How do I interpret an electrocardiogram (ECG)?

Systematic Approach to ECG Interpretation

To properly interpret an electrocardiogram (ECG), you should follow a systematic approach that evaluates rate, rhythm, axis, intervals, and waveform morphology, as this methodical analysis is essential for accurate diagnosis of cardiac conditions. 1

Step-by-Step ECG Interpretation Process

1. Assess Heart Rate

• Calculate heart rate by counting the number of large squares between consecutive R waves or by counting QRS complexes in a 6-second strip and multiplying by 10 1
• Normal heart rate ranges from 60-100 beats per minute 1

2. Evaluate Rhythm

• Examine R-R intervals for consistency to determine if the rhythm is regular or irregular 1
• Identify the underlying rhythm (sinus, atrial, junctional, ventricular) based on P wave morphology and relationship to QRS complexes 1

3. Analyze Intervals and Segments

• Measure PR interval (normal: 120-200 ms or 3-5 small squares) to assess AV conduction 1
• Evaluate QRS duration (normal: <120 ms or <3 small squares) to assess ventricular conduction 1
• Calculate QT interval and correct for heart rate (QTc, normal: <450 ms for men, <460 ms for women) 1

4. 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: positive deflection in lead I and negative deflection in aVF 1
• Right axis deviation: negative deflection in lead I and positive deflection in aVF 1

5. Analyze Waveform Morphology

• P waves: Evaluate morphology (normal: upright in I, II, aVF; biphasic in V1), duration (<120 ms), and amplitude (<2.5 mm) 1
• QRS complexes: Look for pathologic Q waves (>1 mm wide and >1/3 the height of the R wave) suggesting myocardial infarction 1
• Assess R wave progression across precordial leads (normally increasing from V1 to V4, then decreasing toward V6) 1
• ST segments: Check for elevation or depression measured at the J point, which may indicate ischemia or injury 1
• T waves: Normally upright in leads I, II, V3-V6, inverted in aVR, and variable in III, aVL, aVF, V1, and V2 1

6. Look for Additional Features

• U waves: May indicate hypokalemia, bradycardia, or other conditions 1
• Signs of chamber enlargement: P wave abnormalities and QRS voltage criteria 1
• Conduction abnormalities: PR prolongation and QRS widening 1

Common Pitfalls and Considerations

• Computer interpretations should always be verified by a qualified physician as errors in computer interpretation are common 2, 1
• Technical factors like electrode placement and filtering can affect ECG appearance and interpretation 1
• Age and gender affect normal ECG parameters (e.g., QT intervals are typically longer in women) 1
• Certain medications can affect ECG findings (e.g., antiarrhythmics, psychotropics) 1
• When analyzing pacemaker ECGs, apply the same systematic approach used for any other ECG, checking for effectiveness of pacing and potential malfunctions 3

Training Requirements

• Proficiency in ECG interpretation requires a combination of knowledge, skill, and practical clinical experience 2
• The American College of Cardiology/American Heart Association recommends interpretation of 500 ECGs under supervision to attain initial competency 2
• To maintain competency in ECG interpretation, reading approximately 100 ECGs yearly is advised 2

Remember that while major interpretation errors are common (4% to 33%), adverse patient outcomes resulting from these errors are relatively rare, typically occurring in less than 1% of interpretations 2.