A patient ventilation system has a flow regulating and gas mixing assembly for providing oxygen to a first gas inlet and at least a second gas to a second gas inlet. The first and second gas inlets are connected to an inspiratory channel for conveying a delivered gas mixture including the oxygen and the at least second gas to a proximal tubing. The proximal tubing in turn is connected to an expiratory channel and connectable to a patient. The patient ventilation system further has a gas identification unit with which the at least second gas can be identified. The gas identification unit is arranged to measure actively a first value which is dependent on the characteristics of the at least second gas and attempt to identify the at least second gas based on said first value. The flow regulating and gas mixing assembly changes, at least temporarily, the concentration of the at least second gas in the delivered gas mixture if the gas identification attempt fails. The gas identification unit is further arranged to, subsequent to such a change in concentration, actively measure a second value which is dependent on the characteristics, and identify the at least second gas based on the second value alone or in combination with the first value.
