A gas delivery assembly comprising a nasal sub-system to be connected to a nose of a patient for delivering a respiratory gas to the patient, and a measurement module, wherein the nasal sub-system, comprises: - a hollow body with an inner chamber and an inlet for receiving a respiratory gas, and - a pair of nasal prongs (23, 24), in fluid communication with the inner chamber of the hollow body, each nasal prong (23, 24) comprising a pair of inner channels (23b, 23c; 24b, 24c), each pair of inner channels (23b, 23c; 24b, 24c) comprising a first channel (23b; 24b) and a second channel (23c; 24c) arranged in parallel, each first channel (23b; 24b) fluidly connecting in use the internal chamber of the hollow body with a nostril (13, 14) of the patient, and each second channel (23c, 24c) fluidly connecting in use a nostril (13, 14) with a vent conduit arranged in the hollow body and in fluid communication with the atmosphere via at least one venting port, characterized in that a supplementary conduit is arranged in one of the two prongs (23, 24) of the nasal sub-system, said supplementary conduit comprising an inlet port port located within one of the second channels (23c, 24c) of the nasal prongs (23, 24), and an outlet port located into the vent conduit, and wherein the measurement module comprises a main module body in fluid communication with the nasal sub-system, said main module body comprising: - a first port comprising a first exit orifice for a first gas flow from the measurement module, and a second and a third port (44b, 44c) comprising a second and a third exit orifice (244b, 244c), - a first inner gas conduct in fluid communication with the outlet port of the nasal sub-system and further fluidly connected to the first port of the main module body, and - a hollow expansion portion comprising an inner volume in fluid communication with the vent conduit of the nasal sub-system, and with the second and third ports (44b, 44c) of the main module body. - a hollow body with an inner chamber and an inlet for receiving a respiratory gas, and - a pair of nasal prongs (23, 24), in fluid communication with the inner chamber of the hollow body, each nasal prong (23, 24) comprising a pair of inner channels (23b, 23c; 24b, 24c), each pair of inner channels (23b, 23c; 24b, 24c) comprising a first channel (23b; 24b) and a second channel (23c; 24c) arranged in parallel, each first channel (23b; 24b) fluidly connecting in use the internal chamber of the hollow body with a nostril (13, 14) of the patient, and each second channel (23c, 24c) fluidly connecting in use a nostril (13, 14) with a vent conduit arranged in the hollow body and in fluid communication with the atmosphere via at least one venting port, - a first port comprising a first exit orifice for a first gas flow from the measurement module, and a second and a third port (44b, 44c) comprising a second and a third exit orifice (244b, 244c), - a first inner gas conduct in fluid communication with the outlet port of the nasal sub-system and further fluidly connected to the first port of the main module body, and - a hollow expansion portion comprising an inner volume in fluid communication with the vent conduit of the nasal sub-system, and with the second and third ports (44b, 44c) of the main module body.