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Posterior mi indications

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Posterior MI - ECG Indications & Clinical Features

Anatomy & Epidemiology

Posterior MI accounts for 15-20% of all AMIs and usually occurs with inferior or inferolateral infarctions. Isolated posterior MI (showing ST elevation only in posterior leads V7-V9) accounts for ~5% of AMI cases.
Culprit vessels:
  • Right coronary artery (RCA) or its posterior descending branch
  • Left circumflex artery (LCx) - nondominant; isolated posterior MI is typically a nondominant LCx occlusion
Note: Cardiac MRI evidence suggests these so-called "posterior" infarctions actually involve the lateral left ventricular wall. A consensus document now recommends reclassifying them as inferobasal MI.

ECG Indications (12-Lead)

Because no standard leads directly face the posterior wall, diagnosis relies on reciprocal (mirror-image) changes in right precordial leads V1-V3:
ECG FindingLeadsSignificance
Horizontal ST depressionV1-V3Most important sign; mirror of posterior ST elevation
Upright (tall) T waveV1-V3Posterior T-wave inversion seen as upright anteriorly
Tall, broad R wave (≥0.04 sec in V1-V2)V1-V2Mirror image of posterior Q wave; may be delayed
R/S ratio ≥1 with positive T waveV1-V2Suggests prior posterior MI (absent RVH or RBBB)
The combination of horizontal ST depression + upright T wave in V1-V3 increases diagnostic accuracy significantly.
The ECG above shows a classic inferior MI with posterior extension - note the ST depression in V1-V3.

Posterior Leads (V7-V9) - Confirmatory

Extending the ECG with posterior leads V7-V9 increases sensitivity for detecting LCx-related injury:
  • ST elevation ≥0.5 mm in V7-V9 = direct evidence of posterior wall injury
  • Leads are placed at the posterior left chest (V7: posterior axillary line, V8: tip of scapula, V9: left paraspinal)
  • Tintinalli recommends placing V7-V9 specifically when posterior MI is suspected from circumflex lesion

Key Clinical Indications to Suspect Posterior MI

  1. ST depression in V1-V3 without anterior ischemic symptoms - think posterior MI
  2. Inferior STEMI (II, III, aVF) with V1-V3 depression - posterior extension increases infarct size, lowers EF, worsens outcomes
  3. Tall dominant R in V1 without other explanation (no RVH, no RBBB, no WPW)
  4. Isolated LCx territory ischemia - the LCx is "ECG silent" on the standard 12-lead; posterior leads essential

Treatment Implications

  • Posterior MI qualifies as STEMI equivalent - immediate reperfusion (PCI preferred)
  • Fibrinolytic therapy is generally contraindicated in ST-depression UNLESS posterior MI is suspected (Washington Manual: fibrinolytics are contraindicated "in patients with ECG evidence of ST-segment depressions, unless posterior MI suspected")
  • Posterior MI with inferior involvement: order right-sided leads (V4R) simultaneously to rule out RV infarction (affects management - avoid nitrates, preload-dependent state)

Association with Right Ventricular Infarction

Proximal RCA occlusion can cause inferior + posterior + RV infarction simultaneously. In patients with inferior STEMI + posterior extension:
  • Larger infarct zone, lower ejection fraction
  • Higher cardiovascular morbidity and mortality than isolated inferior MI
  • ST elevation in V1 with inferior STEMI + V4R elevation = concomitant RV infarction
Sources: Rosen's Emergency Medicine, p. 1006 | Goldman-Cecil Medicine | Washington Manual of Medical Therapeutics

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Posterior MI ECG Quick Reference

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What's inside the guide (8 sections):
  1. 12-Lead ECG Findings - Color-coded table of all 4 reciprocal changes in V1-V3 (horizontal STD, tall R, upright T, R/S ratio ≥1) with mechanisms
  2. Posterior Leads V7-V9 - Placement guide and the ≥0.5 mm ST elevation threshold
  3. Culprit Vessel Patterns - RCA proximal vs RCA PDA vs nondominant LCx with ECG clues for each
  4. Clinical Triggers - 4 scenarios when to suspect posterior MI (including the "ECG silent" LCx)
  5. Management Pearls - Reperfusion strategy, fibrinolytic exception, nitrate caution in RV infarction
  6. Differential Diagnosis - 6 conditions that mimic tall R in V1
  7. Quick Memory Aid - Large visual 3-box triad: ↓ ST / ↑ T / ↑ R
  8. Step-by-Step Diagnostic Algorithm - 5 color-coded steps from 12-lead to cath lab activation
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