A nasal apparatus comprising a first nasal cannula system configured to engage a nose of a subject, the nose including a first nostril, a second nostril, a first nasal passage, and a second nasal passage; the first nasal cannula system comprising: a first breathing conduit configured to conduct a pressurized flow of breathable gas into one or both nasal passages of the subject through a first nasal port during inhalation, and conduct exhaled gas out of one or both nasal passages to the ambient environment through a first ambient environment port during exhalation, the first nasal port open to one or both nasal passages of the subject, the first ambient environment port open to the ambient environment; a first pressure conduit configured to conduct the pressurized flow of breathable gas from a first pressure port to the first breathing conduit during inhalation, the first pressure port configured to receive the pressurized flow of breathable gas during inhalation; a first sealing surface surrounding the first nasal port configured to removably seal the first nasal cannula system with one or both nostrils to prevent gas from escaping between the first sealing surface and one or both nostrils; and characterized in that the first nasal cannula system further comprises a first flow deflection surface configured to deflect at least a portion of the exhaled gas from the first breathing conduit into the first pressure conduit during exhalation such that one or more gas parameters of the gas in the first pressure conduit are impacted sufficiently to facilitate quantification of such one or more gas parameters in one or both nasal passages of the subject from measurements taken on the pressurized flow of breathable gas upstream from delivery of the first breathing conduit into one or both nasal passages of the subject. a first breathing conduit configured to conduct a pressurized flow of breathable gas into one or both nasal passages of the subject through a first nasal port during inhalation, and conduct exhaled gas out of one or both nasal passages to the ambient environment through a first ambient environment port during exhalation, the first nasal port open to one or both nasal passages of the subject, the first ambient environment port open to the ambient environment; a first pressure conduit configured to conduct the pressurized flow of breathable gas from a first pressure port to the first breathing conduit during inhalation, the first pressure port configured to receive the pressurized flow of breathable gas during inhalation; a first sealing surface surrounding the first nasal port configured to removably seal the first nasal cannula system with one or both nostrils to prevent gas from escaping between the first sealing surface and one or both nostrils; and characterized in that the first nasal cannula system further comprises a first flow deflection surface configured to deflect at least a portion of the exhaled gas from the first breathing conduit into the first pressure conduit during exhalation such that one or more gas parameters of the gas in the first pressure conduit are impacted sufficiently to facilitate quantification of such one or more gas parameters in one or both nasal passages of the subject from measurements taken on the pressurized flow of breathable gas upstream from delivery of the first breathing conduit into one or both nasal passages of the subject.