Estimating Cardiac Axis
February 15th, 2021
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
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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.
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.
To identify which quadrant the axis falls into, look at the direction of the QRS complex in leads I and aVF:
| I | aVF | II | |
|---|---|---|---|
| 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.
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.
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.
Significance
- Extreme axis deviation is a strong indicator of ventricular tachycardia.
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