An implantable, battery-operated cardioverting/defibrillating pacemaker device with the ability to sense and respond to haemodynamic compromise is described. The ventricular pressure (VP) and electrical activity of the heart are sensed via one or more transvenous or epicardial devices. The filtered peak-to-peak amplitude (VFPPA) and a peak pressure function (VPPF) based upon the right and/or left VP waveform(s) are derived. One of the VFPPA and VPPF, and electrically derived heart rate information, are used in the diagnosis of arrhythmic states. Bradycardia support pacing is initiated and terminated by reference to the VP signal, and the pacing pulse voltage used during bradycardia support pacing is optimized in order to extend battery life of the device. This is done by using the haemodynamic sensor to confirm the presence of an appropriate haemodynamic response to the delivery of bradycardia support pacing, and adjusting the pacing voltage to its lowest efficacious level. In the preferred embodiment the high pass filtered right ventricular pressure is monitored to confirm pacing capture.