How to Interpret an ECG for a Patient
A systematic approach to ECG interpretation is essential for accurate diagnosis, with all ECGs requiring verification by a qualified physician regardless of computer interpretation, as incorrect readings can significantly impact patient morbidity and mortality. 1
Systematic Approach to ECG Interpretation
Step 1: Technical Assessment
- Verify proper lead placement and recording quality
- Check calibration (standard: 10 mm/mV, 25 mm/s)
- Ensure adequate bandwidth (high-frequency cutoff ≥150 Hz for adults/adolescents, ≥250 Hz for children) 1
- Identify any technical artifacts that may affect interpretation
Step 2: Rate and Rhythm Analysis
- Calculate heart rate (normal: 60-100 beats/min)
- Identify the underlying rhythm (sinus vs. non-sinus)
- Assess for regularity or irregularity of rhythm
- Note any bradycardia (<60 beats/min) or tachycardia (>100 beats/min) 1
Step 3: Intervals and Axes Measurement
- PR interval (normal: 120-200 ms)
- QRS duration (normal: <120 ms)
- QT interval (use corrected QT - QTc)
- Normal QTc: <450 ms in men, <460 ms in women 1
- Determine QRS axis (normal: -30° to +90°)
Step 4: Chamber Analysis
- Atrial assessment: P wave morphology (normal duration <120 ms, amplitude <2.5 mm)
- Ventricular assessment:
- Left ventricular hypertrophy: Sokolow-Lyon criteria (S in V1 + R in V5 or V6 >35 mm)
- Right ventricular hypertrophy: R in V1 >7 mm or R/S ratio in V1 >1 1
Step 5: Ischemia/Infarction Evaluation
- Look for ST elevation or depression
- Identify pathological Q waves (>40 ms or >25% of R wave amplitude)
- Note T wave inversions
- Determine location based on lead groups:
- Anterior: V1-V4
- Lateral: I, aVL, V5-V6
- Inferior: II, III, aVF
- Posterior: tall R waves and ST depression in V1-V2 1
Important Considerations
Clinical Context Integration
- ECG findings must always be interpreted in light of the patient's clinical situation 1
- A 12-lead ECG has a sensitivity of 76% and specificity of 88% for diagnosing acute cardiac ischemia in patients with chest pain 2
- For diagnosing acute myocardial infarction, ECG has a sensitivity of 68% and specificity of 97% 2
Computer Interpretation Limitations
- Computer algorithms have significant error rates (40.7% for acute myocardial infarction and 75% for high-degree AV blocks) 1
- Only 0-94% of disorders are classified correctly by computer algorithms 1
- Always verify computer interpretations manually
Common Pitfalls to Avoid
- Misplacement of electrodes, especially V1/V2 placed too high 1
- Relying solely on computer interpretation
- Failing to compare with previous ECGs when available
- Not considering normal variants in specific populations:
- Athletes: sinus bradycardia, early repolarization, increased QRS voltage
- Young individuals: first-degree AV block, Mobitz Type I second-degree AV block
- Newborns/infants: higher heart rates, shorter PR intervals, right ventricular dominance 1
Competency Requirements
- Interpreting at least 500-800 ECGs under supervision is recommended to develop competency 1
- Maintaining proficiency requires interpreting at least 100 ECGs yearly 1
- Residency training in internal medicine with Advanced Cardiac Life Support instruction is considered sufficient for bedside interpretation of resting 12-lead ECGs 1
- Continuing medical education through seminars or self-assessment programs is encouraged to maintain ECG interpretation skills 2
Remember that a positive ECG (showing ST elevation/depression, T wave inversion, left ventricular hypertrophy, left bundle branch block, paced rhythm, or new Q waves) is associated with higher risk of complications in patients with suspected myocardial ischemia 3. Always integrate ECG findings with clinical presentation for optimal patient care.