An implantable heart monitoring system includes a housing configured for implantation in a subject, at least one sensor member implanted relative to the heart of the subject to detect an electrical signal related to cardiac activity of the heart, a control circuit in the housing, and a memory accessible by the control circuit. The control circuit, from said electrical signal, derives a first value related to the volume of a first part of the heart and derives a second value related to the volume of a second part of the heart, and monitors variation of said first value over time in at least a substantial portion of a heart cycle, and monitors variation of said second value over time in said substantial portion of said heart cycle, and stores in the memory information representing the monitoring of said first and second values that represents a relationship between the first and second values in the substantial portion of the heart cycle and variation of the relationship in the substantial portion of the heart cycle.