Acute Coronary Syndromes (ACS) & Myocardial Infarction (MI)
Overview
Acute coronary syndromes (ACS) encompass a clinical spectrum caused by acute myocardial ischaemia: unstable angina (UA), non‑ST‑segment elevation myocardial infarction (NSTEMI) and ST‑segment elevation myocardial infarction (STEMI).
Emergency priorities are rapid recognition, ECG‑directed triage to reperfusion when indicated, early risk stratification for NSTEMI/UA and prompt institution of antithrombotic therapy in line with current UK guidance (NICE NG185; NICE HTG552).
This section summarises pathophysiology, diagnosis (ECG and high‑sensitivity troponin), immediate ED priorities and initial management algorithms relevant to practice and exams.
Definitions
- Myocardial infarction (MI): myocyte necrosis due to sustained ischaemia; diagnosis requires a rise and/or fall in cardiac troponin (hs‑cTn) with ≥1 value above the assay 99th percentile together with evidence of myocardial ischaemia (typical symptoms, ECG changes, imaging or coronary thrombus).
- Unstable angina (UA): new or worsening angina at rest without biochemical evidence of myocyte necrosis.
- NSTEMI: biochemical evidence of myocardial necrosis without persistent ST‑segment elevation.
- STEMI: MI with persistent ST‑segment elevation (or new typical LBBB) indicating transmural ischaemia and usually an acute coronary occlusion.
Pathophysiology and ECG correlates
Acute plaque rupture with thrombus formation causes variable degrees of coronary occlusion. The depth and distribution of ischaemia determine ECG changes: