What is a systematic step-by-step approach for interpreting an electrocardiogram (ECG)?

Systematic Approach to ECG Interpretation

Use a structured, step-by-step method to interpret every ECG, beginning with technical quality verification, followed by rate and rhythm analysis, interval measurements, axis determination, and systematic evaluation of waveform morphology across all leads. 1, 2

Step 1: Verify Technical Quality and Recording Standards

Before interpreting any ECG findings, confirm the recording meets diagnostic standards:

• Check that the recording bandwidth is at least 0.5 Hz to 250 Hz for adults (150 Hz minimum for adolescents, 250 Hz for children) to preserve diagnostic information 3, 2
• Verify the baseline is stable without excessive wander from respiration or electrode movement 3
• Confirm absence of significant electrical interference or power-line artifact 3
• Identify any technical artifacts from electrode misplacement, muscle tremor, or cable movement that could mimic pathology 4
• Never accept computer-generated interpretations without physician verification—automated systems produce frequent errors, particularly with arrhythmias (86.4% of computer errors), conduction disorders, and pacemakers 2, 5

Step 2: Calculate Heart Rate

• Count the number of QRS complexes in a 6-second strip and multiply by 10, or use the formula 300 divided by the number of large boxes between consecutive R waves 2
• Normal sinus rhythm: 60-100 beats per minute in adults 2
• Sinus bradycardia: <60 bpm (may be normal in athletes, acceptable down to 30 bpm) 2
• Sinus tachycardia: >100 bpm (may be physiologic during stress) 2

Step 3: Identify the Underlying Rhythm

• Confirm sinus rhythm by verifying a P wave precedes each QRS complex with a consistent PR interval 2
• Note any irregularities: premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation 2
• For arrhythmia diagnosis, the ECG is the only practical noninvasive method, but recognize that computer interpretation of arrhythmias has a 40.7% error rate for life-threatening conditions like acute MI and 75% error rate for high-degree AV blocks 2, 5

Step 4: Measure Intervals and Assess Conduction

• PR interval (normal: 120-200 ms): Assess AV conduction; prolongation suggests first-degree AV block 2
• QRS duration (normal: <120 ms): Measure in a lead with an initial Q wave for accuracy; prolongation indicates ventricular conduction delay or bundle branch block 2
• QTc interval: Calculate using Bazett's formula; normal <450 ms for men, <460 ms for women 2
• Identify conduction abnormalities: AV blocks, bundle branch blocks, or pre-excitation patterns 2

Step 5: Determine the Electrical Axis

• Examine leads I and aVF to determine axis quadrant 2:
  • Normal axis (+90° to -30°): Positive in both leads I and aVF 2
  • Left axis deviation (-30° to -90°): Positive in lead I, negative in aVF 2
  • Right axis deviation (+90° to +180°): Negative in lead I, positive in aVF 2
  • Extreme axis deviation (+180° to -90°): Negative in both leads I and aVF 2
• In neonates, normal axis ranges 55-200° at birth, decreasing to ≤160° by 1 month 2

Step 6: Evaluate for Chamber Enlargement

• Left ventricular hypertrophy: Apply Sokolow-Lyon criterion (S in V1 + R in V5 or V6 >3.5 mV) or Cornell voltage (S in V3 + R in aVL) 2
• Recognize that QRS voltage criteria decline with age and vary by population 2
• Assess for atrial enlargement patterns 2

Step 7: Assess for Ischemia, Injury, and Infarction

• ST-segment elevation: >0.1 mV in limb leads or >0.15-0.2 mV in precordial leads suggests acute injury 2
• T-wave abnormalities: Inversion, hyperacute morphology, or flattening may indicate ischemia 2
• Pathological Q waves: >0.04 seconds or >25% of R wave amplitude suggests prior myocardial infarction 2
• Note the location of abnormalities to determine affected coronary territory 2
• In children older than 1 month, T-wave inversions in V1-V3 are normal variants 2

Step 8: Identify Special Patterns

• Brugada pattern: Coved-type ST elevation in V1-V2 with downsloping morphology indicates arrhythmia risk 2
• Early repolarization: J-point elevation with upward-concave ST segment, generally benign in active individuals 2
• Pre-excitation (WPW): Shortened PR interval with delta wave indicates accessory pathway 2
• Hypertrophic cardiomyopathy: Deep Q waves, markedly inverted T waves, and high voltage amplitudes 2

Step 9: Integrate Clinical Context

• Always interpret ECG findings in conjunction with the patient's clinical presentation—the same pattern has different implications depending on symptoms 2
• Consider known conditions: coronary artery disease, cardiomyopathy, hypertension, or diabetes influence interpretation 2
• Compare with previous ECGs when available to identify new changes 2

Critical Pitfalls to Avoid

• Electrode misplacement (particularly precordial leads) significantly alters interpretation and causes false diagnoses 2
• Inadequate high-frequency response (<150 Hz) systematically underestimates QRS amplitude and smooths Q waves and notches 3, 2
• Inadequate low-frequency response distorts repolarization abnormalities 3
• Excessive filtering (1-30 Hz bandwidth) produces artifact-free tracings but is unacceptable for diagnosis because it distorts critical high and low-frequency components 3
• Interpreting ECG findings in isolation without clinical context leads to inappropriate management 2
• Physician interpretation varies substantially even among experts—formal ECG training significantly improves accuracy 6, 7