A breathable gas supply apparatus for a patient features a breathable gas supply system and a respiratory mask assembly in communication with the breathable gas supply system. The breathable gas supply system provides a supply of breathable gas to the respiratory mask assembly and removes a supply of exhaled gas from the respiratory mask assembly. The breathable gas supply system includes a housing having an inspiration chamber and an exhalation chamber and a movable structure in communication with the inspiration and exhalation chambers. The movable structure moves in phase with a respiratory cycle of a patient. In one preferred embodiment, the breathable gas supply system includes a second movable structure, which moves in response to a phase of the respiratory cycle of the patient. In another one preferred embodiment, the breathable gas supply system is powered to assist the patients respiratory effort.
