An apparatus for determining a patients circulatory fill status is adapted to provide a dilution curve and is capable to derive the ratio between the patients global end-diastolic volume GEDV and the patients intra thoracic thermo volume ITTV from the dilution curve. Further, a computer program for determining a patients circulatory fill status has instructions adapted to carry out the steps of generating the dilution curve on basis of provided measurement data of dilution versus time, deriving the ratio between the patients global end-diastolic volume GEDV and the patients intra thoracic thermo volume ITTV from the dilution curve, and determining the patients circulatory fill status on basis of the ratio between the patients global end-diastolic volume GEDV and the patients intra thoracic thermo volume ITTV, when the computer program is run on a computer. A method is also provided.
