Xenon Inhalation Therapy for Major Depressive Disorder and Bipolar Disorder

The investigators will test the hypothesis that inhaled xenon will produce a rapid improvement in depressive symptoms in patients suffering from treatment-resistant depression. Specifically, the investigators will conduct a parallel randomized, double-blind crossover study that will compare the effects of xenon-oxygen (35:65 ratio by volume) added to treatment as usual (X-TAU group) to the effects of nitrogen-oxygen (35:65 ratio by volume) added to treatment as usual (N-TAU group). A total of 20 severely depressed patients, 10 with major depressive disorder (MDD) and 10 with Bipolar Depression (BP), will be exposed in random order to N-TAU and X-TAU in a double-blind protocol.