A mask assembly for delivering pressurized gas to a patient comprising a mask having an inspiratory port and an expiratory port located on generally opposite sides, wherein said ports are sized, oriented, positioned, and/or spaced apart a sufficient distance to allow a cross-flow of pressurized gas to flow through the mask assembly; an outlet limb connected to the expiratory port and having an aperture in pneumatic communication with the breathing chamber; the aperture size being variable between a first, open configuration and at least one second configuration that is different from the first. A ventilation system for delivering pressurized gas to a patient comprising a seal formed with the patient's airways and in pneumatic communication with a plenum chamber; an exchanger positioned at least partially within the plenum chamber, and in pneumatic communication with inspiratory and expiratory flow paths, to recover heat and/or moisture from gas exhaled by the patient.