12-Lead ECG Basics for ACS

ST elevation in leads II, III, and aVF tells you exactly which coronary artery is occluding and which complications to anticipate — if you know how to read the map.

A 12-lead ECG is the first diagnostic tool in ACS because it localizes ischemia to a specific myocardial wall and guides the treatment pathway. The ECG must be obtained within 10 minutes of arrival. ST-segment elevation ≥1 mm in two or more contiguous leads defines STEMI and triggers the cath lab. Contiguous leads map to coronary territories: leads II, III, aVF reflect the inferior wall (right coronary artery); V1–V4 reflect the anterior wall (left anterior descending); I, aVL, V5–V6 reflect the lateral wall (left circumflex). Reciprocal ST depression in leads opposite the injury confirms the diagnosis and increases specificity. ST depression or deep T-wave inversions without ST elevation suggest NSTEMI or unstable angina — the ECG alone cannot differentiate these two, which is why biomarkers are needed. Right-sided leads (V4R) should be obtained when inferior STEMI is identified to rule out right ventricular involvement, because nitrates and volume depletion are dangerous in RV infarction. New left bundle branch block in the setting of chest pain is treated as a STEMI equivalent.

Key Distinctions

Don't confuse ST elevation (acute injury, STEMI) with ST depression (ischemia without full occlusion). Students mix up lead groups: II, III, aVF is inferior — not anterior. A common error is forgetting that a new LBBB with ACS symptoms is a STEMI equivalent and requires the same urgent response as classic ST elevation.

Clinical Pearl

Inferior STEMI → think right side. Always get V4R before giving nitro — an RV infarct plus nitrates equals profound hypotension.