a stimulation pulse generator adapted to generate electric stimulation pulses and connected or connectable to at least a ventricular stimulation electrode configured to deliver electric stimulation pulses to at least said ventricle of said heart;

a sensing stage connected or connectable to an electrode configured to pick up electric potentials inside at least said ventricle of said heart, said sensing stage adapted to sense an excitation or a contraction of a heart chamber;

a control unit, which is connected to said sensing stage and to said stimulation pulse generator and which is adapted to trigger said stimulation pulse generator in a safe anti-tachycardia pacing time zone before a start or after an end of a vulnerable period in a heart cycle and prior to an expected intrinsic excitation of said heart chamber to be stimulated when said heart stimulator is in an anti-tachycardia pacing mode of operation;

memory connected to said control unit that contains patient specific data that characterizes a time relation between an intrinsic heart rate as characterized by an RR-interval and a QT-interval characterized by a time period from a beginning of the heart cycle to an end of the vulnerable period in that heart cycle wherein said patient specific data comprises an RT-RR or a QT-RR regression model to estimate the vulnerable period at high heart rates where no corresponding RT-RR or QT-RR calibration data exists at said high heart rates; and,

wherein said control unit is adapted to calculate an RVp interval based on a patient specific relation of a heart rate and said QT-interval such that a stimulation pulse is triggered shortly before said start or after said end of said vulnerable period for an actual heart rate.