U wave variability in the surface ECG.

A 72-year-old man with heart failure, left ventricular dysfunction (ejection fraction 20%), prior ischemic stroke, COPD, and exacerbation of chronic renal failure was admitted in our unit. Serum potassium was 6.1 mmol/L, calcium concentration was at the lower normal range 2.15 mmol/L, and NT-pro-BNP was 28,900 pg/mL. The surface 12-lead electrocardiogram (ECG) showed sinus rhythm at 60 bpm, PR interval 160 ms, QRS duration 115 ms, QT interval 460 ms, and left ventricular hypertrophy criteria. Negative T waves in leads I, II, aVL, and V4 -V6 were also seen. In leads V4 -V6 , negative U waves were observed in concordance with negative T waves. In all precordial leads, beat-to-beat U-wave polarity variability was observed as a polarity variation from negative to positive with associated and stable negative T waves, in a beat-to-beat alternate morphology.