Can a giant U-wave be innocent?

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Authors: Sinead Curran, Waleed Arshad, Arvinder Kurbaan, Han B Xiao

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).

Curren - Figure 1. A 12-lead electrocardiogram (ECG) showing sinus rhythm, normal time intervals and normal QRS-T morphology. There are dominant U-waves in leads II, III, avF, V4 to V6. The U-wave is of similar size, or greater than the preceding T-wave in the same leads (arrow) — Figure 1. A 12-lead electrocardiogram (ECG) showing sinus rhythm, normal time intervals and normal QRS-T morphology. There are dominant U-waves in leads II, III, avF, V4 to V6. The U-wave is of similar size, or greater than the preceding T-wave in the same leads (arrow)

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

None declared.

Funding

None.

Patient consent

The patient has given formal consent for us to report the case.

References

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