What is Cardiac Axis?
The electrical activity of the heart starts at the sinoatrial (SA) node then spreads to the atrio-ventricular (AV) node.
It then spreads down the bundle of his and then purkinje fibres to cause ventricular contraction.
So when viewing the heart from the front, the direction of depolarisation is 11 o’clock to 5 o’clock.
The general direction of depolarisation is known as the cardiac axis.
Normal cardiac axis
In healthy individuals you would expect a normal direction of spread (11 o’clock to 5 o’clock).
Therefore the spread of depolarisation would be heading towards leads I,II and III.
As a result you would see a positive deflection in all of these leads.
With lead II being the most positive (it’s at 5 o’clock).
You would expect to see the most negative deflection in aVR.
This is due to aVR looking at the heart in the opposite direction to lead II.
Right axis deviation
Right axis deviation (RAD) is usually caused by right ventricular hypertrophy.
In right axis deviation the direction of depolarisation is distorted to the right (1-7 o’clock).
Extra heart muscle causes a stronger signal to be generated by the right side of the heart.
This causes the deflection in lead I to become negative and deflection in lead aVF / III to be more positive.
RAD is associated with pulmonary conditions as they put strain on the right side of the heart.
It can however be a normal finding in very tall individuals.
Left axis deviation
In left axis deviation (LAD) the general direction of depolarisation becomes distorted to the left.
This causes the deflection in lead III to become negative.
It is only considered significant if the deflection of Lead II also becomes negative.
LAD is usually caused by conduction defects and not by increased mass of the left ventricle.