Why ACS Is a Priority OSCE Topic
Acute coronary syndrome encompasses the spectrum from unstable angina through NSTEMI to STEMI. It is one of the most examined acute medical presentations. OSCEs test recognition, immediate prescribing, risk stratification, ECG interpretation in context, and secondary prevention counselling. The STEMI pathway has a strict time target — examiners will mark you on whether you know it.
⚠️ Red Flag
Door-to-balloon time target for primary PCI in STEMI: within 90 minutes of hospital arrival. State this in any STEMI management scenario. If PCI is not achievable within 120 minutes, thrombolysis should be considered.
The ACS Spectrum
| Diagnosis | Troponin | ECG | Mechanism |
|---|---|---|---|
| Unstable Angina (UA) | Normal | ST depression / T-wave inversion / normal | Partial occlusion; no myocyte death |
| NSTEMI | Elevated | ST depression / T-wave inversion / normal | Partial occlusion with myocyte necrosis |
| STEMI | Elevated | ST elevation in 2 or more contiguous leads | Complete occlusion — full-thickness ischaemia |
ECG Localisation of STEMI
| Territory | ECG leads affected | Artery |
|---|---|---|
| Anterior | V1-V4 | Left anterior descending (LAD) |
| Lateral | I, aVL, V5-V6 | Left circumflex (LCx) |
| Inferior | II, III, aVF | Right coronary artery (RCA) |
| Posterior | Dominant R wave in V1-V2; ST depression V1-V3 | RCA or LCx |
| Right ventricular | ST elevation in V4R | RCA proximal |
⚠️ Red Flag
Right ventricular infarction (inferior STEMI with RV involvement) causes a unique management challenge. These patients are preload-dependent — GTN and diuretics cause severe hypotension. Always do right-sided ECG (V4R) in inferior STEMI and avoid vasodilators if RV involvement confirmed.
ECG Criteria for STEMI
- New ST elevation at the J-point in 2 or more contiguous leads:
- 1 mm or more in all leads except V2-V3
- 2 mm or more in V2-V3 in men (1.5 mm in women)
- New left bundle branch block (LBBB) with typical symptoms — treat as STEMI until proven otherwise
Immediate Management — MONA + Dual Antiplatelet
🧠 Mnemonic
MONA — immediate ACS treatment:
- M orphine — 2.5-5 mg IV titrated for pain (but note: may delay P2Y12 absorption — give with antiemetic)
- O xygen — only if SpO2 below 94% (oxygen is harmful above 94% in ACS — causes vasoconstriction)
- N itrates — GTN sublingual (300-600 micrograms) or IV for ongoing pain (avoid if systolic below 90 or RV infarction)
- A spirin 300 mg orally — stat, chewed for faster absorption
Plus:
- P2Y12 inhibitor (ticagrelor 180 mg loading dose for NSTEMI/STEMI; or clopidogrel 300-600 mg loading dose)
- Anticoagulation: fondaparinux 2.5 mg SC for NSTEMI (OASIS-5); unfractionated heparin IV for STEMI going to primary PCI
STEMI Pathway
- 1ECG within 10 minutes of arrival
- 2Confirm STEMI criteria
- 3Activate cath lab immediately — "primary PCI call"
- 4Dual antiplatelet loading (aspirin 300 mg + ticagrelor 180 mg or prasugrel 60 mg)
- 5Anticoagulation (unfractionated heparin)
- 6Transfer to cath lab for primary PCI within 90 minutes
- 7Ongoing monitoring post-PCI in CCU
If primary PCI not available within 120 minutes:
- Thrombolysis with tenecteplase (dose by weight)
- Transfer to PCI-capable centre afterwards (pharmaco-invasive strategy)
NSTEMI/UA Pathway — GRACE Score Risk Stratification
The GRACE (Global Registry of Acute Coronary Events) score predicts 6-month mortality and guides timing of invasive strategy.
| GRACE risk | Predicted 6-month mortality | Strategy |
|---|---|---|
| Low (below 109) | Below 3% | Conservative or early invasive at 72 hours |
| Intermediate (109-140) | 3-6% | Invasive strategy within 24 hours |
| High (above 140) | Above 6% | Immediate invasive strategy (within 2 hours) |
Very high risk features requiring immediate invasive strategy:
- Haemodynamic instability or cardiogenic shock
- Recurrent or refractory chest pain
- Life-threatening arrhythmia
- Mechanical complications (VSD, acute MR)
- Acute heart failure
Troponin and Diagnosis
| Parameter | Detail |
|---|---|
| High-sensitivity troponin I or T | Measured at 0 and 1 hour (or 0 and 3 hours) |
| Normal value | Below the 99th percentile for the assay |
| NSTEMI diagnosis | Rise and/or fall pattern with at least one value above the 99th percentile, in the context of ischaemic symptoms or ECG changes |
| Troponin rise without ACS | Myocarditis, PE, sepsis, renal failure, subarachnoid haemorrhage, demand ischaemia (Type 2 MI) |
💎 Clinical Pearl
Troponin elevation alone does not diagnose ACS. It requires a consistent clinical context and a rise-fall pattern. Always interpret in context — troponin is elevated in many acute conditions. Documenting the delta (change between serial measurements) is essential.
Secondary Prevention Post-ACS
| Intervention | Drug and dose | Duration |
|---|---|---|
| Antiplatelet 1 | Aspirin 75 mg OD | Lifelong |
| Antiplatelet 2 | Ticagrelor 90 mg BD or clopidogrel 75 mg OD | 12 months post-ACS |
| ACE inhibitor | Ramipril 2.5 mg titrated to 10 mg OD | Lifelong |
| Beta-blocker | Bisoprolol 1.25 mg titrated to 10 mg OD | At least 12 months; consider lifelong if LV impairment |
| Statin | Atorvastatin 80 mg OD | Lifelong |
| Aldosterone antagonist | Eplerenone 25-50 mg OD | If LVEF below 40% and HF or diabetes post-MI |
Frequently Asked Questions
"What is the difference between Type 1 and Type 2 MI?"
Type 1 MI results from atherosclerotic plaque rupture causing thrombotic coronary occlusion — classic ACS. Type 2 MI results from myocardial oxygen supply-demand mismatch without plaque rupture — caused by tachyarrhythmia, hypotension, severe anaemia, or vasospasm. Type 2 MI does not require dual antiplatelet therapy or PCI — treat the underlying cause. Distinguishing the two is important and increasingly examined.
"Why is oxygen potentially harmful in ACS?"
In normoxic patients (SpO2 94% or above), supplemental oxygen causes coronary vasoconstriction and increases oxidative stress in the ischaemic myocardium, potentially expanding infarct size. The AVOID and DETO2X-AMI trials showed no benefit and possible harm from routine high-flow oxygen in ACS. NICE and ESC guidelines now recommend oxygen only if SpO2 falls below 94%.
"What are the contraindications to thrombolysis in STEMI?"
Absolute: prior haemorrhagic stroke (any time), ischaemic stroke within 6 months, central nervous system surgery or trauma within 3 months, active bleeding (excluding menstruation), aortic dissection, uncontrolled hypertension above 180/110. Relative: TIA within 6 months, oral anticoagulation, pregnancy, non-compressible puncture sites, refractory hypertension, advanced liver disease, infective endocarditis.
"How do you calculate the GRACE score in practice?"
The GRACE score uses eight variables: age, heart rate, systolic BP, creatinine, cardiac arrest at presentation, ST deviation, elevated troponin, and Killip class (degree of heart failure). It is calculated using an online calculator or smartphone app — not memorised. In an OSCE, state that you would use the GRACE score to guide timing of angiography and explain which risk group the patient falls into.
"What is Dressler's syndrome?"
Dressler's syndrome is an autoimmune pericarditis occurring 2-12 weeks after myocardial infarction (or cardiac surgery). It presents with fever, pleuritic chest pain, and a pericardial rub. ECG shows saddle-shaped ST elevation. It is caused by an immune reaction to myocardial antigens released at the time of infarction. Treatment: NSAIDs or aspirin (in high dose), colchicine. Steroids for refractory cases.
Related Posts
- Chest Pain History OSCE — systematic approach to the patient presenting with chest pain
- ECG Interpretation OSCE — interpreting the 12-lead ECG in acute ACS presentations
- Cardiovascular Examination OSCE — examining for haemodynamic compromise in ACS