An implantable heart stimulator has an impedance measurement a cardiogenic impedance waveform using an impedance configuration arranged to measure myocardial contractility of the heart. The heart stimulator further has a calculating unit that calculates an estimate value being related to at least two impedance values of the waveform, or of an average waveform of several consecutive waveforms, during a predetermined time period of the waveform, or average waveform, the calculated estimate value being an estimate of the left ventricular (LV) systolic pressure.