Anxiety Symptoms of PTSD Increased With CO2 Inhalation

closeup of oxygen mask
closeup of oxygen mask
Researchers examined if inhalation order, sex, or age affected CO2 response in PTSD.

According to the results of a study published in the Journal of Psychiatric Research, patients with posttraumatic stress disorder (PTSD) had elevated anxiety, panic, dissociative symptoms, and PTSD symptoms following inhalation of 35% carbon dioxide (CO2) relative to placebo gas.

In this placebo-controlled crossover trial, 20 patients with PTSD based on Diagnostic and Statistical Manual of Mental Disorders-IV criteria (60% women; mean severity on Posttraumatic Diagnostic Scale: 32.9) were enrolled. Participants inhaled both a mixture of 35% CO2 with 65% oxygen and a placebo mixture of 35% nitrogen with 65% oxygen 1 hour apart on the same day. The change in preinhalation and postinhalation panic, anxiety, and dissociative and PTSD symptoms were compared between CO2 gas and placebo gas.

The primary end point of mean change in panic scores, evaluated by the Acute Panic Inventory, was significantly greater for the CO2 inhalation compared with the placebo gas inhalation (13.85 vs 0.65). Mean change in panic score, measured by the Panic Symptoms Scale, was also greater after CO2 inhalation compared with placebo inhalation (12.55 vs -0.10).

The secondary end points of posttreatment changes in mean anxiety, dissociative symptoms, and PTSD symptoms were all greater following CO2 inhalation compared with placebo gas inhalation. Order of inhalation, age, and sex did not appear to affect the changes in scores for any of the measured variables.

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According to the study authors, the increases in panic and anxiety following CO2 inhalation were “striking” when compared with the change in values for healthy control participants from a former study.

The authors concluded that more research will be critical to “further characterize if this [CO2 reactivity] probe is of clinical relevance for differential diagnosis, differential therapy or prognosis in patients at risk for PTSD or suffering from the burden of PTSD.”


Kellner M, Muhtz C, Nowack S, Leichsenring I, Wiedemann K, Yassouridis A. Effects of 35% carbon dioxide (CO2) inhalation in patients with post-traumatic stress disorder (PTSD): A double-blind, randomized, placebo-controlled, cross-over trial. J Psychiatr Res. 2018;96:260-264.