U wave

The U wave is a wave on an electrocardiogram (ECG). It comes after the T wave of ventricular repolarization and may not always be observed as a result of its small size. 'U' waves are thought to represent repolarization of the Purkinje fibers. However, the exact source of the U wave remains unclear. The most common theories for the origin are:

• Delayed repolarization of Purkinje fibers
• Prolonged re-polarisation of mid-myocardial M-cells
• After-potentials resulting from mechanical forces in the ventricular wall
• The repolarization of the papillary muscle.