An apparatus for determining a patients volemic status is adapted to make use of a physiological heart-lung interaction during spontaneous breathing or mechanical ventilation. Further, a computer program for determining the patients volemic status has instructions adapted to carry out the steps of generating data of a physiological heart-lung interaction during spontaneous breathing or mechanical ventilation, and determining the patients volemic status when making use of the data of the physiological heart-lung interaction, when run on a computer.
