A nitric oxide delivery system for delivering therapeutic gas into breathing gas in the inspiratory limb of a patient 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; an injector module (107; 400) 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 (107; 400) to the inspiratory limb of a breathing circuit enabling patient breathing gas in the breathing circuit to flow through the first opening and second opening; a therapeutic gas inlet (110; 410) in the injector body, the therapeutic gas inlet (110; 410) being configured to receive the flow of therapeutic gas and enable injection of the therapeutic gas into the injector module (107; 400), and in turn into patient breathing gas in the inspiratory limb of the breathing circuit; characterized in that the nitric oxide delivery system further comprises: at least one NO flow sensor in fluid communication with the therapeutic gas inlet (110; 410); and a control system in communication with the NO flow sensor to receive flow information from the NO flow sensor, wherein the control system: (i) detects use of the high frequency ventilator; (ii) compensates for false NO flow phenomenon by treating flow information generated by the high frequency ventilator as noise and filtering this noise out and/or using noise reduction techniques; (iii) identifies flow information as missing and interpolates the missing flow information and/or (iv) identifies flow information as reverse flow and inverts the value of the flow information. at least one control valve for providing a flow of therapeutic gas; an injector module (107; 400) 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 (107; 400) to the inspiratory limb of a breathing circuit enabling patient breathing gas in the breathing circuit to flow through the first opening and second opening; a therapeutic gas inlet (110; 410) in the injector body, the therapeutic gas inlet (110; 410) being configured to receive the flow of therapeutic gas and enable injection of the therapeutic gas into the injector module (107; 400), and in turn into patient breathing gas in the inspiratory limb of the breathing circuit; 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 (107; 400) to the inspiratory limb of a breathing circuit enabling patient breathing gas in the breathing circuit to flow through the first opening and second opening; a therapeutic gas inlet (110; 410) in the injector body, the therapeutic gas inlet (110; 410) being configured to receive the flow of therapeutic gas and enable injection of the therapeutic gas into the injector module (107; 400), and in turn into patient breathing gas in the inspiratory limb of the breathing circuit; at least one NO flow sensor in fluid communication with the therapeutic gas inlet (110; 410); and a control system in communication with the NO flow sensor to receive flow information from the NO flow sensor, wherein the control system: (i) detects use of the high frequency ventilator; (ii) compensates for false NO flow phenomenon by treating flow information generated by the high frequency ventilator as noise and filtering this noise out and/or using noise reduction techniques; (iii) identifies flow information as missing and interpolates the missing flow information and/or (iv) identifies flow information as reverse flow and inverts the value of the flow information. (i) detects use of the high frequency ventilator; (ii) compensates for false NO flow phenomenon by treating flow information generated by the high frequency ventilator as noise and filtering this noise out and/or using noise reduction techniques; (iii) identifies flow information as missing and interpolates the missing flow information and/or (iv) identifies flow information as reverse flow and inverts the value of the flow information.