Prominent U wave
Also calledprominent U waves · large U wave · tall U wave · giant U wave
Definition
An abnormally tall U wave, the small, low-frequency deflection that follows the T wave. A normal U wave is about 1 mm or less, roughly 11 percent of the preceding T wave, and is most evident in leads V2 and V3; in hypokalemia a U wave taller than 0.5 mm in lead II or 1 mm in V3 is described as prominent, and with advanced hypokalemia it can exceed the T wave. Its origin is still debated: repolarization of the Purkinje fibers, of the papillary muscles or of mid-myocardial M cells, and mechanoelectrical feedback from stretch of the ventricular wall, have all been proposed. Prominent U waves are classically associated with hypokalemia, and also with hypomagnesemia, bradycardia, and quinidine-like and class III antiarrhythmic drugs.
Clinical noteWhen a large U wave fuses with the T wave, the end of the T wave is hard to identify and the QT interval can be misjudged. T-U fusion that becomes more exaggerated after a pause is a recognized warning sign for torsades de pointes.
Sources
- Perez Riera AR, Ferreira C, Filho CF, et al. The enigmatic sixth wave of the electrocardiogram: the U wave. Cardiol J. 2008;15(5):408-421. pubmed.ncbi.nlm.nih.gov/18810715/
- Drew BJ, Ackerman MJ, Funk M, et al. Prevention of torsade de pointes in hospital settings: a scientific statement from the American Heart Association and the American College of Cardiology Foundation. Circulation. 2010;121(8):1047-1060. doi.org/10.1161/CIRCULATIONAHA.109.192704
- Rautaharju PM, Surawicz B, Gettes LS. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part IV: the ST segment, T and U waves, and the QT interval. Circulation. 2009;119(10):e241-e250. doi.org/10.1161/CIRCULATIONAHA.108.191096
Last reviewed 2 October 2026. Every definition in the EKG Dictionary cites the guideline, consensus statement or original paper it comes from.