What is the size of a Q wave to be considered significant on an electrocardiogram (ECG)?

Significant Q Waves on Electrocardiogram

According to current guidelines, a significant Q wave on an electrocardiogram (ECG) is defined as having a duration ≥0.03 seconds (30 ms) and amplitude ≥0.1 mV (1 mm) deep, or a QS complex, in at least two contiguous leads of a lead grouping. 1

Detailed Criteria for Pathological Q Waves

The Third Universal Definition of Myocardial Infarction provides specific criteria for Q waves that are considered pathological:

1. For leads V2-V3:

   • Any Q wave ≥0.02 seconds (20 ms) in duration
   • OR a QS complex in these leads 1

2. For all other leads (I, II, aVL, aVF, V1, V4-V6):

   • Q wave ≥0.03 seconds (30 ms) in duration AND
   • ≥0.1 mV (1 mm) in depth
   • OR a QS complex
   • Present in any two leads of a contiguous lead grouping (anterior, inferior, lateral) 1

3. For posterior MI (represented as R waves in V1-V2):

   • R wave ≥0.04 seconds (40 ms) in V1-V2
   • R/S ratio ≥1 with a concordant positive T wave
   • In the absence of conduction defect 1

Clinical Significance and Interpretation

Q waves of pathological significance typically indicate myocardial necrosis and are often associated with prior myocardial infarction. However, several important considerations should guide interpretation:

• Normal variants: Isolated Q waves in lead III may be normal, especially without repolarization abnormalities in other inferior leads 1
• Septal Q waves: Small, non-pathological Q waves <0.03 sec and <25% of the R-wave amplitude in leads I, aVL, aVF, and V4–V6 are considered normal septal Q waves 1
• Lead-specific considerations: A QS complex in lead V1 is normal 1
• Axis-dependent normal Q waves:
  • 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°
  • Q wave may be normal in aVL if frontal QRS axis is between 60° and 90° 1

Conditions That May Cause Pathological Q Waves Besides MI

Pathological Q waves are not exclusively associated with myocardial infarction. Other conditions that may produce Q waves include:

• Cardiomyopathies (hypertrophic, dilated, stress)
• Pre-excitation syndromes
• Cardiac amyloidosis
• Left bundle branch block
• Left anterior hemiblock
• Left ventricular hypertrophy
• Right ventricular hypertrophy
• Myocarditis
• Acute cor pulmonale
• Hyperkalemia 1

Pitfalls in Q Wave Interpretation

• Lead placement errors: Improper lead placement can create pseudo-Q waves, particularly in V1-V2 1
• Small Q waves in V2-V3: Even small Q waves (<40 ms duration, <0.5 mV amplitude) in these leads may predict coronary artery disease, especially left anterior descending artery stenosis 2
• Left anterior fascicular block: May produce Q waves in precordial leads that are benign, typically around 0.02 seconds in duration and restricted to one or two leads 3
• Q waves in athletes: In asymptomatic athletes, pathological Q waves should be defined using a Q/R ratio ≥0.25 or duration ≥40 ms in two or more contiguous leads (except III and aVR) 1

By understanding these criteria and potential confounders, clinicians can more accurately interpret Q waves on ECG to guide appropriate management decisions for patients with suspected myocardial injury or infarction.