What are the basics of reading an electrocardiogram (ECG)?

How to Read an ECG: A Systematic Approach

Reading an ECG requires a systematic, step-by-step evaluation of rate, rhythm, intervals, axis, and waveform morphology, always interpreted by a qualified physician rather than relying solely on computer interpretation. 1

Understanding What the ECG Represents

The standard 12-lead ECG records electrical potential differences between prescribed sites on the body surface that vary during the cardiac cycle, reflecting transmembrane voltage changes in myocardial cells during depolarization and repolarization. 1 The electrical activity originates from a time-dependent dipole source (the heart vector), with voltage in each lead representing the projection of this vector onto the lead axis. 1

The Systematic 5-Step Approach

Step 1: Calculate Heart Rate

• Count the number of large boxes (5 mm) between R waves and divide 300 by this number for regular rhythms 2
• Normal heart rate: 60-100 beats per minute 2
• For irregular rhythms, count the number of QRS complexes in a 6-second strip and multiply by 10 3

Step 2: Assess Rhythm and P Waves

• Identify P waves before each QRS complex to confirm sinus rhythm 3
• P waves should be upright in leads I, II, and aVF in normal sinus rhythm 1
• Absent or irregular P waves suggest atrial fibrillation or other arrhythmias 1

Step 3: Measure Critical Intervals

• PR interval: Normal is 120-200 ms (3-5 small boxes) to assess AV conduction 2
  • Prolonged PR interval (>200 ms) indicates first-degree AV block 3
  • Short PR interval (<120 ms) suggests pre-excitation or accelerated conduction 3
• QRS duration: Normal is <120 ms (<3 small boxes) to evaluate ventricular conduction 2
  • Wide QRS (≥120 ms) indicates bundle branch block or ventricular origin 3
• QT interval: Must be corrected for heart rate (QTc) 2
  • Normal QTc: <450 ms for men, <460 ms for women 2
  • Prolonged QTc predisposes to torsades de pointes 4

Step 4: Determine Axis

• Assess the QRS axis using leads I and aVF 3
• Normal axis: -30° to +90° (positive QRS in both leads I and aVF) 3
• Left axis deviation: -30° to -90° (positive in I, negative in aVF) 3
• Right axis deviation: +90° to +180° (negative in I, positive in aVF) 3

Step 5: Evaluate Waveform Morphology

• Examine each waveform component systematically across all 12 leads 1
• Look for Q waves (pathologic if >40 ms or >25% of R wave height) suggesting prior infarction 3
• Assess R wave progression across precordial leads V1-V6 3
• Evaluate ST segments for elevation (>1 mm) or depression (>0.5 mm) indicating ischemia 5
• Analyze T wave morphology for inversion, flattening, or hyperacute changes 5, 4

Critical Technical Considerations

Proper signal acquisition is essential for accurate interpretation. 1 The fundamental frequency for the QRS complex is 10 Hz, with most diagnostic information contained below 100 Hz in adults, though high-frequency components up to 500 Hz exist. 1

Inadequate high-frequency response systematically underestimates signal amplitude and smooths critical features like Q waves and notched QRS components. 1, 2 The minimum bandwidth should be 150 Hz for adults and 250 Hz for pediatric patients to preserve diagnostic accuracy. 1, 2

Common Pitfalls to Avoid

• Never rely solely on computer interpretation without physician verification - automated systems still produce frequent errors that can lead to misdiagnosis 2, 6
• Avoid misplacement of precordial electrodes - incorrect positioning significantly alters interpretation and causes false diagnoses 2
• Do not interpret the ECG in isolation - always correlate findings with clinical presentation, symptoms, and physical examination 1, 3
• Recognize that filtering between 1-30 Hz produces a stable, artifact-free tracing but is unacceptable for diagnostic recording as it distorts both high and low-frequency signal components 1

Clinical Context Integration

The ECG is essential for diagnosing arrhythmias and is the first-choice test for patients presenting with chest pain, dizziness, syncope, or symptoms predictive of sudden death or myocardial infarction. 1 However, the ECG records electrical activity, so diagnoses of structural changes (myocardial infarction, hypertrophy) or pathophysiologic changes (electrolyte disturbances, drug effects) are made by inference and subject to error. 1

The ECG has varying sensitivity and specificity for different conditions - each waveform component has its own diagnostic characteristics and may be influenced differently by pathologic and physiologic factors. 1 Understanding these limitations prevents overinterpretation and guides appropriate clinical decision-making. 3