If you'd like to support us and get something great in return, check out our PDF OSCE Checklist Booklet containing over 100 OSCE checklists in PDF format.
What is Cardiac Axis?
- 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.
- 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, 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 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 -30° and -90°).
- 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: Michael Rosengarten BEng, MD.McGill – Right axis deviation – via Wikimedia Commons – Licence: CC BY-SA 3.0
- Author: Michael Rosengarten BEng, MD.McGill – Left axis deviation – via Wikimedia Commons – Licence: CC BY-SA 3.0
