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Figure 1 Spectral computation of RPA. Left panel, The first two QRS-aligned and baseline-corrected beats from a series of 64 beats at 120 beats/min (500 ms CL pacing). JT alternation is computed from interbeat power spectra at successive time points within 1) the entire JT interval (R); and windows of half of this duration in 2) early (RE); 3) middle (RM); and 4) late (RL) repolarization. Right panel, Summation of spectra at each time point reveals RPA as the magnitude at 0.5 cycles/beat, 
T. T is related to noise (0.33 to 0.48 cycles/beat) as TWAR, where TWAR 
3 is significant (TWAR = 67.6 here for R). The mean voltage of JT alternation was Valt = 7.67 µV.