Sensitivity of the respiratory center to carbon dioxide during prolonged exposure of humans to gas environment with unchanging level of hypercapnia

: Effects of moderate CO2 concentrations (2.2-3.2%) on external respiration were studied in a series of extended (30-d) continuous tests with volunteered human subjects. In one test, PCO2 in the gas mixture was maintained at 15-19 mm Hg (2.0-2.5%), in the other, at 20-23 mm Hg (2.7-3.0%). Partial pressure of oxygen was controlled at 144-152 mm Hg (19-20%) with the barometric pressure, temperature and humidity at the norm. As compared with the onset, at the end of the first test parameters of the external respiration were changed neither at rest nor during exercises. At the end of the second test, the minute pulmonary ventilation showed a steady rise in synch with the duration of exposure growing in 2.5 times at rest and in 1.7-2 times during physical exercises suggesting an increase in the respiratory center sensitivity to the CO2 level under study. Hence, breathing CO2 at 2.7-3.0% for many days did not sensitize the respiratory center of the human subjects.