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!
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-The Plague Doctor