Abstract: The invention relates to a device to determine effective pulmonary blood flow (PBF) by means of partial CO.sub.2 rebreathing. The device is characterized in that it comprises an endotracheal tube whose conduit leading from the patient to the respirator is divided up into two lanes between a controllable three-way valve (4) and a Y-piece; one lane builds a larger dead area (6) for CO.sub.2 rebreathing whereby, in order to measure CO.sub.2 elimination and end expiratory partial CO.sub.2 pressure a CO.sub.2 sensor (3) and a respiration flow sensor (1) are provided on the endotracheal tube of the patient. The calculation of effective pulmonary blood flow is provided by a microprocessor/controller (7) which also controls the three-way valve (4) which provides the switching between both lanes.