Yu.A. Shulagin, A.I. Dyachenko, E.S. Ermolaev, A.O. Goncharov

Сhemoreflex control of breathing plays a major role in adjustment of ventilation to metabolic demand and changes of CO2 and O2 contents in the inspired air. Studies in space revealed changes of human breathing control. The hypoxic ventilatory response (VR) was reduced, but hypercapnic VR was steepened somewhat in both short spaceflights and supine subjects. There was no any study of VR during long space missions. To further understand effect of microgravity on control of breathing it is necessary to perform measurements of both hypoxic and hypercapnic VR in long space flights. Various modifications of experimental techniques could be used for studies of VR. Standard ground-based equipment and methods are not well suitable for this purpose because usually used balloons with oxygen are dangerous in space. Another limitation is a demand for minimal dimensions of equipment. All these means that a special apparatus and methods should be developed for the study of chemoreflex control of breathing in space. This small apparatus could be useful for clinical evaluation of chemoreflex control of breathing. The purpose of this work was a development of modified methods aimed for future studies of VR during long space missions and a study of VR in sitting and supine subjects by these methods. An experimental setup included bag in box for rebreathing with a set of tubes and valves, computer, sensors and software. End-tidal СО2 and О2 were registered respectively by custom-made CO2-meter (Triton, Russia) and O2-meter (Beckman OM-11, USA). Eight normal subjects in this study were volunteers with age from 20 to 25 years, who had no history of lung diseases. Before the study an informed consent was obtained from every subject. The experimental protocol of respiration via rebreathing apparatus included: 5 min of a normal breathing with room air, 1 min of hyperventilation to obtain PETCO2 about 20 - 25 mmHg, rebreathing in the bag during 10 - 12 min or up to obtaining PETCO2 about 60 mmHg, breathing with room air. Each subject performed 4 different runs: 1) Hypercapnic ventilatory response starting breathing room air in the upright seated position. 2) Hypercapnic ventilatory response starting breathing room air in the supine head-down position - 12,5°. 3) Hypercapnic ventilatory response starting breathing hyperoxic gas mixture in the upright seated position. 4) hypercapnic ventilatory response starting breathing hyperoxic gas mixture in the supine head-down position - 12,5°. Ventilatory responses were analyzed in terms of a linear line of best fit of ventilation against PETCO2. Tidal volume responses were also obtained and analyzed respectively in the same runs 1-4 in terms of a linear line of best fit of tidal volume against PETCO2. Experimental results demonstrate that tidal volume response on СО2, in progressive hypercapnic - hypoxic rebreathing was more in supine head-down position than in sitting position (p<0.01). Progressive hypercapnic - hypoxic rebreathing revealed posture changes in chemoreflex control, so it is reasonable to use this method in space studies as well as the methods of hyperoxic rebreathing and of isocapnic rebreathing. Chemoreflex control of tidal volume response, has a greater sensitivity to posture than ventilation response, so it is recommended to include VT in parameters of study in long space flights. A modified method aimed for future studies of chemoreflex control of breathing during long space missions is developed.