What is an inferior ST-elevation myocardial infarction (STEMI)?

What is an Inferior STEMI?

An inferior ST-elevation myocardial infarction (STEMI) is a heart attack caused by complete thrombotic occlusion of the coronary artery supplying the inferior wall of the heart—most commonly the right coronary artery (85% of cases) or less frequently the left circumflex artery (11% of cases)—resulting in myocardial necrosis and characteristic electrocardiographic changes. 1

Diagnostic Criteria

STEMI is defined as ST-segment elevation greater than 0.1 mV (1 mm) in at least two contiguous leads 1. For inferior STEMI specifically, you must see this elevation in the inferior leads:

• Lead II
• Lead III
• Lead aVF

The ECG may also show reciprocal ST-depression in leads I and aVL 1. When evaluating subtle cases where ST elevation appears borderline, carefully examine lead aVL for ST depression—this is the key finding that confirms true inferior wall ischemia 2.

Pathophysiology and Culprit Vessels

The inferior wall receives blood supply from:

• Right coronary artery (RCA): 85% of inferior STEMIs, with 36% involving proximal RCA occlusion 3
• Left circumflex artery (LCx): 11% of cases, with only 3% involving proximal LCx 3

Complete thrombotic occlusion develops from atherosclerotic plaque rupture, ulceration, or erosion with superimposed thrombus formation, blocking myocardial blood flow and causing cell necrosis 4, 5.

Critical Clinical Considerations

Unique Hemodynamic Complications

Inferior STEMI carries specific risks that distinguish it from anterior infarctions:

• High-grade atrioventricular block (the RCA typically supplies the AV node)
• Sinus bradycardia
• Hypotension
• Hemodynamic collapse and syncope 6

These complications occur because the RCA supplies the conduction system in most patients. When you see inferior STEMI, immediately obtain a right-sided ECG (leads V3R-V4R) to assess for right ventricular involvement, which occurs in up to 50% of inferior STEMIs and dramatically worsens prognosis 1.

Right Ventricular Infarction

Right ventricular (RV) STEMI is a critical pitfall to avoid missing. The guidelines explicitly state: "EKG (right-sided EKG if inferior or RV STEMI is suspected)" 1. RV infarction presents with:

• Hypotension disproportionate to left ventricular dysfunction
• Clear lung fields despite low cardiac output
• Elevated jugular venous pressure
• Requires aggressive IV fluid resuscitation, NOT diuretics
• Contraindication to nitrates (can cause catastrophic hypotension)

Immediate Management Algorithm

Time-Critical Reperfusion Strategy

The single most important determinant of mortality and morbidity is time to reperfusion. 1, 7

1. Immediate aspirin 162-325 mg (chewed, non-enteric coated) 1, 4

2. Determine reperfusion strategy based on time:

   • Primary PCI preferred if door-to-balloon time ≤90 minutes from first medical contact 1, 7
   • Fibrinolysis if PCI cannot be achieved within 90-120 minutes (goal: drug administration within 30 minutes of diagnosis) 1, 7

3. Antiplatelet therapy:

   • Prasugrel or ticagrelor preferred over clopidogrel for primary PCI 1, 7
   • Clopidogrel acceptable if high bleeding risk or bradycardia concerns 8

4. Anticoagulation:

   • Unfractionated heparin (or enoxaparin/bivalirudin as alternatives) 7, 6

Special Circumstances

If cardiogenic shock is present, immediately transport to a facility capable of mechanical circulatory support and emergent revascularization, regardless of time delays 1. Shock trumps all time considerations.

If severe congestive heart failure develops, an invasive strategy is reasonable even outside typical time windows 1.

Prognosis and Complications

Inferior STEMI is associated with:

• Mortality risk (though generally lower than anterior STEMI)
• Reinfarction
• Heart failure
• Myocardial rupture 1
• Conduction abnormalities requiring temporary pacing

The extent of myocardial damage directly correlates with mortality 9, 5. Early, complete reperfusion is the only intervention proven to limit infarct size and reduce these complications 5, 10.

Common Pitfalls

1. Missing RV involvement: Always obtain right-sided leads in inferior STEMI
2. Giving nitrates in RV infarction: Can cause profound, refractory hypotension
3. Dismissing subtle ST elevation: If lead aVL shows ST depression with borderline inferior ST elevation, this represents true occlusion 2
4. Delayed recognition in atypical presentations: Elderly, diabetics, and women may present without chest pain 11
5. Assuming LCx occlusion based on ECG alone: ECG algorithms cannot reliably distinguish RCA from LCx occlusion—angiography is definitive 3