JOINT NASA-ESA-DARA STUDY - PART 3 - CARDIORESPIRATORY RESPONSE TO ELEVATED CO2 LEVELS DURING SLEEP

Long-term exposure to elevated ambient CO2-levels is a common conditio n for living in a closed environment such as a spacecraft. In this stu dy, the cardio-respiratory system response to CO2-levels of 0.7% and 1 .2% was assessed. The response was investigated during non-REM sleep w hen the sensitivity of the respiratory system to ambient CO2 is low an d 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 ana lyzed by a repeated measure ANOVA. As a response to CO2 exposure two d ynamic effects were observed. Heart rate increased initially with a pe ak between the second and the sixth night. Over the period of the expo sure, 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, equi librium in the physiological state had not been reached. Though the ex periment did not show severe effects from CO2, it is too early to stat e that a long-term exposure does not have any consequences for health and well-being.