The electrical activity of the heart starts at the sinoatrial node then spreads to the atrioventricular(AV) node.
It then spreads down the bundle of His and then Purkinje fibres to cause ventricular contraction.
Whenever the direction of electrical activity is towards a lead you get a positive deflection in that lead.
Whenever the direction of electrical activity is away from a lead you get a negative deflection in that lead.
Cardiac axis gives us an idea of the overall direction of electrical activity.
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Normal cardiac axis
In healthy individuals, you would expect the axis to lie between -30° and +90º.
The overall direction of electrical activity is towards leads I, II and III (the yellow arrow below). As a result, you see a positive deflection in all of these leads, with lead II showing the most positive deflection as it is the most closely aligned to the overall direction of electrical spread.
You would expect to see the most negative deflection in aVR. This is due to aVR looking at the heart in the opposite direction.
Right axis deviation
Right axis deviation (RAD) involves the direction of depolarisation being distorted to the right (between +90º and +180º).
The most common cause of RAD is right ventricular hypertrophy. Extra right ventricular tissue results in a stronger electrical signal being generated by the right side of the heart. This causes the deflection in lead I to become negative and the deflection in lead aVF/III to be morepositive.
RAD is commonly associated with conditions such as pulmonaryhypertension, as they cause right ventricular hypertrophy. RAD can, however, be a normalfinding in verytallindividuals.
Left axis deviation
Left axis deviation (LAD) involves the direction of depolarisation being distorted to the left (between -30° and -90°).
This results in the deflection of leadIII becoming negative (this is only considered significant if the deflection of lead II also becomes negative).
LAD is usually caused by conduction abnormalities.
Dr Matthew Jackson
Consultant Interventional Cardiologist
Michael Rosengarten BEng, MD.McGill – Right axis deviation. Licence: CC BY-SA 3.0
Michael Rosengarten BEng, MD.McGill – Left axis deviation. Licence: CC BY-SA 3.0