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Lesson 10 of 21 · Axis & Interpretation

Axis Determination

The electrical axis is the average direction of ventricular depolarization in the frontal plane. Normal is −30° to +90°. Two leads, I and aVF, get you there in seconds.

01The quadrant method

  • I ⊕ aVF ⊕: normal axis
  • I ⊕ aVF ⊖: left axis deviation (check lead II: positive = still normal, −30° to 0°)
  • I ⊖ aVF ⊕: right axis deviation
  • I ⊖ aVF ⊖: extreme ('northwest') axis, think VT or lead reversal
Axis: the two-lead method
I (0°)aVF (+90°)−90°±180°NormalLADRADExtremeLEAD IaVF↑↑Normal (−30° to +90°)↑↓Left axis deviation*↓↑Right axis deviation↓↓Extreme axis*If lead II is positive, axis is still normal (0° to −30°)

02Causes

  • LAD: left anterior fascicular block, inferior MI, LVH, LBBB, WPW
  • RAD: RVH, left posterior fascicular block, lateral MI, acute PE, normal in children and tall thin adults
  • Extreme: VT, hyperkalemia, lead misplacement

Clinical pearl. A new right axis shift in a dyspneic, tachycardic patient: think PE.

Key takeaways

  • Look at I and aVF
  • Both up = normal
  • Thumbs pointing toward each other (I ⊕, aVF ⊖) = left; away = right