What are the implications of a prominent deep Q wave on an electrocardiogram (EKG)?

Prominent Deep Q Waves on EKG: Clinical Implications and Management

A prominent deep Q wave on EKG is usually pathognomonic of prior myocardial infarction, but you must systematically rule out normal variants, technical errors, and non-ischemic cardiomyopathies before concluding the patient has had an MI. 1

Diagnostic Criteria for Pathological Q Waves

Pathological Q waves are defined by:

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

Immediate Evaluation Algorithm

Step 1: Verify Technical Accuracy

• Check lead placement first - high placement of precordial leads commonly causes pseudo-septal infarct patterns with Q waves in V1-V2 2, 3
• Repeat the EKG with careful attention to proper lead positioning if Q waves are isolated to V1-V2 3

Step 2: Identify Normal Variants (Do NOT Treat These)

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

Step 3: Distinguish Acute vs. Chronic MI

Acute or evolving infarction is suggested by:

• Q waves WITH accompanying ST-segment elevation (≥0.2 mV in V1-V3 or ≥0.1 mV in other leads) or depression 2
• Elevated cardiac biomarkers (troponin) indicating recent myocardial necrosis 2
• Q waves appearing during hospitalization - up to 25% of NSTEMI patients develop Q waves during their hospital stay 2

Prior MI is suggested by:

• Q waves without acute ST changes 1
• Normal cardiac biomarkers 2
• Established Q waves ≥0.04 sec suggest prior MI and high likelihood of significant coronary artery disease 1, 2

Step 4: Check for QRS Confounders That Invalidate Q Wave Interpretation

• Left bundle branch block (LBBB) 1, 2
• Right bundle branch block with left anterior fascicular block 4
• Pre-excitation/accessory pathways (Wolff-Parkinson-White syndrome) 1, 3
• Paced rhythm 1

Step 5: Obtain Prior EKGs for Comparison

• Obtaining prior EKGs dramatically improves diagnostic accuracy 2
• When three or more ECGs are available, at least two consecutive ECGs should demonstrate the abnormality to confirm evolution 2

Non-Ischemic Causes of Pathological Q Waves (Critical Differential)

You must exclude these conditions before attributing Q waves to MI:

• Hypertrophic cardiomyopathy (HCM) 2, 3, 5
• Arrhythmogenic right ventricular cardiomyopathy (ARVC) 2, 3
• Infiltrative myocardial diseases (amyloidosis, sarcoidosis) 2, 3
• Accessory pathways 2, 3
• Myocardial fibrosis without coronary artery disease 3
• Circumscribed hypertrophy, especially in younger patients 5

Mandatory Workup

Immediate Testing

• Measure cardiac biomarkers (troponin) to distinguish acute from chronic changes 2
• Perform echocardiography as minimum evaluation to exclude cardiomyopathy and assess wall motion abnormalities 2, 3
• Carefully examine the ECG for accessory pathways if Q waves are present 3

Additional Testing Based on Context

• In patients ≥30 years with suspicion of prior MI or risk factors for coronary artery disease, stress testing may be warranted 3
• For inferior Q waves in apparently healthy subjects with high atherosclerotic risk, repeat EKG during deep inspiration - persistence during inspiration has 80% sensitivity and 95% specificity for silent MI 6
• Cardiac magnetic resonance imaging can definitively identify myocardial scar when diagnosis remains uncertain 6

Prognostic Implications

When Q waves represent true MI:

• Q waves on admission in acute MI patients are associated with longer time from symptom onset (208 vs. 183 minutes), higher peak creatine kinase, and increased mortality in anterior MI (odds ratio 1.61) 7
• Abnormal Q waves on admission in inferior MI are NOT associated with adverse prognosis 7
• Early Q waves in evolving MI may represent severely ischemic but salvageable myocardium, not irreversibly damaged tissue - these can disappear with successful thrombolytic therapy 8

Common Pitfalls to Avoid

• Do not assume all Q waves indicate MI - benign Q waves occur in 5.3% of patients with left anterior fascicular block, typically 0.02 sec in duration and restricted to V2 and/or V3 4
• Lead misplacement is a frequent cause of pseudo-infarct patterns 2, 3
• Failure to recognize normal limits for Q waves in specific leads leads to overdiagnosis 1
• In younger patients with atypical chest pain, circumscribed hypertrophy should be considered to avoid unnecessary invasive procedures 5