A gas measurement module configured to monitor composition of a flow of breathable gas within a respiratory circuit that is in fluid communication with an airway of a subject, the gas measurement module being configured to assemble with an airway adapter in fluid communication with the airway, the gas measurement module comprising: an emitter configured to emit infrared electromagnetic radiation; a detector configured to generate output signals conveying information related to one or more parameters of electromagnetic radiation that becomes incident thereon; and a plurality of optical elements configured to guide the electromagnetic radiation emitted by the emitter through a sampling chamber of the airway adapter to the detector, wherein the sampling chamber has a first opening and a second opening, and is configured to form a flow path between the first opening and the second opening such that the flow of breathable gas within the respiratory circuit passes therethrough, and wherein the plurality of optical elements comprise a first optical element configured to guide the electromagnetic radiation emitted by the emitter through the sampling chamber via first and second airway adapter windows formed in substantially opposite sides of the airway adapter, a second optical element configured to reflect the electronic radiation from the second airway adapter window of the sampling chamber back through the sampling chamber via the second and first airway adapter windows, and a third optical element configured to reflect the electronic radiation emitted from the first airway adapter window of the sampling chamber to the detector such that the output signals generated by the detector convey information related to one or more parameters of the electromagnetic radiation that has traversed the sampling chamber and the flow of breathable gas therein twice, and wherein at least one of the optical elements is a toric element. an emitter configured to emit infrared electromagnetic radiation; a detector configured to generate output signals conveying information related to one or more parameters of electromagnetic radiation that becomes incident thereon; and a plurality of optical elements configured to guide the electromagnetic radiation emitted by the emitter through a sampling chamber of the airway adapter to the detector, wherein the sampling chamber has a first opening and a second opening, and is configured to form a flow path between the first opening and the second opening such that the flow of breathable gas within the respiratory circuit passes therethrough, and wherein the plurality of optical elements comprise a first optical element configured to guide the electromagnetic radiation emitted by the emitter through the sampling chamber via first and second airway adapter windows formed in substantially opposite sides of the airway adapter, a second optical element configured to reflect the electronic radiation from the second airway adapter window of the sampling chamber back through the sampling chamber via the second and first airway adapter windows, and a third optical element configured to reflect the electronic radiation emitted from the first airway adapter window of the sampling chamber to the detector such that the output signals generated by the detector convey information related to one or more parameters of the electromagnetic radiation that has traversed the sampling chamber and the flow of breathable gas therein twice, and wherein at least one of the optical elements is a toric element.