The segment volume of a cardiac chamber or blood vessel of a patient is determined by injecting a first indicator in the blood stream of a patient, which influences the conductance of the blood. The electrical conductance in the cardiac chamber or blood vessel is measured. An injected quantity of indicator is determined and the development of the concentration of this indicator in the blood is measured wherein the cardiac output is calculated from the injected quantity of indicator and the development of the concentration in the blood. Subsequently, the segment volume and electrical parallel conductance of a cardiac chamber or blood vessel are calculated from the calculated cardiac output, the injected quantity of conductance indicator and the measured conductance.