Abnormal U waves occur in several situations some of which are listed here.
Hypopotassemia
Ischemia
Some cardio-active drugs
Congenital long QT syndromes
It is important to recognize that the presence of such abnormal U waves has diagnostic, prognostic and therapeutic importance. Such U waves often merge with the T wave and complicate the measurement of the QT interval. For these reasons, It is important to be able to recognize the U wave on the electrocardiogram and to interpret its presence within the clinical context. Negative or inverted U waves, although rarely observed, are always abnormal. The are most frequently a manifestation of myocardial ischemia or severely depressed ventricular function