How do you interpret an electrocardiogram (ECG)?

How to Interpret an Electrocardiogram (ECG)

A systematic approach to ECG interpretation requires assessment of rate, intervals, rhythm, and ischemic changes (RIRI approach), with careful attention to proper lead placement and adequate bandwidth to ensure accurate diagnosis. 1

Step-by-Step ECG Interpretation Algorithm

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) 2, 1
• Note: Misplacement of electrodes, especially V1/V2 placed too high, can lead to incorrect ECG readings 1

2. Rate Assessment

• Count the number of QRS complexes in a 6-second strip and multiply by 10
• Normal sinus rhythm: 60-100 beats/min
• Sinus bradycardia: <60 beats/min
• Sinus tachycardia: >100 beats/min 1

3. Rhythm Analysis

• Assess P waves: Are they present before each QRS? Are they uniform in shape?
• Evaluate P-P intervals and R-R intervals for regularity
• Determine if the rhythm is regular or irregular
• Look for patterns of irregularity (e.g., grouped beating) 1

4. Interval Measurements

• PR interval: 120-200 ms (3-5 small squares)
• QRS duration: <120 ms (3 small squares)
• QT interval: Varies with heart rate, use corrected QT (QTc)
  • Normal QTc: <450 ms in men, <460 ms in women 1

5. Axis Determination

• Look at leads I and aVF:
  • If both positive: Normal axis (0° to +90°)
  • If I positive, aVF negative: Left axis deviation (-90° to 0°)
  • If I negative, aVF positive: Right axis deviation (+90° to +180°)
  • If both negative: Extreme right axis deviation (-90° to -180°)

6. P Wave Analysis

• Normal duration: <120 ms
• Normal amplitude: <2.5 mm
• Look for P wave abnormalities (e.g., left atrial enlargement: wide, notched P waves in lead II) 1

7. QRS Complex Evaluation

• Assess for pathological Q waves (>40 ms or >25% of R wave amplitude)
• Evaluate R wave progression across precordial leads (should increase from V1 to V5)
• Check for bundle branch blocks or fascicular blocks
• Look for signs of ventricular hypertrophy:
  • 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

8. ST Segment Analysis

• Evaluate for ST elevation or depression
• ST elevation ≥0.1 mV (1 mm) in two contiguous leads suggests myocardial injury
• ST depression may indicate subendocardial ischemia 2, 3
• Note: ST elevation in V2-V3 has different thresholds by gender:
  • ≥0.2 mV (2 mm) in men ≥40 years
  • ≥0.25 mV (2.5 mm) in men <40 years
  • ≥0.15 mV (1.5 mm) in women 2

9. T Wave Assessment

• Normal T waves are upright in leads I, II, and V3-V6
• Inverted T waves may indicate ischemia, strain, or other pathology
• T wave alternans (alternating T wave amplitude) may indicate electrical instability 2

10. U Wave Examination

• Low-amplitude deflection after the T wave
• Most evident in leads V2 and V3
• Amplitude approximately 11% of the T wave
• More prominent with bradycardia 2

Common Pitfalls in ECG Interpretation

• Electrode misplacement: Particularly V1/V2 placed too high in the second (rather than fourth) intercostal space, distorting R-wave progression 2
• Inadequate filtering: Inappropriate low-pass filtering (<250 Hz) reduces R wave amplitude; inappropriate high-pass filtering (>0.05 Hz) can introduce artifactual ST segment deviation 2
• Over-reliance on computer interpretation: Computer algorithms have error rates up to 40.7% for acute myocardial infarction, requiring physician overreading 1
• Failure to compare with previous ECGs: Changes over time are often more significant than a single tracing 3
• Ignoring clinical context: ECG findings must be interpreted in the context of the patient's symptoms and clinical presentation 1, 3

Clinical Applications of ECG Interpretation

• Acute coronary syndromes: ST elevation (and ST depression in V1-V3) with active symptoms usually indicates acute epicardial artery occlusion requiring emergent reperfusion 3
• Arrhythmias: Systematic rhythm analysis helps identify potentially life-threatening conditions like ventricular tachycardia or complete heart block 1, 4
• Structural heart disease: ECG can provide evidence of chamber enlargement, hypertrophy, and other structural abnormalities 1
• Electrolyte disturbances: Changes in T wave morphology, QT interval, and U waves may indicate electrolyte abnormalities 1

Remember that ECG interpretation requires practice and experience. The American College of Cardiology recommends interpreting at least 500-800 ECGs under supervision to develop competency and maintaining proficiency by interpreting at least 100 ECGs yearly 1.