A nitric oxide delivery system comprising: an injector module for delivering therapeutic gas 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; a therapeutic gas inlet configured to receive therapeutic gas and enable injection of the therapeutic gas into breathing circuit gas flowing through the injector module; and at least one bi-directional breathing circuit gas, flow sensor capable of measuring breathing circuit gas flow in a forward direction and in a reverse direction; and a control module for providing the therapeutic gas to the therapeutic gas inlet, and the control module being in communication with the at least one bi-directional BCG flow sensor, wherein when the at least one bi-directional BCG flow sensor measures flow in the reverse direction, therapeutic gas is not delivered into the breathing circuit via the therapeutic gas inlet, and wherein when the at least one bi-directional BCG flow sensor measures flow in the forward direction after the at least one bi-directional BCG flow sensor measures flow in the reverse direction, therapeutic gas is delivered into the breathing circuit after compensating for at least a portion of the flow in the reverse direction. an injector module for delivering therapeutic gas 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; a therapeutic gas inlet configured to receive therapeutic gas and enable injection of the therapeutic gas into breathing circuit gas flowing through the injector module; and at least one bi-directional breathing circuit gas, flow sensor capable of measuring breathing circuit gas flow in a forward direction and in a reverse direction; and 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; a therapeutic gas inlet configured to receive therapeutic gas and enable injection of the therapeutic gas into breathing circuit gas flowing through the injector module; and at least one bi-directional breathing circuit gas, flow sensor capable of measuring breathing circuit gas flow in a forward direction and in a reverse direction; and a control module for providing the therapeutic gas to the therapeutic gas inlet, and the control module being in communication with the at least one bi-directional BCG flow sensor, wherein when the at least one bi-directional BCG flow sensor measures flow in the reverse direction, therapeutic gas is not delivered into the breathing circuit via the therapeutic gas inlet, and wherein when the at least one bi-directional BCG flow sensor measures flow in the forward direction after the at least one bi-directional BCG flow sensor measures flow in the reverse direction, therapeutic gas is delivered into the breathing circuit after compensating for at least a portion of the flow in the reverse direction.