Assessment of the autonomic response to a high dose CO2 inhalation challenge based on heart rate variability and skin conductance in a healthy population

Stress induction tests such as carbon dioxide (CO₂) inhalation challenges, are often used in research of anxiety and panic disorders. Physiological parameters of the autonomic response, e.g., heart rate variability (HRV) and skin conductance (SC), are often measured alongside questionnaires for evaluation. Previous studies have shown varied results on CO₂ inhalation-induced physiological reactivity and further knowledge is of interest. The aim of this study was to explore the effects of a 35 % CO₂ inhalation test on HRV frequencies and SC, in healthy subjects; and to set those into relation with subjective psychological ratings. In this single-blinded, repeated measures study, healthy subjects underwent a 35 % CO₂ inhalation challenge, whereof the first and third double vital capacity inhalations were with normal air and the second with 35 % CO₂. HRV (low and high frequency) and SC (as electrodermal activity (EDA)) were measured throughout. CO₂ inhalation resulted in a significant increase of HRV's high and very high frequencies compared to the first air inhalation (median difference + 0.0001633; + 0.0000348) and of HRV's very high and very low frequencies compared to the last air inhalation (+ 0.0000321; + 0.0000154). Mean and maximum SC increased significantly during the CO₂ inhalation compared to both air inhalations (mean difference to first air inhalation + 1.151; + 1.964; to last air inhalation + 0.5751; +1.484), but also between the separate air inhalations (+ 0.5754; + 0.4799). The HRV results indicate increased parasympathetic activity, while the SC results indicate increased sympathetic activity during CO₂ inhalation. SC only minorly correlated with provoked panic symptoms (EDA minimum to VAS minimum r = -0.559, and not between any other EDA and VAS/PSL measure). While those results seem contradictory, this study confirms that a 35 % CO₂ inhalation challenge in young healthy adults, provokes a physiological as well as psychological reaction.