What ECG findings indicate an anterior myocardial infarction (MI) on a 12-lead electrocardiogram (ECG)?

ECG Findings Indicating an Anterior Myocardial Infarction

ST-segment elevation in leads V1-V6, particularly V2-V4, is the hallmark ECG finding that indicates an anterior myocardial infarction, with the most diagnostic changes typically appearing in leads V3 and V4.

Primary ECG Findings in Anterior MI

The following ECG findings are characteristic of anterior wall myocardial infarction:

• ST-segment elevation:

  • Present in at least 2 contiguous precordial leads (V1-V6)
  • Most prominent in leads V2-V4
  • Must meet specific threshold criteria 1:
    • Men ≥40 years: ≥0.2 mV (2 mm) in V2-V3 and ≥0.1 mV (1 mm) in other leads
    • Men <40 years: ≥0.25 mV (2.5 mm) in V2-V3
    • Women: ≥0.15 mV (1.5 mm) in V2-V3 and >0.1 mV (1 mm) in other leads

• Q waves:

  • Development of pathological Q waves in leads V1-V6
  • Defined as Q waves ≥0.03 sec and ≥0.1 mV deep or QS complex in leads V1-V6 1
  • Indicates myocardial necrosis and often develops after the acute phase

• T-wave inversion:

  • Follows ST-segment elevation in the evolution of MI
  • Deep T-wave inversions (>0.5 mV) in leads V2-V4 with QT prolongation may indicate severe proximal LAD stenosis 1

Localization of Anterior MI Based on ECG Findings

Different patterns of ST elevation can help identify the specific location of the anterior MI:

1. Proximal LAD occlusion:

   • ST elevation in lead aVL and precordial leads
   • ST depression in inferior leads (II, III, aVF)
   • ST elevation in lead aVR may be present 2, 3
   • Associated with worse prognosis due to larger area of myocardium at risk

2. Mid or distal LAD occlusion:

   • ST elevation limited to precordial leads
   • Less likely to have ST depression in inferior leads 4
   • Better prognosis than proximal LAD occlusion

3. Anteroseptal MI:

   • ST elevation predominantly in V1-V3
   • Usually indicates LAD occlusion proximal to first septal perforator

4. Anterolateral MI:

   • ST elevation in V3-V6, I, and aVL
   • May indicate LAD occlusion or left circumflex involvement

Additional ECG Findings and Clinical Correlations

• Reciprocal changes:

  • ST depression in leads opposite to the area of infarction
  • In anterior MI, reciprocal ST depression often appears in inferior leads (II, III, aVF)
  • Indicates larger infarct size and worse prognosis 5

• Right ventricular involvement:

  • In patients with anterior MI and small conus branch, ST elevation may be seen in right-sided leads (V3R, V4R)
  • Associated with higher rate of heart failure and adverse cardiac events 6

• QRS distortion:

  • Distortion of the terminal portion of the QRS complex
  • Important indicator of poor outcome 3

Clinical Implications and Pitfalls

• The extent of ST elevation correlates with infarct size and prognosis
• Patients with anterior MI due to proximal LAD occlusion have the worst short and long-term prognosis 3
• Be aware that some patients with anterior MI (1-6%) may present with a normal ECG initially 1
• Deep T-wave inversions with QT prolongation in leads V2-V4 without other ECG evidence of evolving infarction should raise suspicion for severe proximal LAD stenosis requiring urgent evaluation 1
• Avoid confusing anterior MI with other causes of ST elevation such as pericarditis, early repolarization, LV aneurysm, or Takotsubo cardiomyopathy 1

Remember that early recognition of anterior MI on ECG is critical for timely reperfusion therapy, which significantly reduces mortality and improves outcomes in these patients.