What does the U wave represent in an ECG?

The U wave in an ECG typically represents the repolarization of the Purkinje fibers, which are specialized fibers in the ventricular conduction system. During the cardiac cycle, after the QRS complex (which represents ventricular depolarization), the U wave may appear as an additional positive deflection. This is particularly significant in cases where there is fulminant repolarization of the ventricular myocardium or when the heart is under certain conditions, such as electrolyte imbalances.

While atrial repolarization does occur, it is usually not distinctly visible on a standard ECG tracing because it happens concurrently with the QRS complex and is thus considered part of that complex. The ventricular depolarization is shown by the QRS complex itself, and the electrical activity of the Sinoatrial node is represented by the P wave. Therefore, identifying the U wave as a representation of Purkinje fiber repolarization is accurate and reflects its role in the overall cardiac cycle.