A method for processing a cardiac signal represented as a function of time includes providing a number n of different threshold levels Ni, with i=1 to n and n being greater than or equal to three; detecting, from a given time T and per threshold level Ni, at least two successive intersections of the cardiac signal with the threshold level Ni, considering a crossing per increasing and/or decreasing value of the cardiac signal with the threshold level Ni; and determining at least one statistical parameter for the cardiac signal from the intersections of the cardiac signal with the at least three different threshold levels Ni. The method may be implemented by a subcutaneous active implantable medical device having a control circuit configured to process a cardiac signal.