U waves represent prolonged repolarisation of myocardial M cells. Under normal conditions the M cells are electrically coupled to adjacent cell layers. However, in conditions that produce electrical uncoupling, these M cells have significant effects on repolarisation producing prolonged rounded ST-T waves or U waves. The finding of U wave inversion on an ECG at rest or baseline is a non-specific finding. It is significant when observed post exercise, being originally described in early 1940s. Subsequent studies in the late 1970s, demonstrated a significant correlation of U wave inversion with either left main or LAD disease.