A ventilator includes first and second pathways, a conduit and a controller. The first pathway (120) is configured to supply a first gas and the second pathway (140) is configured to supply a second gas, where the second gas is mixed with the first gas to produce mixed gas having a predetermined percentage of the second gas. The conduit (166) is configured to provide the mixed gas from the first and second pathways to an access port during an inspiratory phase, and to provide discharged gas from the access port to the first pathway during an expiratory phase. The controller (180) is configured to delay supply of the second gas from the second pathway for a delay time in order to maintain the predetermined percentage of the second gas in the mixed gas provided to the access port during a subsequent inspiratory phase.