An apparatus and a method for determining the volume amount of a physiological volume (EVLW) from the system response to two successive system disturbances, taking into account that an intrinsic physical property of the physiological volume is influenced by the first (or previous, respectively) system disturbance, is described. By introducing a cold bolus to the central venous blood stream, a flowed-by volume, such as extravascular lung water, is cooled down. The driving temperature gradient for heat transfer is reduced when a second cold bolus is introduced. From the difference of the system response to the first bolus injection and the second bolus injection EVLW can be determined.
