Gas washout vent for patient interface

A gas washout vent for a patient interface, comprising one or more outlet orifices (3402); a diffusing member (3406) covering each of the one or more outlet orifices so that flow exiting the one or more outlet orifices flows at least partially into the diffusing member; and a blocking member (3408) having an air-impermeable material. The diffusing member is in contact with a surface (3404) wherein the one or more outlet orifices are formed. The blocking member is configured to cause the flow to change direction with respect to the direction through each of the one or more outlet orifices before exiting the diffusing member. The invention provides a washout vent suitable for use with CPAP devices configured to maintain a pressure in a range of 4 cmH2O to 30 cmH2O above ambient air pressure, throughout a patient's respiratory cycle, while the patient is sleeping, to ameliorate sleep disordered breathing.