Imagine this: It’s 2 AM, you’re the on-call Medical Officer or night shift House Officer, and the nurse hands you an ECG. The patient complained of sudden onset of chest pain and palpitations. Worst Case Scenario: You hesitate, unsure of what you’re seeing. The delay leads to missed intervention, and suddenly, your patient crashes.

(1) Check ECG details

• Confirm patient name & date to ensure is the right ECG
• Check calibration. Standard settings: 25 mm/s, 10 mm/mV

(2) Rhythm – Is it sinus rhythm or something else?

• P waves before every QRS?
  • If Yes = Sinus rhythm
• No P waves or irregular rhythm?
  • Consider atrial fibrillation, atrial flutter, or junctional rhythms
• PR interval: 0.12-0.20 sec

(3) Calculate the heart rate (based on rhythm regularity)

• Regular rhythm: Use the 300 Rule: 300 ÷ number of large squares between two R waves.
• Irregular rhythm: Count R waves in 6 seconds (30 big boxes) and multiply by 10.
• Normal range: 60-100 beats per minute

(4) Determine axis

Lead I & aVF QUADRANT approach:

• Normal axis: both positive
• Left axis deviation: Lead I positive, aVF negative
• Right axis deviation: Lead I negative, aVF positive

(5) ST Segment & T Waves — look for ischaemic changes

• Any ST Elevation?
  • Consider STEMI (especially if it occurs at contiguous leads). Look for reciprocal changes.
• Any ST Depression?
• Any T wave inversion?

(6) Look for heart block

Bundle branch block

• Left Bundle Branch Block (LBBB): “WiLLiaM”
  • V1: W-shaped (deep S wave)
  • V6: M-shaped (broad, notched R wave)
• Right Bundle Branch Block (RBBB): “MaRRoW”
  • V1: M-shaped (rSR’ pattern, “rabbit ears”)
  • V6: W-shaped (deep S wave)

Atrioventricular block

• First degree heart block: PR interval >200 ms (5 small boxes), but all P waves are followed by QRS complexes; regular ventricular rate
• Second-degree heart block
  • Mobitz Type I: Progressive lengthening of PR interval, followed by a non-conducted P wave (a QRS complex is dropped), i.e., a beat is skipped
  • Mobitz Type II: Sudden dropped QRS complexes without prior PR prolongation. (PR interval is constant, with intermittent non-conducted P waves)
• Third-Degree Heart Block (Complete heart block): No relationship between P waves and QRS complexes