What are the ECG (electrocardiogram) criteria using the Q wave to confirm myocardial infarction (MI) and differentiate between old and new infarction?

ECG Q Wave Criteria for Myocardial Infarction

Pathologic Q waves are defined as ≥0.03 seconds in duration and ≥0.1 mV (1 mm) deep in at least two contiguous leads, and these are pathognomonic of prior myocardial infarction in patients with ischemic heart disease. 1, 2

Specific Q Wave Criteria for Confirming Infarction

Standard Pathologic Q Wave Definition

• Duration ≥0.03 seconds (30 ms) AND depth ≥0.1 mV (1 mm) in any two contiguous leads of a lead group 1, 2
• QS complex (complete absence of R wave) in leads V2-V3 with duration ≥0.02 seconds is also pathologic 2
• The specificity for MI diagnosis is greatest when Q waves occur in several leads or lead groupings 1, 2

Minor Q Wave Criteria

• Q waves with duration 0.02-0.03 seconds that are 0.1 mV deep are suggestive of prior MI if accompanied by inverted T waves in the same lead group 1, 2
• This combination of minor Q waves with T wave inversion increases diagnostic accuracy 1, 2

Normal Q Waves That Should NOT Be Interpreted as Pathologic

You must recognize these common pitfalls to avoid false-positive diagnoses:

• QS complex in lead V1 is normal 1, 2
• Q wave in lead III is normal if <0.03 sec and <25% of R wave amplitude when frontal QRS axis is between 30° and 0° 1, 2
• Q wave in aVL is normal if frontal QRS axis is between 60° and 90° 1, 2
• Septal Q waves (<0.03 sec and <25% of R-wave amplitude in leads I, aVL, aVF, and V4-V6) are physiologic 1, 2

Differentiating Old (Prior) vs New (Acute) Infarction

Key Approach: Compare Serial ECGs

The most reliable method to differentiate old from new infarction is comparison with previous ECGs - if Q waves are present on a prior tracing, they represent old infarction. 1, 3

Characteristics of Acute/New Infarction with Q Waves

When Q waves are present in the context of acute MI, you will see:

• Associated ST-segment elevation in the same leads where Q waves appear 1, 3
• Hyperacute T waves (tall, peaked) may precede Q wave development 1, 3
• Dynamic ECG changes - serial ECGs at 15-30 minute intervals show evolution 3
• Reciprocal ST depression in opposite leads 3
• Q waves may develop within hours of coronary occlusion and can be transient if successful reperfusion occurs 4
• Clinical correlation: ongoing chest pain, elevated cardiac biomarkers (troponin) 1

Characteristics of Old/Prior Infarction with Q Waves

When Q waves represent old MI, you will see:

• Isolated Q waves without ST elevation 1
• Stable T wave inversions (not dynamic) in the same leads, or normalized T waves 1
• No reciprocal changes 1
• Unchanged appearance on serial ECGs obtained 15-30 minutes apart 3
• Clinical correlation: no acute chest pain, normal or baseline cardiac biomarkers 1
• May be discovered as "silent MI" on routine ECG follow-up 1

Evolution Timeline of Q Waves

Understanding the temporal evolution helps differentiate timing:

• Hyperacute T waves: appear within minutes of coronary occlusion 3
• ST-segment elevation: develops within hours 3
• Q waves: develop over hours to days in many (but not all) patients 3
• T-wave inversion: may persist for weeks to months after acute event 3
• Q wave regression: occurs in approximately 40% of patients by 24 months, particularly with successful early reperfusion 5

Critical Diagnostic Algorithm

When you encounter deep Q waves on ECG, follow this approach:

1. Obtain prior ECGs immediately - if Q waves were present previously, this is old MI 1, 3

2. If no prior ECG available, assess for acute features:

   • ST elevation in same leads as Q waves = likely acute/evolving MI 1, 3
   • ST depression, T wave changes = likely acute MI 1, 3
   • Isolated Q waves without ST-T changes = likely old MI 1

3. Obtain serial ECGs at 15-30 minute intervals if patient is symptomatic - dynamic changes indicate acute process 3

4. Check cardiac biomarkers (troponin) - elevated levels indicate acute or recent MI 1

5. Consider cardiac imaging (echocardiography or CMR) to assess wall motion abnormalities and differentiate acute from chronic changes 1

Conditions That Mimic Pathologic Q Waves

Be aware that Q waves can occur without MI in these conditions:

• Pre-excitation syndromes (Wolff-Parkinson-White) 1, 2
• Cardiomyopathies (obstructive, dilated, stress, hypertrophic) 1, 2, 6
• Cardiac amyloidosis 1, 2
• Left bundle branch block (LBBB) - obscures Q waves 1
• Left ventricular hypertrophy 1, 2
• Myocarditis 1
• Acute cor pulmonale 1

Enhanced Diagnostic Accuracy

Combining Q waves with other ECG findings significantly increases diagnostic accuracy:

• Q waves + ST deviations in same leads = higher likelihood of MI 1, 2
• Q waves + T wave inversions in same leads = higher likelihood of MI 1, 2
• Pathologic Q waves in multiple lead groupings = highest specificity for MI 1, 2

Special Considerations

Q Wave Regression After Reperfusion

• Early Q waves in acute MI may represent severely ischemic (not irreversibly damaged) myocardium that can be salvaged with thrombolytic therapy or primary PCI 4
• Q wave regression occurs in approximately 40% of patients by 24 months and is associated with the largest improvement in left ventricular ejection fraction 5
• Transient Q waves during acute phases may represent stunned myocardium 4

Clinical Significance

• Pathologic Q waves indicate transmural myocardial infarction with necrosis 2
• Larger infarct size (typically >6.2% of left ventricular mass) is associated with pathologic Q waves 2
• Silent Q wave MI accounts for 9-37% of all non-fatal MI events and carries significantly increased mortality risk 1, 2