Respiration and gas exchange under negative pressure breathing during simulated microgravity

The research considers the effect of inspiratory negative pressure breathing (NPBin, -20 cmH ₂ O relative to barometric pressure) on respiration and gas exchange in healthy humans under various conditions: in the sitting and supine positions as well as during simulating the physiological effects of a long stay in conditions of microgravity (dry immersion and head-down bed rest). Under NPBin, respiratory rate significantly decreased (by 7 min ^-1 on average, in some volunteers up to two or three cycles per minute), tidal volume increased (by an average of 0.3 l), while minute ventilation decreased (on average by 1.8 l/min), and respiratory exchange ratio increased (on average by 0.09). The response of respiratory and gas exchange parameters to NPBin in all the considered conditions was almost the same (there were no significant differences in parameters changes induced by NPBin between the conditions). Despite the decrease in minute ventilation, the results indicate CO ₂ washout from the body under NPBin.