What are the criteria for determining a pathological vs normal Q wave in an electrocardiogram (ECG) and how to differentiate between a recent and previous cardiac change?

Criteria for Pathological Q Waves on ECG

Pathological Q waves are defined by specific duration and depth criteria: Q/R ratio ≥0.25 or duration ≥40 ms in two or more contiguous leads (except III and aVR), or Q waves ≥30 ms with depth ≥0.1 mV (1 mm) in two contiguous leads. 1, 2

Specific Diagnostic Criteria

The American College of Cardiology provides multiple definitions that have evolved over time:

• Q/R ratio ≥0.25 or duration ≥40 ms in two or more contiguous leads (except III and aVR) - this is the most current criterion that reduces false positives in athletes with physiological left ventricular hypertrophy 2, 1

• Any Q wave ≥0.02 sec or QS complex in leads V2-V3 1

• 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 leads of a contiguous lead grouping 1, 2

• Established Q waves ≥0.04 sec suggest prior MI and indicate high likelihood of significant coronary artery disease 2

Normal Q Waves That Should NOT Be Considered Pathological

Critical pitfall: Not all Q waves are pathological. The following are normal variants 1:

• Small septal Q waves <0.03 sec and <25% of R-wave amplitude in leads I, aVL, aVF, and V4-V6 1

• QS complex in lead V1 is normal 1

• Q wave in lead III <0.03 sec and <25% of R wave amplitude when frontal QRS axis is between 30° and 0° 1

• Isolated Q waves in lead III without repolarization abnormalities in other inferior leads 2

• Q wave in aVL when frontal QRS axis is between 60° and 90° 1

Differentiating Recent vs. Previous Cardiac Change

The key to determining timing is comparing serial ECGs and correlating with cardiac biomarkers and associated ST-T wave changes. 2

Indicators of Acute/Recent MI:

• Q waves with accompanying ST-segment elevation or depression suggest acute or evolving infarction 2

• New Q waves at presentation (within 4 hours of symptom onset) independently predict worse outcomes and are associated with larger infarct size, lower ejection fraction (37% vs 61%), and increased cardiac mortality 3

• Q waves with elevated cardiac biomarkers (troponin, CK-MB) indicate recent myocardial necrosis 2

• Q waves appearing during hospitalization: Up to 25% of NSTEMI patients with elevated CK-MB develop Q waves during their hospital stay 2

• Serial ECG changes: When three or more ECGs are obtained, at least two consecutive ECGs should demonstrate the abnormality to confirm evolution 2

Indicators of Old/Previous MI:

• Isolated Q waves without ST-T wave changes suggest prior infarction 2

• Q waves without elevated biomarkers indicate old myocardial scar 2

• Stable Q waves on comparison with prior ECGs confirm chronicity 2

• Q wave regression: Approximately 40% of patients with initial Q-wave MI show regression by 24 months, which is associated with improved left ventricular ejection fraction 4

Clinical Algorithm for Evaluation

When pathological Q waves are identified, follow this systematic approach:

1. First, verify lead placement - pseudo-septal infarct patterns with Q waves in V1-V2 commonly result from high lead placement 2, 1

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

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

   • Left bundle branch block (obscures Q waves) 2
   • Right bundle branch block (does not interfere with Q wave diagnosis) 2
   • Left ventricular hypertrophy 2
   • Wolff-Parkinson-White syndrome 2

4. Assess for acute changes:

   • ST-segment elevation ≥0.2 mV in V1-V3 or ≥0.1 mV in other leads 2
   • ST-segment depression >0.05 mV 2
   • T-wave inversions ≥1 mm in two contiguous leads 2

5. Measure cardiac biomarkers - troponin is essential to distinguish acute from chronic changes 2

6. Perform echocardiography as minimum evaluation to:

   • Exclude cardiomyopathy (HCM, ARVC, infiltrative diseases) 2, 1
   • Assess wall motion abnormalities
   • Evaluate left ventricular function 3, 5

7. Consider cardiac MRI if echocardiography is inconclusive - CMR can definitively determine infarct size, transmural extent, and presence of microvascular obstruction 6, 5, 4

Non-Ischemic Causes of Pathological Q Waves

Important caveat: Q waves do not always indicate myocardial infarction. Other causes include 2, 1:

• Hypertrophic cardiomyopathy 2, 1
• Arrhythmogenic right ventricular cardiomyopathy 2, 1
• Infiltrative myocardial diseases (amyloidosis, sarcoidosis) 2, 1
• Accessory pathways (Wolff-Parkinson-White) 2, 1
• Myocardial fibrosis without coronary artery disease 1

Prognostic Significance

Q waves carry independent prognostic value regardless of other clinical findings:

• Patients with Q waves at presentation have significantly larger infarct size (24% vs 17% of LV mass), lower ejection fraction (37-49% vs 45-61%), and higher rates of microvascular obstruction (68% vs 34%) 3, 5, 4

• New Q waves independently predict increased cardiac mortality at 30 days (7% vs 2%) and long-term follow-up (17% vs 7%) 3

• Q waves at presentation predict higher risk of heart failure development (5.2% vs 2.5% at 1 year) with accelerated onset 7

• The QW/NQW distinction is determined by total infarct size rather than transmural extent, with area under ROC curve of 0.90 for anterior and 0.77 for inferior MI 6