Posterior STEMI: why standard 12-lead ECG shows ST depression, not elevation
Posterior STEMI is the most commonly missed STEMI pattern on both nursing and physician ECG review. The reason: the standard 12-lead ECG has no leads that directly face the posterior wall of the left ventricle. V1–V3 look at the posterior wall from the anterior direction — the opposite side.
The result: posterior STEMI produces ST DEPRESSION (not elevation) in V1–V3, representing the reciprocal view of posterior wall ST elevation. The pattern mimics NSTEMI or subendocardial ischemia — leading to missed cath lab activation.
Key recognition features: (1) New ST depression in V1–V3, especially in a patient with typical chest pain presentation. (2) Tall, broad R wave in V1–V2 — the Q-wave equivalent for posterior infarction (the Q wave, seen as a positive deflection when viewed from the posterior direction, appears as a dominant R from the anterior V1–V2 view). (3) Upright T waves in V1–V2 (normally negative or biphasic).
Posterior leads V7–V9: confirming the missed STEMI
Standard rule: any patient with ST depression in V1–V3 and clinical chest pain presentation should have posterior leads applied BEFORE STEMI is excluded.
Posterior lead placement: V7 — left posterior axillary line, same horizontal level as V4–V6. V8 — left mid-scapular line. V9 — left paravertebral border, same level. Apply like standard precordial leads, same horizontal level, patient supine.
Diagnostic criterion: ST elevation ≥0.5mm in V7–V9 = posterior STEMI. This is a lower threshold than anterior STEMI (2mm) because posterior leads have inherently lower voltage. Posterior STEMI is predominantly from circumflex artery (LCx) occlusion — often accompanied by inferior or lateral changes.