A patient interface (3000) to provide breathable gas to a patient (1000), the patient interface comprising a seal-forming structure (3100) and a plenum chamber (3200) formed in one piece, the seal-forming structure having a nasal opening and a nasal sling (3213) dividing the nasal opening into a pair of nare ports (3214), each of the pair of nare ports corresponding to one of the patient's nares to provide pressurized gas to the corresponding nare, the plenum chamber including a plenum connection region (3240), and the seal-forming structure is configured to seal around an inferior periphery of the patient's nose； and a frame (3310) comprising a frame connection region (3312), a connection port (3600) for connection of the patient interface to a gas delivery tube (4180), and a headgear connection region； headgear (3300) connected to the headgear connection region, the headgear being configured to maintain the cushion member in sealing contact with the patient； and a gas washout vent (3400) configured to allow a flow of patient exhaled CO2 to an exterior of the patient interface to minimise rebreathing of exhaled CO2 by the patient, wherein the frame connection region is configured for attachment to the plenum chamber at the plenum connection region, wherein the plenum chamber comprises a sealing lip (3250) adapted to form a pneumatic seal between the plenum connection region and the frame connection region.
