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ECG Basics
 
 

Estimating Cardiac Axis

 
 
 
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Overview

The cardiac axis represents the overall direction of depolarisation of the cardiac conduction system, and is assessed by looking at the depolarisation of the individual leads.
    • Normal Axis

    • -30° to +90°
  • Einthoven’s Triangle

Quadrant Method

An ECG lead will be positive if the direction of depolarisation is in the same direction as that lead, and will be negative if the direction of depolarisation is in the opposite direction.
  • Quadrant Method
Lead I has a purely leftward direction, and aVF has a purely inferior direction. Therefore, if leads I and aVF are positive then the direction of depolarisation is inferior and leftward, and the cardiac axis is normal.
  • Quadrant Method
To identify which quadrant the axis falls into, look at the direction of the QRS complex in leads I and aVF:
IaVFII
Normal Axis
0° to 90°
+++
Normal Axis
-30° to 0°
+-+
Left axis deviation
-30° to -90°
+--
Right axis deviation
90° to 180°
-++/-
Extreme axis deviation
-90° to 180°
---

Left Axis Deviation

Left axis deviation occurs when the direction of cardiac depolarisation lies between -30° and -90°.
  • Look For

  • Positive lead I with negative leads II and aVF.
  • Left Axis Deviation
  • Causes of Left Axis Deviation

  • Left ventricular hypertrophy
  • Myocardial infarction
  • Left anterior fascicular block
  • Aortic stenosis

Right Axis Deviation

Right axis deviation occurs when the direction of cardiac depolarisation lies between 90° and 180°.
  • Look For

  • Negative lead I with positive lead aVF.
  • Right Axis Deviation
Lead II will be positive between 90-150° and negative between 150-180°.
  • Causes of Right Axis Deviation

  • Right ventricular hypertrophy
  • Right heart strain - PE, COPD
  • Lateral myocardial infarction
  • Left posterior fascicular block
  • Dextrocardia

Extreme Axis Deviation

Extreme axis deviation (also known as ‘northwest axis) occurs when the direction of cardiac depolarisation lies between -90° and 180°.
  • Look For

  • Negative leads I, II and aVF.
  • Extreme Axis Deviation
  • Significance

  • Extreme axis deviation is a strong indicator of ventricular tachycardia.
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