Introduction
Context
Acute chest pain accounts for approximately 700,000 presentations to the emergency department (ED) per year in England and Wales and for 25% of emergency medical admissions [1].
Acute coronary syndromes (ACS) encompass a broad range of presentations including:
- Unstable angina (UA)
- Non-ST segment elevation myocardial infarction (NSTEMI)
- ST segment elevation myocardial infarction (STEMI)
A clear understanding of the pathophysiology, classification and clinical presentation of these conditions is a pre-requisite for effective subsequent risk stratification and therapy.
Definition of Myocardial Infarction
Myocardial infarction (MI) is defined pathologically as myocardial cell death following prolonged ischaemia. Myocardial necrosis releases proteins (troponins, myoglobin, creatine kinase, etc.) into the circulation, which can be measured biochemically, and also gives rise to a clinical syndrome with characteristic symptoms and electrocardiographic changes. The criteria for acute, evolving or recent MI [2] are listed in the panel on the right.
Classification
In the context of cardiac marker rise, ST segment changes on the ECG define either STEMI or NSTEMI.
At the time of presentation, however, cardiac marker status is unknown, so the classification of patients presenting with ischaemic chest pain is based largely on the ECG.
Most patients with ST elevation at presentation have acute total coronary artery occlusion and progress to STEMI. However, many patients without ST elevation may not have a subsequent cardiac marker rise and are collectively termed non-ST elevation acute coronary syndromes (NSTE-ACS) until their markers define them as NSTEMI (marker rise) or UA (no marker rise) [3].
The image illustrates the classification of acute coronary syndromes. Click on the image to enlarge.