<unk>  is a beat-to-beat alternation in the repolarization cycle of the heartbeat.  It can be observed in the electrocardiogram (ECG) as a difference in the amplitude and morphology of the ST-segment and/or the T wave among successive odd and even beats in an ABAB pattern (Figure).
T wave alternans
U wave alternans
PR interval alternans
T wave inversion
Sine wave pattern
Low QRS voltage
Tombstone ST elevation
Peaked T waves