Approach to ECG Interpretation

Step 1: confirm ECG setup

  • Patient name, date, time of recording
  • Paper speed: 25 mm/s
  • Calibration: 10 mm/mV
  • Check lead placement and rule out artifacts

Step 2: determine heart rate

  • Normal: 50–100 bpm
  • Regular rhythm
    • use the 300 rule: 300 / number of large squares between two R waves
  • Irregular rhythm
    • count the number of R waves in a 10-second strip and multiply by 6
  • Escape rhythms
    • atrial: 60–80 bpm
    • junctional: 40–60 bpm
    • ventricular: 20–40 bpm

Step 3: assess rhythm

  • Regular vs. irregular vs. irregularly irregular
  • Identify the P wave before every QRS and the QRS after every P wave
  • Sinus rhythm: P upright in leads I, II, and aVF + rate 50–100 bpm
  • Irregularly irregular = think AFib

Step 4: evaluate axis

  • Look at leads I and aVF:
    • lead I +, aVF + → Normal axis
    • lead I +, aVF − → Left axis deviation (LAD)
    • lead I −, aVF + → Right axis deviation (RAD)
    • lead I −, aVF − → Extreme axis
  • Normal axis: −30° to +90°

Step 5: measure intervals

  • PR interval: 120–200 ms (3–5 small squares)
  • QRS duration: ≤120 ms
  • QT interval:

Step 6: assess P wave morphology

  • Left atrial enlargement (LAE)
    • bifid P wave in lead II
    • biphasic P in V1 (terminal negative portion >1 mm)
  • Right atrial enlargement (RAE)
    • tall, peaked P waves in II and V1 (>2.5 mm)

Step 7: examine QRS morphology

  • Bundle Branch Blocks (BBB)
    • LBBB: Broad, notched R in V5/V6; deep S in V1
    • RBBB: rsR’ in V1; wide S in V6
  • LVH criteria
    • S in V1 + R in V5 or V6 >35 mm
  • RVH criteria
    • RAD + R/S >1 in V1

Step 8: review ST segment and T waves

  • Ischemia: ST depression, T wave inversion (V1–V6)
  • Infarction: ST elevation in the affected area
  • Reciprocal changes in opposite leads
  • T wave changes: hyperacute (early), inverted (later)

*All images are from ecgwaves.com

Step 9: look for pathologic Q waves

  • 1 small square wide
  • 25% of R wave height
  • Present in ≥2 contiguous leads

Step 10: check for miscellaneous patterns

  • Electrolytes
    • hyperK: tall T waves → sine wave
    • hypoK: U waves, ST depression, long QT
    • hypoCa: long QT; HyperCa: short QT
  • Drugs: digoxin (scooped ST), antipsychotics (QT prolongation)
  • Others
    • osborne J waves (hypothermia)
    • diffuse ST elevation + PR depression (pericarditis)

🚀 Final step: clinical correlation

  • summarize: rate, rhythm, axis, intervals, major abnormalities
  • compare to previous ECGs
  • correlate with symptoms and clinical scenario
  • for related cases, explore approach to palpitations and chest pain to connect ECG findings with real clinical decisions

📚 References & further reading

  1. Life in the Fast Lane – ECG Library
    Huge, easy-to-navigate library with annotated ECG examples and clinical pearls
    🔗 https://litfl.com/ecg-library/
  2. ECG Waves – Clinical ECG Interpretation
    Detailed step-by-step explanations, beautiful visuals, and case examples
    🔗 https://ecgwaves.com/
  3. Geeky Medics – ECG Interpretation Guide
    Concise and OSCE-friendly; great for beginners and quick refreshers
    🔗 https://geekymedics.com/how-to-read-an-ecg/
  4. BMJ ECG Basics Article
    Evidence-based clinical approach in a structured review
    🔗 https://www.bmj.com/content/357/bmj.j1720
  5. American Heart Association (AHA) – ECG-Related Guidelines
    Searchable hub for official ECG-related guidance (e.g., STEMI, blocks, arrhythmias)
    🔗 https://www.ahajournals.org/
    (Tip: search “ECG” or “STEMI criteria” in the site search bar)
MOC