What does a Q wave on an electrocardiogram (ECG) indicate and how is it managed?

Q Waves on ECG: Clinical Significance and Management

What Q Waves Indicate

Pathologic Q waves (≥0.03 seconds duration and ≥0.1 mV depth in two or more contiguous leads) are pathognomonic of prior myocardial infarction in patients with ischemic heart disease, indicating transmural myocardial necrosis and typically representing infarct size >6.2% of left ventricular mass. 1, 2

Defining Pathologic vs. Normal Q Waves

Pathologic criteria include: 1, 2

• Duration ≥0.03 seconds AND depth ≥0.1 mV in any two contiguous leads of a lead group 3, 1
• Q/R ratio ≥0.25 in two or more contiguous leads 2
• QS complex (complete absence of R wave) in leads V2-V3 with duration ≥0.02 seconds 1, 2

Normal Q waves that should NOT be misinterpreted as pathologic: 1, 2

• QS complex in lead V1 is normal 3, 1
• Q wave <0.03 sec and <25% of R wave amplitude in lead III when frontal QRS axis is between 30° and 0° 3, 2
• Q wave in aVL when frontal QRS axis is between 60° and 90° 2
• Small septal Q waves (<0.03 sec and <25% of R-wave amplitude) in leads I, aVL, aVF, and V4-V6 1, 2

Clinical Context Matters

In acute presentations with ST elevation: 4

• Q waves present at initial presentation independently predict 44% higher 30-day mortality (adjusted OR 1.44,95% CI 1.25-1.65) compared to those without Q waves 4
• Early Q waves may represent severely ischemic but potentially salvageable myocardium rather than irreversible necrosis, particularly if they appear within 6 hours of symptom onset 5, 6
• More aggressive reperfusion therapy is warranted when Q waves are present at presentation 4

In chronic/stable presentations: 3, 1

• Q waves indicate prior myocardial infarction with associated increased mortality risk, even in "silent" Q-wave MIs 1
• The specificity for MI diagnosis is greatest when Q-waves occur in several leads or lead groupings 3
• Minor Q waves (0.02-0.03 sec that are 0.1 mV deep) become suggestive of prior MI when accompanied by inverted T-waves in the same lead group 3, 1

Differential Diagnosis: Non-Ischemic Causes

Critical pitfall: Q waves can occur without coronary artery disease. 3, 1, 2

Non-ischemic conditions that mimic pathologic Q waves include: 1, 2

• Hypertrophic cardiomyopathy (HCM) 1, 2
• Arrhythmogenic right ventricular cardiomyopathy (ARVC) 1, 2
• Cardiac amyloidosis and other infiltrative myocardial diseases 1, 2
• Pre-excitation syndromes (accessory pathways) 1, 2
• Left ventricular hypertrophy 3
• Bundle branch blocks 1
• Myocardial fibrosis in the absence of coronary disease 3, 1

Management Algorithm

Step 1: Verify Technical Accuracy

• Confirm proper lead placement, as high placement of precordial leads can cause pseudo-septal infarct patterns with Q waves in V1-V2 1, 2
• Compare with previous ECGs if available, particularly valuable in patients with left ventricular hypertrophy or previous MI 3
• Obtain ECG during symptoms if possible and compare with asymptomatic tracing 3

Step 2: Assess Clinical Context

If acute presentation with chest pain/symptoms: 3

• Obtain cardiac troponin T or I (preferred over CK-MB) at presentation and 6-9 hours later to demonstrate rise/fall exceeding 99th percentile 3
• Assess for ST-segment elevation or depression in contiguous leads 3
• ST-segment depression >1 mm in two or more contiguous leads with Q waves is highly suggestive of acute coronary syndrome 3
• Consider immediate reperfusion therapy if ST elevation present, as early Q waves may represent salvageable myocardium 5, 4

If chronic/incidental finding: 1, 2

• Examine ECG carefully for possibility of accessory pathway 2
• If Q waves isolated to V1-V2, repeat ECG with careful attention to lead placement 2
• If pathologic Q waves persist in two or more contiguous leads, obtain echocardiography to assess for regional wall motion abnormalities and exclude cardiomyopathy 1, 2

Step 3: Further Investigation Based on Echocardiography

If echocardiography shows regional wall motion abnormality consistent with prior MI: 1

• In patients ≥30 years with risk factors for coronary artery disease, consider stress testing 2
• Assess for secondary prevention measures and optimize medical therapy

If echocardiography shows alternative diagnosis (HCM, ARVC, infiltrative disease): 1, 2

• Pursue diagnosis-specific workup and management
• Consider cardiac MRI when diagnosis remains uncertain 1

If echocardiography is normal but Q waves persist: 1

• Consider cardiac MRI to detect myocardial fibrosis or subtle structural abnormalities 1
• Reassess for technical factors and normal variants

Key Prognostic Implications

• Larger infarct size and increased mortality risk are associated with pathologic Q waves 1
• In anterior MI with Q waves on admission, expect higher peak creatine kinase, greater prevalence of heart failure (13.8% vs 7.0%), and higher hospital mortality (8.0% vs 4.6%) compared to those without Q waves 6
• In inferior MI, Q waves on admission do not confer the same adverse prognosis as in anterior MI 6
• Combining Q waves with other ECG findings (ST-T changes) increases diagnostic accuracy 1