STUDIES OF THE ACTIVITY OF THE CENTRAL RESPIRATORY MECHANISM IN LONG-TERM SPACE MISSIONS

Interrelation between blood redistribution toward the upper part of the body in microgravity and changes in sensitivity of the central respiratory mechanism was studied with participation of 16 Russian members of the International space station crews (40-57 year-old males). Time of maximal voluntary breath-holding during inspiration (MVHins) and expiration (MVHexp) was measured prior to and following the 15-min low-body negative pressure test (LBNP, -25 mm Hg). Before launch, the baseline MVHins and MVHexp durations in the vertical position made up 61.9 ± 3.9 s and 24.5 ± 1.5 s, respectively. In the supine position, the parameters increased to 81.1 ± 5.6 s and 31.2 ± 1.9 s (p < 0.01), respectively. More significant increases of breath-holding were observed in space microgravity. On flight months 1-5, MVHins and MVHexp rose by 25–45.5 % and by 17.5–54.2 %, respectively. After return from space the initial values of the parameters were recovered. Comparison of the measurements made prior to and within 5-10 minutes after LBNP revealed a further gain in both MVHins and MVHexp. Breath-holding extension in the supine position (at 1 G) and in microgravity suggests attenuation of the respiratory center sensitivity to the physiological chemoreceptor stimuli, namely, increased РаСО2 and lowered РаО2. This attenuation is driven by the blood shift toward the upper part of the body that causes increase of the vascular pressure in the sinus carotid region and activation of baroreceptors. The conclusion results from the significantly greater time of breath-holding during LBNP in microgravity and supine position, as well as time reduction to the initial values on return from space mission.