Differences Between Significant Q Waves and Pathologic Q Waves on ECG
A pathologic Q wave is defined by specific duration and amplitude criteria (≥0.03 sec and ≥0.1 mV deep) and indicates myocardial necrosis from prior myocardial infarction, while a significant Q wave typically refers to any Q wave that has clinical relevance but may not meet the strict pathologic criteria. 1
Pathologic Q Waves
Pathologic Q waves are characterized by specific measurements and indicate myocardial necrosis:
- Duration ≥0.03 seconds (30 ms) and depth ≥0.1 mV (1 mm) in any two contiguous leads of a lead group 1
- QS complex (absence of R wave) in leads V2-V3 with duration ≥0.02 seconds 1
- Pathognomonic of prior myocardial infarction in patients with ischemic heart disease, regardless of symptoms 1
- Highest specificity for MI diagnosis when present in several leads or lead groupings 1
- Represent areas of myocardial necrosis and inert myocardium incapable of depolarization 2
- May persist for years after an infarction, though can regress or even disappear over time 3, 4
Significant Q Waves
Significant Q waves have clinical relevance but may not meet strict pathologic criteria:
- Include Q waves that are smaller than pathologic criteria but still have diagnostic significance 5
- May include Q waves with duration <0.03 seconds and depth <25% of R wave amplitude 1
- Minor Q waves (0.02-0.03 sec that are 0.1 mV deep) are suggestive of prior MI if accompanied by inverted T waves in the same lead group 1
- Small Q waves in V2 or V3 (<40 ms duration and <0.5 mV amplitude) can predict coronary artery disease, particularly LAD stenosis 5
- May represent early myocardial damage before full-thickness necrosis has occurred 2
Clinical Significance and Interpretation
What Pathologic Q Waves Indicate:
- Transmural myocardial infarction with necrosis 1, 2
- Larger infarct size (typically >6.2% of left ventricular mass) 4
- Increased mortality risk, even in "silent" Q-wave MIs 1
- May occur due to myocardial fibrosis in the absence of coronary artery disease (e.g., in cardiomyopathy) 1
What Significant (Non-Pathologic) Q Waves May Indicate:
- Smaller infarctions that don't meet full pathologic criteria 4
- Early stages of infarction before complete necrosis 2
- Coronary artery disease with less extensive damage 5
- Altered ventricular depolarization due to myocardial scarring 3
Important Caveats and Pitfalls
Normal Q Waves (Non-Pathologic):
- QS complex in lead V1 is normal 1
- Q wave <0.03 sec and <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 are small, non-pathological Q waves <0.03 sec and <25% of R-wave amplitude in leads I, aVL, aVF, and V4-V6 1
Conditions That Can Mimic Pathologic Q Waves:
- Pre-excitation syndromes 1
- Cardiomyopathies (obstructive, dilated, stress) 1
- Cardiac amyloidosis 1
- Bundle branch blocks (especially LBBB) 1
- Left anterior hemiblock (may produce small Q waves in V2-V3) 6
- Left ventricular hypertrophy 1
- Myocarditis or acute cor pulmonale 1
- Hyperkalemia 1
Evolution Over Time:
- Q waves may regress or disappear over time in up to 44% of patients by 5 years post-MI 4
- Non-Q-wave infarcts are significantly smaller than Q-wave infarcts 4
- Fragmented QRS complexes may be more sensitive markers of prior MI than Q waves alone 3
Diagnostic Accuracy
- Pathologic Q waves have high specificity (99.2%) but lower sensitivity (36.3%) for detecting myocardial scar 3
- Combining Q waves with other ECG findings (ST-T changes) increases diagnostic accuracy 1
- Location accuracy varies: anterior Q-wave infarcts are more reliably localized by ECG than non-anterior infarcts 4
- Diagnostic accuracy decreases with time post-infarction 4