induced the phosphorylation of eNOS (Ser(1177)) as well as a transient increase of NO production in rat cardiomyoblast cell line (H9c2 cells). When 11,12-EET or 14,15-EET was administered at 5min prior to reperfusion, infarct size was also reduced to 42.8±2.2% and 42.6±1.9%, respectively. Interestingly, L-NAME (1.0mg/kg) and a mitochondrial KATP channel blocker, 5-HD (10mg/kg) did not abolish while a sarcolemmal KATP channel blocker, HMR 1098 (6.0mg/kg) and a mitochondrial permeability transition pore (MPTP) opener, atractyloside (5.0mg/kg) completely abolished the cardioprotection produced by the EETs. 14,15-EET (1.5mg/kg) with an inhibitor of MPTP opening, cyclosporin A (CsA, 1.0mg/kg) produced a greater reduction of infarct size than their individual administration. Conversely, an EET antagonist 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EEZE, 2.5mg/kg) completely abolished the cardioprotective effects of CsA, suggesting a role of MPTP in mediating the EET actions. Taken together, these results suggest that the cardioprotective effects of the EETs in an acute ischemia-reperfusion model are mediated by distinct mediators depending on the time of EET administration. The cardioprotective effects of EETs administered prior to ischemia were regulated by the activation of eNOS and increased NO production, while sarcKATP channels and MPTP were involved in the beneficial effects of the EETs when administered just prior to reperfusion.

Postconditioning and protection from reperfusion injury: where do we stand? Position paper from the Working Group of Cellular Biology of the Heart of the European Society of Cardiology

Ischaemic postconditioning (brief periods of ischaemia alternating with brief periods of reflow applied at the onset of reperfusion following sustained ischaemia) effectively reduces myocardial infarct size in all species tested so far, including humans. Ischaemic postconditioning is a simple and safe manoeuvre, but because reperfusion injury is initiated within minutes of reflow, postconditioning must be applied at the onset of reperfusion. The mechanisms of protection by postconditioning include: formation and release of several autacoids and cytokines; maintained acidosis during early reperfusion; activation of protein kinases; preservation of mitochondrial function, most strikingly the attenuation of opening of the mitochondrial permeability transition pore (MPTP). Exogenous recruitment of some of the identified signalling steps can induce cardioprotection when applied at the time of reperfusion in animal experiments, but more recently cardioprotection was also observed in a proof-of-concept clinical trial. Indeed, studies in patients with an acute myocardial infarction showed a reduction of infarct size and improved left ventricular function when they underwent ischaemic postconditioning or pharmacological inhibition of MPTP opening during interventional reperfusion. Further animal studies and large-scale human studies are needed to determine whether patients with different co-morbidities and co-medications respond equally to protection by postconditioning. Also, our understanding of the underlying mechanisms must be improved to develop new therapeutic strategies to be applied at reperfusion with the ultimate aim of limiting the burden of ischaemic heart disease and potentially providing protection for other organs at risk of reperfusion injury, such as brain and kidney.

Protective ischaemia in patients: preconditioning and postconditioning

Infarct size can be limited by reducing the determinants of infarct size or increasing collateral blood flow by treatment initiated before the ischaemic event. Reperfusion is the definitive treatment for permanently reducing infarct size and restoring some degree of contractile function to the affected myocardium. Innate survival mechanisms in the heart can be stimulated by short, non-lethal periods of ischaemia and reperfusion, applied either before or after the ischaemic event. Preconditioning, a series of transient intervals of ischaemia and reperfusion applied before the lethal 'index' ischaemic event, sets in motion molecular and cellular mechanisms that increase cardiomyocyte survival to a degree that had not hitherto been seen before. The cardioprotective ischaemic-reperfusion protocol applied at onset of reperfusion, termed 'postconditioning' (Postcon), is also associated with significant cardioprotection that can be applied at the point of reperfusion treatment in the catheterization laboratory or operating room. Both preconditioning and Postcon have been successfully applied to the clinical setting and have been found to reduce infarct size and other attributes of post-ischaemic injury. This review will summarize the physiological preclinical data on preconditioning and Postcon that are relevant to their translation to clinical therapeutics and treatment.

Postconditioning: a mechanical maneuver that triggers biological and molecular cardioprotective responses to reperfusion

Infarct size is determined not only by the duration and severity of ischemia, but also by pathological processes initiated at reperfusion (reperfusion injury). Numerous pharmacological strategies have been reported which administer drugs at or just before the onset of reperfusion, with subsequent salubrious effects, notably a reduction in infarct size. However, few if any of these strategies have become standard of care in the catheterization laboratory setting. Postconditioning, defined as repeated brief cycles of reperfusion interrupted by ischemia (or hypoxia) applied at the onset of reperfusion, was recently introduced as a mechanical strategy to attenuate reperfusion injury. Postconditioning intervenes only during the first few minutes of reperfusion. However, it reduces endothelial activation and dysfunction, the inflammatory response to reperfusion, necrosis, and apoptosis both acutely and long-term. Cardioprotection has been demonstrated by multiple independent laboratories and in multiple species. Postconditioning stimulates G-protein coupled receptors by their cognate endogenously released ligands and surprisingly activates survival kinases that may converge on mitochondrial K(ATP) channels and the permeability transition pore. Postconditioning has been shown in two clinical studies to reduce infarct size in patients undergoing percutaneous coronary intervention in the catheterization laboratory, and at least five other studies are in some phase of implementation. This significant reduction in infarct size has implications for reduction in heart failure as a consequence of myocardial infarction, but this link has yet to be demonstrated. The salubrious effects of postconditioning are an indirect validation of the experimental and clinical importance of reperfusion injury in the setting of coronary artery occlusion.

Preconditioning and postconditioning: innate cardioprotection from ischemia-reperfusion injury

Reperfusion is the definitive treatment to salvage ischemic myocardium from infarction. A primary determinant of infarct size is the duration of ischemia. In myocardium that has not been irreversibly injured by ischemia, reperfusion induces additional injury in the area at risk. The heart has potent innate cardioprotective mechanisms against ischemia-reperfusion that reduce infarct size and other presentations of postischemic injury. Ischemic preconditioning (IPC) applied before the prolonged ischemia exerts the most potent protection observed among known strategies. It has been assumed that IPC exerts protection during ischemia. However, recent data suggest that cardioprotection is also exerted during reperfusion. Postconditioning (PoC), defined as brief intermittent cycles of ischemia alternating with reperfusion applied after the ischemic event, has been shown to reduce infarct size, in some cases equivalent to that observed with IPC. Although there are similarities in mechanisms of cardioprotection by these two interventions, there are key differences that go beyond simply exerting these mechanisms before or after ischemia. A significant limitation of IPC has been the inability to apply this maneuver clinically except in situations where the ischemic event can be predicted. On the other hand, PoC is applied at the point of service in the hospital (cath-lab for percutaneous coronary intervention, coronary artery bypass grafting, and other cardiac surgery) where and when reperfusion is initiated. Initial clinical studies are in agreement with the success and extent to which PoC reduces infarct size and myocardial injury, even in the presence of multiple comorbidities.

Inflammation, proinflammatory mediators and myocardial ischemia-reperfusion Injury

Ischemic myocardium must be reperfused to terminate the ischemic event; otherwise the entire myocardium involved in the area at risk will not survive. However, there is a cost to reperfusion that may offset the intended clinical benefits of minimizing infarct size, postischemic endothelial and microvascular damage, blood flow defects, and contractile dysfunction. There are many contributors to this reperfusion injury. Targeting only one factor in the complex web of reperfusion injury is not effective because the untargeted mechanisms induce injury. An integrated strategy of reducing reperfusion injury in the catheterization laboratory involves controlling both the conditions and the composition of the reperfusate. Mechanical interventions such as gradually restoring blood flow or applying postconditioning may be used independently in or conjunction with various cardioprotective pharmaceuticals in an integrated strategy of reperfusion therapeutics to reduce postischemic injury.

Postconditioning via stuttering reperfusion limits myocardial infarct size in rabbit hearts: role of ERK1/2

Emerging evidence suggests that restoration of blood flow in a stuttering manner may limit lethal myocardial ischemia-reperfusion injury. However, the mechanisms contributing to this phenomenon, termed postconditioning (post-C), remain poorly defined. Our aim was to test the hypothesis that activation of classic “survival kinases,” phosphatidylinositol 3-kinase (PI3-kinase) and/or extracellular signal-regulated kinase (ERK)1/2, may play a role in post-C-induced cardioprotection. In protocol 1, isolated buffer-perfused rabbit hearts underwent 30 min of sustained coronary artery occlusion and were randomized to receive abrupt reperfusion (controls) or four cycles of 30 s of reperfusion and 30 s of reocclusion before full restoration of flow (post-C). Protocol 2 was identical except control and postconditioned hearts received the PI3-kinase inhibitor LY-294002 ( protocol 2A) or the ERK1/2 antagonist PD-98059 ( protocol 2B) throughout the first 25 min of reperfusion, whereas in protocol 3, myocardial samples were obtained during the early minutes of reflow from additional control, postconditioned, and nonischemic sham hearts for the assessment, by standard immunoblotting, of phospho-Akt (downstream target of PI3-kinase) and phospho-ERK. Protocols 1 and 2 corroborated that infarct size (delineated by tetrazolium staining and expressed as a percent of risk region) was reduced in postconditioned hearts vs. control hearts and also revealed that post-C-induced cardioprotection was maintained despite LY-294002 treatment but was abrogated by PD-98059. These pharmacological data were supported by protocol 3, which showed increased immunoreactivity of phospho-ERK but not phospho-Akt with post-C. Thus our results implicate the involvement of ERK1/2 rather than PI3-kinase/Akt in the reduction of infarct size achieved with post-C.

Pulmonary Histopathology after Cardiopulmonary Bypass with Pall LG6 Leukocyte-Depleting Filter

There is increasing interest in the use of leukocyte-depleting filters to reduce the harmful effects of neutrophils during cardiopulmonary bypass. Pulmonary histopathology was studied in 7 patients before and after cardiopulmonary bypass. The arterial line leukocyte-depleting filter (Pall LG6) was used in 5 cases and the conventional arterial line filter (Pall Autovent) was used in 2 patients. The results showed no significant influence of the leukodepletion filter on pulmonary histopathologic changes associated with cardiopulmonary bypass.

Milestones in the development of cardioplegia

Long before there was a possibility for the application of cardioplegia to clinical situations, physiologists and surgeons studied the influence of various cations on function of the myocardial cell. These monumental studies helped set the stage for the eventual practical application of various means of myocardial preservation once open cardiac surgical techniques became available. This presentation attempts to summarize early research effort from the work of Sidney Ringer in 1878 until the present time. The historical progression of myocardial protective strategies is reviewed in a sequential fashion, highlighting major contributions and practices of investigators at the time. It is hoped that such a review of the accomplishments of our colleagues past and present will improve our perspective regarding current applications of such knowledge and that it may stimulate further inquiry toward improving present-day myocardial preservation strategies.

Beneficial effects of prostacyclin treatment on reperfusion of the myocardium

A total of 20 patients with coronary artery disease were studied in order to assess the benefits of prostacyclin administration on reperfusion of the ischaemic myocardium after cardiopulmonary bypass. Ten received prostacyclin (25 ng/kg per min) while ten were untreated controls. There was no difference between groups with regard to age, preoperative ejection fraction and aortic cross-clamping times. There were no in-hospital deaths in either group. The administration of prostacyclin significantly altered the metabolic side effects of reperfusion followed by hypothermic cardioplegic arrest. Myocardial oxygen consumption after cardiopulmonary bypass was significantly higher in the prostacyclin-treated group than in controls (18.5 ml versus 13 ml; P < 0.01). Prostacyclin treatment significantly reduced the leucocyte activity: leukotriene B4 concentrations were 58 pmol/l in prostacyclin-treated patients compared with 93 pmol/l in controls (P < 0.01). Such recovery of metabolic status during reperfusion resulted in better haemodynamic function in patients receiving prostacyclin.

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)

Postischemic [Ca2+] repletion improves cardiac performance without altering oxygen demands

The positive inotropism expected with correction of postischemic hypocalcemia might be counterbalanced by potential aggravation of reperfusion injury, in particular by calcium overload. We evaluated the effect of normalizing blood calcium concentration ([Ca2+]) on postischemic left ventricular systolic and diastolic mechanics using oxygen consumption and indices derived from pressure-diameter relations. In 10 open-chest dogs on cardiopulmonary bypass, the hearts underwent 30 minutes of normothermic global ischemia followed by one hour of multidose hypothermic (4 degrees C), hypocalcemic (0.3 mmol/L) blood cardioplegia. After reperfusion, systemic [Ca2+] had decreased to 70% of control (p = 0.017). The left ventricular inotropic state was significantly depressed from baseline (control) values, but was restored to baseline levels by resumption of normocalcemia after one hour of reperfusion. Chamber stiffness increased by 308% (p = 0.006) after hypocalcemic reperfusion but decreased significantly after [Ca2+] correction. Recovery of left ventricular performance with [Ca2+] correction did not augment myocardial oxygen consumption from the postischemic uncorrected state (5.0 +/- 0.3 mL O2/min/100 g versus 5.3 +/- 0.3 mL O2/min/100 g). We conclude that normalizing [Ca2+] after blood cardioplegia improves postischemic left ventricular performance without adversely affecting compliance or oxygen consumption.

Reperfusion of infarcting myocardium: benefit of surgical reperfusion in a chronic model

Surgical reperfusion of experimental infarction leads to improved recovery of regional function compared with medical reperfusion, but sustained myocardial salvage has not been demonstrated. Twenty-two dogs were subjected to two hours of anterior descending occlusion and divided into three groups: group P (n = 7), no reperfusion; group M (n = 8), medical reperfusion; and group S (n = 7), controlled surgical reperfusion. Ischemia caused systolic bulging (-36% of control systolic shortening, p less than 0.01) and decreased regional work (9% of control pressure-length loop area, p less than 0.05). Thirty minutes after reperfusion group M had persistent systolic bulging (-9% of control systolic shortening) and decreased regional work (9% of control pressure-length loop area), whereas group S had +17% of control systolic shortening and 33% of control pressure-length loop area. After 1 week, regional function improved in all three groups (percent of control systolic shortening: group P, 26%; group M, 19%; group S, 52%), but systolic shortening was significantly better in group S (p less than 0.05 versus group M). Surgical reperfusion also resulted in one half of the eventual myocardial necrosis found in the other groups (group P, 45% of area at risk; group M, 38%; group S, 19%; p less than 0.05, group S versus group P or M). In this model, medical reperfusion offered no demonstrable benefit, whereas controlled surgical reperfusion led to a sustained (1 week) improvement in regional function and significant myocardial salvage.

Right ventricular free wall ischemia: correlation of ischemic duration with extent of infarction in dogs

The time dependence of myocardial necrosis has not been established for the right ventricle. In 16 dogs, we studied the temporal extension of ischemia-induced right ventricular free wall infarction for one hour (n = 8) and two hours (n = 8) followed by two hours of reperfusion, and quantitated segmental shortening, area at risk from ischemia, and area of necrosis. Stroke volume decreased 7.3 +/- 0.5 mL/beat (+/- standard error of the mean) in one hour (p less than 0.05) and 7.4 +/- 1.3 mL/beat in two hours (p less than 0.05). Segmental shortening was replaced by paradoxical motion in both groups during ischemia, and remained depressed throughout reperfusion. Right ventricular end-diastolic pressure increased with ischemia and then returned to normal during reperfusion. The extent of infarction appeared proportionate to the duration of ischemia, 13.2% +/- 4.1% at one hour and 66.9% +/- 4.5% at two hours (p less than 0.01). We conclude that the wave front of necrosis in the right ventricle progresses with duration of ischemia and that clinical techniques capable of limiting the duration of ischemia might salvage substantial amounts of right ventricular myocardium.