U wave

The U wave is a small, rounded bump that can appear just after the T wave on an ECG, a gentle final flourish at the tail end of each heartbeat. It is often so faint that it goes unnoticed, and in many normal tracings it is not visible at all.

Its exact origin is still debated, but it is generally thought to reflect a late phase of ventricular recovery, possibly from the Purkinje fibers or from particular layers of heart muscle finishing their reset slightly behind the rest. A small upright U wave, in the same direction as the T wave, is considered a normal variant.

The U wave becomes clinically interesting when it grows prominent or turns the wrong way. Tall U waves are a classic clue to low blood potassium and can also accompany certain medicines or other electrolyte shifts; inverted U waves may hint at ischemia or strain. Because U waves can blur into the following T or P wave, they are easy to misread, so they are interpreted cautiously and confirmed when they seem to matter.