Where on the electrocardiogram (ECG) can an old myocardial infarction (MI) be seen?

ECG Findings of Old Myocardial Infarction

Old myocardial infarctions are primarily identified on ECG by pathologic Q waves or QS complexes in specific lead groupings that correspond to the anatomical location of the prior infarct. 1

Specific Q Wave Criteria for Old MI

The following ECG findings are pathognomonic of a prior MI in patients with ischemic heart disease:

• In leads V2-V3:

  • Q wave ≥0.02 seconds in duration, or
  • QS complex 1

• In leads I, II, aVL, aVF, or V1-V6:

  • Q wave ≥0.03 seconds and ≥0.1 mV deep, or
  • QS complex in at least two leads of a contiguous lead grouping 1

• In leads V1-V2:

  • R wave ≥0.04 seconds with R/S ratio ≥1 and a concordant positive T wave (in the absence of conduction defect) 1

Anatomical Correlation of Q Wave Location

The location of pathologic Q waves corresponds to the anatomical region of the prior infarct:

• Anterior MI: Q waves in V1-V4
• Lateral MI: Q waves in I, aVL, V5-V6
• Inferior MI: Q waves in II, III, aVF
• Posterior MI: Tall R waves in V1-V2 (equivalent to Q waves) with R/S ratio ≥1 and upright T waves 1

Enhanced Diagnostic Value

The specificity of ECG diagnosis for prior MI increases when:

1. Q waves appear in several leads or lead groupings 1
2. Q waves are associated with ST deviations or T wave changes in the same leads 1
3. Even minor Q waves (0.02-0.03 sec) that are ≥0.1 mV deep are more suggestive of prior MI when accompanied by inverted T waves in the same lead group 1

Potential Confounders

Several normal variants and pathological conditions can produce Q waves that mimic MI:

• A QS complex in lead V1 is normal 1
• Q wave <25% of R wave amplitude in lead III is normal if frontal QRS axis is between 30° and 0° 1
• Q wave may be normal in aVL if frontal QRS axis is between 60° and 90° 1
• Septal Q waves (small, <0.03 sec and <25% of R-wave amplitude) in leads I, aVL, aVF, and V4-V6 are non-pathological 1

Conditions That May Produce False-Positive Q Waves

Be cautious of Q waves or QS complexes that may appear in the absence of MI due to:

• Pre-excitation syndromes
• Cardiomyopathies (obstructive, dilated, stress)
• Cardiac amyloidosis
• Bundle branch blocks (especially LBBB)
• Left anterior hemiblock
• Ventricular hypertrophy
• Myocarditis
• Acute cor pulmonale
• Hyperkalemia 1

Clinical Implications

Recognizing old MI on ECG has important prognostic implications:

• Q waves from anterior MI are associated with higher mortality compared to those from inferior MI 2
• Silent Q wave MIs (asymptomatic) account for 9-37% of all non-fatal MI events and carry significantly increased mortality risk 1

When interpreting Q waves, always consider the clinical context, as proper identification of old MI guides secondary prevention strategies that can significantly impact morbidity and mortality.