Repolarization alternans detection using the KL transform and the beatquency spectrum

Electrical alternans at the repolarization phase of the cardiac cycle have been reported to be related with increased risk for ventricular arrhythmias (VA). Here, the authors present and study a method for detecting ST-T complex alternans based on the Karhunen-Loeve (KL) transform and a beatquency transform (Fourier transform of KL series with beat order as temporal reference). The ratio of the power in the band around 0.5 cycles/beat with respect to the total power is then used as indicator of the period 2 alternans presence. This is similarly done for period 4 alternans. To study the performance of this detection system the authors have generated a simulated alternating ECG with added noise (moving artifacts, EMG and baseline wander noise). The results for SNR around 15 dB for the ST-T complex (normal value for ECG signals) give a sensitivity and specificity for period 2 alternans detection of over 95%. This method applied to the entire European ST-T database gives 25 alternating episodes, 56% corresponding to ischemic episodes, showing that 5.3% of the ischemic episodes include alternans episodes.