A non-sealing patient interface (132) comprising:- a first gas passageway (126) configured to deliver a continuous positive flow of gas from a positive gas flow source to a subject's airway in use,- a second gas passageway (130) configured to simultaneously provide a negative flow of gas to draw gas from the subject's airway to a negative gas flow source in use,the patient interface allowing to:- provide a flow rate of the negative flow of gas in use that is sufficient to reduce a concentration of an exhaled gas in a physiological deadpace of the subject's airway, and- provide the flow rate of the negative flow of gas in use set to a function of the rate of the positive flow of gas.
