U Wave. The clinical significance of U wave is limited to the occasional obfuscation of the end of T wave and an inadequately explained U wave inversion associated with myocardial ischemia, infarction, and ventricular hypertrophy and dilatation. Lengthening of QT interval often interferes with the recognition of U wave. The characteristics of U wave are not compatible with the Purkinje or ventricular muscle repolarization hypotheses. The timing of the U wave during ventricular relaxation and the links between U wave and mechanical events favor the mechanoelectrical hypothesis of U wave genesis. Unfortunately, little research has been done to test this hypothesis.