Let us now discuss the morphology of the U wave. The U wave is a small deflection that follows the T wave. The U wave is asymmetric, with the initial deflection steeper than the terminal deflection. It is best visualized in the mid-precordial leads V3-V4, and it is very unusual in leads I or aVL. It should be upright and no higher in amplitude than 25% of the preceding T wave voltage. An abnormal U wave is most helpful in the diagnosis of hypokalemia.