There is considerable controversy about the precise mechanisms producing the normal and abnormal U waves. Suffice it to say that low amplitude positive waves < 1.5 mm tall that range from 160-200 msec in duration that are best seen in leads V2 or V3 are present in many normal hearts. Leads V2 and V3 are close to the ventricular mass and small amplitude signals may be best seen in these leads. All observers call these normal U waves.

It is the area of abnormal U waves which produces the most controversy. It is likely that the abnormal U wave is not really a U wave at all, but an abnormal T wave. Antzelevitch and co-workers have described a sub-population of cells located in the mid-myocardial layer, called M-cells, that have action potentials much longer than those of either the epicardial or endocardial cells. During normal conditions, these cells are closely coupled electrically to adjacent cell layers and may not be manifest in the ECG. However, during conditions which produce electrical uncoupling from adjacent cell layers, the M cells may have profound effects on the morphology of repolarization, producing ST-T waves that are rounded and prolonged in duration, as well as apparent U waves. These abnormal "U waves" are waves of increased amplitude or ones which merge with abnormal T waves and produce T-U fusion. Criteria include an amplitude > 5 mm tall or a U wave that is as tall as the T wave that immediately precedes it.

Causes of abnormal U waves:
These are most commonly due to hypokalemia and/or digitalis, but they are commonly seen in patients taking cardiac antiarrhythmic drugs. Prominent U waves are common in bradycardias, where they are a normal response.

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