Apparatus for monitoring and storing heart activity signals is disclosed as including a data processor illustratively in the form of a digital microprocessor. The monitoring and storing apparatus comprises a memory comprising first and second pluralities of storage locations, a circuit operating in a real time frame for sampling at a regular rate the patient's heart activity and for storing such sample signals in corresponding locations of said first plurality, an the data processor for processing the signals stored in the first plurality and for storing the processed signals in selected locations of said second plurality. The storage of the signals in the second plurality is performed in a second computer time frame and at a rate faster than the real time sampling rate. A sampling clock provides a sampling clock signal to the sampling circuit whereby the samples of the patient's heart activity are taken. A second computer clock provides computer clock signals to at least a portion of the data processor whereby the processing of the heart activity is effected in the relatively fast computer time frame. The data processor comprises a first portion or means that operates in response to the sample clock signal for evaluating the heart activity signals to determine the existence of a valid heartbeat and a second portion operating in the computer time frame for processing the detected heartbeat and in particular for determining the interval therebetween, which is evaluated to determine whether the heartbeat is occurring with a regular rhythm or not.