A device for the analysis of the alternans cycle to cycle and/or the variability of the ventricular repolarization wave in an ECG signal. This device extracts from an ECG signal, for each cardiac beat, a temporal segment of T samples of the ventricular repolarization wave, considered as from a moment t following a predetermined temporal origin. The T samples for B consecutive beats are aggregated and a local variance factor is evaluated. The square root of the local variance is obtained and weighted by a factor of local alternans. The device delivers at the output an index of alternans and variability TVar.sub.b function, for a given beat, local variance factor weighted by the factor of local alternans.