portion 32 is inserted into the trachea of the patient by intubation procedures and gas is provided to the patient from a ventilator machine not shown via an inspiratory hose 34 which is interconnected by a y piece 36 to an expiratory hose 38 an additional length of hose 40 is provided between the tubular portion 32 and the y piece 36 which acts as a deadspace for receiving exhaled gas a three way valve 42 generally positioned between the y piece 36 and the opening to the additional length of hose 40 is constructed for intermittent actuation to selectively direct the flow of gas that is at one setting the valve 42 allows inspiratory gas to enter the tubular portion 32 while preventing movement of the gas into the additional length of hose 40 in a second setting the valve 42 allows exhaled gas to enter into the expiratory hose 38 while preventing movement of gas into the additional length of hose 40 in a third setting the thee way valve 42 directs exhaled air to enter into the additional length of hose 40 and causes the patient to re breath the exhaled air on the following breath to thereby cause a change in effective ventilation the change in v co 2 and end tidal co 2 caused by the change in ventilation in the prior art system of fig2 can then be used to calculate cardiac output sensing and or monitoring devices may be attached to the re breathing ventilation circuit 30 between the additional length of hose 40 and the tubular portion 32 the sensing and or monitoring devices may include for example means for detecting co.