A metabolic measurement system comprising: (a) an airway adapter (20, 100, 200, 334) adapted to be coupled in series with a mainstream gas flow comprising: a housing having a bore defined therethrough to carry the mainstream gas flow through the airway adapter, a window defined in the housing providing optical access to the gas flow through the airway adapter, and an opening defined in the housing providing optical access to the gas flow through the airway adapter; (b) a sensor head (22, 334) adapted to be removably attached to the airway adapter, the sensor head comprising: an infrared sensing system adapted to transmit or receive infrared radiation through the window, and a luminescence quenching system adapted to (i) transmit excitation radiation through the opening, (ii) receive emitted radiation from a luminescence material through the opening, or both (i) and (ii); and (c) a flow measurement system (310, 352) adapted to be coupled in series with such a mainstream gas flow; characterized in that the system further comprises: (d) a gas monitoring module adapted to receive a signal from the infrared sensing system and the luminescence quenching system, wherein the gas monitoring module includes a first processor adapted to determine a carbon dioxide waveform based on the signal from the infrared sensing system and an oxygen waveform based on the output of the luminescence quenching system; (e) a flow processing module adapted to receive a signal from the flow measurement system, wherein the flow processing module includes a second processor adapted to determine a rate, a volume, flow waveform, or any combination thereof for the mainstream gas flow; and (f) a metabolic parameter processing module, wherein the metabolic parameter processing module includes a third processor adapted to receive signals from the infrared sensing system, the luminescence quenching system, and the flow measurement system, either directly or via the gas monitoring module and the flow monitoring module, wherein the third processor is adapted to determine a metabolic parameter including oxygen consumption, carbon dioxide production, respiratory quotient, resting energy expenditure, or any combination thereof; and wherein the gas monitoring module, the flow monitoring module, and the metabolic parameter processing module are configured such that each module is capable of physically joining another module and, in so joining, creating a communication and/or power link between the joined modules. (a) an airway adapter (20, 100, 200, 334) adapted to be coupled in series with a mainstream gas flow comprising: a housing having a bore defined therethrough to carry the mainstream gas flow through the airway adapter, a window defined in the housing providing optical access to the gas flow through the airway adapter, and an opening defined in the housing providing optical access to the gas flow through the airway adapter; a housing having a bore defined therethrough to carry the mainstream gas flow through the airway adapter, a window defined in the housing providing optical access to the gas flow through the airway adapter, and an opening defined in the housing providing optical access to the gas flow through the airway adapter; (b) a sensor head (22, 334) adapted to be removably attached to the airway adapter, the sensor head comprising: an infrared sensing system adapted to transmit or receive infrared radiation through the window, and a luminescence quenching system adapted to (i) transmit excitation radiation through the opening, (ii) receive emitted radiation from a luminescence material through the opening, or both (i) and (ii); and an infrared sensing system adapted to transmit or receive infrared radiation through the window, and a luminescence quenching system adapted to (i) transmit excitation radiation through the opening, (ii) receive emitted radiation from a luminescence material through the opening, or both (i) and (ii); and (c) a flow measurement system (310, 352) adapted to be coupled in series with such a mainstream gas flow; (d) a gas monitoring module adapted to receive a signal from the infrared sensing system and the luminescence quenching system, wherein the gas monitoring module includes a first processor adapted to determine a carbon dioxide waveform based on the signal from the infrared sensing system and an oxygen waveform based on the output of the luminescence quenching system; (e) a flow processing module adapted to receive a signal from the flow measurement system, wherein the flow processing module includes a second processor adapted to determine a rate, a volume, flow waveform, or any combination thereof for the mainstream gas flow; and (f) a metabolic parameter processing module, wherein the metabolic parameter processing module includes a third processor adapted to receive signals from the infrared sensing system, the luminescence quenching system, and the flow measurement system, either directly or via the gas monitoring module and the flow monitoring module, wherein the third processor is adapted to determine a metabolic parameter including oxygen consumption, carbon dioxide production, respiratory quotient, resting energy expenditure, or any combination thereof; and