Criteria of Culprit Artery in Inferior Wall STEMI

Identifying the culprit artery (RCA vs LCX) in inferior wall STEMI from surface ECG helps anticipate complications, guide cath strategy, and assess myocardial area at risk.

This post summarizes ECG-based criteria commonly used to differentiate RCA from LCX occlusion.

ST Elevation in Leads III and II

Key principle: Lead III reflects RCA territory more strongly than lead II

Findings favoring RCA occlusion

ST elevation in lead III > lead II

ST depression in aVL greater than lead I

These patterns indicate an injury vector directed inferiorly and rightward, consistent with RCA involvement.

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Role of Lateral Precordial Leads (V5–V6)

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V5 and V6 have limited value in differentiating RCA from LCX occlusion

Presence of ST elevation in V5–V6 suggests:

Larger myocardial area at risk

Possible extension beyond isolated inferior infarction

They should be interpreted as markers of infarct size rather than culprit artery.

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ST-Segment Behavior in Lead I

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Lead I provides useful lateral wall information.

Patterns and interpretation

ST depression in lead I → RCA occlusion

ST elevation in lead I → LCX occlusion

Isoelectric ST in lead I with STE II > III → LCX occlusion

LCX infarctions tend to generate a leftward injury vector, explaining ST elevation or neutrality in lead I.

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STD in V3 / STE in Lead III Ratio

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This ratio helps localize proximal vs distal RCA and differentiate from LCX.

Interpretation

< 0.5 → Proximal RCA occlusion

0.5 – 1.2 → Distal RCA occlusion

> 1.2 → LCX occlusion

Higher ratios indicate greater posterior or lateral involvement, favoring LCX.

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Practical Summary

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Findings favoring RCA

STE III > II

STD in aVL > I

STD in lead I

Low V3/III ratio (<1.2)

Findings favoring LCX

STE in lead I

Isoelectric lead I with STE II > III

High V3/III ratio (>1.2)

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Clinical Takeaway

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No single ECG criterion is absolute. Accuracy improves when multiple ECG features are integrated with:

Clinical presentation

Hemodynamic status

Echocardiographic findings

Surface ECG remains a powerful bedside tool for early culprit artery prediction—especially before angiography.

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