Libby: Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 8th ed

Shifts in the site of initial activation within or away from the SA node to other ectopic sites can lead to major changes in the pattern of atrial activation and, hence, in the morphology of P waves. These shifts can occur either as escape rhythms if the normal SA nodal pacemaker fails or as accelerated ectopic rhythms if the automaticity of an ectopic site is enhanced Chap. 35 . The resulting electrocardiographic abnormalities most commonly include negative P waves in the leads in which P waves are normally upright (leads I, II, aVF, and V4 through V6), with or without shortening of the PR interval.