How do I interpret an electrocardiogram (ECG)?

Systematic Approach to ECG Interpretation

To accurately interpret an electrocardiogram (ECG), follow a systematic approach that evaluates rate, rhythm, intervals, axis, and morphology while considering the clinical context, as this methodology significantly reduces diagnostic errors and improves patient outcomes. 1

Step-by-Step ECG Interpretation Algorithm

1. Assess Technical Quality

• Evaluate the quality of the recording and identify any technical artifacts that may affect interpretation 1
• Ensure proper lead placement, as misplaced electrodes can significantly alter ECG interpretation and lead to false diagnoses 1

2. Calculate Heart Rate

• Count the number of QRS complexes in a 6-second strip and multiply by 10, or use the formula 300 divided by the number of large boxes between consecutive R waves 1
• Normal sinus rhythm has a rate between 60-100 beats per minute in adults 1

3. Determine Rhythm

• Identify the underlying rhythm by examining if there's a P wave before each QRS complex with consistent PR interval (sinus rhythm) 1
• Note any irregularities such as premature beats, pauses, or completely irregular patterns that might suggest atrial fibrillation 1

4. Measure Intervals

• PR interval: Normal 120-200 ms (3-5 small boxes) 1
• QRS duration: Normal <120 ms (less than 3 small boxes) 1
• QT interval: Calculate QTc using Bazett's formula, with normal values <450 ms for men and <460 ms for women 1

5. Determine Electrical Axis

• Examine leads I and aVF: 1
  • Normal axis: +90° to -30° (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)

6. Assess Chamber Enlargement

• Look for voltage criteria for left ventricular hypertrophy: S in V1 + R in V5 or V6 >3.5 mV 1
• Assess for atrial enlargement patterns 1

7. Evaluate for Ischemia, Injury, and Infarction

• Examine ST-segment deviations: ST elevation (>0.1 mV in limb leads or >0.15-0.2 mV in precordial leads) may indicate acute injury 1
• Look for T-wave abnormalities (inversion, hyperacute, flattening) 1
• Identify pathological Q waves (>0.04 seconds or >25% of the R wave amplitude) suggesting myocardial infarction 1
• Note the location of abnormalities to determine the affected coronary territory 1

Common Pitfalls to Avoid

• Over-reliance on computer interpretation without physician verification, as errors in computer interpretation are still common 1, 2
• Interpreting ECG findings in isolation without considering clinical context 1
• Failure to compare with previous ECGs when available 1
• Inadequate filtering settings, which can distort waveforms and affect measurements 1

Maintaining ECG Interpretation Competency

• Continuing medical education through seminars or self-assessment programs is essential, especially for physicians who read ECGs infrequently 2
• Consider periodic self-study to ensure skills remain current 2
• Utilize available resources such as quality improvement programs with ECG overreading and feedback by expert electrocardiographers, self-paced interactive computer modules, and workshops 2

Clinical Context Importance

• Always interpret the ECG in the context of the patient's clinical presentation, as the same ECG finding may have different implications depending on symptoms 1, 3
• Noncardiologists are more affected by the context of the patient's clinical history and automated computer interpretation than cardiologists when interpreting an ECG 2
• Cardiologists demonstrate greater accuracy than noncardiologists in ECG interpretation during standardized examinations when minimal history is given 2

By following this systematic approach to ECG interpretation, you can improve diagnostic accuracy and enhance patient care through proper identification of cardiac abnormalities.