U wave on ECG

U wave on ECG

U wave on ECG occurs after the T wave and is usually seen in the mid precordial leads. In hypokalemia, T wave becomes flattened and U wave becomes prominent (or apparently so because of near absence of T waves). Important conditions associated with U waves are systemic hypertension, aortic and mitral regurgitation and coronary artery disease [1].

Theories about genesis of U waves

One theory about the genesis of U wave is that it is due to repolarization of the Purkinje fibres. Another possibility is that it is due to after potentials caused by mechanical forces in the ventricular wall. A third hypothesis suggests that it is due to prolonged repolarization of the mid myocardial M cells [2]. Yet another hypothesis is the delayed repolarization of papillary muscles [3]. None of these theories have been widely accepted.

Importance of negative U wave

An inverted U wave usually has the same significance as an inverted T wave. A negative U wave is considered to be highly specific for heart disease and is associated with other abnormal findings on ECG in 90% of cases [1].

A discordant U wave inversion (with upright T wave) is also thought to represent myocardial ischemia.

An interesting case of U wave due to compression of the right ventricle by a reconstructed stomach tube after surgery for esophageal cancer has been reported [4].

Persistent giant U wave inversion was noted in a case after anoxic brain injury [5].

Exercise induced U wave inversion

Exercise induced U wave inversion in anterior leads has been associated with stenosis of proximal left anterior descending coronary artery in multiple case reports [6,7].

U wave alternans

U wave alternans along with pulsus alternans has been described in left ventricular failure [8].

References

1. Kishida H, Cole JS, Surawicz B. Negative U wave: a highly specific but poorly understood sign of heart disease. Am J Cardiol. 1982 Jun;49(8):2030-6.
   
2. Ritsema van Eck HJ, Kors JA, van Herpen G. The elusive U wave: a simple explanation of its genesis. J Electrocardiol. 2003;36 Suppl:133-7.
   
3. Pérez Riera AR, Ferreira C, Filho CF, Ferreira M, Meneghini A, Uchida AH, Schapachnik E, Dubner S, Zhang L. The enigmatic sixth wave of the electrocardiogram: the U wave. Cardiol J. 2008;15(5):408-21.
   
4. Yamagata K, Uno K, Mori K, Seto Y. Unusual U wave induced by reconstructed retrosternal esophagus. Clin Case Rep. 2015 Nov;3(11):949-951.
5. Peters MN, Katz MJ, Howell LA, Moscona JC, Turnage TA, Delafontaine P. Persistent giant U wave inversion with anoxic brain injury. Proc (Bayl Univ Med Cent). 2013 Apr;26(2):177-8.
   
6. Costantini M, Capone S, Tondo A, Oreto G. Is exercise-induced U-wave inversion predictive of proximal left anterior descending coronary artery disease? J Electrocardiol. 2008 Mar-Apr;41(2):99-101.
7. Raveendran S, Hadfield R, Petkar S, Malik N. Significance of exercise induced U wave inversion as a marker for coronary artery disease. BMJ Case Rep. 2012 Feb 10;2012. pii: bcr0420114132.
   
8. Eyer KM. U wave alternans: an electrocardiographic sign of left ventricular failure. Am Heart J. 1974 Jan;87(1):41-5.