What is the difference between pathological Q (pathological Q waves) waves and physiological Q (physiological Q waves) waves on an electrocardiogram (ECG)?

Pathological vs Physiological Q Waves on ECG

Key Distinction

Pathological Q waves indicate myocardial necrosis or structural heart disease and require investigation, while physiological Q waves are normal variants that do not warrant further evaluation. 1, 2

Defining Pathological Q Waves

Pathological Q waves meet specific criteria that distinguish them from normal septal Q waves:

• Q/R ratio ≥0.25 OR duration ≥40 ms in two or more contiguous leads (excluding leads III and aVR) 3, 1, 2
• Any Q wave ≥0.02 sec or QS complex in leads V2-V3 1, 2
• Q wave ≥0.03 sec AND ≥0.1 mV deep or QS complex in leads I, II, aVL, aVF, or V4-V6 in any two contiguous leads 1, 2

These criteria represent the most current consensus from the American College of Cardiology and are used to identify abnormal myocardial tissue. 3

Defining Physiological (Normal) Q Waves

Normal septal Q waves are small and should NOT trigger further evaluation:

• Small Q waves <0.03 sec duration AND <25% of R-wave amplitude in leads I, aVL, aVF, and V4-V6 3, 1, 2
• QS complex in lead V1 is always normal 3, 1, 2
• Q wave in lead III when frontal QRS axis is between 30° and 0° (if <0.03 sec and <25% of R wave) 3, 1, 2
• Q wave in aVL when frontal QRS axis is between 60° and 90° 3, 2
• Isolated Q waves in lead III without repolarization abnormalities in other inferior leads 1

Clinical Significance and Outcomes

Pathological Q waves carry prognostic implications that directly impact mortality and morbidity:

• Presence of pathological Q waves on admission ECG independently predicts increased cardiac mortality (odds ratio 1.61,95% CI 1.04-2.49) 4, 5
• Q waves correlate with larger infarct size and lower left ventricular ejection fraction (24% vs 17% LV mass, LVEF 37% vs 45%) 6
• Persistent Q waves after myocardial infarction confer 4-fold increased risk of death or heart failure compared to non-Q-wave MI (HR 4.7, p=0.03) 7
• Q-wave regression is associated with significant LVEF improvement (9% improvement vs 2-3% in persistent or non-Q-wave groups) 6, 7

Algorithmic Approach to Q Wave Evaluation

When encountering Q waves on ECG, follow this systematic approach:

1. Verify proper lead placement first - high precordial lead placement commonly causes pseudo-septal infarct patterns with Q waves in V1-V2 1, 2

2. Obtain prior ECGs for comparison - this dramatically improves diagnostic accuracy 1

3. Check for QRS confounders that invalidate Q wave interpretation:

   • Left bundle branch block 3, 1
   • Right bundle branch block 3
   • Ventricular pre-excitation 3, 2
   • Left ventricular hypertrophy 3

4. Assess for acute changes suggesting active infarction:

   • ST-segment elevation ≥0.2 mV in V1-V3 or ≥0.1 mV in other leads 1
   • ST-segment depression ≥0.5 mm 3
   • Elevated cardiac biomarkers (troponin) 1

5. Perform echocardiography as minimum evaluation to exclude cardiomyopathy and assess wall motion abnormalities 3, 1, 2

6. Consider stress testing in patients ≥30 years with suspicion of prior MI or coronary artery disease risk factors 2

Non-Ischemic Causes of Pathological Q Waves

Critical pitfall: Q waves do not always indicate myocardial infarction. 1, 2

Alternative diagnoses to consider:

• Hypertrophic cardiomyopathy - commonly associated with pathological Q waves and non-voltage LVH criteria 3, 1, 2
• Arrhythmogenic right ventricular cardiomyopathy 3, 1, 2
• Infiltrative myocardial diseases (amyloidosis) 3, 1, 2
• Accessory pathways (pre-excitation) 3, 1, 2
• Myocardial fibrosis without coronary disease 1, 2

Special Populations

In athletes, the interpretation differs significantly:

• Pathological Q waves in athletes are abnormal findings requiring investigation for structural heart disease 3
• Athletes with pathological Q waves need echocardiography regardless of symptoms or family history 3
• Isolated voltage criteria for LVH without Q waves is physiological in athletes and requires no evaluation 3

The lack of pathological Q waves has specific clinical utility:

• Absence of Q waves is a specific marker (72% specificity) for myocardial viability in chronic coronary disease with ventricular dysfunction 8
• NoQ status should alert clinicians to potential myocardial hibernation and reversible dysfunction 8