A system for delivering a flow of breathing gas to a patient , the system comprising: an ambient air dispenser and a supplemental gas supply cooperating to supply a breathing gas having an adjustable breathing gas flowrate set-point that is initially set to an initial breathing gas flowrate set-point and an adjustable fraction of inspired oxygen to the patient , a predetermined fraction of inspired oxygen corresponding to a weaning threshold being defined, each one of the ambient air dispenser and the supplemental gas supply including at least one actuator to respectively vary the ambient air flowrate supplied by the ambient air dispenser and the supplemental flowrate supplied by the supplemental gas supply ; an oxygen saturation sensor measuring an oxygen saturation level of the patient , the fraction of inspired oxygen of the breathing gas being automatically adjusted on the basis of the measured oxygen saturation level of the patient and the breathing gas flow rate set-point; a respiratory rate sensor determining a respiratory rate of the patient ; and a control unit operatively connected to the oxygen saturation sensor , the respiratory rate sensor , the actuator of the ambient air dispenser and the actuator of the supplemental gas supply , the control unit automatically adjusting the ambient air flowrate and the supplemental gas flowrate according to the measured oxygen saturation level and the breathing gas flowrate set-point and automatically adjusting the breathing gas flowrate set-point according to the determined respiratory rate, wherein the breathing gas flowrate set-point is adjusted solely when the fraction of inspired oxygen meets the predetermined fraction of inspired oxygen threshold, the breathing gas flowrate set-point remaining at the initial breathing gas flowrate set-point until the predetermined fraction of inspired oxygen threshold is met. an ambient air dispenser and a supplemental gas supply cooperating to supply a breathing gas having an adjustable breathing gas flowrate set-point that is initially set to an initial breathing gas flowrate set-point and an adjustable fraction of inspired oxygen to the patient , a predetermined fraction of inspired oxygen corresponding to a weaning threshold being defined, each one of the ambient air dispenser and the supplemental gas supply including at least one actuator to respectively vary the ambient air flowrate supplied by the ambient air dispenser and the supplemental flowrate supplied by the supplemental gas supply ; an oxygen saturation sensor measuring an oxygen saturation level of the patient , the fraction of inspired oxygen of the breathing gas being automatically adjusted on the basis of the measured oxygen saturation level of the patient and the breathing gas flow rate set-point; a respiratory rate sensor determining a respiratory rate of the patient ; and a control unit operatively connected to the oxygen saturation sensor , the respiratory rate sensor , the actuator of the ambient air dispenser and the actuator of the supplemental gas supply , the control unit automatically adjusting the ambient air flowrate and the supplemental gas flowrate according to the measured oxygen saturation level and the breathing gas flowrate set-point and automatically adjusting the breathing gas flowrate set-point according to the determined respiratory rate, wherein the breathing gas flowrate set-point is adjusted solely when the fraction of inspired oxygen meets the predetermined fraction of inspired oxygen threshold, the breathing gas flowrate set-point remaining at the initial breathing gas flowrate set-point until the predetermined fraction of inspired oxygen threshold is met. <EOS>