A nitric oxide delivery system comprising: an injector module for delivering therapeutic gas comprising nitric oxide into breathing gas in a breathing circuit, the injector module comprising: an injector body having a first opening and a second opening, the first opening and the second opening being configured to couple the injector module to a patient breathing circuit; and a therapeutic gas inlet (110; 410) configured to receive therapeutic gas and enable injection of the therapeutic gas into breathing circuit gas flowing through the injector module ; at least one sensor (119; 402) capable of measuring flow of the breathing circuit gas in a forward direction and in a reverse direction; and a control module for providing the therapeutic gas to the therapeutic gas inlet (110; 410), and the control module being in communication with the at least one sensor (119; 402) and operable to determine a volume of forward flow and reverse flow of the breathing gas based on the measurements of the at least one sensor (119; 402), wherein the therapeutic gas flow is controlled by a control valve in communication with the therapeutic gas inlet (110; 410); wherein the control module compensates delivery of therapeutic gas based on the measurement from the at least one sensor such that the control module closes the control valve when the at least one sensor (119; 402) measures reverse flow of the breathing gas and opens the control valve after the at least one sensor (119; 402) measures resumed forward flow of the breathing gas and the resumed volume of forward flow is at least equal to the volume of reverse flow. an injector module for delivering therapeutic gas comprising nitric oxide into breathing gas in a breathing circuit, the injector module comprising: an injector body having a first opening and a second opening, the first opening and the second opening being configured to couple the injector module to a patient breathing circuit; and a therapeutic gas inlet (110; 410) configured to receive therapeutic gas and enable injection of the therapeutic gas into breathing circuit gas flowing through the injector module ; an injector body having a first opening and a second opening, the first opening and the second opening being configured to couple the injector module to a patient breathing circuit; and a therapeutic gas inlet (110; 410) configured to receive therapeutic gas and enable injection of the therapeutic gas into breathing circuit gas flowing through the injector module ; at least one sensor (119; 402) capable of measuring flow of the breathing circuit gas in a forward direction and in a reverse direction; and a control module for providing the therapeutic gas to the therapeutic gas inlet (110; 410), and the control module being in communication with the at least one sensor (119; 402) and operable to determine a volume of forward flow and reverse flow of the breathing gas based on the measurements of the at least one sensor (119; 402), wherein the therapeutic gas flow is controlled by a control valve in communication with the therapeutic gas inlet (110; 410); wherein the control module compensates delivery of therapeutic gas based on the measurement from the at least one sensor such that the control module closes the control valve when the at least one sensor (119; 402) measures reverse flow of the breathing gas and opens the control valve after the at least one sensor (119; 402) measures resumed forward flow of the breathing gas and the resumed volume of forward flow is at least equal to the volume of reverse flow. <EOS>