Heat and moisture exchanger for a patient interface

A patient interface system configured 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 system is configured to maintain a therapeutic 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 system comprising: a patient interface comprising: a plenum chamber configured to be pressurised at the therapeutic pressure in use; a seal forming structure connected to the plenum chamber and configured to seal with an area around the entrance to the patient's airways including at least the patient's nares, the seal forming structure being constructed from a soft, flexible, resilient material; a positioning and stabilising structure comprising at least one strap and being configured to maintain the seal forming structure in sealing contact with the area surrounding the entrance to the patient's airways while maintaining the therapeutic pressure at the entrance to the patient's airways; a connection port configured to connect to an air circuit; and a gas washout vent configured to allow a flow of patient exhaled gas to escape to ambient air to minimise rebreathing of exhaled carbon dioxide by the patient; and a heat and moisture exchanger (HME), positioned inside of the plenum chamber and that includes a rigid frame and an HME material, the HME material being moisture-absorbent and positioned inside of the rigid frame, the rigid frame including at least one engagement member configured to releasably engage the plenum chamber such that the HME is removably connectable to the plenum chamber.