U wave manifesting in alternating beats: The bigeminal U wave

The U wave is a deflection of the electrocardiogram whose genesis and clinical significance are yet unclear. It usually appears at the end of each ventricular complex in a selected lead. This study presents a unique case where the U wave appeared exclusively in beat-to-beat alternating cardiac cycles, so as to give rise to a pattern of " bigeminal U wave". Moreover, the presence of the U wave appeared to be related to the previous duration of the cardiac cycle correlating with lengthening of the cardiac cycle, while it did occur after shortening of the cardiac cycle. We can hypothesize that prolonged action potentials in ventricular M cells might contribute to this phenomenon.