Myocardial infarction

Published on 01/03/2015 by admin

Last modified 01/03/2015

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Myocardial infarction

Infarction is defined as the process by which necrosis (cell or tissue death) results from ischaemia (loss of blood supply). Infarction of cardiac muscle (myocardial infarction or MI) is one of the commonest causes of morbidity and mortality in adults living in industrialized societies.

Pathology

The underlying pathology in MI is atherosclerosis, an inflammatory process located within the arterial wall in the form of atheromatous plaques (Fig 27.1). These cause narrowing of the arterial lumen, resulting in reduced coronary perfusion, the clinical manifestation of which is chest pain (angina pectoris). If an unstable plaque ruptures, the released contents precipitate the formation of a clot. This process, known as thrombosis, may result in sudden complete occlusion of the affected artery and infarction of the area of myocardium it supplies.

Fig 27.1 Development of atheroma in coronary arteries, with histopathological section (bottom right).

Definitions

Acute coronary syndrome

The term acute coronary syndrome (ACS) refers to chest pain and other related symptoms attributed to impaired blood supply to the heart. It encompasses ST-segment elevation myocardial infarction (STEMI), non–ST-segment elevation myocardial infarction (NSTEMI) and unstable angina. The ST segment refers to part of the electrical tracing of the heart beat recorded on the electrocardiogram or ECG (Fig 27.2). Pathologically, it is almost always associated with rupture of an atherosclerotic plaque and partial or complete thrombosis of a coronary artery. In some instances ACS may occur from increased demands on the heart, e.g. with severe blood loss, anaemia, tachycardia or severe infections.

Fig 27.2 ECG changes following an MI. (a) Normal ECG. (b) Two hours after onset of chest pain. Note elevated ST segment. (c) Twenty-four hours later the patient had a further episode of chest pain.

Myocardial infarction

The universal definition of acute myocardial infarction (AMI) has undergone changes in recent years in tandem with newer developments in the assays for diagnostic biomarkers. Table 27.1 briefly outlines the Experts Consensus Document on behalf of the Joint Task Force of European and American Cardiology Societies for the Redefinition of Myocardial Infarction.

Table 27.1

Definition of myocardial infarction (MI)

Criteria for acute myocardial infarction

The term myocardial infarction should be used when there is evidence of myocardial necrosis in a clinical setting consistent with myocardial ischaemia. Under these conditions the following meets the diagnosis for myocardial infarction:

Detection of rise and/or fall of cardiac biomarkers (preferably troponin) with at least one value above the 99^th percentile of the upper reference limit, together with evidence of myocardial ischaemia with at least one of the following: