Patient interface with volume reducing member

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 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;

• said patient interface comprising:
• a cushion assembly adapted to form a seal around the patient's nose and/or mouth, the cushion assembly defining a plenum chamber pressurised at a pressure above ambient pressure in use;
• a gas washout vent configured to allow a flow of patient exhaled CO2 to an exterior of the patient interface to minimise rebreathing of exhaled CO2 by the patient;
• and
• at least one volume reducing member provided within the plenum chamber to reduce an internal volume of the plenum chamber,
• wherein the at least one volume reducing member is arranged to separate the plenum chamber into a main chamber and at least one peripheral chamber, the main chamber comprising a main volume of the plenum chamber for delivery of the therapy pressure, and the at least one peripheral chamber comprising a deadspace volume of the plenum chamber,
• wherein the main chamber is adapted to be in direct fluid communication with at least the entrance to the patient's nares and the gas washout vent, and
• wherein the least one peripheral chamber includes an upper or superior peripheral chamber adapted to be positioned superior the entrance to the patient's nares.