Acute coronary syndrome covers the spectrum from unstable angina through NSTEMI to STEMI. For the examination, almost every mark turns on two decisions: is the ECG diagnostic of ST elevation, and can reperfusion be delivered in time.
Definitions and classification
Acute coronary syndrome (ACS) describes the clinical spectrum resulting from acute myocardial ischaemia. It is separated by the initial ECG and subsequent troponin result.
| Entity | ECG | Troponin | Immediate priority |
|---|---|---|---|
| STEMI | ST elevation or new LBBB | Rises (do not wait) | Reperfusion within 120 minutes |
| NSTEMI | ST depression, T inversion or normal | Elevated | Risk stratify, early invasive if high risk |
| Unstable angina | Often normal | Normal | Risk stratify, antiplatelet therapy |
Recognition
Classic presentation is central crushing chest pain radiating to the jaw or left arm with autonomic features. The examination reliably tests the atypical presentations, because those are the ones missed in practice.
- Diabetic patients may present with breathlessness or collapse without pain, due to autonomic neuropathy.
- Older patients present with delirium, falls or non-specific deterioration in around a third of cases.
- Women more frequently report nausea, fatigue and interscapular pain rather than crushing central pain.
- Posterior infarction produces ST depression in V1–V3 with tall R waves — read it as a mirror image.
If a question describes ST depression in V1 to V3 with dominant R waves, obtain posterior leads V7 to V9 before concluding it is an NSTEMI. This is the single most common trap in ACS questions.
Immediate management
Initial treatment is common to the whole spectrum, then diverges on the reperfusion decision.
- Aspirin 300 mg orally, chewed or dispersed.
- Oxygen only if saturations fall below 94% — routine oxygen worsens outcomes in normoxic patients.
- Analgesia with intravenous morphine plus an antiemetic; sublingual GTN unless hypotensive or right ventricular infarction is suspected.
- Second antiplatelet agent per local protocol.
- Continuous cardiac monitoring — ventricular fibrillation risk is highest in the first hour.
The reperfusion decision in STEMI
Primary percutaneous coronary intervention is the reperfusion strategy of choice where it can be delivered within 120 minutes of the time thrombolysis could otherwise have been given. Beyond that window, thrombolysis is given first and the patient is transferred for angiography.
High-yield exam points
- Inferior STEMI (II, III, aVF) — always obtain right-sided leads. RV infarction is preload dependent, so nitrates can cause profound hypotension.
- New left bundle branch block with a convincing history is treated as STEMI.
- Do not delay reperfusion for troponin in a diagnostic ECG. The diagnosis is already made.
- Anticoagulation excludes thrombolysis but does not exclude primary PCI.
- Wellens' syndrome — deep T inversion in V2–V3 during pain-free periods — signals critical LAD stenosis despite an apparently reassuring patient.
Complications to anticipate
Examiners frequently move a stable ACS stem into a complication in the second half of the question.
- Arrhythmia — VF in the first hour, complete heart block in inferior infarction.
- Cardiogenic shock — occurs in roughly 7%, mortality above 40%.
- Mechanical complications — papillary muscle rupture, ventricular septal defect, free wall rupture, typically days 2 to 7.
- Pericarditis — early or as Dressler's syndrome weeks later.
Guideline update, June 2026. The revised RCEM position now recommends against routine high-flow oxygen in normoxic ACS. Nine questions in this chapter were rewritten to reflect this.