Left axis deviation: a reassessment.

Abstract

Ramifications of the concept of left axis deviation are described. In early life, the leftward shift of the frontal plane QRS axis is determined chiefly, if not solely, by the relative weights of the ventricles. Once adult [human] ventricular weight ratios are reached, there is a long period of axis stability, then a gradual leftward drift of the QRS, governed principally by left anterior fascicular conduction. The normal QRS axis is age-dependent and left axis deviation must be considered accordingly.