A control system provides automated control of gas washout of a patient interface, such as a mask or nasal prongs. A gas washout vent assembly of the system may include a variable exhaust area, such as one defined by gears, radial exhaust revolvers and/or flow diverters for a conduit having a variable gas passage channel. The vent assembly may be attached substantially near or included with the patient interface. An actuator of the assembly, such as a solenoid, motor or voice coil, manipulates the vent assembly. The actuator may be configured for control by a processor to change the exhaust area of the vent assembly based on various methodologies including, for example, sleep detection, disordered breathing event detection and/or leak detection.
