Disclosed is a respiratory apparatus for distinguishing between closed and open types of apnoeas. The apparatus is comprised of a flow generator (34), a patient interface (30), an air delivery tube (32) coupled between the flow generator (34) and the patient interface (30), a flow sensor (50), a pressure sensor (48) and a controller (62). The flow generator (34) is configured to provide a supply of pressurized gas to a patient via the patient interface (30) which is adapted to sealing engage with a patient's face to deliver the pressurized gas to the patient when in use. The controller (62) is coupled to the flow sensor (50) and the pressure sensor (48) and configured to detect the occurrence of an apnoea. When the controller (62) detects the presence of an apnoea it is configured to perform the following steps: apply an alternating pressure waveform at the flow generator (34)； provide the alternating pressure waveform to the patient interface (30) via the air delivery tube (32) for delivery to the patient； measure a flow rate and air pressure using the flow sensor (50) and pressure sensor (48)； and calculate a complex quantity representing a patient admittance. The processor (62) is configured to then determine whether the apnoea is an open or closed airway apnoea based on said patient admittance.
