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Axis

The heart has several axes, but the most common by far is the axis of the QRS complex (references to "the axis" imply the QRS axis).Each axis can be computationally determined to result in a number representing degrees of deviation from zero, or it can be categorized into a few types.

The QRS axis is the general direction of the ventricular depolarization wavefront (or mean electrical vector) in the frontal plane.It is often sufficient to classify the axis as one of three types: normal, left deviated, or right deviated.Population data shows that a normal QRS axis is from −30° to 105°, with 0° being along lead I and positive being inferior and negative being superior (best understood graphically as the hexaxial reference system). Beyond +105° is right axis deviation and beyond −30° is left axis deviation (the third quadrant of −90° to −180° is very rare and is an indeterminate axis).A shortcut for determining if the QRS axis is normal is if the QRS complex is mostly positive in lead I and lead II (or lead I and aVF if +90° is the upper limit of normal).

The normal QRS axis is generally down and to the left, following the anatomical orientation of the heart within the chest. An abnormal axis suggests a change in the physical shape and orientation of the heart or a defect in its conduction system that causes the ventricles to depolarize in an abnormal way.

The extent of a normal axis can be +90° or 105° depending on the source.