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The article you requested is

Sensitivity to 35% Carbon Dioxide in Patients With Generalized Anxiety Disorder.

J Clin Psychiatry 1999;60:379-384
Copyright 1999 Physicians Postgraduate Press, Inc.

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Background: Panic disorder and generalized anxiety disorder (GAD) are both characterized by severe anxiety, but there is evidence that indicates a qualitative difference between these 2 anxiety disorders. To investigate the specificity of the association between carbon dioxide (CO2) hypersensitivity and panic disorder and the possible relationships between panic disorder and GAD, the responses to inhalation of a gas mixture of 35% CO2 and 65% oxygen (O2) were assessed.

Method: Fifteen patients with panic disorder, 13 patients with GAD, and 10 patients with comorbid GAD and panic disorder according to a consensus diagnosis using Diagnostic Interview Schedule Version III-R (DIS-R) and DSM-IV criteria, and 12 healthy controls inhaled 2 vital capacities: 1 of 35% CO2 and 1 of compressed air. A double-blind, randomized, crossover design was used.

Results: GAD patients showed reactions to 35% CO2 that were similar to those of healthy controls and significantly weaker than that of panic disorder patients. Patients with comorbid panic disorder and GAD had anxiogenic reactions similar to those of subjects with panic disorder.

Conclusion: The results of the present study support the idea that panic disorder and GAD are separate disorders that have at least some differences in pathogenetic mechanisms and suggest that the 35% CO2 test might be a valid tool for discriminating between these 2 disorders.