Abstract: A process and a device for determining a patient's circulatory fill status, particularly the global end-diastolic volume of the heart (GEDV), intrathoracic blood volume (ITBV), pulmonary blood volume (PBV), extravascular lung water volume (EVLW), and/or global cardiac function index (CFI) by means of thermo-dilution; with this process intrathoracic thermo-volume (ITTV) and pulmonary thermo-volume (PTV), for example, are obtained and the global end-diastolic volume of the heart (GEDV) is determined from the equationGEDV=ITTV-PTV.The other volumes can be calculated by means of this parameter, which is specific for the fill status of the heart, as well as other species-specific characteristics (a, a', b, b').