Spectral computation of repolarization alternans

Two consecutive QRS beats from a sequence of 64 are aligned, allowing corresponding timepoints within repolarization to be compared between beats (panel A). STU power spectra between beats are computed at successive timesamples and the 0.5 cycles/beat magnitude indicates alternate-beat periodicity (panel B). Summating these spectra across all timesamples reveals repolarization alternans (RPA) as ST, distinct from respiratory modulation (0.1 cycles/beat). T wave alternans ratio (TWAR) relates ST to noise (0.33 to 0.48 cycles/beat) and TWAR >3 predicts ventricular arrhythmia (here TWAR = 67.6). Panel C shows RPA temporal distribution (time domain reconstruction) showing ST for each STU timesample. T is the timepoint of the center-of-area of this reconstruction normalized to STU duration, and may be early (T<0.5), late (T>0.5), or symmetric within repolarization (T = 0.5, as here). V_alt, voltage difference of alternation.