The present invention relates to an apparatus for automated control of gas washout of a patient interface (30, 330, 430) of a respiratory treatment apparatus comprising: a vent assembly (60, 360, 460) associated with a patient interface (30, 330, 430) to vent expiratory gas and positioned in an air delivery conduit proximal to the patient interface； an actuator to manipulate the vent assembly (60, 360, 460)； and a controller, including a processor, that is coupled to the actuator； wherein the vent assembly (60, 360, 460) has a variable exhaust area defined by overlapping apertures (90, 590, 1090, 1290； 95, 595, 1295), wherein the overlapping apertures (90, 590, 1090, 1290； 95, 595, 1295) allow air to pass through the exhaust area； and the controller is configured to operate the actuator to change the variable exhaust area of the vent assembly (60, 360, 460).
