Five percent carbon dioxide challenge: valid analogue and marker of panic disorder?

The administration of 5% carbon dioxide (CO2) to patients with panic disorder (PD) induces a behavioral response similar to a naturally occurring panic attack. This article reviews the literature on the nature and incidence of this response. We conclude that the 5% CO2 challenge test is a valid and useful laboratory analogue of naturally occurring panic attacks, and shows promise as a marker to identify a subset of PD patients. Though further research on reliability, validity, and dose-response effects must be conducted, the CO2 challenge test provides important information regarding the phenomenology of panic states.