A method, arrangement and apparatus for assessing fluid balance status of a subject are disclosed. At least one physiological parameter responsive to blood volume changes in the subject is determined (21) and a predetermined change in the ventilation of the subject is produced (22). The response that the predetermined change of ventilation causes in at least one of the at least one physiological parameter is then determined (23), thereby to obtain at least one measure of change. The fluid balance status of the subject may be assessed (24) based on the at least one measure of change. The subject is typically connected to a ventilator and the predetermined change is produced bv controlling the ventilator.
