An opponent-process interpretation of the anxiolytic effects of single inhalations of large concentrations of carbon dioxide.

Ten anxiety-disorder outpatients and 10 controls were monitored for heart rate and self reports of anxiety approximately 1 min before and 1 min after a single full-capacity inhalation of a gas consisting of either 65% CO2 + 35% O2 or compressed air. Under the CO2 + O2 condition, patients and controls showed almost identical reductions in anxiety and parallel reductions in heart rate. From an opponent-process point of view, these reductions were explained by the acute negative affective state being followed by the hedonically pleasant state that follows dissipation of CO2. Under the compressed air condition, only patients showed reductions of anxiety presumably because in them breathing an unknown gas via an inhalation mask, raised anxiety, followed by an opponent-process to anxiety after withdrawal of the mask.