The processing of gas mixtures, in particular, of respiration gases for ventilated patients. The processing according to the invention relates, in particular, to the use of selective gas separation membranes for the retention of noble gases in the exhaled air of ventilated patients. The gas separation membrane is an active separator which is integrated in a ventilator. The separation membrane separates the noble gases from the remainder of the residual of the exhaled air by selectively retaining the noble gases. Thus, it is possible to provide a ventilator which enables the application of noble gases, in particular xenon, as an anesthetic preferably with low loss and as simple as possible.