Stable angina is a chronic condition that occurs on a relatively predictable basis on exertion when cardiac ischaemia develops due to the inability of a narrowed coronary artery to meet an increased cardiac oxygen demand. Conversely, the acute coronary syndromes (ACS), including in ascending order of severity, unstable angina (UA), non-ST segment elevation myocardial infarction (NSTEMI) and ST segment elevation myocardial infarction (STEMI), represent a spectrum of dangerous conditions in which myocardial ischaemia results from a sudden decrease in the flow of blood through a coronary vessel. This decrease is almost always initiated by the rupture of an atherosclerotic plaque, resulting in the formation of an intracoronary thrombus that diminishes or abolishes the flow of blood.

When a patient presents with suspected ACS, serial ECGs are immediately carried out. The hallmark of STEMI is sustained elevation of the ST segments of the ECG (Figure 42, upper left). This indicates that a large area of the myocardium, probably involving the full thickness of a ventricular wall, has developed a lesion as a result of prolonged ischaemia. Myocardial damage releases intracellular proteins, such as troponins T and I into the blood. These serve as important markers of myocardial injury and as a prognostic tool. STEMI is confirmed when elevated levels of these markers are found in addition to the requisite ECG changes. STEMI typically occurs when a thrombus has completely occluded a coronary artery for a significant period of time, and usually causes more severe symptoms than do unstable angina or NSTEMI.

Incomplete or temporary coronary occlusion, or the existence of collateral coronary arteries that can maintain some supply of blood to the affected region, may result in a smaller degree of myocardial infarction (MI) and necrosis. This may not result in ST segment elevation, but does cause increased levels of cardiac markers of damage in the plasma. Patients with ACS who are found to have elevated levels of these markers, but who do not exhibit ST segment elevation, are deemed to have suffered an NSTEMI.

Patients who demonstrate symptoms associated with ACS, but who have neither ST segment elevation nor raised levels of troponins, are deemed to have unstable angina. In this case, it is likely that the coronary obstruction has been of limited extent and/or duration (<20 min), and is thus sufficient to cause ischaemia but not detectable injury. Both NSTEMIs and UA may be associated with ECG changes other than ST elevation, for example ST segment depression and T-wave inversion.

Both NSTEMI and STEMI are grouped together as acute MIs, but are managed differently in the acute phase, in that reperfusion therapies, either pharmacological (thrombolysis), or preferably percutaneous coronary intervention

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