U wave
Also calledU-wave
Definition
A small, low-amplitude deflection that may follow the T wave, most evident in leads V2 and V3 and often absent in the limb leads. Its origin is debated: the AHA/ACCF/HRS standards describe it as a mechanoelectric phenomenon, while late repolarization of the Purkinje fibers, M-cell activity and other mechanisms have also been proposed. It normally has the same polarity as the preceding T wave, its amplitude is usually 5% to 25% of the T wave, and it is seen more often at slow heart rates. Prominent U waves are classically associated with hypokalemia, while an inverted U wave in V2 to V5 is abnormal and may appear during acute ischemia or with hypertension.
Sources
- 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
- Samesima N, God EG, Kruse JCL, et al. Brazilian Society of Cardiology guidelines on the analysis and issuance of electrocardiographic reports - 2022. Arq Bras Cardiol. 2022;119(4):638-680. doi.org/10.36660/abc.20220623
Last reviewed 2 October 2026. Every definition in the EKG Dictionary cites the guideline, consensus statement or original paper it comes from.