How do I interpret this ECG?

How to Interpret an ECG: A Systematic Approach

Follow a standardized, stepwise method every time you interpret an ECG to ensure accuracy and avoid missing critical findings. 1, 2

Step 1: Verify Technical Quality First

Before interpreting any ECG, you must confirm the recording is adequate:

• Check electrode placement is correct – misplaced precordial leads are the most common technical error and can create false diagnoses of infarction or conduction abnormalities 2, 3
• Verify filtering settings are appropriate – ensure minimum high-frequency response of 150 Hz for adults (250 Hz for children) to maintain diagnostic precision 4, 2
• Identify artifacts from baseline wander, muscle tremor, or electrical interference that could distort your interpretation 4, 3
• Compare with previous ECGs when available – this is critical for detecting dynamic changes and avoiding misinterpretation 1, 2

Step 2: Calculate Heart Rate

• Count QRS complexes in a 6-second strip and multiply by 10, or use 300 divided by the number of large boxes between consecutive R waves 1, 2
• Normal sinus rhythm: 60-100 bpm 1, 2
• Bradycardia: <60 bpm (may be normal in athletes if ≥30 bpm) 2
• Tachycardia: >100 bpm 1, 2

Step 3: Identify the Rhythm

• Confirm sinus rhythm by finding a P wave before each QRS complex with consistent PR interval 1, 2
• Note any irregularities: premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation 1, 2

Step 4: Measure Intervals and Assess Conduction

• PR interval (normal: 120-200 ms) – identifies AV conduction delays or blocks 1, 2
• QRS duration (normal: <120 ms) – prolongation indicates bundle branch blocks or ventricular conduction delays 1, 2
• QTc interval using Bazett's formula – normal <450 ms for men, <460 ms for women; prolongation increases risk of torsades de pointes 1, 2

Step 5: Determine Electrical Axis

• Use leads I and aVF to quickly determine axis 1, 2:
  • Normal axis (+90° to -30°): both leads I and aVF positive
  • Left axis deviation (-30° to -90°): lead I positive, aVF negative
  • Right axis deviation (+90° to +180°): lead I negative, aVF positive
  • Extreme axis deviation: both negative

Step 6: Assess for Hypertrophy

• Left ventricular hypertrophy: S wave in V1 + R wave in V5 or V6 >3.5 mV (Sokolow-Lyon criteria) 2
• Note that voltage criteria decline with age and vary by population 2

Step 7: Identify Ischemia, Injury, or Infarction (Most Critical)

This is where you identify life-threatening conditions requiring immediate action:

• ST-segment elevation >0.1 mV in limb leads or >0.15-0.2 mV in precordial leads indicates acute injury requiring emergent reperfusion therapy 1, 2, 5
• Pathological Q waves (>0.04 seconds or >25% of R wave amplitude) suggest prior myocardial infarction 1, 2
• T-wave abnormalities: inversion, hyperacute changes, or flattening may indicate ischemia 1, 2
• Note the location of abnormalities to determine affected coronary territory for catheterization lab activation 1, 2
• Up-sloping ST-depression with positive T waves indicates severe LAD obstruction 5
• Widespread ST-depression with ST-elevation in aVR suggests diffuse subendocardial ischemia from severe multi-vessel disease requiring urgent angiography 5

Step 8: Integrate with Clinical Context

Never interpret the ECG in isolation – this is the most critical pitfall to avoid:

• The ECG has 76% sensitivity and 88% specificity for acute cardiac ischemia in chest pain patients 1
• Clinical signs and symptoms alone have only 35-38% sensitivity without ECG and biomarkers 1
• The same ECG pattern may have different implications depending on symptoms – always correlate with patient presentation 1, 2, 6
• Serial ECGs combined with cardiac biomarkers improve diagnostic accuracy 1

Critical Pitfalls to Avoid

• Do not accept computer interpretation without physician verification – computer accuracy ranges from 0-94%, with arrhythmias being most problematic 4, 1, 6
• Do not fail to compare with previous ECGs – you will miss important dynamic changes 1, 2
• Do not over-rely on pattern recognition alone – understand the underlying pathophysiology 7
• Recognize that noncardiologists are more influenced by patient history than cardiologists when interpreting ECGs, making systematic approach even more critical 2, 6

Special Populations

• Athletes: Sinus bradycardia ≥30 bpm, increased QRS voltages, and early repolarization are common physiological findings, not pathological 4
• Children: T waves often inverted in V1-V3 after 1 month of age; normal axis ranges 55-200° at birth, decreasing to ≤160° by 1 month 2

Competency Maintenance

• Interpret at least 100 ECGs yearly to maintain competency 6
• Continuing medical education through seminars or self-assessment programs is essential for physicians who read ECGs infrequently 2, 6