Joint NASA-ESA-DARA Study. Part three: cardiorespiratory response to elevated CO2 levels during sleep

Long-term exposure to elevated ambient CO2-levels is a common condition for living in a closed environment such as a spacecraft. In this study, the cardio-respiratory system response to CO2-levels of 0.7% and 1.2% was assessed. The response was investigated during non-REM sleep when the sensitivity of the respiratory system to ambient CO2 is low and only subject to the metabolic respiratory drive. Four subjects were exposed to 0.7% and 1.2% CO2 for 23 d each. Respiration rate and heart rate were determined for the first two phases of slow wave sleep. In addition, the occurrence of central apneas was assessed. Data were analyzed by a repeated measure ANOVA. As a response to CO2 exposure two dynamic effects were observed. Heart rate increased initially with a peak between the second and the sixth night. Over the period of the exposure, respiration rate and heart rate decreased steadily. At least two mechanisms with different time constants must be considered for this dynamic behavior: an uncompensated respiratory acidosis, followed by a phase of relative compensation. At the end of the 23-d exposure, equilibrium in the physiological state had not been reached. Though the experiment did not show severe effects from CO2, it is too early to state that a long-term exposure does not have any consequences for health and well-being.