Ischemia/Reperfusion Injury following Acute Myocardial Infarction: A Critical Issue for Clinicians and Forensic Pathologists

Abstract

Acute myocardial infarction (AMI) is a leading cause of morbidity and mortality. Reperfusion strategies are the current standard therapy for AMI. However, they may result in paradoxical cardiomyocyte dysfunction, known as ischemic reperfusion injury (IRI). Different forms of IRI are recognized, of which only the first two are reversible: reperfusion-induced arrhythmias, myocardial stunning, microvascular obstruction, and lethal myocardial reperfusion injury. Sudden death is the most common pattern for ischemia-induced lethal ventricular arrhythmias during AMI. The exact mechanisms of IRI are not fully known. Molecular, cellular, and tissue alterations such as cell death, inflammation, neurohumoral activation, and oxidative stress are considered to be of paramount importance in IRI. However, comprehension of the exact pathophysiological mechanisms remains a challenge for clinicians. Furthermore, myocardial IRI is a critical issue also for forensic pathologists since sudden death may occur despite timely reperfusion following AMI, that is one of the most frequently litigated areas of cardiology practice. In this paper we explore the literature regarding the pathophysiology of myocardial IRI, focusing on the possible role of the calpain system, oxidative-nitrosative stress, and matrix metalloproteinases and aiming to foster knowledge of IRI pathophysiology also in terms of medicolegal understanding of sudden deaths following AMI.

Figure 1

Schematic representation of calpain activation during myocardial IRI. Ca²⁺ overload and pH recovery in reperfusion phase are crucial in the activation of the calpain system. Increased sarcolemmal fragility may lead to membrane rupture and cell death. In addition, both the death-receptor and mitochondrial mediated apoptotic pathways seem to be affected by calpain activation. The degradation of myofibrillar proteins and the loss/disorganization of T-tubules structure are key factors in post-MI heart failure development.

Figure 2

Schematic representation of oxidative stress contributing to tissue injury and cell death in IRI. Following ischemia, hypoxia results in reduction of ATP production, ion pump function unbalance, leading to overload of Na⁺ and Ca²⁺, activation of anaerobic glycolysis, and, finally, reduction of pH. During the initial ischemic phase, the activation and upregulation of enzymes (such as NADPH oxidase, a superoxide-generating enzyme comprising a membrane-bound catalytic subunit) occurs, that are capable of producing ROS, when molecular oxygen is reintroduced in the reperfusion phase. ROS induces cell dysfunction and death via other mechanisms: activation of metalloproteinases and calpains, mitochondrial permeability transition pore (MPTP) opening which contributes to swelling and lysis of cells. This may elicit the release of proapoptotic factors in the cytosol, thus contributing to cell death. ROS indirectly interact with nitric oxide (NO) production, partly mediated by the inducible NOS (iNOS), the high-capacity NO-producing enzyme. Unlike the other two NOS isoforms, iNOS is not constitutively expressed in cells, and its production is elicited by several stimuli like IRI. NO cytotoxic effects are both direct and indirect mediated by NO reaction with superoxide to form the potent oxidant peroxynitrite which in turn exerts cytotoxicity.

Figure 3

Histomorphological pictures showing the phenotypic results of altered pathways in IRI. (a) Mild calpain 1 expression in the left ventricle cardiac tissue of a patient who died following early reperfused AMI (calpain 1, antibody anti-calpain 1, Santa Cruz, USA). (b) NOX2 expression in the left ventricle cardiac tissue of a patient who died following prompt fibrinolysis in acute STEMI. (c) Strong immunopositivity to anti-nitrotyrosine antibody (Abcam, Cambridge, UK). (d) Mild immunopositivity to anti-iNOS (inducible nitric oxide synthase) antibody (Santa Cruz, CA, USA) in the left ventricle sample of a patient who died following reperfusion therapy in STEMI.

Figure 4

The matrix metalloproteinases system. MMPs activity results from different levels of regulation: transcription, activation, and inhibition by tissue inhibitors of metalloproteinases (TIMPs). During ischemia/reperfusion, oxidative stress stimulates the activity of MMPs, like MMP-2. Several biological activities of MMPs may contribute to myocardial contractile dysfunction and cell death. MMPs can both degrade extracellular matrix (ECM) and modulate different cellular mechanisms, thus leading to contractile dysfunction and modulation of cardiac remodelling and healing.