Acute Coronary Syndrome (STEMI, NSTEMI, Unstable Angina)
Acute coronary syndrome (ACS) is an umbrella term for the spectrum of conditions caused by a sudden reduction in coronary blood flow, usually from the disruption of an atherosclerotic plaque and overlying thrombosis. It encompasses ST-elevation myocardial infarction (STEMI), non-ST-elevation myocardial infarction (NSTEMI), and unstable angina, which are distinguished by the electrocardiogram and cardiac biomarkers.
Definition
Acute coronary syndrome refers to a group of clinical presentations of acute myocardial ischemia caused by an abrupt reduction in coronary flow, classified into ST-elevation myocardial infarction, non-ST-elevation myocardial infarction, and unstable angina according to electrocardiographic findings and the presence or absence of myocardial necrosis indicated by cardiac biomarkers.
Scope
This topic covers the shared pathophysiology of acute coronary syndromes, the way they are categorised using the electrocardiogram and troponin, and the conceptual relationships between STEMI, NSTEMI, and unstable angina. It is a reference entry on how the syndrome is defined and classified, not a treatment protocol.
Key concepts
- Plaque rupture or erosion with thrombosis
- ST-elevation myocardial infarction (STEMI)
- Non-ST-elevation myocardial infarction (NSTEMI)
- Unstable angina
- Role of the electrocardiogram in triage
- Cardiac troponin and detection of necrosis
- Occlusive versus non-occlusive thrombus
Mechanisms
Most acute coronary syndromes share a common origin: an atherosclerotic plaque ruptures or erodes, exposing thrombogenic material that triggers platelet aggregation and thrombus formation. A fully occlusive thrombus that interrupts flow typically produces ST-segment elevation on the electrocardiogram and a transmural infarction (STEMI). A partially occlusive or intermittently occlusive thrombus produces ischemia without persistent ST elevation; when this causes detectable myocardial necrosis it is NSTEMI, and when it does not, it is unstable angina. The classification therefore reflects the degree and persistence of flow limitation and whether necrosis has occurred.
Clinical relevance
Acute coronary syndrome is a common cardiac emergency and a major focus of cardiovascular care and research. This entry explains the conceptual framework that links its forms for educational reference; it does not provide triage criteria, diagnostic thresholds, or management recommendations for individual patients.
Epidemiology
Acute coronary syndromes are a leading cause of hospital admission and cardiovascular death worldwide, and their incidence tracks the prevalence of atherosclerotic risk factors. The relative proportion of STEMI and non-ST-elevation presentations has shifted over time, in part with changes in biomarker sensitivity and population risk profiles.
Evidence & guidelines
Professional-society guidelines and the universal definition of myocardial infarction provide the classification and diagnostic framework for acute coronary syndromes referenced in this entry. They are cited to convey how the syndrome is defined and subdivided rather than as actionable clinical instructions.
History
The concept of an acute coronary syndrome emerged as it became clear that unstable angina, NSTEMI, and STEMI form a continuum arising from a shared mechanism of plaque disruption and thrombosis rather than being wholly separate diseases. This unifying view, together with the electrocardiographic distinction between ST-elevation and non-ST-elevation presentations, shaped modern classification.
Key figures
- Peter Libby
- Eugene Braunwald
- Robert A. Byrne
- Kristian Thygesen
Related topics
Seminal works
- libby-2013
- byrne-2023
- thygesen-2018
Frequently asked questions
- What distinguishes STEMI, NSTEMI, and unstable angina?
- STEMI shows persistent ST-segment elevation on the electrocardiogram and reflects an occlusive thrombus with transmural infarction; NSTEMI lacks persistent ST elevation but shows myocardial necrosis indicated by elevated troponin; unstable angina has ischemia without detectable necrosis.
- Are all acute coronary syndromes caused by plaque rupture?
- Plaque rupture is the most common trigger, but acute coronary syndromes can also result from plaque erosion or, less often, other mechanisms; the shared feature is an acute reduction in coronary blood flow producing myocardial ischemia.