The U-wave on electrocardiogram (ECG) is a small deflection following the T-wave, the sixth wave. It is 25% or less of the preceding T-wave in amplitude.1,2 While the genesis of the U-wave is uncertain, it is said to represent repolarisation of the Purkinje fibres.1,2 Disproportionally large U-waves may indicate underlying cardiac or non-cardiac pathology. A relatively frequent cause for a large U-wave is hypokalaemia. It is observed in patients with bradycardia, ventricular hypertrophy, hypothyroidism, hypocalcaemia, hypomagnesaemia, mitral valve prolapse, hypothermia, increased intracranial pressure, or patients on anti-arrhythmic medicine.2 A negative U-wave, on the other hand, may represent early myocardial ischaemia, specifically in the context of a lesion in the left main or proximal left anterior descending coronary artery.2,3
We recorded an ECG (figure 1) in a 50-year-old Caucasian woman when she attended our clinic for atypical chest pain and a history of familial hypercholesterolaemia. Her medication included atorvastatin and ferrous sulphate only. Cardiovascular examination was unremarkable. The ECG shows a very large U-wave, but was otherwise normal. Because of the disproportionally large U-wave, she underwent extensive investigations. Her echocardiogram, exercise stress echocardiogram and 24-hour Holter monitor were all normal. Her blood tests showed normal thyroid function, normal serum potassium (4.4 mmol/L) and calcium (2.4 mmol/L).
We found no evidence of possible causes for the large U-wave in this patient. This makes it possible that a giant U-wave can be innocent.
Conflicts of interest
The patient has given formal consent for us to report the case.
1. Papp C. The sixth wave of the electrocardiogram. Br Heart J 1940;2:9–24. https://doi.org/10.1136/hrt.2.1.9
2. Pérez Riera AR, Ferreira C, Filho CF et al. The enigmatic sixth wave of the electrocardiogram: the U wave. Cardiol J 2008;15:408–21. Available from: https://journals.viamedica.pl/cardiology_journal/article/view/21565
3. Verma N, Figueredo V, Greenspan A, Pressman G. Giant U waves: an important clinical clue. Research Reports in Clinical Cardiology 2011;2:51–5. https://doi.org/10.2147/RRCC.S18459