Can you interpret the ECG?

Systematic Approach to ECG Interpretation

ECG interpretation requires a methodical, step-by-step analysis of rate, rhythm, axis, intervals, waveforms, and ST-T segments, always verified by a qualified physician and never relying solely on computer-generated interpretations. 1, 2

Step 1: Verify Technical Quality First

Before interpreting any ECG findings, you must assess the recording quality to avoid misdiagnosis from technical artifacts 2, 3:

• Check for proper electrode placement – precordial lead misplacement occurs in over one-third of routine recordings and can produce false diagnoses 4, 3
• Identify baseline wander, electrical interference, or poor electrode contact that may compromise automated measurements 4
• Verify adequate filtering settings – inadequate high-frequency response (minimum 150 Hz for adults, 250 Hz for children) causes systematic underestimation of signal amplitude and smoothing of Q waves 1, 2
• Never accept computer interpretation without physician verification – automated systems produce frequent errors, particularly with arrhythmias 1, 2

Step 2: Calculate Heart Rate

• Count the number of large boxes between consecutive R waves and divide 300 by this number, or count QRS complexes in a 6-second strip and multiply by 10 2, 5
• Normal sinus rhythm: 60-100 bpm in adults; sinus bradycardia <60 bpm (can be normal in athletes down to 30 bpm); sinus tachycardia >100 bpm 2

Step 3: Determine Rhythm

• Identify if a P wave precedes each QRS complex with consistent PR interval – this defines sinus rhythm 2, 5
• Examine R-R intervals for regularity – completely irregular rhythm suggests atrial fibrillation 2, 5
• Note any premature beats, pauses, or irregular patterns 2

Step 4: Measure Critical Intervals

• PR interval: 120-200 ms (3-5 small squares) – prolongation indicates AV conduction delay 2, 5
• QRS duration: <120 ms (<3 small squares) – widening suggests bundle branch block or ventricular conduction delay 1, 2, 5
• QT interval corrected for heart rate (QTc) using Bazett's formula: normal <450 ms for men, <460 ms for women 2, 5

Step 5: Determine Electrical Axis

• Examine leads I and aVF simultaneously to quickly determine axis quadrant 2, 5:
  • Normal axis (-30° to +90°): positive in both leads I and aVF 2
  • Left axis deviation (-30° to -90°): positive in lead I, negative in aVF 2, 5
  • Right axis deviation (+90° to +180°): negative in lead I, positive in aVF 2, 5
  • Extreme axis deviation (+180° to -90°): negative in both leads I and aVF 2

Step 6: Analyze P Waves

• Normal P wave morphology: upright in leads I, II, aVF; biphasic in V1 5
• Duration <120 ms and amplitude <2.5 mm 5
• Look for signs of atrial enlargement – broad, notched P waves suggest left atrial enlargement; tall, peaked P waves suggest right atrial enlargement 2, 5

Step 7: Examine QRS Complexes

• Identify pathological Q waves – >0.04 seconds (1 mm) wide AND >25% of the R wave amplitude (or >1/3 the height) suggests prior myocardial infarction 2, 5
• Assess R wave progression across precordial leads – R wave amplitude should increase from V1 to V4, then decrease toward V6 5
• Apply voltage criteria for left ventricular hypertrophy: S wave in V1 + R wave in V5 or V6 >3.5 mV (Sokolow-Lyon criterion) 2

Step 8: Evaluate ST Segments and T Waves

• ST elevation >0.1 mV (1 mm) in limb leads or >0.15-0.2 mV (1.5-2 mm) in precordial leads indicates acute myocardial injury requiring immediate intervention 2
• ST depression or T wave inversion may indicate ischemia, but must be interpreted in clinical context 2, 5
• Note the location of ST-T abnormalities to determine affected coronary territory 2
• T waves normally upright in leads I, II, V3-V6; inverted in aVR; variable in III, aVL, aVF, V1, V2 5

Step 9: Recognize Special Patterns

• Brugada pattern: coved ST-segment elevation in V1-V2 with downsloping morphology – confers risk of ventricular arrhythmia 2
• Early repolarization: J-point elevation with upward-concave ST segment – generally benign, common in athletes 2
• Pre-excitation (WPW): shortened PR interval with delta wave indicating accessory pathway 2
• Hypertrophic cardiomyopathy: deep Q waves, markedly inverted T waves, unusually high voltage 2

Step 10: Integrate Clinical Context

• Always interpret ECG findings in conjunction with patient symptoms – the same ECG pattern has different implications depending on clinical presentation 1, 2
• Consider patient age, gender, medications, and comorbidities – QT intervals are longer in women; T wave inversions in V1-V3 are normal in children; sinus bradycardia to 30 bpm is physiologic in athletes 2, 5
• Compare with previous ECGs when available – failure to compare with prior tracings can miss important changes 2

Critical Pitfalls to Avoid

• Over-reliance on computer interpretation – errors in automated analysis remain common, particularly for arrhythmias, and can lead to diagnostic errors 1, 2
• Misplaced electrodes – particularly precordial leads, which significantly alter interpretation and produce false diagnoses 2, 4, 3
• Interpreting ECG in isolation without clinical context – may lead to inappropriate management decisions 2
• Failure to recognize age-specific and population-specific variants – T wave inversions in V1-V3 are normal in children over 1 month; voltage criteria for LVH decline with age 2, 5
• Inadequate filtering – distorts waveforms and affects measurements 2