The qualitative definition of repolarization alternans (RA) as an every-other-beat alternation of the repolarization amplitude allows several possible quantitative characterizations of RA. In the absence of a standardization, any correct comparison among quantitative outputs by different automatic methods requires knowledge of the differences in the RA parameterization at the basis of their algorithms. Thus, aim of the present study was to investigate the kind of information provided by five methods, namely the fast Fourier spectral method (FFTSM), the complex demodulation method (CDM), the modified moving average method (MMAM), the Laplacian likelihood ratio method (LLRM) and the heart-rate adaptive match filter method (AMFM) when characterizing RA in terms of its amplitude and location. Eight synthetic ECG recordings affected by stationary RA with uniform and triangular profiles localized along the ST segment, over the T wave, at the end of the T wave and all along the JT segment, respectively, were considered. Results indicate that quantitative RA characterization is method dependent. More specifically, the FFTSM and the LLRM provide a measure that matches the root mean square of the RA profile over the JT segment. Instead, the CDM and the AMFM compute RA amplitude as the mean value of the RA profile over the JT segment. Eventually, the MMAM provides the maximum amplitude difference between consecutive beats along repolarization. RA location is homogeneously among methods, since they all provide the time instant in correspondence of which the center of mass of the alternation occurs. 
Quantitative characterization of repolarization alternans in terms of amplitude and location: What information from different methods?