A user interface has a non-sealing nasal cannula and a mask arranged about the nasal cannula, the mask including a seal configured with a user's face to allow the interface to be pressurized, the cannula configured to deliver breathing gases to the nares of a user at a flow rate exceeding the intended user's peak inspiratory flow requirements so that the mask and the user's pharynx are flushed continuously with fresh breathing gases to reduce dead space.