A Practical, Clinically Oriented Guide
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Why ECG Localization Matters
Accurate localization of MI on ECG helps to:
Identify the culprit coronary artery
Predict complications (heart block, papillary muscle rupture, RV infarction)
Guide urgent reperfusion strategy
Correlate with echocardiography and angiography findings
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Basic ECG Principles in MI Localization
ST-segment elevation reflects acute transmural injury
ST-segment depression may represent reciprocal change or subendocardial ischemia
T-wave inversion indicates evolving or prior ischemia
Pathological Q waves suggest established infarction
Key rule: Always interpret ECG in clinical context and with serial tracings.
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ECG Leads and Myocardial Territories
Inferior Wall MI
Leads: II, III, aVF
Common artery: Right coronary artery (RCA)
Key points:
ST elevation often greater in III than II → favors RCA
Check for associated RV infarction
Reciprocal ST depression in I and aVL
Complications:
AV block
RV infarction
Hypotension
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Anterior Wall MI
Leads: V1–V4
Common artery: Left anterior descending (LAD)
Sub-localization:
Septal: V1–V2
Anterior: V3–V4
Key points:
Large myocardial territory
Poor prognosis if extensive
Look for bundle branch blocks
Complications:
Cardiogenic shock
Ventricular arrhythmias
Ventricular septal rupture
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Extensive Anterior MI
Leads: V1–V6, I, aVL
Artery: Proximal LAD
ECG clues:
ST elevation in precordial + high lateral leads
ST depression in inferior leads
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Lateral Wall MI
Leads: I, aVL, V5, V6
Artery:
Left circumflex (LCx)
Diagonal branches of LAD
High lateral MI:
I, aVL
Low lateral MI:
V5, V6
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Posterior Wall MI
Standard ECG limitation: Posterior wall not directly visualized
Clues in V1–V3:
ST depression
Tall R waves
Upright T waves
Confirm with posterior leads:
V7, V8, V9 → ST elevation confirms posterior MI
Artery:
RCA (most common)
LCx
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Right Ventricular MI
Suspect in:
Inferior MI with hypotension
Clear lungs + elevated JVP
ECG:
ST elevation in V1
ST elevation in right-sided leads (V3R, V4R)
Artery:
Proximal RCA
Clinical pearl:
Avoid nitrates and aggressive diuresis
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Reciprocal Changes: An Important Clue
Reciprocal ST depression:
Confirms acute STEMI
Helps localization
Examples:
Inferior MI → ST depression in I, aVL
Anterior MI → ST depression in II, III, aVF
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NSTEMI and Localization
NSTEMI usually shows:
ST depression
T-wave inversion
Localization is less precise but patterns still help:
Widespread ST depression + ST elevation in aVR → left main or severe multivessel disease
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ECG Localization Summary Table
MI Location Leads Likely Artery
Inferior II, III, aVF RCA
Anterior V1–V4 LAD
Extensive anterior V1–V6, I, aVL Proximal LAD
Lateral I, aVL, V5–V6 LCx / Diagonal
Posterior V7–V9 (or V1–V3 changes) RCA / LCx
Right ventricle V3R–V4R Proximal RCA
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Common Pitfalls
Missing posterior MI without posterior leads
Mislabeling early repolarization as STEMI
Ignoring reciprocal changes
Failing to record right-sided leads in inferior MI
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Practical Clinical Tips
Always obtain serial ECGs
Add posterior and right-sided leads when indicated
Correlate ECG with echo wall motion abnormalities
Treat the patient, not the ECG alone
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Key Takeaway
Systematic ECG analysis allows accurate MI localization, early recognition of complications, and better patient outcomes. Mastery of lead–territory correlation is a core skill for every clinician managing acute coronary syndromes.
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