U wave

An electrocardiogram (ECG) low-amplitude deflection following the T wave, thought to represent delayed repolarization of the Purkinje fibers or mid-myocardial M-cells. Possible causes of prominent (tall) U waves are hypokalemia, hypomagnesemia, hypocalcemia, bradycardia, medications (quinidine, sotalol, digoxin), and hypothermia. Causes of inverted U waves are coronary artery disease, hypertension, cardiomyopathy, hyperthyroidism, and valvular heart disease.