Vent arrangement for respiratory mask

Disclosed is an air or gas ventilation system a gas washout vent arrangement which allows for adequate venting of carbon dioxide while permitting efficient air delivery to the patient. A control system provides automated control of gas washout of a patient interface (30), such as a mask or nasal prongs. A gas washout vent assembly (60) of the system may include a variable exhaust area such as one defined by overlapping apertures of the assembly or a conduit having a variable gas passage channel. The vent assembly may be formed by nested structures, such as conic or cylindrical members, each having an opening of the overlapping apertures. The vent assembly may be attached substantially near or included with the patient interface. A controller including a processor, the controller coupled with the actuator, the controller configured to (a) control operation of a flow generator and (b) operate the actuator to change the exhaust area of the vent assembly to change a vent flow rate of the expiratory gas through the exhaust area. The controller is configured to control an operation of the flow generator to change flow supply, such that a set pressure at the patient interface is maintained.