Patient interface comprising a gas washout vent

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 a patient's airways including at least the 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, said patient interface comprising:

• a sealing structure configured to seal against an area of the patient's face surrounding an entrance to the patient's airways;
• a plenum chamber connected to the sealing structure and configured to be pressurized to the therapy pressure in use;
• a positioning and stabilizing structure configured to maintain the sealing structure in sealing contact with the patient's face in use;
• a gas washout vent configured to allow a vent flow of gas to exit the plenum chamber to atmosphere to minimize rebreathing of exhaled gas by the patient, said gas washout vent comprising:
• a first layer having a plurality of first ellipsoidal structures arranged in rows and columns to form a plurality of first openings in a first grid;
• and
• a second layer having a plurality of second ellipsoidal structures arranged in rows and columns to form a plurality of second openings in a second grid,
• wherein the first layer and the second layer are permanently attached to one another such that the first openings and the second openings are offset relative to each other to form a plurality of constricted passages extending through the first openings and the second openings of the first layer and the second layer, respectively, to allow the vent flow of gas to pass through the gas washout vent to atmosphere.