A system for controlling the delivery of medical gases during respiratory support of a patient, the system comprising: a breathing circuit including a patient connection configured to deliver respiratory support to the patient; a mixer pneumatically connected to the breathing circuit, the mixer being configured to provide at least oxygen and a balance gas to the breathing circuit; a mechanical ventilator pneumatically connected to the breathing circuit, the mechanical ventilator being configured to create fluid pressure waveforms within the breathing circuit to provide respiratory support to the patient; a respiratory gas monitor pneumatically connected to the breathing circuit, the respiratory gas monitor being configured to produce signals indicative of an oxygen concentration inhaled and exhaled by the patient; a digital signal processor communicatively connected to the mixer, the mechanical ventilator, and the gas monitor; an input device operable by a clinician to input to the digital signal processor at least one ventilation parameter value change; and a graphical display communicatively connected to the digital signal processor; and characterised by: the digital signal processor being configured to calculate a fresh oxygen flow rate into the breathing circuit and a total fresh gas flow rate into the breathing circuit, based on the at least one ventilation parameter value change, and to calculate a reference oxygen flow rate as the fresh oxygen flow rate into the breathing circuit that is required to achieve a delivery of a predetermined referenced oxygen concentration to the patient at the calculated total fresh gas flow rate, wherein the graphical display is configured to present the calculated fresh oxygen flow rate, the total fresh gas flow rate, and the reference oxygen flow rate. a breathing circuit including a patient connection configured to deliver respiratory support to the patient; a mixer pneumatically connected to the breathing circuit, the mixer being configured to provide at least oxygen and a balance gas to the breathing circuit; a mechanical ventilator pneumatically connected to the breathing circuit, the mechanical ventilator being configured to create fluid pressure waveforms within the breathing circuit to provide respiratory support to the patient; a respiratory gas monitor pneumatically connected to the breathing circuit, the respiratory gas monitor being configured to produce signals indicative of an oxygen concentration inhaled and exhaled by the patient; a digital signal processor communicatively connected to the mixer, the mechanical ventilator, and the gas monitor; an input device operable by a clinician to input to the digital signal processor at least one ventilation parameter value change; and a graphical display communicatively connected to the digital signal processor; and characterised by: the digital signal processor being configured to calculate a fresh oxygen flow rate into the breathing circuit and a total fresh gas flow rate into the breathing circuit, based on the at least one ventilation parameter value change, and to calculate a reference oxygen flow rate as the fresh oxygen flow rate into the breathing circuit that is required to achieve a delivery of a predetermined referenced oxygen concentration to the patient at the calculated total fresh gas flow rate, wherein the graphical display is configured to present the calculated fresh oxygen flow rate, the total fresh gas flow rate, and the reference oxygen flow rate. the digital signal processor being configured to calculate a fresh oxygen flow rate into the breathing circuit and a total fresh gas flow rate into the breathing circuit, based on the at least one ventilation parameter value change, and to calculate a reference oxygen flow rate as the fresh oxygen flow rate into the breathing circuit that is required to achieve a delivery of a predetermined referenced oxygen concentration to the patient at the calculated total fresh gas flow rate, wherein the graphical display is configured to present the calculated fresh oxygen flow rate, the total fresh gas flow rate, and the reference oxygen flow rate.