Acute coronary syndrome is among the highest-yield cardiology topics on the USMLE, testing ECG recognition, biomarkers, and a time-critical reperfusion decision — and the management was refreshed by the 2025 ACC/AHA guideline. This review covers ACS the way the boards test it, reflecting current US practice. Follow current guidelines and institutional protocols clinically; this reflects guidance as of mid-2026.
What ACS is
Acute coronary syndrome is a spectrum of myocardial ischemia from acute plaque rupture and thrombosis in a coronary artery. It spans unstable angina (ischemic symptoms without a biomarker rise), non-ST-elevation myocardial infarction (NSTEMI; a biomarker rise without ST elevation), and ST-elevation myocardial infarction (STEMI; complete occlusion with ST elevation). The distinction drives both urgency and reperfusion strategy. The underlying event is usually rupture or erosion of an atherosclerotic plaque with superimposed thrombus: a fully occlusive thrombus produces transmural ischemia with ST elevation, while a partially occlusive one produces subendocardial ischemia without ST elevation. Type 2 MI, from supply-demand mismatch (severe anemia, tachyarrhythmia, hypotension) without acute plaque rupture, is a distinct and tested entity.
Presentation
The classic presentation is crushing substernal chest pressure radiating to the arm or jaw, with diaphoresis, dyspnea and nausea. Boards reward awareness of atypical presentations — women, older adults and people with diabetes may have dyspnea, fatigue or epigastric discomfort rather than chest pain. A high index of suspicion is essential. Time matters from first medical contact, because myocardium is lost continuously during coronary occlusion — the principle that “time is muscle.”
Diagnosis
Two tests anchor the diagnosis. The ECG distinguishes STEMI (ST elevation, or a new left bundle branch block) from NSTE-ACS (ST depression, T-wave inversion, or no changes), and should be obtained and read within 10 minutes of arrival. High-sensitivity cardiac troponin, trended over serial draws, confirms myocardial injury and separates NSTEMI from unstable angina. Specific patterns are tested: ST depression in V1 to V3 with tall R waves suggests a posterior MI, and an inferior STEMI (II, III, aVF) should prompt right-sided leads to detect right ventricular involvement, in which nitrates are dangerous. Reciprocal ST changes support a true infarction, and serial ECGs help when the initial tracing is equivocal. Troponin can also rise in non-ACS conditions such as pulmonary embolism, myocarditis, sepsis and renal failure, so it is interpreted with the clinical picture, and a single normal troponin does not exclude early ACS.
Management
Initial therapy and reperfusion are the core of the answer.
- Antiplatelets: give an aspirin loading dose immediately, plus an oral P2Y12 inhibitor. The 2025 guideline prefers ticagrelor or prasugrel over clopidogrel for ACS undergoing PCI, with clopidogrel reserved for when these are contraindicated, not tolerated or unavailable, and with prasugrel avoided after a prior stroke or transient ischemic attack.
- Anticoagulation: heparin (unfractionated or low-molecular-weight) is standard; bivalirudin is a reasonable alternative in STEMI PCI.
- Anti-ischemic and supportive care: nitrates for ongoing pain (avoiding them in right ventricular infarction or recent phosphodiesterase inhibitor use), a beta-blocker, and oxygen only if the saturation is below 90% — routine oxygen is not beneficial.
- Reperfusion in STEMI: primary PCI is preferred, with a door-to-balloon goal of 90 minutes or less. If PCI cannot be delivered within 120 minutes of first medical contact, give fibrinolysis (with concurrent clopidogrel), then transfer for PCI.
- NSTE-ACS: risk-stratify, for example with the TIMI or GRACE score, to time an invasive strategy; high-risk features prompt early angiography. Morphine may be given for refractory pain but is no longer routine, since it can blunt and delay P2Y12 absorption.
Secondary prevention
After ACS, the boards expect the durable regimen: dual antiplatelet therapy for at least 12 months (when bleeding risk is not high), a high-intensity statin for every patient, an ACE inhibitor or ARB, a beta-blocker, an aldosterone antagonist where the ejection fraction is reduced, and cardiac rehabilitation with risk-factor control. These measures reduce reinfarction and mortality, and lifestyle change — smoking cessation, diet and exercise — is reinforced at every visit.
High-yield exam points and traps
- ECG within 10 minutes; a STEMI (or new LBBB) goes to immediate reperfusion, NSTE-ACS to risk stratification.
- Primary PCI within 90 minutes of arrival; fibrinolysis if PCI is more than 120 minutes away.
- Ticagrelor or prasugrel is preferred over clopidogrel for PCI; avoid prasugrel after a prior stroke or TIA.
- Oxygen only if the saturation is below 90% — routine supplemental oxygen does not help.
- Avoid nitrates in right ventricular infarction (inferior STEMI with hypotension) — they can cause profound hypotension.
- A high-intensity statin is for every ACS patient, and dual antiplatelet therapy runs for at least 12 months at standard bleeding risk.
- A new left bundle branch block with ischemic symptoms is treated as a STEMI-equivalent.
- Troponin elevation is not specific to ACS — interpret it alongside the ECG and the clinical context.
Common questions
What is the door-to-balloon goal in STEMI? Primary PCI within 90 minutes of arrival; if PCI cannot be delivered within 120 minutes of first medical contact, fibrinolysis is given and the patient transferred.
Which P2Y12 inhibitor is preferred? Ticagrelor or prasugrel over clopidogrel for ACS undergoing PCI, per the 2025 ACC/AHA guideline; clopidogrel is used when these are unsuitable, and prasugrel is avoided after a prior stroke or TIA.
How do you distinguish NSTEMI from unstable angina? Both lack ST elevation; NSTEMI has a rise in cardiac troponin, whereas unstable angina does not — the troponin is the discriminator.
Why avoid nitrates in inferior MI? Nitrates are avoided in inferior MI because an inferior STEMI can involve the right ventricle, which is preload-dependent: nitrates reduce preload and can cause severe hypotension.
Who gets a statin after ACS? Every patient — a high-intensity statin is recommended for all ACS patients as part of secondary prevention, regardless of the baseline LDL level.
Practice acute coronary syndrome and other high-yield topics with free sample questions on iatroX →