How to Master ECG Interpretation

A Systematic Approach for OSCEs and Finals

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The Golden Rule

In an exam setting, panic is your enemy. The only way to combat panic is with a rigid system. Do not look at an ECG and shout "Anterior STEMI!" immediately. If you are wrong, you have nowhere to go. Instead, state the clinical context, then follow the system below.

1. Rate

Calculate the ventricular rate.

• Regular Rhythm: 300 / number of large squares between R waves.
• Irregular Rhythm: Count R waves in the rhythm strip (10 seconds) and multiply by 6.
• Normal: 60 - 100 bpm.
• Bradycardia: < 60 bpm.
• Tachycardia: > 100 bpm.

2. Rhythm

Look at the rhythm strip (usually Lead II at the bottom).

• Is it regular? Use a piece of paper to mark R-R intervals.
• Are P-waves present? Is there a P for every QRS, and a QRS for every P?
• Morphology: "Sawtooth" indicates Atrial Flutter. Chaotic baseline indicates Atrial Fibrillation.

3. Axis

Look at leads I and II.

• Normal Axis: Positive (up) in I and II.
• Left Axis Deviation: Positive in I, Negative in II. (Think: "Leaving each other"). Causes: LVH, Inferior MI, LBBB.
• Right Axis Deviation: Negative in I, Positive in II. (Think: "Reaching for each other"). Causes: RVH, PE, Cor Pulmonale.

4. Hypertrophy & Intervals

• PR Interval: Normal is 120-200ms (3-5 small squares). Prolonged = Heart Block. Short = Wolff-Parkinson-White (look for Delta wave).
• QRS Duration: Normal < 120ms (3 small squares). Wide = Bundle Branch Block or Ventricular rhythm.
• LBBB: "WiLLiam" - W shape in V1, M shape in V6.
• RBBB: "MaRRoW" - M shape in V1, W shape in V6.

5. Ischemia & Infarction

Group the leads to localise the pathology: