A subject interface appliance (10) configured to deliver a breathable substance to a subject. The subject interface appliance includes a primary interface (12) configured to deliver a breathable substance to the subject, and a secondary interface (14) configured to obtain gas samples from the airway of the subject. The secondary interface is resiliently held at a default position with respect to the primary interface. The default position is located such that installation of the primary and secondary interfaces on the subject results in the application of a bias force to the secondary interface that holds the secondary interface in place.