How do I interpret an electrocardiogram (ECG)?

How to Interpret an ECG: A Systematic Approach

Use a systematic, stepwise method to interpret every ECG, examining rate, rhythm, axis, intervals, and waveform morphology in sequence, always correlating findings with the patient's clinical presentation. 1, 2

Step 1: Assess Technical Quality First

• Verify proper electrode placement and adequate signal quality before proceeding with interpretation, as misplaced electrodes (particularly precordial leads) can significantly alter the ECG and lead to false diagnoses. 1
• Check for artifacts from baseline wander, muscle tremor, or electromagnetic interference that may distort waveforms and affect measurements. 3
• Ensure the recording meets technical standards with appropriate filtering (high-frequency response ≥150 Hz for adults, ≥250 Hz for children) to avoid systematic underestimation of signal amplitude and smoothing of critical features like Q waves. 1

Step 2: Calculate Heart Rate

• Count the number of large boxes (0.20 seconds each) between consecutive R waves and divide 300 by this number, or count QRS complexes in a 6-second strip and multiply by 10. 1, 2
• Normal sinus rhythm: 60-100 beats per minute in adults. 1
• Sinus bradycardia (<60 bpm) may be normal in athletes; sinus tachycardia (>100 bpm) may occur with physiological stress. 1

Step 3: Determine Rhythm and Regularity

• Identify if P waves precede each QRS complex with a consistent PR interval, which characterizes normal sinus rhythm. 1
• Examine R-R intervals for consistency to assess rhythm regularity. 2
• Note any irregularities: premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation. 1
• Identify the underlying rhythm origin (sinus, atrial, junctional, or ventricular) based on P wave morphology and relationship to QRS complexes. 2

Step 4: Measure Critical Intervals

• PR interval (normal: 120-200 ms or 3-5 small squares): Measure from the beginning of the P wave to the beginning of the QRS complex to assess AV conduction. 1, 2
• QRS duration (normal: <120 ms or <3 small squares): Measure to identify ventricular conduction delays or bundle branch blocks. 1, 2
• QT interval: Calculate and correct for heart rate using Bazett's formula (QTc); normal values are <450 ms for men and <460 ms for women. 1, 2

Step 5: Determine Electrical Axis

• Examine leads I and aVF to quickly determine the axis quadrant: 1, 2
  • Normal axis (-30° to +90°): Positive in both leads I and aVF
  • Left axis deviation (-30° to -90°): Positive in lead I, negative in aVF
  • Right axis deviation (+90° to +180°): Negative in lead I, positive in aVF
  • Extreme axis deviation (+180° to -90°): Negative in both leads I and aVF

Step 6: Analyze P Waves

• Examine P wave morphology: Normal P waves are upright in leads I, II, and aVF; biphasic in V1. 2
• Measure P wave duration (<120 ms) and amplitude (<2.5 mm) to assess for atrial enlargement. 2

Step 7: Evaluate QRS Complexes

• Look for pathological Q waves (>0.04 seconds wide or >25% of R wave amplitude) suggesting prior myocardial infarction. 1, 2
• Assess R wave progression across precordial leads (V1-V6), with R wave amplitude normally increasing from V1 to V4 then decreasing toward V6. 2
• Apply voltage criteria for left ventricular hypertrophy: S wave in V1 + R wave in V5 or V6 >3.5 mV (Sokolow-Lyon criterion). 1, 2

Step 8: Examine ST Segments and T Waves

• Measure ST segments at the J point for elevation or depression: 1, 2
  • ST elevation >0.1 mV in limb leads or >0.15-0.2 mV in precordial leads may indicate acute injury
  • ST depression suggests subendocardial ischemia or other pathology
• Assess T wave morphology: Normally upright in leads I, II, and V3-V6; inverted in aVR; variable in III, aVL, aVF, V1, and V2. 2
• Look for T wave abnormalities (inversion, hyperacute appearance, flattening) that may indicate ischemia. 1

Step 9: Identify Location of Abnormalities

• Note which leads show abnormalities to determine the affected coronary territory: 1
  • Inferior leads (II, III, aVF): Right coronary artery
  • Lateral leads (I, aVL, V5-V6): Circumflex artery
  • Anterior leads (V1-V4): Left anterior descending artery

Step 10: Integrate Clinical Context

• Always interpret the ECG in conjunction with the patient's symptoms, as the same ECG finding may have different implications depending on clinical presentation. 1, 2
• Compare with previous ECGs when available to identify new changes. 1
• Consider age, gender, medications (antiarrhythmics, psychotropics), and population-specific factors that affect normal ECG parameters. 1, 2

Critical Pitfalls to Avoid

• Never rely solely on computer interpretation without physician verification, as computer-generated diagnoses remain prone to errors and are not recognized as properly interpreted ECGs. 1, 2
• Avoid interpreting ECG findings in isolation without clinical context, which may lead to inappropriate management decisions. 1
• Do not overlook technical artifacts or electrode misplacement that can mimic pathology. 1
• Remember that ECG diagnosis of structural or pathophysiologic changes is made by inference and subject to error, as the same pattern may occur in different disease states. 2
• Be aware that QRS voltage criteria for hypertrophy decline with age and vary by population. 1

Special Populations

• Children: T waves are often inverted in leads V1, V2, and V3 after 1 month of age; higher frequency response (250 Hz) is needed for accurate interpretation. 1
• Athletes: Sinus bradycardia ≥30 beats/min is a common normal variant. 1
• Women: QT intervals are typically longer than in men. 2