A nitric oxide delivery system for delivering a therapeutic gas into a breathing gas in an inspiratory limb of a breathing circuit affiliated with a high frequency ventilator or ventilation techniques which provide reverse and/or oscillations in inspiratory pressure or flow, the nitric oxide delivery system comprising: at least one control valve for providing a flow of therapeutic gas containing nitric oxide to an injector module configured to inject the therapeutic gas into the breathing gas in the inspiratory limb of the 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 the inspiratory limb of the breathing circuit enabling breathing gas in the breathing circuit to flow through the first opening and the second opening; and a therapeutic gas inlet in the injector body, the therapeutic gas inlet being configured to receive the flow of therapeutic gas and enable injection of the therapeutic gas into the injector module, and in turn in to the breathing gas in the inspiratory limb of the breathing circuit; at least one NO flow sensor for measuring NO flow at the at least one control valve, wherein the NO flow sensor is in fluid communication with the therapeutic gas inlet of the injector module; a control system in communication with the at least one NO flow sensor to receive flow information from the at least one NO flow sensor; wherein the control system is operable to: (i) detect use of the high frequency ventilator; and wherein the control system is further operable to: (ii) identify the flow information as missing and interpolate the missing flow information; and/or (iii) identify the flow information as reverse flow. 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 the inspiratory limb of the breathing circuit enabling breathing gas in the breathing circuit to flow through the first opening and the second opening; and a therapeutic gas inlet in the injector body, the therapeutic gas inlet being configured to receive the flow of therapeutic gas and enable injection of the therapeutic gas into the injector module, and in turn in to the breathing gas in the inspiratory limb of the breathing circuit; at least one NO flow sensor for measuring NO flow at the at least one control valve, wherein the NO flow sensor is in fluid communication with the therapeutic gas inlet of the injector module; a control system in communication with the at least one NO flow sensor to receive flow information from the at least one NO flow sensor; wherein the control system is operable to: (i) detect use of the high frequency ventilator; and wherein the control system is further operable to: (ii) identify the flow information as missing and interpolate the missing flow information; and/or (iii) identify the flow information as reverse flow. (i) detect use of the high frequency ventilator; and wherein the control system is further operable to: (ii) identify the flow information as missing and interpolate the missing flow information; and/or (iii) identify the flow information as reverse flow.