The RR series, comprising a number of samples respectively defining the time interval between two successive heartbeats, is filtered by means of a digital band-pass filter (Fk), having a given band-pass [fc; f′c] and preferably by means of a recursive elective filter (RII). A variability index (Ik), is calculated which is a function of the instantaneous amplitude [vs(n)] of the discrete signal (Sk) coming from said band-pass filter. The variability index (Ik) is preferably a function of and particularly equal to the effective value of the discrete signal (Sk) from the band-pass filter.