Abstract: A device for monitoring cardiac pacing rate having a measuring unit for receiving an electrical signal representing the patient's cardiac demand, and a computing unit for determining the myocardial energy balance by calculating energy consumed by the myocardium for both an external dynamic work for pumping blood into a vascular system, and an internal static work of the myocardium. Volume and time based measurements are used, and in one embodiment, volumes are estimated and volume ratios are calculated from volume estimates. In another embodiment, volumes are estimated from bioimpedance measurements. A further aspect is a rate adaptive pacemaker, wherein the maximum pacing rate is determined from the myocardial energy balance such that the energy supplied to the myocardium approximately equals the energy consumed by the myocardium for both an external dynamic work for pumping blood into a vascular system and an internal static work of the myocardium.