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.