A device for calculating and monitoring an oxygenation index during treatment with delivery of nitric oxide comprising: a control module configured to control the supply of therapeutic gas from a therapeutic gas supply: a breathing gas delivery means configured to supply a breathing gas to a patient; a first inlet to be placed in fluid communication with the therapeutic gas supply comprising nitric oxide or a nitric oxide-releasing agent; a second inlet to be placed in fluid communication with the flow of breathing gas; an outlet to be placed in fluid communication with the first inlet, the second inlet, and the patient for supplying a gas mixture of the breathing gas and the therapeutic gas; a proximal pressure transducer for determining a mean airway pressure of the combined flow of the therapeutic gas and the breathing gas; and a sample line connected to a respiratory tube (11, 12) which delivers the gas mixture to the patient; a FiO 2 measurement means for measuring fraction of inspired oxygen (FiO 2); an oxygen measurement means for measuring one or more oxygen measurements selected from the group consisting of arterial oxygen saturation (SaO 2), peripheral oxygen saturation (SpO 2) and partial pressure of oxygen in arterial blood (PaO 2); a signal processor capable of calculating oxygenation index based upon the mean airway pressure measurement obtained from the proximal pressure transducer, the oxygen measurement obtained from the oxygen measurement means, and the FiO 2 measurement obtained from the FiO 2 measurement means, wherein the oxygenation index is an indicator to one of decrease or increase the administration of nitric oxide; a display; characterised in that the device further comprises: a flow transducer to measure the flow of the breathing gas; and wherein the signal processor is in communication with the flow transducer and the breathing gas delivery means, wherein the signal processor is capable of determining when the oxygenation index is below or above a predetermined limit and sending a signal to the breathing gas delivery means to adjust the breathing gas flow rate so that the oxygenation index is within the predetermined limit; wherein the proximal pressure transducer is located along the sample line; and wherein the control module is configured to present a representation of the oxygenation index on the display. a breathing gas delivery means configured to supply a breathing gas to a patient; a first inlet to be placed in fluid communication with the therapeutic gas supply comprising nitric oxide or a nitric oxide-releasing agent; a second inlet to be placed in fluid communication with the flow of breathing gas; an outlet to be placed in fluid communication with the first inlet, the second inlet, and the patient for supplying a gas mixture of the breathing gas and the therapeutic gas; a proximal pressure transducer for determining a mean airway pressure of the combined flow of the therapeutic gas and the breathing gas; and a sample line connected to a respiratory tube (11, 12) which delivers the gas mixture to the patient; a FiO 2 measurement means for measuring fraction of inspired oxygen (FiO 2); an oxygen measurement means for measuring one or more oxygen measurements selected from the group consisting of arterial oxygen saturation (SaO 2), peripheral oxygen saturation (SpO 2) and partial pressure of oxygen in arterial blood (PaO 2); a signal processor capable of calculating oxygenation index based upon the mean airway pressure measurement obtained from the proximal pressure transducer, the oxygen measurement obtained from the oxygen measurement means, and the FiO 2 measurement obtained from the FiO 2 measurement means, wherein the oxygenation index is an indicator to one of decrease or increase the administration of nitric oxide; a display; characterised in that the device further comprises: a flow transducer to measure the flow of the breathing gas; and wherein the signal processor is in communication with the flow transducer and the breathing gas delivery means, wherein the signal processor is capable of determining when the oxygenation index is below or above a predetermined limit and sending a signal to the breathing gas delivery means to adjust the breathing gas flow rate so that the oxygenation index is within the predetermined limit; wherein the proximal pressure transducer is located along the sample line; and wherein the control module is configured to present a representation of the oxygenation index on the display. a flow transducer to measure the flow of the breathing gas; and wherein the signal processor is in communication with the flow transducer and the breathing gas delivery means, wherein the signal processor is capable of determining when the oxygenation index is below or above a predetermined limit and sending a signal to the breathing gas delivery means to adjust the breathing gas flow rate so that the oxygenation index is within the predetermined limit; wherein the proximal pressure transducer is located along the sample line; and wherein the control module is configured to present a representation of the oxygenation index on the display.