ECG Diagnosis and Management of Inferior Myocardial Infarction
Primary Inferior STEMI Criteria
- ST‑segment elevation ≥ 0.1 mV in leads II, III, and aVF defines an acute inferior myocardial infarction. 1
- Reciprocal ST‑segment depression in leads I and aVL is common, especially when the right coronary artery (RCA) is the culprit vessel. 3
- ST elevation greater in lead III than lead II points toward RCA occlusion rather than left circumflex (LCx) occlusion. 4
Right Ventricular (RV) Involvement
- ST‑segment elevation ≥ 0.05 mV in right‑sided chest leads V3R and V4R indicates right‑ventricular infarction. 5
- ST elevation may also appear in lead V1 in the setting of RV infarction. 6
- The RV changes are transient and disappear sooner than inferior‑lead elevations; therefore V3R/V4R should be recorded immediately after symptom onset. 5
- AHA/ACC/Canadian Cardiovascular Society give a Class I, Level A recommendation to obtain right‑sided leads V3R and V4R in every patient with ECG evidence of acute inferior wall ischemia/infarction. 2
Posterior / Posterolateral Extension
- ST‑segment depression in precordial leads V1‑V3 together with inferior STEMI suggests concomitant posterior or posterolateral ischemia. 4
- When posterior involvement is suspected, record posterior leads V7‑V9; a cut‑point of 0.05 mV ST elevation (or 0.1 mV in men ≥ 40 years) is diagnostic. 7
Differentiating the Culprit Vessel
- RCA occlusion pattern: ST elevation in lead III > lead II plus reciprocal ST depression in leads I and aVL. 4
- Proximal RCA occlusion: adds ST elevation in right‑sided leads V3R, V4R and possibly V1. 3
- LCx occlusion pattern: the overall ST‑segment vector is directed less rightward than in RCA occlusion. 4
Clinical Pitfalls & Additional Indicators
- Pseudo‑normalization of previously inverted T waves during chest discomfort can be an early sign of acute myocardial ischemia. 7