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What is Cardiac Axis?
- The electrical activity of the heart starts at the sinoatrialΒ node then spreads to the atrioventricular (AV)Β node.
- It then spreads down the bundle of HisΒ 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.
- The cardiac axis gives us an idea of the overall direction of electrical activity.
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, IIΒ andΒ 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 overallΒ 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 +90ΒΊΒ and +180ΒΊ).
- 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 inΒ lead I to becomeΒ negative and the deflection inΒ lead aVF/III to beΒ more positive.
- RAD is commonly associated with conditions such as pulmonary hypertension, as they cause right ventricular hypertrophy.
- RAD can, however, be a normal finding in very tall individuals.

Left Axis Deviation
- Left axis deviation (LAD) involves the direction of depolarisation being distorted to the left (between -30Β° and -90Β°).
- This results in the deflection of lead III becoming negative (this is only considered significant if the deflection of lead II also becomes negative).
- LAD is usually caused by conduction abnormalities.

Reviewer
Dr Matthew Jackson
Consultant Interventional Cardiologist
References
- Author:Β Michael Rosengarten BEng, MD.McGill –Β Right axis deviation –Β via Wikimedia Commons – Licence:Β CC BY-SA 3.0
- Author:Β Michael Rosengarten BEng, MD.McGillΒ – Left axis deviation – via Wikimedia Commons – Licence: CC BY-SA 3.0