Gas washout vent for patient interface

A patient interface for sealed delivery of a flow of air at a continuously positive pressure with respect to ambient air pressure to an entrance to the patient's airways including at least an entrance of a patient's nares, wherein the patient interface is configured to maintain a therapy pressure in a range of about 4 cmH2O to about 30 cmH2O above ambient air pressure in use, throughout the patient's respiratory cycle, while the patient is sleeping, to ameliorate sleep disordered breathing, the patient interface comprising:

• a sealing structure configured to seal around the entrance to the patient's airways;
• a positioning and stabilising structure to maintain the sealing structure in sealing contact with an area surrounding the entrance to the patient's airways while maintaining a therapeutic pressure at the entrance to the patient's airways;
• a plenum chamber configured to be pressurised at a pressure above ambient pressure in use;
• a gas washout vent configured to allow patient-exhaled CO2 to flow to an exterior of the plenum chamber to minimise rebreathing of exhaled CO2 by the patient, the gas washout vent including at least one outlet orifice;
• a diffusing member at least partly covering the outlet orifice;
• and
• a blocking member having an air-impermeable material, the blocking member preventing gas exiting from the outlet orifice from flowing straight through the diffusing member.