cardio-respiratory interactions during sleep on earth and in microgravity

We studied respiratory mechanics during sleep in 5 astronauts before, during and after space ﬂight in two shuttle missions (STS-90 Neurolab and STS-95). A total of 77 full night (8h) polysomnographic studies were performed, and respiratory abdominal and thoracic movements recorded in parallel using respiratory inductive plethysmography. Breath-by-breath analysis of respiratory variables was performed for each sleep condition ( awake , light sleep , deep sleep and Rapid Eye Movement - REM ). Abdominal contribution to tidal breathing increased for all sleep stages in µ G, with the ﬁrst measure in space being signiﬁcantly higher than pre-ﬂight values ( awake , light sleep , REM , p < 0 . 05, deep sleep , 0 . 05 < p < 0 . 10), with a return towards pre-ﬂight values on the following days. µ G induced the most striking changes in breathing pattern during REM sleep, during which there was an increase in abdominal contribution in space (64.8 ± 3.7 % pre-ﬂight, 70.6 ± 4.4 % in-ﬂight, p < 0 . 05). Pre-ﬂight, thoracic and abdominal movements were found to be in phase for all but REM sleep, for which an abdominal lead was observed. Removal of the gravitational load increased the abdominal leading role during REM sleep, increasing asynchrony from 3.22 ± 0.73 % (pre-ﬂight) to 5.21 ± 1.43 % (in-ﬂight) ( p < 0 . 05). Deep sleep showed the opposite behavior during µ G, with the thorax taking a leading role (-2.17 ± 2.11 %) in space. T I /T Tot decreased in microgravity ( µ G) for all sleep stages and was found to be increased above pre-ﬂight values in early post-ﬂight. Normalized ˙ V ins increased in space for light sleep , deep sleep and REM . Post-ﬂight it returned to pre-ﬂight values for all but REM sleep. Relative minute ventilation, expressed as % of awake values increased in µ G for all sleep classes, and remained elevated post-ﬂight. Inspiratory time decreased in space for REM and NREM sleep classes, while expiratory time increased in the awake class. Respiratory period was shorter during sleep post-ﬂight for all classes, with the change being due mostly to a shortening of the expiratory time. These results suggest altered respiratory mechanics during sleep in microgravity. However the preservation or even increase of total ventilation suggests that short-term adaptive mechanisms of ventilatory control compensate for these mechanical changes. Changes in abdominal–rib cage displacements probably result from a less eﬃcient operating point for the diaphragm and a less eﬃcient coupling between the diaphragm and the lower rib cage in µ G. Abstract This chapter focuses on cardio-respiratoy interactions during sleep, and more speciﬁcally on the inﬂuence of gravity. We have studied beat-to-beat interval, heart-rate variability, respiratory sinus arrhythmia in parallel with respiration during sleep before, during and after 2 space-shuttle ﬂights (STS-90 Neurolab and STS-95). The diﬀerent frequency components of heart-rate variability were quantiﬁed continuously throughout the night using a continuous wavelet transform-based algorithm, speciﬁcally tailored for heart rate variability analysis during sleep (presented in chapter 6). The gain and phase delay between the heart-period oscillations at the respiratory frequency and respiration was computed using the same algorithm. Sleep was partitioned into 4 classes: light sleep (NREM sleep stages 1 and 2), deep sleep (NREM stages 3 and 4), REM sleep, and periods when the subject was awake. Heart period increased in the early days spent in microgravity ( µ G), and this was observed for all classes considered. This initial increase was followed by a return towards pre-ﬂight values, observed in late ﬂight recordings. Post-ﬂight, heart period was shorter than pre-ﬂight for all classes. µ