Effect of Simulated Microgravity and Lunar Gravity on Human Inspiratory Muscle Function: 'Selena-T' 2015 Study.

As a part of the multi-disciplinary "SELENA-T"-2015 Bed Rest Study, we investigated the pattern of inspiratory muscles fatigue in 22 healthy male subjects during incremental exercise test to exhaustion before and after 21 days of hypokinesia evoked by bed rest. Hypokinesia consisted of head-down bed rest (HDBR) at a minus 6° angle, simulating microgravity present on orbiting spacecraft, in 10 subjects. The remaining 12 subjects spent the first 5 days of hypokinesia in HDBR position and the subsequent 16 days in head-up bed rest (HUBR) at a plus 9.6° angle, as a presumed analog of lunar gravity that is six times less than Earth's gravity. Maximal inspiratory pressure (MIP) and electromyograms (EMG) of the diaphragm (D), parasternal (PS), sternocleidomastoid (SCM), and scalene (S) muscles served as indices of inspiratory muscle function. Before both HDBR and HUBR, exercise decreased MIP and centroid frequency (fc) of EMG (D, PS, SCM, and S) power spectrum (p < 0.05). After 3 weeks of HDBR, but not HUBR, inspiratory muscles fatigue was more expressed compared with control (p < 0.05). We conclude that HDBR lowers inspiratory muscles resistance to fatigue during high-intensity exercise while HUBR has no such effect. These changes may limit maximal ventilation and may contribute to exercise intolerance observed after prolonged simulated microgravity. The physiological mechanisms of respiratory muscle dysfunction after HDBR consist primarily of postural effects, and are not due only to hypokinesia.

Effect of central hypervolemia on respiratory function.

Effects of central hypervolemia on respiratory function and compensatory capabilities of the respiratory system were studied in the anesthetized, vagally intact or vagotomized rats. Central hypervolemia was induced by a head-down tilt on -30 degree rotation. The tilt evoked an elevation of central venous pressure (from -2+/-0.4 cmH2O to 3.9+/-0.8 cmH2O). At 30 min after tilting, airway resistance and negative intrathoracic pressure (indirect measure of respiratory effort) significantly increased, whereas inspiratory flow, tidal volume, and minute ventilation decreased. Load compensatory response was strongly weakened. The tilt-induced esophageal pressure augmentation was suppressed by transection of the vagal nerves. In vagotomized animals inspiratory swings of the intrathoracic pressure increased barely to 116+/-15%, whereas they increased to 216+/-17% of control in vagally intact animals (P<0.05). We conclude that central hypervolemia increases mechanical loading and weakens compensatory capabilities of the respiratory system. Vagal afferents have a part in the realization of the respiratory response to central hypervolemia.