What are the signs of an old inferior myocardial infarct (MI) on an electrocardiogram (ECG)?

Signs of Old Inferior Myocardial Infarction on ECG

Pathological Q waves in leads II, III, and aVF are the most specific ECG findings for an old inferior myocardial infarction. 1

Primary Diagnostic Criteria

The following ECG findings indicate an old inferior myocardial infarction:

• Q waves or QS complexes in leads II, III, and aVF:

  • Q waves ≥0.03 seconds (30 ms) in duration 1, 2
  • Q waves ≥0.1 mV deep 2
  • QS complexes in these leads 1
  • The specificity is greatest when Q waves occur in multiple leads (II, III, and aVF) 1

• Associated ST-T changes:

  • Persistent T wave inversions in the same leads as the Q waves 1
  • Resolution of acute ST-segment elevation (which would be present in acute MI) 2
  • Return of ST segments to baseline in chronic phase 2

Enhanced Diagnostic Value

The likelihood of inferior MI is increased when:

• Q waves are accompanied by inverted T waves in the same lead group 1
• Minor Q waves (0.02-0.03 sec) that are ≥0.1 mV deep with associated T wave inversions 1
• Q waves occur in several leads or lead groupings (II, III, aVF) rather than in isolation 1

Special Considerations

• Distinguishing normal from pathological Q waves:

  • A Q wave ≥25% of the R wave amplitude in lead III is considered normal if the frontal QRS axis is between -30° and 0° 1
  • Isolated Q waves in lead III may be a normal finding, especially without repolarization abnormalities in other inferior leads 1

• Confounding factors:

  • Left anterior hemiblock can mask inferior MI by causing initial r waves in inferior leads, but a q wave in lead II remains an important sign of inferior infarction even with left anterior hemiblock 3
  • Pre-excitation, cardiomyopathies, and other conditions may produce Q waves that mimic infarction 1

Additional Diagnostic Clues

• In cases where the diagnosis is uncertain, comparison with previous ECGs is extremely valuable 1, 2
• Persistent Q waves generally indicate irreversible myocardial damage and fibrosis 1
• The presence of reciprocal changes in other leads (such as tall R waves in V1-V2) may support the diagnosis of inferior MI 4

Clinical Implications

• Silent Q wave MI (asymptomatic patients with new pathologic Q waves) accounts for 9-37% of all non-fatal MI events and carries significantly increased mortality risk 1
• The presence of Q waves in inferior leads without abnormal Q waves in anterior leads suggests isolated inferior wall damage, typically from right coronary artery occlusion 2

Common Pitfalls

• Improper lead placement can result in what appear to be new Q waves 1
• When in doubt, confirmation with cardiac imaging (echocardiography, cardiac MRI) is recommended 1
• ECG changes in the lateral and posterolateral regions may be insensitive, requiring additional diagnostic studies for proper localization 5

By systematically evaluating for these specific ECG findings, clinicians can accurately identify old inferior myocardial infarctions and distinguish them from normal variants or other cardiac conditions.