Reactivity of the hypothalamicpituitary-adrenal (HPA) and sympathetic-adrenal-medullary (SAM) axes following inhalation of 35 per cent CO2

Background: Hypercapnia has anxiogenic effects and is, therefore, an aversive stimulus to healthy individuals. The inhalation of carbon dioxide (CO2), therefore, provides a good paradigm to test as a possible stressor to cardiovascular, endocrine and psychological reactivity. Methods: Healthy participants (n=24) inhaled a practice breath of air and then a single vital capacity breath of a mixture of CO2 (35 per cent) and oxygen (65 per cent). Blood pressure and heart rate were recorded for five minutes before and after the test and blood and saliva samples were taken immediately before and two, 10, 20 and 30 minutes post-inhalation for the measurement of adrenaline, noradrenaline plasma and salivary cortisol respectively. In addition, psychosomatic symptoms were recorded immediately before and after the test. Results: A single inhalation of CO2 elicited increases in blood pressure, adrenaline, noradrenaline, cortisol and psychosomatic symptoms, but decreases in heart rate. Sympathetic, parasympathetic and psychosomatic reactivity were stable over a one-month follow-up period, although HPA reactivity was reduced. Subsequent analyses of salivary cortisol data revealed that 70 per cent of the sample could be reliably (i.e. responder status remained consistent across test sessions) classified as either low or high responders. These groups differed in terms of their psychological and endocrine reactivity. Conclusions: These data indicate that a single inhalation of CO2 simultaneously stimulates the HPA axis (increases in cortisol), the sympathetic (increases in blood pressure, adrenaline and noradrenaline) and parasympathetic (reduction in heart rate) nervous systems, as well as inducing perceived feeling of stress. The test also elicited stable individual differences in HPA reactivity. The test is safe, easy to administer and can readily be applied to a wide range of populations.