The invention relates to an apparatus (200) comprising: a flow generator (210) generating a flow of gas for delivery to a patient, at least one sensor (230, 232) for measuring properties of the flow of gas being delivered to the patient, a controller (224) operatively connected to the flow generator and connected to the at least one sensor, the controller adapted to operate according to: a pressure control mode in which the controller controls the flow generator to output a substantially steady pressure, or a flow control mode in which the controller controls the flow generator to output a substantially steady flow, wherein in the flow control mode, the controller is configured to monitor the flow of gas, and if the flow of gas indicates CO2 rebreathing, adjust operation of the flow generator.