How do I interpret an electrocardiogram (ECG)?

How to Interpret an ECG: A Systematic Approach

A systematic approach to ECG interpretation is essential for accurate diagnosis of cardiac conditions that affect morbidity and mortality, requiring careful analysis of rate, rhythm, axis, intervals, and waveform morphology. 1

Step 1: Assess Heart Rate and Rhythm

• Calculate heart rate by counting the number of large squares between consecutive R waves (300 divided by number of large squares) or by counting the number of QRS complexes in a 6-second strip and multiplying by 10 1
• Evaluate rhythm regularity by examining R-R intervals for consistency 1
• Identify the presence of P waves and their relationship to QRS complexes to determine if the rhythm is sinus or non-sinus 1
• Look for patterns suggesting specific arrhythmias such as atrial fibrillation (irregular rhythm with no discernible P waves) or atrial flutter (regular "saw-tooth" P waves) 1, 2

Step 2: Analyze Intervals and Segments

• Measure PR interval (normal 120-200 ms or 3-5 small squares) to assess AV conduction 1
• Measure QRS duration (normal <120 ms or <3 small squares) to evaluate ventricular conduction 1
• Calculate QT interval and correct for heart rate (QTc, normal <450 ms for men, <460 ms for women) to assess repolarization 1
• Examine ST segments for elevation or depression that might indicate ischemia, injury, or infarction 1

Step 3: Determine Electrical Axis

• Examine leads I and aVF to quickly determine the quadrant of the axis 1
• Normal axis is between -30° and +90° 1
• Left axis deviation (-30° to -90°) may indicate left anterior fascicular block or lateral wall MI 1
• Right axis deviation (+90° to +180°) may suggest right ventricular hypertrophy or posterior wall MI 1

Step 4: Analyze P Waves, QRS Complexes, and T Waves

• Examine P wave morphology (normal: upright in I, II, aVF; biphasic in V1) to assess atrial conduction 1
• Analyze QRS complex morphology for pathologic Q waves (>1 mm wide and >1/3 the height of the R wave) suggesting myocardial infarction 1
• Evaluate R wave progression across precordial leads (R waves should increase in size from V1-V4, then decrease) 1
• Assess T wave morphology and polarity (normally upright in I, II, V3-V6) 1

Step 5: Look for Signs of Specific Pathologies

• Myocardial ischemia/infarction: ST elevation/depression, pathologic Q waves, T wave inversions 1, 3
• Ventricular hypertrophy: increased QRS voltage, secondary ST-T changes 1, 3
• Bundle branch blocks: widened QRS with specific morphology patterns 1
• Electrolyte abnormalities: prolonged QT (hypokalemia), shortened QT (hypercalcemia), etc. 1
• Pre-excitation syndromes: short PR interval with delta wave 1, 4

Common Pitfalls to Avoid

• Relying solely on computer interpretations without physician overreading 1
• Misinterpreting normal variants as pathologic findings 1, 5
• Failing to compare with previous ECGs when available 1
• Not considering clinical context when interpreting ECG findings 1
• Overlooking subtle but significant abnormalities due to hasty interpretation 1, 5

Special Considerations

• Age and gender affect normal ECG parameters (e.g., QT intervals are typically longer in women) 1
• Athletes may have normal variants including sinus bradycardia, early repolarization, and increased voltage 4, 2
• Certain medications can affect ECG findings (e.g., antiarrhythmics, psychotropics) 1
• Technical factors such as improper lead placement can cause misleading ECG patterns 1
• Computerized ECG interpretations should always be verified by a qualified physician 1