Dictionary.net

Premature contractions of the heart arising from an ectopic atrial focus. With ventricular premature complexes, they represent one of the most common causes of irregular pulse. They are more apt to occur if there is atrial or conduction system disease such as left atrial enlargement in mitral stenosis. In community prospective studies, atrial premature complexes are not related to sudden death, as are ventricular premature beats in coronary disease. (From Stedman, 25th ed; Braunwald, Heart Disease, 4th ed, p679; Sokolow, et al., Clinical Cardiology, 5th ed, p472)