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Figure 4 (A) Spectral computation of T-wave alternans (TWA). In the original aligned ECG beats, alternans at each time point within the T-wave (vertical arrows) results in down-up-down oscillations. Fast Fourier transformation (FFT) yields a spectrum, in which the alternating component of these oscillations causes the 0.5 cycles/beat peak (T). In the final spectrum (summated for all time points), T is related to spectral noise to compute V_alt and k-score (see panel B).

(B) Positive TWA (from commercial system) shows (i) V_alt 1.9 µV in two precordial or one vector lead (here V_alt4 to 6 µV in V₃ to V₆) with (ii) k-score 3 (gray shading) for >1 min (here 5 min), at (iii) onset rate <110 beats/min (here 100 beats/min), with (iv) <10% bad beats and <2 µV noise, without (v)artifactual alternans (see text). Black horizontal bars indicate periods when conditions for positive TWA are met.