A blood treatment machine includes a blood pump; an arterial line in fluid communication with the blood pump; a venous line; and a heartbeat evaluation system including (i) a first electrode coupled to the arterial line or to a patient, (ii) a second electrode coupled to the venous line, (iii) electronic circuitry communicating electrically with the first electrode and the second electrode to sense an electrical heartbeat signal generated by the patient, and (iv) signal processing configured to use the sensed heartbeat signal to calculate at least one of (a) heart rate, (b) respiration, (c) stroke volume, (d) cardiac output, or (e) central blood volume.