Pathophysiology Of Acute Myocardial Infarction
Infarction is tissue death caused by ischaemia. Acute myocardial infarction (MI) occurs when localized myocardial ischaemia causes the development of a defined region of necrosis. MI is most often caused by rupture of an atherosclerotic lesion in a coronary artery. This causes the formation of a thrombus that plugs the artery, stopping it from supplying blood to the region of the heart that it supplies.
Pivotal studies by DeWood and colleagues showed that coronary thrombosis is the critical event resulting in MI. Of patients presenting within 4 h of symptom onset with ECG evidence of transmural MI, coronary angiography showed that 87% had complete thrombotic occlusion of the infarct-related artery. The incidence of total occlusion fell to 65% 12–24 h after symptom onset due to spontaneous fibrinolysis. Fresh thrombi on top of ruptured plaques have also been demonstrated in the infarct-related arteries in patients dying of MI.
Mechanisms and consequences of plaque rupture
Coronary plaques that are prone to rupture are typically small and non-obstructive, with a large lipid-rich core covered by a thin fibrous cap. These ‘high-risk’ plaques typically contain abundant macrophages and T lymphocytes which are thought to release metalloproteases and cytokines that weaken the fibrous cap, rendering it liable to tear or erode due to the shear stress exerted by the blood flow.
Plaque rupture reveals subendothelial collagen, which serves as a site of platelet adhesion, activation and aggregation. This results in:
1 release of substances such as thromboxane A2 (TXA2), fibrinogen, 5-hydroxytryptamine (5-HT), platelet activating factor and adenosine diphosphate (ADP), which further promote platelet aggregation.
2 Activation of the clotting cascade, leading to fibrin formation and propagation and stabilization of the occlusive thrombus.
The endothelium is often damaged around areas of coronary artery disease. The resulting deficit of antithrombotic factors such as thrombomodulin and prostacyclin enhances thrombus formation. In addition, the tendency of several platelet-derived factors (e.g. TXA2, 5-HT) to cause vasoconstriction is increased in the absence of endothelial-derived relaxing factors. This may promote the development of local vasospasm, which worsens coronary occlusion.
Sudden death and acute coronary syndrome onset show a circadian variation (daily cycle), peaking at around 9 a.m. with a trough at around 11 p.m. Levels of catecholamines peak about an hour after awakening in the morning, resulting in maximal levels of platelet aggregability, vascular tone, heart rate and blood pressure, which may trigger plaque rupture and thrombosis. Increased physical and mental stress can also cause MI and sudden death, supporting a role for increases in catecholamines in MI pathophysiology. Furthermore, chronic β-adrenergic receptor blockade abolishes the circadian rhythm of MI.
Autopsies of young subjects killed in road accidents often show small plaque ruptures in susceptible arteries, suggesting that plaque rupture does not always have pathological consequences. The degree of coronary occlusion and myocardial damage caused by plaque rupture probably depends on systemic catecholamine levels, as well as local factors such as plaque location and morphology, the depth of plaque rupture and the extent to which coronary vasoconstriction occurs.
Severe and prolonged ischaemia produces a region of necrosis spanning the entire thickness of the myocardial wall. Such a transmural infarct usually causes ST segment elevation (i.e. STEMI; see Chapter 45). Less severe and protracted ischaemia can arise when:
· Coronary occlusion is followed by spontaneous reperfusion
· The infarct-related artery is not completely occluded
· Occlusion is complete, but an existing collateral blood supply prevents complete ischaemia
· The oxygen demand in the affected zone of myocardium is smaller.
Under these conditions, the necrotic zone may be mainly limited to the sub endocardium, typically causing non-ST segment elevation MI.
The classification of acute MI according to the presence or absence of ST segment elevation is designed to allow rapid decision-making concerning whether thrombolysis should be initiated (see Chapter 43). This classification replaces the previous one, based on the presence or absence of Q waves on the ECG, which was less useful for guiding immediate therapy.
Evolution of the infarct
Both infarcted and unaffected myocardial regions undergo progressive changes over the hours, days and weeks following coronary thrombosis. This process of postinfarct myocardial evolution leads to the occurrence of characteristic complications at predict- able times after the initial event (see Chapter 45).
Ischaemia causes an immediate loss of contractility in the affected myocardium, a condition termed hypokinesis. Necrosis starts to develop in the subendocardium (which is most prone to ischaemia; see Chapter 2), about 15–30 min after coronary occlusion. The necrotic region grows outward towards the epicardium over the next 3–6 h, eventually spanning the entire ventricular wall. In some areas (generally at the edges of the infarct) the myocardium is stunned (reversibly damaged) but will eventually recover if blood flow is restored. Contractility in the remaining viable myocardium increases, a process termed hyperkinesis.
A progression of cellular, histological and gross changes develop within the infarct. Although alterations in the gross appearance of infarcted tissue are not apparent for at least 6 h after the onset of cell death, cell biochemistry and ultrastructure begin to show abnormalities within 20 min. Cell damage is progressive, becomingly increasingly irreversible over about 12 h. This period there- fore provides a window of opportunity during which percutaneous coronary intervention (PCI) or thrombolysis leading to reperfusion may salvage some of the infarct (see Chapter 43).
Between 4 and 12 h after cell death starts, the infarcted myocardium begins to undergo coagulation necrosis, a process characterized by cell swelling, organel lebreak down and protein denaturation. After about 18 h, neutrophils (phagocytic lymphocytes) enter the infarct. Their numbers reach a peak after about 5 days, and then decline. After 3–4 days, granulation tissue appears at the edges of the infarct zone. This consists of macrophages, fibroblasts, which lay down scar tissue, and new capillaries. The infarcted myocardium is especially soft between 4 and 7 days, and is therefore maximally prone to rupturing. This event is usually fatal, may occur at any time during the first 2 weeks, and is responsible for about 10% of MI mortality. As the granulation tissue migrates inward toward the centre of the infarct over several weeks, the necrotic tissue is engulfed and digested by the macrophages. The granulation tissue then progressively matures, with an increase in connective (scar) tissue and loss of capillaries. After 2–3 months, the infarct has healed, leaving a non-contracting region of the ventricular wall that is thinned, firm and pale grey.
Infarct expansion, the stretching and thinning of the infarcted wall, may occur within the first day or so after an MI, especially if the infarction is large or transmural, or has an anterior location. Over the course of several months, there is progressive dilatation, not only of the infarct zone, but also of healthy myocardium. This process of ventricular remodelling is caused by an increase in end diastolic wall stress. Infarct expansion puts patients at a substantial risk for the development of congestive heart failure, ventricular arrhythmias and free wall rupture.