A nasal cannula for supplying a respiratory gas to a patient and a method of treating a patient with sleep disorder. The nasal cannula comprises a pair of spaced apart supply lines which each have a head at one end thereof with a discharge opening located therein. The opposite end of each supply line is connectable to a high flow respiratory gas source. Each head is sized to be snugly received and retained within one of the nasal cavities of the patient while forming a sufficient leakage passage, between a portion of inwardly facing nasal cavity skin of a patient and a portion of an exterior surface of the head, to facilitate exhausting of any excess respiratory gas supplied to the patient through the leakage passage and also facilitate inhalation of any room air required in excess of the respiratory gas to be supplied to the patient.
