A valved nasal cannula, said valved nasal cannula being insertable in a nose of an intended user and usable to selectively restrict a flow of air flowing through said valved nasal cannula, said nose having a nostril defining a nostril inlet and a nostril vestibule extending inwardly into said nose from said nostril inlet, said valved nasal cannula comprising: a cannula body (18), said cannula body defining a cannula passageway extending therethrough, said cannula passageway defining a passageway longitudinal axis, said cannula body also defining a cannula proximal end and a substantially longitudinally opposed cannula distal end, said cannula body being, at least in part, insertable in said nostril with said cannula proximal end positioned inside said nostril vestibule substantially spaced apart from said nostril inlet and said cannula distal end positioned substantially adjacent said nostril inlet； a valve (34) operatively coupled to said cannula body for selectively restricting said flow of air through said cannula passageway； wherein said valve has dimensions, configurations and deformation properties such that a substantially constant backpressure is exerted by said valve upon said intended user expiring air from said cannula proximal end towards said cannula distal end for a predetermined range of expiration flow rate.
