Inferior Myocardial Infarction (MI) on ECG

Introduction

Inferior myocardial infarction (MI) refers to a heart attack that affects the inferior (bottom) wall of the heart, typically involving the right coronary artery (RCA) or, less commonly, the left circumflex artery.

Identifying an inferior MI on an electrocardiogram (ECG) is crucial for early diagnosis and treatment.

ECG Findings of Inferior MI

To diagnose an inferior MI, the following changes should be observed on an ECG:

1. ST-Segment Elevation

• Found in leads II, III, and aVF.
• ST elevation in lead III is often greater than in lead II.

2. Reciprocal ST-Segment Depression

• Seen in the lateral leads (I, aVL, sometimes V2-V3).
• This finding supports the diagnosis.

3. T-Wave Changes

• Typically inverted T-waves can appear in the affected leads.
• Hyperacute (peaked) T-waves may be seen early in MI.

4. Pathologic Q Waves

• Develop within hours to days after infarction.
• Indicate myocardial necrosis.

Important Considerations

1. Right Ventricular Involvement

• If the inferior MI is RCA-dominant, the right ventricle (RV) may also be affected.
• Lead V4R should be checked: ST elevation here suggests RV infarction.
• RV infarction can lead to hypotension and requires careful fluid management.

2. Bradycardia and AV Block

• Inferior MI is frequently associated with sinus bradycardia due to involvement of the SA node (60% RCA dominance).
• AV block (first-degree, second-degree, and complete heart block) may also occur.

3. Differential Diagnosis

• Pericarditis: Diffuse ST elevation rather than limited to leads II, III, and aVF.
• Early repolarization: ST elevation without reciprocal changes.

Management of Inferior MI

1. Emergency Treatment

• Aspirin and P2Y12 inhibitors (e.g., clopidogrel, ticagrelor) to prevent further clot formation.
• Heparin (unfractionated or LMWH) for anticoagulation.
• Nitroglycerin (avoid if RV infarct is suspected!).
• Oxygen only if hypoxic.
• Morphine for pain relief (use cautiously).

2. Reperfusion Therapy

• Primary Percutaneous Coronary Intervention (P-PCI) is the preferred treatment.
• Fibrinolytic therapy (tPA, streptokinase) if PCI is unavailable within 120 min.

3. Supportive Care

• Monitor for arrhythmias (especially bradyarrhythmias and AV block).
• Check for complications, such as mitral valve dysfunction due to papillary muscle involvement.

Conclusion

Inferior MI is a serious but treatable condition that requires careful ECG interpretation and rapid intervention. Identifying ST elevations in leads II, III, and aVF, along with reciprocal changes in lead aVL, is key. Additionally, right ventricular infarction should always be considered and managed accordingly.

Source recommendations

1. American College of Cardiology/American Heart Association (ACC/AHA) Guidelines for the Management of ST-Elevation Myocardial Infarction (STEMI)

1. https://www.ahajournals.org/doi/10.1161/cir.0b013e3182742cf6
2. https://professional.heart.org/en/science-news/2021-acc-aha-scai-guideline-for-coronary-artery-revascularization
3. https://www.ahajournals.org/doi/10.1161/01.cir.0000134791.68010.fa
4. https://www.acc.org/Guidelines
5. https://www.sciencedirect.com/science/article/pii/S073510972106157X

2. European Society of Cardiology (ESC) Guidelines for the Management of 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://www.acc.org/Latest-in-Cardiology/ten-points-to-remember/2023/08/29/14/01/2023-esc-guidelines-acs-esc-2023
5. https://pubmed.ncbi.nlm.nih.gov/26320110/