Abstract
The abdominal and rib cage contributions to tidal breathing differ between rapid-eye-movement (REM) and non-NREM sleep. We hypothesized that abdominal relative contribution during NREM and REM sleep would be altered in different directions when comparing sleep on Earth with sleep in sustained microgravity (microG), due to conformational changes and differences in coupling between the rib cage and the abdominal compartment induced by weightlessness. We studied respiration during sleep in five astronauts before, during, and after two Space Shuttle missions. A total of 77 full-night (8 h) polysomnographic studies were performed; abdominal and rib cage respiratory movements were recorded using respiratory inductive plethysmography. Breath-by-breath analysis of respiration was performed for each class: awake, light sleep, deep sleep, and REM sleep. Abdominal contribution to tidal breathing increased in microG, with the first measure in space being significantly higher than preflight values, followed by a return toward preflight values. This was observed for all classes. Preflight, rib cage, and abdominal movements were found to be in phase for all but REM sleep, for which an abdominal lead was observed. The abdominal leading role during REM sleep increased while deep sleep showed the opposite behavior, the rib cage taking a leading role in-flight. In microG, the percentage of inspiratory time in the overall breath, the duty cycle (T(I)/T(Tot)), decreased for all classes considered when compared with preflight, while normalized inspiratory flow, taking the awake values as reference, increased in-flight for light sleep, deep sleep, and REM. Changes in abdominal-rib cage displacements probably result from a less efficient operating point for the diaphragm and a less efficient coupling between the abdomen and the apposed portion of the rib cage in microG. However, the preservation of total ventilation suggests that short-term adaptive mechanisms of ventilatory control compensate for these mechanical changes.