EKG Signs of Old Anterior Myocardial Infarction
The hallmark EKG finding of an old anterior MI is the presence of pathological Q waves (≥0.03 seconds duration and ≥0.1 mV depth) in at least two contiguous anterior leads (V1-V4), often accompanied by persistent T-wave inversions in these same leads. 1
Primary Diagnostic Criteria
Pathological Q waves are the most specific finding for prior anterior MI and typically appear in the following pattern: 1
- Q waves ≥0.03 seconds (30 milliseconds) in duration 1
- Q wave depth ≥0.1 mV (1 mm) 1
- Present in at least two contiguous anterior leads (V1, V2, V3, V4) 1
- QS complexes (absence of any R wave) may be present instead of Q waves, particularly in leads V1-V3 1, 2
The specificity of Q waves for diagnosing prior MI is greatest when they occur in several leads or lead groupings, making the diagnosis more certain. 1
Associated Findings in Old Anterior MI
Beyond Q waves, several other EKG abnormalities commonly persist after anterior MI:
- Persistent T-wave inversions in the anterior leads (V1-V4), which may remain for weeks to months or indefinitely after the acute event 1, 2
- Loss of R-wave progression across the precordial leads, reflecting loss of anterior forces 2
- Reduced R-wave amplitude in anterior leads compared to what would be expected normally 1
- ST-segment abnormalities may persist if there is residual ischemia or LV aneurysm formation 1
Lead-Specific Patterns
The distribution of Q waves helps localize the extent of the old anterior MI:
- Anteroseptal MI: Q waves predominantly in V1-V3 3, 4
- Anterolateral MI: Q waves extending into V4-V6, and possibly leads I and aVL 4
- Extensive anterior MI: Q waves across V1-V6, often including leads I and aVL 4, 5
Research demonstrates that anterior Q waves reliably predict MI location, size, and transmural extent, with the number of anterior Q waves correlating strongly (r=0.70) with anterior MI size on cardiac MRI. 4
Critical Diagnostic Pitfalls
Several important caveats must be considered when interpreting Q waves:
- A QS complex in lead V1 alone can be normal and should not automatically be interpreted as evidence of septal infarction 2, 3
- Small septal Q waves (≤0.03 seconds and ≤25% of R-wave amplitude) may be normal variants in certain leads 3
- Isolated Q waves in lead III may be normal, especially without repolarization abnormalities in other inferior leads 1
- Bundle branch blocks can obscure or mimic Q waves; right bundle branch block can induce Q waves that simulate anterior MI 6
- LV hypertrophy and other conditions can produce Q waves in the absence of coronary artery disease 1
Comparison with Prior EKGs
Always compare with previous EKGs when available, as this dramatically improves diagnostic accuracy for distinguishing old from new findings. 1, 2 The presence of established Q waves ≥0.04 seconds suggests prior MI and indicates a high likelihood of significant coronary artery disease, even if they are less helpful for diagnosing acute ischemia. 1
When Q Waves May Be Absent Despite Prior MI
Importantly, not all anterior MIs develop Q waves. Approximately 75% of NSTEMIs result in non-Q-wave infarctions, and the absence of Q waves does not exclude prior myocardial damage. 1 In cases of extensive anterior MI with extraordinarily long LAD wrapping around the apex, abnormal Q waves may be absent in leads I or aVL due to cancellation effects from inferoapical involvement. 5
Clinical Context
The EKG findings must be interpreted alongside clinical history and cardiac biomarkers, as the EKG by itself is often insufficient to diagnose myocardial infarction, since ST deviation and Q waves may be observed in other conditions. 1, 2 When Q waves occur in the same leads as ST-segment or T-wave abnormalities, the likelihood of myocardial infarction is increased. 1