A subject interface appliance configured to deliver a breathable substance to the airway of a subject, the subject interface appliance comprising: a primary interface including a mask barrier that encloses one or more external orifices of the airway of a subject including a first external orifice, wherein the mask barrier is shaped to form a delivery opening through which a breathable substance can be communicated to the one or more external orifices of the subject that are enclosed by the mask barrier; a secondary interface, the secondary interface comprising: a hollow prong having a first end and a second end, the hollow prong being configured to be inserted into the first external orifice of the airway of the subject such that the first end of the hollow prong is positioned inside of the first external orifice and the second end of the hollow prong is positioned outside of the first external orifice, and a main conduit connected to the hollow prong at or near the second end to receive gas that has entered the first end of the hollow prong within the airway of the subject, the main conduit being configured to guide the received gas toward a sensor configured to generate an output signal conveying information related to one or more parameters of the sampled gas received into the secondary interface through the hollow prong; characterized in that it further comprises a resilient member extending from the primary interface to the secondary interface, the resilient member being configured to suspend and hold the secondary interface spatially apart from the mask barrier within the delivery opening of the primary interface at a default position near a plane defined by a seal portion with respect to the primary interface such that displacement of the secondary interface from the default position, in response to installation of the primary interface to enclose the one or more external orifices of the airway of the subject, results in the application of a bias force by the resilient member on the secondary interface toward the default position, wherein the interface appliance is configured such that responsive to the primary interface being installed on the face of the subject, the resilient member resiliently holds the hollow prong in place on the face of the subject with the first end of the hollow prong inside of the first external orifice of the airway of the subject. a primary interface including a mask barrier that encloses one or more external orifices of the airway of a subject including a first external orifice, wherein the mask barrier is shaped to form a delivery opening through which a breathable substance can be communicated to the one or more external orifices of the subject that are enclosed by the mask barrier; a secondary interface, the secondary interface comprising: a hollow prong having a first end and a second end, the hollow prong being configured to be inserted into the first external orifice of the airway of the subject such that the first end of the hollow prong is positioned inside of the first external orifice and the second end of the hollow prong is positioned outside of the first external orifice, and a main conduit connected to the hollow prong at or near the second end to receive gas that has entered the first end of the hollow prong within the airway of the subject, the main conduit being configured to guide the received gas toward a sensor configured to generate an output signal conveying information related to one or more parameters of the sampled gas received into the secondary interface through the hollow prong; a hollow prong having a first end and a second end, the hollow prong being configured to be inserted into the first external orifice of the airway of the subject such that the first end of the hollow prong is positioned inside of the first external orifice and the second end of the hollow prong is positioned outside of the first external orifice, and a main conduit connected to the hollow prong at or near the second end to receive gas that has entered the first end of the hollow prong within the airway of the subject, the main conduit being configured to guide the received gas toward a sensor configured to generate an output signal conveying information related to one or more parameters of the sampled gas received into the secondary interface through the hollow prong; it further comprises a resilient member extending from the primary interface to the secondary interface, the resilient member being configured to suspend and hold the secondary interface spatially apart from the mask barrier within the delivery opening of the primary interface at a default position near a plane defined by a seal portion with respect to the primary interface such that displacement of the secondary interface from the default position, in response to installation of the primary interface to enclose the one or more external orifices of the airway of the subject, results in the application of a bias force by the resilient member on the secondary interface toward the default position, wherein the interface appliance is configured such that responsive to the primary interface being installed on the face of the subject, the resilient member resiliently holds the hollow prong in place on the face of the subject with the first end of the hollow prong inside of the first external orifice of the airway of the subject.