Embodiments relate to a heart monitor, which is connected to or can be connected to at least one sensor for pressure and volume data or equivalent substitute variables and which comprises an evaluation unit for processing at least one input signal reflecting the temporal course of pressure and volume data or equivalent substitute variables of the heart. The evaluation unit is configured such that it segments the input signal in accordance with individual completed cardiac cycles and examines segments of the input signal obtained in this way as to whether a particular segment of the input signal represents a PV diagram, which corresponds to specified quality conditions regarding the direction of circulation, morphology and distance between a starting and end values.