The 35% carbon dioxide test in stress and panic research: Overview of effects and integration of findings

The carbon dioxide test—a vital capacity breath of air containing 35% carbon dioxide (CO 2 )—provokes panic attacks in many individuals with panic disorder (PD). It has thus been extensively used as an experimental model of panic and less frequently as a clinical method of provoking symptoms for interoceptive exposure treatment. Recently, stress researchers have suggested another use for the CO 2 test: that of an acute physiological stressor indexing the human stress response. The purpose of this review is to synthesize findings about the effects of the CO 2 test from both the panic and stress literatures in order to advance understanding about this increasingly popular test. Both panic and stress researchers have examined the fleeting effects of the CO 2 test, finding that the test engenders transient breathlessness, dizziness, and minor anxiety in most participants and panic attacks in those with or at risk for PD. Physiological measurements after the test indicate a brief homeostatic disruption in many bodily systems, including increased respiration, systolic blood pressure, and noradrenaline, and decreased heart rate. Most studies indicate increased cortisol. Possible benefits of integrating findings from the panic and stress research lines, given their common use of the CO 2 test, are discussed.