Understanding Posterior ECG Leads

Introduction

When evaluating a standard 12-lead ECG, the posterior part of the heart is not directly visualized. However, some conditions, such as posterior myocardial infarction (MI), require additional ECG leads to assess the posterior wall.

What is a Posterior ECG?

A Posterior ECG is an extension of a 12-lead ECG where additional electrodes are placed on the back (posterior aspect) of the chest to better visualize infarctions affecting the posterior wall of the left ventricle.

Why is it Important?

• Detecting Posterior Myocardial Infarction (MI): Standard ECG leads (V1-V3) may show subtle reciprocal changes (ST depression) that suggest posterior MI, but direct ST-elevation can only be seen using posterior leads.
• Avoiding Misdiagnosis: Without posterior leads, posterior infarctions can be misclassified as anterior ischemia due to reciprocal ST-segment changes.

How is a Posterior ECG Recorded?

• Electrodes are placed at V7, V8, and V9 positions:
  • V7: Left posterior axillary line, at the same level as V6.
  • V8: Left mid-scapular region.
  • V9: Left paraspinal region.
• If ST elevation ≥0.5 mm (or ≥1 mm in men under 40 years) is present in two consecutive posterior leads, it suggests posterior MI.

Key ECG Changes in Posterior MI

1. Standard 12-lead ECG Findings:
   • ST depression in V1-V3 (reciprocal change for posterior wall injury).
   • Tall, broad R waves in V1-V2 (opposite of Q waves seen in anterior MI).
   • Upright T waves in V1-V3 (posterior reciprocal change).
2. Posterior Leads (V7-V9):
   • Direct ST elevation confirming posterior involvement.

Clinical Implications and Treatment

• Reperfusion Therapy: If confirmed, urgent percutaneous coronary intervention (PCI) or thrombolysis is crucial.
• Monitoring for Right Ventricular Involvement: A posterior infarction may be associated with right ventricular MI, requiring extra caution with preload-reducing medications (e.g., nitrates).

Conclusion

Posterior ECG leads (V7-V9) play a crucial role in diagnosing posterior myocardial infarction, which can otherwise be missed on a standard ECG. Early detection leads to more timely and effective treatment.

Recommended Clinical Guidelines

Source recommendations

1. American Heart Association Guidelines on STEMI Management

1. https://www.ahajournals.org/doi/10.1161/01.cir.0000134791.68010.fa
2. https://www.heart.org/en/professional/quality-improvement/mission-lifeline/mission-lifeline-hospital-stemi-and-nstemi-recognition
3. https://professional.heart.org/en/science-news/2021-acc-aha-scai-guideline-for-coronary-artery-revascularization
4. https://www.ahajournals.org/doi/10.1161/cir.0b013e3182742cf6
5. https://www.acc.org/Guidelines

2. European Society of Cardiology Guidelines on Acute Coronary Syndromes

1. https://www.escardio.org/Guidelines/Clinical-Practice-Guidelines/Acute-Coronary-Syndromes-ACS-Guidelines
2. https://academic.oup.com/eurheartj/article/44/38/3720/7243210
3. https://pubmed.ncbi.nlm.nih.gov/37622654/
4. https://academic.oup.com/eurheartj/article/42/14/1289/5898842
5. https://www.ahajournals.org/doi/10.1161/CIR.0000000000001309