Myocardial infarction
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.
Definitions
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.
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 99th percentile of the upper reference limit, together with evidence of myocardial ischaemia with at least one of the following:
ECG changes indicative of new ischaemia (new ST-T changes or new left bundle branch block (LBBB));