The T wave on the body surface is frequently followed by a small deflection called the U wave. The etiology of this wave remains unclear. Postulated mechanisms include the following: 1) repolarization of Purkinje fibers occurring after the ventricular fibers have repolarized; 2) delayed repolarization in some ventricular fibers; and 3) a wave generated in association with ventricular contraction, referred to as mechanico-electrical coupling. This latter mechanism is considered to be the most important of the three