Patient interface with a heat and moisture exchanger

A patient interface comprising:

• a plenum chamber pressurisable to a therapeutic pressure of at least 6 cmH2O above ambient air pressure, said plenum chamber including a plenum chamber inlet port sized and structured to receive a flow of air at the therapeutic pressure for breathing by a patient,
• a seal-forming structure constructed and arranged to form a seal with a region of the patient's face surrounding an entrance to the patient's airways, said seal-forming structure having a hole therein such that the flow of air at said therapeutic pressure is delivered to at least an entrance to the patient's nares, the seal-forming structure constructed and arranged to maintain said therapeutic pressure in the plenum chamber throughout the patient's respiratory cycle in use;
• a positioning and stabilising structure to provide a force to hold the seal-forming structure in a therapeutically effective position on the patient's head, and
• a vent structure to allow a continuous flow of gases exhaled by the patient from an interior of the plenum chamber to ambient, said vent structure being sized and shaped to maintain the therapeutic pressure in the plenum chamber in use;
• wherein the patient interface is configured to allow the patient to breath from ambient through their mouth in the absence of a flow of pressurised air through the plenum chamber inlet port, or the patient interface is configured to leave the patient's mouth uncovered;
• and
• wherein patient interface comprises a flexible heat and moisture exchanger (HME) located within the plenum chamber, the HME comprising a flexible heat and moisture exchange material and a retaining structure formed from a pliable material provided around an outer perimeter of the heat and moisture exchange material,
• wherein:
• the interior of the plenum chamber comprises at least one snap fit feature, and the retaining structure comprises at least one complementary snap fit feature comprising a rib, the at least one complementary snap fit feature being configured to engage, in use, the at least one snap fit feature located on the interior of the plenum chamber.