Apparatus, computer system, computer program and storage medium for determining intrathoracic blood volume (ITBV) and other cardio-vascular parameters of a patient by thermodilution measurements. The apparatus includes: temperature influencing means for introducing a traveling temperature deviation to a patient's blood stream, a temperature sensor device for measuring the local temperature of the patient's blood downstream of the temperature influencing means, a computer system adapted to: record local blood temperature measured as a function of time to determine a thermodilution curve, determine global enddiastolic blood volume (GEDV) and intrathoracic thermovolume (ITTV) from the thermodilution curve, determine intrathoracic blood volume (ITBV) as a function of GEDV, ITTV and the airway pressure inside the patient's lungs (P). By determining ITBV not only from GEDV but also a function of ITTV and P enhanced accuracy can be obtained, especially if the patient suffers from pulmonary edema and/or is mechanically ventilated.