Inferior STEMI: leads II, III, aVF with RV involvement assessment
Inferior STEMI presents with ST elevation in leads II, III, and aVF, reflecting the inferior wall of the left ventricle supplied by the right coronary artery (RCA) in approximately 80% of patients, or the circumflex artery in the remaining 20%.
Reciprocal ST depression in aVL is the most reliable confirmatory sign of inferior STEMI — its presence increases specificity substantially. Reciprocal changes in leads I and aVL occurring simultaneously with inferior ST elevation should prompt immediate STEMI alert before troponin results return.
Right ventricular involvement assessment is mandatory in all inferior STEMIs. Approximately 30–40% of inferior STEMIs have concurrent RV involvement. Apply right-sided leads (V3R–V6R); ST elevation ≥1 mm in V4R indicates RV MI. Nursing implication: avoid nitroglycerin (drops RV preload and can cause profound hypotension); volume-load the RV; maintain heart rate (RV depends on rate to maintain output).
Anterior STEMI: LAD territory, V1–V4, and high-risk features
Anterior STEMI presents with ST elevation in V1–V4 from LAD (left anterior descending) occlusion. This is the highest-mortality STEMI territory because the LAD supplies the anterior wall, most of the interventricular septum, and the anterolateral wall. Large anterior STEMIs can produce ST elevation across V1–V6 (extensive anterior or anterolateral STEMI).
High-risk anterior STEMI features include: ST elevation in V1 with LBBB morphology (may require modified Sgarbossa criteria for diagnosis), ST elevation in aVR with diffuse ST depression (indicates left main or proximal LAD occlusion with global ischemia), and new LBBB in the context of chest pain (requires the same urgency as overt STEMI).
Anterior STEMI complications include cardiogenic shock, acute mitral regurgitation from papillary muscle involvement, ventricular septal rupture, and complete heart block from septal ischemia involving the conduction system.
Posterior and lateral STEMI: the missed patterns
Lateral STEMI presents with ST elevation in I, aVL, V5, and V6 from diagonal branch or circumflex territory involvement. Isolated high-lateral STEMI (ST elevation in I and aVL only) can be subtle and is commonly missed on initial triage ECG review.
Posterior STEMI is the most commonly missed STEMI. The standard 12-lead shows no ST elevation — it shows ST depression in V1–V3 because these leads face opposite the posterior wall. The key recognition pattern is ST depression in V1–V3 with a dominant R wave in V2 (the Q-wave equivalent viewed from the opposite direction). Posterior leads V7–V9 reveal the true ST elevation. Any patient with anterior ST depression in the context of chest pain should have posterior leads placed before STEMI is excluded.