A system configured to determine a volume of carbon dioxide excreted by a subject during respiration, the system comprising: one or more gas parameter sensors configured to generate first output signals conveying information related to pressure and/or flow in a respiratory circuit, wherein the respiratory circuit comprises a non-invasive subject interface appliance configured to non-invasively communicate with an airway of a subject; a carbon dioxide sensor configured to generate second output signals conveying information related to a concentration of carbon dioxide present in the respiratory circuit; a processor configured to execute modules, wherein the modules comprise: a concentration module configured to determine the concentration of carbon dioxide in gas expired from the airway of the subject during expiration based on the second output signals generated by the carbon dioxide sensor; a leak module configured to estimate gas leakage from the respiratory circuit during respiration based on the first output signals generated by the one or more gas parameters sensors; and a carbon dioxide volume module configured to determine volumes of carbon dioxide excreted by the subject during individual breaths based on (i) a determination of the concentration of carbon dioxide by the concentration module, (ii) an estimate of gas leakage from the respiratory circuit by the leak module, and (iii) the first output signals generated by the one or more gas parameters sensors and the second output signals generated by the carbon dioxide sensor, wherein the carbon dioxide volume module is further configured to adjust a determination of a volume of carbon dioxide excreted during a given breath to include an estimate of the carbon dioxide leaked during expiration and an estimate of the carbon dioxide leaked during inspiration. one or more gas parameter sensors configured to generate first output signals conveying information related to pressure and/or flow in a respiratory circuit, wherein the respiratory circuit comprises a non-invasive subject interface appliance configured to non-invasively communicate with an airway of a subject; a carbon dioxide sensor configured to generate second output signals conveying information related to a concentration of carbon dioxide present in the respiratory circuit; a processor configured to execute modules, wherein the modules comprise: a concentration module configured to determine the concentration of carbon dioxide in gas expired from the airway of the subject during expiration based on the second output signals generated by the carbon dioxide sensor; a leak module configured to estimate gas leakage from the respiratory circuit during respiration based on the first output signals generated by the one or more gas parameters sensors; and a carbon dioxide volume module configured to determine volumes of carbon dioxide excreted by the subject during individual breaths based on (i) a determination of the concentration of carbon dioxide by the concentration module, (ii) an estimate of gas leakage from the respiratory circuit by the leak module, and (iii) the first output signals generated by the one or more gas parameters sensors and the second output signals generated by the carbon dioxide sensor, wherein the carbon dioxide volume module is further configured to adjust a determination of a volume of carbon dioxide excreted during a given breath to include an estimate of the carbon dioxide leaked during expiration and an estimate of the carbon dioxide leaked during inspiration. a concentration module configured to determine the concentration of carbon dioxide in gas expired from the airway of the subject during expiration based on the second output signals generated by the carbon dioxide sensor; a leak module configured to estimate gas leakage from the respiratory circuit during respiration based on the first output signals generated by the one or more gas parameters sensors; and a carbon dioxide volume module configured to determine volumes of carbon dioxide excreted by the subject during individual breaths based on (i) a determination of the concentration of carbon dioxide by the concentration module, (ii) an estimate of gas leakage from the respiratory circuit by the leak module, and (iii) the first output signals generated by the one or more gas parameters sensors and the second output signals generated by the carbon dioxide sensor, wherein the carbon dioxide volume module is further configured to adjust a determination of a volume of carbon dioxide excreted during a given breath to include an estimate of the carbon dioxide leaked during expiration and an estimate of the carbon dioxide leaked during inspiration.