ECG Interpretation Basics

The only defense against missing subtle, life-threatening pathology on a 12-lead ECG is a rigid, systematic approach. Looking straight at the ST segments guarantees you will miss important clues in the intervals.

Diagram showing P wave, QRS complex, and T wave of an ECG

The Systematic Approach (Rule of 6)

1. Rate & Rhythm

Rate: 300 / number of large squares between R-R (if regular). Or count number of R waves in a 10-second rhythm strip and multiply by 6.
Rhythm: Is there a P wave before every QRS? Is there a QRS after every P wave? Are the P waves upright in leads I, II, aVF (Sinus Rhythm)?

2. Axis

Look at leads I and aVF.

Positive in I, Positive in aVF = Normal Axis
Positive in I, Negative in aVF = Left Axis Deviation (Check Lead II: if negative, it's pathological LAD)
Negative in I, Positive in aVF = Right Axis Deviation

3. Intervals

PR Interval: 120-200 ms (3-5 small squares). Short = WPW. Long = 1st degree AV block.
QRS Duration: < 120 ms (3 small squares). Wide = Bundle Branch Block, Ventricular rhythm, Hyperkalemia.
QTc: < 440 ms (men), < 460 ms (women). Use our QTc Calculator to correct for heart rate.

4. Hypertrophy

LVH (Sokolow-Lyon): S wave in V1 + R wave in V5 or V6 > 35 mm. Note: Voltage criteria are poorly specific in young, thin individuals.

5. Ischemia/Infarction (ST-T changes)

Assess territorially, not numerically.

Inferior (RCA): II, III, aVF
Lateral (LCx/LAD): I, aVL, V5, V6
Anterior (LAD): V1-V4

Look for ST elevation (injury), ST depression (ischemia/reciprocal change), T wave inversion, and pathological Q waves (infarction).

6. The Rest

Check for miscellaneous findings: U waves (hypokalemia), Osborn waves (hypothermia), Delta waves (WPW).