The present invention relates to a patient ventilation system comprising flow regulating and gas mixing means (3, 4, 5, 6, 16) for providing oxygen to a first gas inlet (1) and at least a second gas to a second gas inlet (2). The first (1) and second (2) gas inlets are connected to an inspiratory channel (7) for conveying a delivered gas mixture comprising the oxygen and the at least second gas to a proximal tubing (8) which proximal tubing in turn is connected to an expiratory channel (9) and connectable to a patient. The patient ventilation system further comprises gas identification means (10, 10a, 10b) by which said at least second gas can be identified. The gas identification means (10, 10a, 10b) is arranged to measure actively a first value which is dependent on the characteristics of theat least second gas and attempt to identify saidat least second gas based on said firstvalue. The flow regulating and gas mixing means (3, 4, 5, 6,16) are arranged to change, at least temporarily, the concentration of said at least second gas in the delivered gas mixture if the gas identification attempt fails. The gas identification means (10, 10a, 10b)is further arranged to, subsequent to such a change in concentration, actively measure a second value which is dependent on said characteristics, and identify said at least second gas based on the second value alone or in combination with said first value.Linvention concerne un système de ventilation de patient comprenant des moyens de régulation de débit et de mélange de gaz (3, 4, 5, 6, 16) destinés à fournir de loxygène à un premier orifice dentrée de gaz (1) et au moins un deuxième gaz à un deuxième orifice dentrée de gaz (2). Les premier (1) et deuxième (2) orifices dentrée de gaz sont reliés à un canal dinspiration (7) destiné à acheminer le mélange de gaz obtenu contenant loxygène et ledit deuxième gaz au moins vers une tubulure proximale (8) reliée à son tour à un canal dexpiration (9) et pouvant être reliée à un patien
