Cardiac axis 101 – A post for EKG beginners

Don’t let anyone tell you the cardiac axis is difficult to understand or that it’s useless. Electricity flows through the heart starting from the sinus node. The direction of the flow creates a vector. Traditionally we look for one specific vector (“the cardiac axis”). If this one main vector is between -30 and 90 degrees then the axis is ok, which means the heart’s conduction system and anatomical structure is fine.

If the main electrical vector flows in a direction less than -30 degrees it is called a left axis deviation. Usually, but not always, a left deviation may mean damage to left heart structures: LV strain, LVH are good examples.

If the main vector flows beyond 90 degrees then there is right axis deviation. This may not only mean there is damage to, or anomalies in the right ventricle, but may also mean pulmonar pathology (which affects the right ventricle): Pulmonary hypertension and PE are good examples.

How do you get the axis? The process is far more complicated than this but you can get a general idea by looking at the QRS. If its positive in left leads and negative in right leads, it’s usually fine. Isoelectric (RS) or negative QRS (deep S wave) in left or inferior leads may mean severe axis modification. A positive AVR (a right lead), which is complete inversion, may also give a clue about a severe deviation and therefore a stuctural anomaly in the heart.

Some of these ideas are illustrated here. Enjoy!

Illustration 1: What is the axis? What’s normal and what is a deviation? – click to enlarge
Illustration 2: QRS complexes according to the leads. Left leads are positive, right leads show negative QRS complexes. The opposite may mean a serious deviation.

Thank you for stopping by the Plague Doctor’s blog. If you like these illustrations and the blog please leave a comment and tell me what you think. You opinion is of most importance. Tell your friends about it and visit us at facebook, twitter, instagram or tumblr. Download the illustrations for free and print it. A3 should be fine for you to put it up your wall and review anytime you like. Thank you!

-The Plague Doctor

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