Effects of 35% carbon dioxide (CO 2 ) inhalation in patients with post-traumatic stress disorder (PTSD): A double-blind, randomized, placebo-controlled, cross-over trial.

Background: In patients with post-traumatic stress disorder (PTSD) two open pilot studies about the effects of 35% carbon dioxide (CO ₂ ) exist. One shows an augmented panicogenic and anxiogenic response (Muhtz et al., 2011), the other does not (Talesnik et al. 2007). We further characterized the CO ₂ reactivity in PTSD using for the first time placebo-controlled and double-blind conditions.
Methods: In 20 patients with PTSD we assessed panic, anxiety, dissociative and PTSD symptoms after a single vital capacity inhalation of 35% CO ₂ compared to a placebo gas condition in a within-participant cross-over, placebo-controlled, double-blind and randomized design.
Results: Inhalation of 35% CO ₂ versus placebo provoked significantly increased panic, anxiety, dissociative and PTSD symptoms. The reaction to placebo gas was minimal. Order of inhalation, patients' sex or age did not influence the results. The panic and anxiety response under CO ₂ was considerably higher in the PTSD patients than in healthy controls from our previous open study.
Conclusions: The results corroborate that our preceding findings of an increased CO ₂ reactivity in patients with PTSD are not false positive due to the open design or the lack of placebo control. Replication in a larger number of PTSD patients and matched control subjects is needed. The potential role of childhood traumatisation, psychiatric comorbidity, psychotropic medication and trait dissociation in prior contradictory reports should be clarified.
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