An acute coronary syndrome occurs when atherosclerotic coronary plaque becomes unstable leading to a series of events eventually resulting in partial or total thrombotic occlusion of a coronary artery. Acute coronary syndromes are categorized into unstable angina, non-ST segment elevation myocardial infarctions and ST segment elevation myocardial infarction. The terms “transmural”, “non-transmural”, “Q wave MI” and “non-Q wave MI” are no longer recommended. The differences between the types of acute coronary syndromes are below:

Unstable angina pectoris: Three different presentations of unstable angina exist.

1. Exertional angina of new onset. Even if relieved with rest and requiring a consistent amount of exertion to procedure symptoms, when angina first occurs it is considered unstable.

2. Exertional angina that was previously stable and now occurs with less physical exertion.

3. Anginal symptoms at rest without physical exertion.

Non-ST segment elevation myocardial infarction: Anginal symptoms at rest that result in myocardial necrosis as identified by elevated cardiac biomarkers (see Cardiac Biomarkers) with no ST segment elevation on the 12-lead electrocardiogram.

ST segment elevation myocardial infarction: Anginal symptoms at rest that result in myocardial necrosis as identified by elevated cardiac biomarkers (see Cardiac Biomarkers) with ST segment elevation on the 12-lead electrocardiogram.