Heart stimulating system for stimulating at least a ventricle of a heart including: stimulation pulse generator adapted to generate stimulation pulses and connected to a ventricular stimulation electrode for delivering stimulation pulses, atrial sensing stage connected to an electrode for picking up potentials inside an atrium and adapted to sense an excitation or contraction of atrial myocardium, ventricular sensing stage connected to an electrode for picking up potentials inside a ventricle and adapted to sense an excitation or contraction of ventricular myocardium, memory for AV-delay values, a control unit adapted to trigger said stimulation pulse generator to generate ventricular stimulation pulses timed based on AV-delay values stored in said memory and to acquire atrioventricular interval samples, and atrioventricular interval timing analyzing unit for receiving atrioventricular interval samples from said control unit and adapted to generate at least one histogram based on said atrioventricular interval samples and analyze said histograms.