What are the causes of Q waves on an electrocardiogram (ECG)?

5 Major Causes of Q Waves on ECG

The five major causes of Q waves on ECG are myocardial infarction, cardiomyopathy, conduction abnormalities, ventricular hypertrophy, and normal physiological variants. 1, 2

1. Myocardial Infarction

Q waves or QS complexes are pathognomonic of prior myocardial infarction in patients with ischemic heart disease, regardless of symptoms 1. These represent the most clinically significant cause of Q waves.

Key characteristics:

• Pathological Q waves defined as:
  • Any Q wave in leads V2–V3 ≥0.02 sec or QS complex in leads V2 and V3
  • Q wave ≥0.03 sec and ≥0.1 mV deep or QS complex in leads I, II, aVL, aVF or V1–V6 in any two leads of a contiguous lead grouping 1, 2
• Specificity increases when Q waves occur in several leads or lead groupings 1
• When associated with ST deviations or T wave changes in the same leads, likelihood of MI increases 1
• May represent "silent MI" in asymptomatic patients 1

2. Cardiomyopathies

Several types of cardiomyopathy can produce Q waves in the absence of coronary artery disease 1:

• Hypertrophic cardiomyopathy
• Dilated cardiomyopathy
• Stress cardiomyopathy (Takotsubo)
• Cardiac amyloidosis

These Q waves typically result from myocardial fibrosis or altered ventricular activation patterns rather than ischemic injury 1.

3. Conduction Abnormalities

Various conduction disturbances can produce Q waves or QS patterns:

• Left bundle branch block (LBBB)
• Left anterior hemiblock
• Pre-excitation syndromes (e.g., Wolff-Parkinson-White)
• Ventricular pacing 1, 2

These abnormalities alter the normal ventricular activation sequence, resulting in abnormal Q wave patterns.

4. Ventricular Hypertrophy

Ventricular hypertrophy can lead to Q waves through altered electrical forces:

• Left ventricular hypertrophy (LVH)
• Right ventricular hypertrophy (RVH)
• Conditions causing pressure or volume overload (e.g., aortic stenosis) 1, 2, 3

In aortic stenosis, Q waves may be distinguished from MI by greater QRS voltage in chest leads and different horizontal QRS axis 3.

5. Normal Physiological Variants

Several normal variants can produce Q waves that may be mistaken for pathology:

• Normal QS complex in lead V1 1
• Q wave <25% of R wave amplitude in lead III (normal if frontal QRS axis between 30° and 0°) 1
• Normal Q wave in aVL if frontal QRS axis between 60° and 90° 1
• Septal Q waves (small, non-pathological Q waves <0.03 sec and <25% of R-wave amplitude in leads I, aVL, aVF, and V4–V6) 1, 2
• "Juvenile pattern" in adolescents <16 years with borderline Q waves in anterior leads 2
• Normal variant in Black athletes, especially when associated with J-point elevation 2

Clinical Pearls and Pitfalls

• Transient Q waves may occur during acute ischemia or early in myocardial infarction and can disappear following successful reperfusion, representing stunned rather than irreversibly damaged myocardium 4, 5
• Misinterpreting normal variants as pathological Q waves can lead to unnecessary investigations and "electrocardiographogenic disease" 2, 6
• Other conditions that can produce Q waves include myocarditis, acute cor pulmonale, and hyperkalemia 1
• Lead misplacement, especially in V1-V2, can result in false positive Q waves 2
• The presence of pathological Q waves in anterior MI is associated with higher peak creatine kinase, higher prevalence of heart failure, and increased mortality 7

When evaluating Q waves, consider the clinical context, associated ECG findings, and patient demographics to distinguish pathological from non-pathological causes.