Changes in the sensitivity of the central respiratory mechanism during a 21-h bed rest

The study deals with the mechanisms of the changes in the sensitivity of the respiratory center during a 21-h bed rest with an orthopedic table tilted at an angle of −15 degrees combined with liquid loss (lasix, 20 mL) and fluid recovery (intravenous injections of infukoll and glucose). The maximal breath-holding times, as well as capillary and venous tensions of O2 and CO2, were measured in the background period, during the bed rest, and shortly after the bed rest. Our data showed a statistically significant increase in both inspiratory and expiratory maximal breath-hold times during the initial 10 min of tilting. The comparison of data on breath-holding during different experimental series showed that infusions of neither glucose nor infukoll affected the duration of voluntary expiratory and inspiratory apnea. Since hour 17 of bed rest, there was a significant increase in the partial tension of O2 in the venous blood, while, in the same samples, there was a significant decrease in the partial tension of CO2. We assume that a decrease in the sensitivity of the respiratory center is triggered by the redistribution of blood to the upper part of the body and the changes in the pressure on the baroreceptors.