A system configured to monitor the breathing of a subject, the system comprising: a sample cell in fluid communication with a conduit that communicates with the airway of a subject, wherein the conduit is placed in fluid communication with the airway of the subject by an interface appliance that engages the airway of the subject, wherein the sample cell is configured to exhaust gas received into the sample cell from the conduit; a composition detector configured to generate output signals conveying information related to the composition of gas received into the sample cell from the conduit; a pressure detector configured to generate output signals conveying information related to pressure within the conduit; computer program modules (40,42,44,46) that comprise: a breath identification module configured to identify breathing based on the output signals generated by the pressure detector; a breathing parameter module configured to determine one or more breathing parameters; one or more processors configured to execute the computer program modules, characterized in that the computer program modules (40,42,44,46) further comprise an interface classification module configured to determine an interface appliance type of the interface appliance based on the output signals generated by the pressure detector, wherein the determination of the interface appliance type is based on a magnitude of a pressure fluctuation experienced by the conduit and/or sample cell and the determination is useable to differentiate between an interface appliance that is configured to create a sealed engagement with the airway of the subject and an interface appliance that is not configured to create a sealed engagement with the airway of the subject, wherein the breathing parameter module is configured to determine the one or more breathing parameters based on the output signals generated by the composition detector, the breathing identified by the breath identification module and the determination by the interface classification module of the interface appliance type of the interface appliance. a sample cell in fluid communication with a conduit that communicates with the airway of a subject, wherein the conduit is placed in fluid communication with the airway of the subject by an interface appliance that engages the airway of the subject, wherein the sample cell is configured to exhaust gas received into the sample cell from the conduit; a composition detector configured to generate output signals conveying information related to the composition of gas received into the sample cell from the conduit; a pressure detector configured to generate output signals conveying information related to pressure within the conduit; computer program modules (40,42,44,46) that comprise: a breath identification module configured to identify breathing based on the output signals generated by the pressure detector; a breathing parameter module configured to determine one or more breathing parameters; one or more processors configured to execute the computer program modules, a breath identification module configured to identify breathing based on the output signals generated by the pressure detector; a breathing parameter module configured to determine one or more breathing parameters; one or more processors configured to execute the computer program modules, characterized in that the computer program modules (40,42,44,46) further comprise an interface classification module configured to determine an interface appliance type of the interface appliance based on the output signals generated by the pressure detector, wherein the determination of the interface appliance type is based on a magnitude of a pressure fluctuation experienced by the conduit and/or sample cell and the determination is useable to differentiate between an interface appliance that is configured to create a sealed engagement with the airway of the subject and an interface appliance that is not configured to create a sealed engagement with the airway of the subject, wherein the breathing parameter module is configured to determine the one or more breathing parameters based on the output signals generated by the composition detector, the breathing identified by the breath identification module and the determination by the interface classification module of the interface appliance type of the interface appliance.