1 2 Microgravity alters respiratory abdominal and rib cage motion 3 during sleep 4 5 Rui

24 The abdominal and rib cage contributions to tidal breathing differ between REM and NREM sleep. We 25 hypothesized that abdominal relative contribution during NREM and REM sleep would be altered in 26 different directions when comparing sleep on Earth with sleep in sustained microgravity (μG), due to 27 conformational changes and differences in coupling between the rib cage and the abdominal compartment 28 induced by weightlessness. We studied respiration during sleep in 5 astronauts before, during and after 29 two Space Shuttle missions. A total of 77 full night (8h) polysomnographic studies were performed; 30 abdominal and rib cage respiratory movements were recorded using respiratory inductive 31 plethysmography. Breath-by-breath analysis of respiration was performed for each class: awake, light 32 sleep, deep sleep and REM sleep. Abdominal contribution to tidal breathing increased in μG, with the first 33 measure in space being significantly higher than pre-flight values, followed by a return towards pre-flight 34 values. This was observed for all classes. Pre-flight, rib cage and abdominal movements were found to be 35 in phase for all but REM sleep, for which an abdominal lead was observed. The abdominal leading role 36 during REM sleep increased while deep sleep showed the opposite behavior, the rib cage taking a leading 37 role in-flight. In μG, TI/TTot decreased for all classes considered when compared with pre-flight, while 38 normalized inspiratory flow – taking the awake values as referenceincreased in-flight for light sleep, 39 deep sleep and REM. Changes in abdominal-rib cage displacements probably result from a less efficient 40 operating point for the diaphragm and a less efficient coupling between the abdomen and the apposed 41 portion of the rib cage in μG. However the preservation of total ventilation suggests that short-term 42 adaptive mechanisms of ventilatory control compensate for these mechanical changes. 43 44