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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- ACS = spectrum from unstable angina to NSTEMI to STEMI, distinguished by ECG and serial troponin
- STEMI = ST elevation + troponin rise — emergent PCI (door-to-balloon <90 min)
- NSTEMI = troponin rise without persistent ST elevation — risk-stratify with TIMI or HEART score, invasive strategy within 24-72 h
- All ACS: aspirin 325 mg + P2Y12 inhibitor (ticagrelor or clopidogrel), anticoagulation (heparin), high-intensity statin
- Key Step 2 CK pearl: new LBBB with chest pain is a STEMI equivalent — treat with emergent reperfusion
Overview
Acute coronary syndromes encompass a spectrum of presentations caused by sudden reduction in coronary blood flow. The unifying pathology is atherosclerotic plaque rupture or erosion with superimposed thrombus formation. Classification depends on ECG findings and troponin dynamics: STEMI shows persistent ST elevation and requires emergent reperfusion; NSTEMI shows troponin rise without persistent ST elevation; unstable angina presents with ischemic symptoms at rest or in a crescendo pattern but without troponin elevation. ACS remains the leading cause of mortality in the United States.
Epidemiology
Coronary heart disease affects approximately 20 million adults in the United States. There are roughly 1.5 million ACS events per year, with NSTEMI now accounting for ~70% of ACS presentations. Major risk factors include smoking, hypertension, diabetes mellitus, hyperlipidemia, family history of premature CAD (first-degree male relative <55 y, female <65 y), obesity, and chronic kidney disease. The in-hospital mortality for STEMI has decreased to ~5-6% with modern reperfusion strategies.
Clinical Features
Symptoms
Substernal chest pressure or heaviness radiating to the left arm, neck, or jaw
Pain at rest or with minimal exertion, lasting >20 minutes
Dyspnea and diaphoresis
Nausea, vomiting, or epigastric discomfort
Syncope or near-syncope
Atypical presentations: isolated dyspnea, fatigue, or silent MI — common in women, elderly, and diabetic patients
Signs
Hemodynamic instability: hypotension, tachycardia, or bradycardia
Signs of acute heart failure: elevated JVP, bibasilar crackles, S3 gallop
Diaphoresis, pallor, distress
New systolic murmur (papillary muscle rupture or VSD)
Examination may be entirely normal
Investigations
First-line
12-lead ECGWithin 10 minutes. ST elevation (>=1 mm in >=2 contiguous limb leads, >=2 mm in >=2 precordial leads), ST depression, T-wave inversions, new LBBB, or Wellens sign (biphasic/deeply inverted T in V2-V3 = critical proximal LAD stenosis)
Serial troponin (hs-cTn preferred)At presentation and 3-6 hours. Rising/falling pattern confirms acute myocardial injury
Basic labsCBC, BMP (creatinine, potassium), glucose, lipid panel, coagulation studies, BNP if HF suspected
Second-line
Chest X-rayPulmonary edema, widened mediastinum (rule out aortic dissection)
EchocardiogramRegional wall motion abnormalities, LVEF, mechanical complications
TIMI or HEART scoreRisk stratification for NSTEMI/UA
Specialist
Coronary angiographyEmergent for STEMI, within 24 h for high-risk NSTEMI, within 72 h for intermediate-risk
CT coronary angiographyLow-to-intermediate risk chest pain (HEART 0-3) to rule out significant CAD
1
Immediate (all ACS)
- Aspirin 162-325 mg (chewed, non-enteric-coated)
- Nitroglycerin SL or IV (avoid if hypotensive, RV infarct, or PDE-5 inhibitor within 24-48 h)
- Morphine IV only if pain unresponsive to nitroglycerin (use cautiously)
- Supplemental O2 only if SpO2 <90%
- Continuous telemetry
- P2Y12 loading: ticagrelor 180 mg or clopidogrel 300-600 mg (prasugrel 60 mg for STEMI going to PCI)
2
STEMI pathway
- Primary PCI: door-to-balloon <90 min (or <120 min if transfer)
- If PCI unavailable within 120 min: fibrinolytics (tenecteplase, alteplase, reteplase) within 30 min
- DAPT: aspirin indefinitely + ticagrelor or prasugrel (preferred over clopidogrel) for 12 months
- UFH bolus + infusion during PCI
- GP IIb/IIIa inhibitors for high thrombus burden
- If fibrinolysis given: assess reperfusion at 60-90 min; rescue PCI if failed
3
NSTEMI / UA pathway
- UFH (weight-based) or enoxaparin 1 mg/kg SC q12h
- DAPT: aspirin + ticagrelor (or clopidogrel if high bleeding risk)
- TIMI >=3 or HEART >=4: early invasive (angiography within 24 h)
- Very high-risk (refractory angina, hemodynamic instability, VT/VF): emergent angiography within 2 h
- Low-risk: ischemia-guided with stress test or CTA
- High-intensity statin: atorvastatin 80 mg regardless of baseline LDL
4
Secondary prevention (all ACS at discharge)
- DAPT 12 months (aspirin 81 mg indefinitely + P2Y12 inhibitor)
- High-intensity statin: atorvastatin 80 mg or rosuvastatin 20-40 mg
- ACEi/ARB if LVEF <=40%, anterior MI, diabetes, or hypertension
- Beta-blocker if LVEF <=40% (carvedilol, metoprolol succinate, bisoprolol)
- Eplerenone/spironolactone if LVEF <=40% + HF or diabetes (Cr <2.5, K+ <5.0)
- Cardiac rehab referral; smoking cessation; BP goal <130/80
Complications
- Arrhythmias: VF/VT (most common cause of early death), AV block (inferior MI), AF
- Heart failure: Acute LV failure, cardiogenic shock (Killip IV, ~50% mortality)
- Mechanical: Papillary muscle rupture (acute MR, days 2-7), VSD, free wall rupture (tamponade, days 3-5)
- Pericarditis: Early (24-72 h) or Dressler syndrome (2-10 weeks, autoimmune)
- Thromboembolism: LV mural thrombus (anterior/apical akinesis)
- Ventricular aneurysm: Persistent ST elevation weeks post-MI
USMLE Step 2 CK Exam Tips
- 1Chest pain + ST elevation + troponin = STEMI. Answer is PCI unless explicitly unavailable (then fibrinolytics)
- 2New LBBB + acute chest pain = STEMI equivalent — activate the cath lab
- 3Wellens sign (biphasic/deep T inversions V2-V3, minimal troponin) = critical LAD stenosis — needs angiography, NOT a stress test
- 4Prasugrel is contraindicated with prior stroke/TIA; avoid if age >=75 or weight <60 kg
- 5RV infarct (ST elevation V4R, hypotension, inferior MI, clear lungs): IV fluids, avoid nitroglycerin/diuretics
- 6Dressler syndrome = autoimmune pericarditis 2-10 weeks post-MI — aspirin/NSAIDs + colchicine, NOT antibiotics
- 7Most common cause of death in first hour after MI = ventricular fibrillation
- 8Post-MI: no verapamil/diltiazem if LVEF reduced — use beta-blocker
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