Can you help me interpret a 12‑lead electrocardiogram (ECG)?

12-Lead ECG Interpretation: A Systematic Approach

Yes, I can help you interpret a 12-lead electrocardiogram using a systematic, evidence-based approach that prioritizes patient safety and accurate diagnosis. 1

Essential Prerequisites Before Interpretation

Never interpret an ECG with missing leads—the test must be repeated immediately with verified correct electrode placement for all 12 leads. 2 Missing leads distort electrical axis, ST-segment analysis, and rhythm interpretation, potentially masking acute coronary syndromes or creating false patterns. 2

Technical Quality Assessment

Before beginning interpretation, verify the following:

• Check for proper electrode placement, particularly precordial leads, as misplacement significantly alters interpretation and leads to false diagnoses 1
• Evaluate signal quality by checking for artifacts, electrical interference, and baseline stability 1
• Verify adequate filtering settings (minimum 150 Hz for adults, 250 Hz for children) to prevent waveform distortion 1
• Never accept computer interpretation without physician verification—automated systems produce frequent errors, particularly with arrhythmias (only 0-94% accuracy) 3, 1

Systematic Interpretation Algorithm

Step 1: Rate and Rhythm

• Calculate heart rate by counting QRS complexes in a 6-second strip and multiplying by 10, or use 300 divided by the number of large boxes between consecutive R waves 1
• Normal sinus rhythm: 60-100 bpm with P wave before each QRS and consistent PR interval 1
• Identify irregularities such as premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation 1

Step 2: Intervals and Conduction

• PR interval: Normal 120-200 ms; prolonged suggests AV block 1
• QRS duration: Normal <120 ms; widened indicates ventricular conduction delays or bundle branch blocks 1
• QTc interval: Use Bazett's formula; normal <450 ms (men), <460 ms (women) 1

Step 3: Axis Determination

• Normal axis: +90° to -30° (positive in both leads I and aVF) 1
• Left axis deviation: -30° to -90° (positive in lead I, negative in aVF)—may indicate left ventricular hypertrophy, conduction disease, or prior inferior MI 1, 4
• Right axis deviation: +90° to +180° (negative in lead I, positive in aVF) 1
• Extreme axis deviation: +180° to -90° (negative in both leads I and aVF) 1

Step 4: Chamber Enlargement

• Left ventricular hypertrophy: S in V1 + R in V5 or V6 >3.5 mV (Sokolow-Lyon criterion) 1
• Left atrial enlargement: Requires evaluation for hypertension, valvular disease, heart failure with preserved ejection fraction, or atrial fibrillation 4
• Note: QRS voltage criteria decline with age and vary by population 1

Step 5: Ischemia, Injury, and Infarction

This is the most critical step for patient safety:

• ST elevation: >0.1 mV in limb leads or >0.15-0.2 mV in precordial leads indicates acute injury requiring immediate action 1
• Pathological Q waves: >0.04 seconds or >25% of R wave amplitude suggests myocardial infarction 1
• T-wave abnormalities: Inversion, hyperacute, or flattening patterns 1
• Identify coronary territory: Match abnormalities to specific leads (V1-V3 for septal/anterior, V4-V6 for lateral, II/III/aVF for inferior) 1, 2

Step 6: Special Lead Considerations

When acute coronary syndrome is suspected:

• Right-sided leads (V3R-V6R): Required during acute inferior-wall MI to assess right ventricular involvement, as treatment varies significantly 2
• Posterior leads (V7-V9): Essential when posterior infarction suspected, as it may show only anterior ST depression on standard leads—missing these leads completely loses the diagnosis 2

Critical Clinical Context Integration

Always interpret the ECG in conjunction with the patient's clinical presentation—the same ECG pattern has different implications depending on symptoms. 1 This is particularly important for noncardiologists, who are more influenced by patient history than cardiologists. 3, 1

Immediate Action for Abnormal Findings

When you identify concerning abnormalities (left axis deviation, left atrial enlargement, possible anteroseptal infarct, or ST changes):

1. Obtain cardiac biomarkers immediately and serially at 3-6 hours 4
2. Initiate continuous cardiac monitoring for dynamic changes or arrhythmias 4
3. Administer aspirin if chest pain or acute coronary syndrome suspected 4
4. Arrange echocardiography to assess left ventricular function and structural disease 4
5. Consider coronary angiography for elevated troponin or ongoing symptoms 4

Common Pitfalls to Avoid

• Do not over-rely on computer interpretation—errors remain common, and physician review is mandatory 3, 1
• Do not interpret in isolation—compare with previous ECGs when available to identify important changes 1
• Do not extrapolate from other leads when a lead is missing—this introduces unacceptable diagnostic error 2
• Do not assume low voltage is benign—it may indicate pericardial effusion, infiltrative cardiomyopathy, obesity, emphysema, or prior MI 4
• Do not miss evolving changes—in acute settings, serial ECGs over 25 minutes increase STEMI detection from 84.6% to 100% 5

Competency Maintenance

Residency training in internal medicine with Advanced Cardiac Life Support instruction is sufficient for bedside interpretation in routine and emergency situations. 3 However, continuing medical education through seminars or self-assessment programs is encouraged to keep skills current, especially for physicians who read ECGs infrequently. 1