Part II
Respiration and
cardio-respiratory interactions during sleep on earth and in
microgravity
201
Chapter 7
Microgravity alters respiratory mechanics during sleep
7.1 Abstract
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 move- ments 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 µG, 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. µG 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 µG, with the thorax taking a leading role (-2.17 ± 2.11 %) in space.
T I /T T ot decreased in microgravity (µG) 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 µG 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 µG.
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7.2 Introduction
Gravity plays a major role in determining respiratory mechanics. Studies on postural changes ([1], [2], [3]), immersion studies ([4], [5]), exposure to transient microgravity ([6], [7], [8], [9], [10], [11]) and sustained microgravity ([12], [13], [14], [15]), as well as exposure to hyper gravity ([16]) have highlighted different aspects of the influence of gravity in respiratory mechanics.
Changing position from seated to the supine posture results in a stiffer rib cage (RC) and a more distensible abdominal (ABD) region ([3]). All else remaining equal, the different changes of ABD and RC compliance would be expected to lead to a different partition of rel- ative abdominal-rib cage contribution to quiet breathing when changing from the seated to supine position. ABD contribution to quiet breathing was observed to increase when chang- ing from seated to supine posture ([2], [3]). Two studies in parabolic flights ([6], [7]), where hypergravity and microgravity (µG) periods alternate, have shown that RC compliance in the seated position is relatively independent of gravity, while ABD compliance is increased in µG by 60% and decreased in hypergravity. ABD contribution to quiet breathing increased (seated position) in µG by almost 50%, and decreased slightly in hypergravity. Wantier et al ([13]) reported similar results for mid (10 days) and long-term (180 days) µG exposure, in seated subjects. Wantier et al argue that these changes are related to changes in the mechanical coupling between the lower rib cage and the diaphragm, and to a de-recruitment of inspiratory rib cage muscles. This has been confirmed by Estenne et al in parabolic flight ([8]): in µG the movement and shape of the apposed and the upper rib cage are altered differently in µG, and the scalene was found to be de-recruited from the inspiratory effort, no longer contributing to inspiration as it does for tidal breathing in seated subject at 1 G.
Sleep on the ground also induces major alterations in respiratory mechanics, altering upper airway resistance (mean increase of 230% during NREM sleep, [17], [18]), changing muscular tone and thus muscle action on breathing ([19]), as well as by inducing changes in the control of respiration, dependent on sleep stage ([20]), which result in a modest decrease in functional residual capacity (FRC), a decrease that is maximal for deep sleep (sleep stages 3 and 4) ([18]). Impact of these changes in respiratory mechanics is not consistent between studies; for NREM sleep, some authors indicating a decrease in abdominal contribution ([19], [21]), others an increase ([22]); however all these sources agree that abdominal contribution is significantly higher during REM sleep when compared to awake, and is associated with an increase in diaphragmatic EMG activity.
Sleep has been consistently reported as being of poor quality in microgravity, both by American astronauts and Russian cosmonauts ([23], [24]) yet few studies have addressed the combined action of microgravity and sleep on respiration and respiratory mechanics.
Elliot et al ([14]), in a study that shares the same experimental data used in the current paper, demonstrated that gravity played an important part in the generation of apneas, hypopneas and snoring in healthy subjects, with the number of these events decreasing dramatically in space. Obstructive apneas were reported to be practically absent in µG, which suggests that removal of gravity prevents the collapse of upper airways during sleep.
In summary, µG produces an increased abdominal contribution to tidal breathing, with a
decrease in inspiratory rib cage muscles’ activity, as well as a less efficient coupling between
the diaphragm and the lower rib cage. Sleep results in a significant increase in the activity
of the rib cage respiratory muscles during NREM periods, and a significant decrease during
REM periods ([17]).
7.3. METHODS 205 Therefore, we expected respiratory mechanics during NREM and REM to change dif- ferently in from Earth to µG. During REM periods, as rib cage muscle activity is small or silent, 1G – µG changes in respiratory mechanics would be expected to be mainly deter- mined by the conformational changes in the rib cage and abdomen. Respiratory mechanics during NREM sleep, on the other hand would be expected to reflect not only the confor- mational changes, but also the µG induced de-recruitment of rib cage inspiratory muscles.
Together, this de-recruitment present during NREM and the decrease in coupling between the lower rib cage and the diaphragm would be expected to result in a delay in the rib cage displacement in relation to the abdominal movements.
7.3 Methods
Data used in the current chapter were obtained in the framework of the study sleep and breathing in microgravity, where cardiac, respiratory and sleep related parameters were recorded during sleep before, during and after two space shuttle flights. Description of data collection for respiratory [14] and sleep [25] variables have been published elsewhere.
The reader is referred to chapter 2 for a complete description. This section will present the data collection and analysis methods relevant to the current work.
7.3.1 Subjects and sessions
Five astronauts, 1 woman and 4 men, participated in the study. Four subjects flew on STS- 90 Neurolab (17 day mission, April 17 - May 3, 1998), and one on STS-95 (10 day mission, October 29 - November 7, 1998). All subjects met NASA comprehensive health criteria for flight assignment, and reported no sleep disorders. All subjects were non-smokers and had a normal respiratory function with FVC and F EV 1 within the predicted normal range.
Average age was 41.0 ± 2.7(SD) yr, height 181 ± 13 cm, weight 79 ± 14 kg and body mass index 24 ± 1.6 kg/m 2 . Prior to flight, astronauts were trained extensively in application of sensors, operating the equipment and quality control of the recorded signals. The experimental protocol was approved by the Institutional Review Board of NASA Johnson Space Center, the University of California San Diego and the Brigham and Women’s Hospital. The subjects provided written informed consent before performing the protocol.
A total of 77 polysomnographic recordings (PSGs) were performed (ranging from 13 to 16 per subject), including, per subject, 9 pre-flight (range 6 to 9), 4 in-flight, and 3 post-flight.
Each PSG lasted approximately 8 h. Pre-flight recordings were performed in groups of 2 or 3 consecutive nights. For the 4 Neurolab subjects the recordings took place on days 102-101, 73-72, 45-44, 7-6-5 before launch. For STS-95 on days 77-76, 50-49 and 38-37 before launch.
The first two recordings for each subject were considered as habituation nights, and were not retained in the following analysis. In-flight recordings were also registered in batches of 2 consecutive nights, between day 3 and 15 of the mission for Neurolab, and between day 4 and 8 for STS-95. Post-flight data collection took place on day 1, 3 and 4 after landing (corresponding to the second, fourth and fifth sleep episode after return to earth). Subjects refrained from consuming caffeine or alcohol in the 12h preceding each recording and were also asked to refrain from hypnotic medication.
Recordings were performed from around 11pm to 7 am local time, except on the first
Neurolab post-flight recording, where sleep started 1 h earlier upon crew’s request due to fa-
tigue. A light sensor incorporated in the overall recording system allowed the determination
of the exact moment of lights off, and the moment when they are turned back on.
All pre and post-flight recordings were performed in Johnson Space Center’s crew quar- ters (or in a local hotel, for STS-95 only). In-flight recordings took place inside the shuttle mid-deck, which was equipped with four sleep compartments. These compartments are 2 m long, 0.75 m high, and wide enough for one.
Cabin atmosphere during flight remained normoxic with an increased CO 2 level (by approximately 0.4%). The two missions were single shift missions, and launch/landing constraints imposed only small deviations from the normal (pre-flight) 24 h cycle (20 minutes per day for Neurolab, 35 minutes per day for STS-95, both shortening the normal cycle).
7.3.2 Polysomnography
Polysomnographic data was digitally recorded using a Vitaport-2 (TEMEC Instruments B.V., The Netherlands), using a sleep monitoring system, developed for this specific ex- periment, and consisting of a custom fitted sleep cap, and a computerized signal-quality assessment system. The same system and procedures were used in all pre and post-flight recordings. In-flight instrumentation was performed by a second astronaut, while on-ground instrumentation was performed by technicians.
Electroencephalogram (EEG) channels O1/A2, O2/A1, C2/A2 and C4/A1 were recorded, and together with two electrooculogram channels (left and right) and two facial EMG sig- nals, were used for sleep stage scoring, according to the standard criteria of Rechtschaffen and Kales ([26]). Sleep scoring using 30-second epochs was performed for prior publication ([25]) and our analysis were based on those detections. Six stages were considered, Non- REM sleep stages 1, 2, 3 and 4 (from lighter sleep to deeper sleep), REM sleep, and windows when the subject is awake.
For the analysis presented here, sleep stages were grouped into 4 classes: Light Sleep, including NREM sleep stages 1 and 2; Deep Sleep, grouping sleep stages 3 and 4; REM sleep (no distinction was made between tonic REM and phasic episodes); and Awake. Events in the awake state were retained only when happening in periods of darkness. This was done in order to avoid standing periods, periods of movement, activity, etc, prior to lights out.
Movements, activity, changes in posture were retained if occurring during lights out period, and respiratory events were still clearly detectable in the recorded signal.
7.3.3 Respiratory data
In addition to the electro-physiological recordings allowing the determination of sleep stage,
measurements of interest for the present work are rib cage and abdominal motion (respi-
ratory inductive plethysmography, 32 Hz), snoring sounds (through a microphone placed
at the level of the larynx), light intensity (detector incorporated in the microphone), as
well as an event marker channel. No body position sensor was used. Most of the sensors
mentioned were integrated into a custom-fitted two-piece (vest and short) lycra body suit
(Blackbottoms, Salt Lake City, UT, USA). Rib cage and abdominal wires for the induc-
tance plethysmography were sewed into the body suit, with the chest band at the level of
the nipples and the abdominal band over the umbilicus. The vest allowed, by adjusting
shoulder straps, to adjust its size - in microgravity the spine lengthens -, thus allowing to
maintain proper location of each plethysmographic band for every gravity condition. Vest
and short were attached to each other through velcro straps to maintain the correct band
location throughout the night. Signal quality was verified prior to sleep onset using a laptop
computer and an interface described in [27].
7.3. METHODS 207
7.3.4 Data analysis and statistics
Data was visualized using TEMEC’s Vitagraph software package (TEMEC Instruments B.V., The Netherlands), and exported in ASCII format. These files were imported into Matlab (the Mathworks Inc., MA, USA). Detection of respiratory events was performed using an artificial neural network based algorithm ([28], also chapter 4). This algorithm detects moments of onset of inspiration and expiration from the respiratory signal. These detections were visually verified for each night studied. Large body movements and other artifacts were removed from the analysis.
Thoracic and abdominal respiratory signals relative gain was determined by performing an isovolume maneuver prior to sleep onset. Each isovolume maneuver was visually identified in the respiratory signals and analyzed for the determination of relative abdominal and thoracic gain. No absolute calibration of volume was performed. The signal obtained from the weighted sum of thoracic and abdominal respiratory movements corresponds thus to respiratory volume measured in arbitrary units (a.u.). Comparisons of volume from one night to the next are not possible in absolute terms, therefore volume was normalized for each night taking the awake values as reference, and are therefore expressed as a percentage of Awake (% Aw).
Breath-by-breath variables : breath period (T Resp ) was calculated on a breath-by-breath basis throughout the entire night as the time difference between two consecutive inspiratory onsets. Each breath was labeled with a time stamp corresponding to the time of occurrence of the first event. Inspiratory time (T I ), expiratory time (T E ) and the percentage of inspiratory time in the overall breath - duty cycle - (T I /T tot ) were calculated and labeled in identical manner. Tidal volume (V T ), minute ventilation ( ˙ V E ), and inspiratory flow ( ˙ V ins = V T /T I ) were also computed based on the same detections.
The abdominal and thoracic relative contribution to breathing (ABD cont and RC cont ) were also computed on a breath-by-breath basis. Relative contribution was calculated by integrating the locally (breath-by-breath) detrended abdominal and thoracic signal, divided by the integral of the volume signal for each breath.
Thoraco-abdominal asynchrony (Async) was calculated once for every 30s window. Thor- aco-abdominal asynchrony expresses the phase lag between abdominal and thoracic move- ments in percentage of breath time. Following Prisk et al ([29]), asynchrony was calculated using the maximum linear correlation method, using 30 s windows synchronous with sleep staging, and a threshold of 0.75 for the acceptable correlation (below that implying an inconsistent phase relationship). All windows presenting lower r values were ignored.
Statistical analysis of the effect of gravity on respiratory variables:
Data from the first pair of sleep recordings, 90 days prior to flight was excluded from the data analysis, in order to eliminate possible effects of the adaptation to the sleep instru- mentation. One in-flight recording for subject B (corresponding to the 2 nd recorded night, chapter 2) was excluded from the present analysis for technical reasons.
As sleep stages were scored using 30s windows, each variable was averaged over 30s windows synchronous with sleep stage scoring, and their coefficient of variation (CV) was calculated inside the same window. Mean values and mean variability values for entire nights were obtained by averaging all 30s windows of the same sleep class throughout each night, thus obtaining the overall mean value and the overall average coefficient of variability.
Individual average values were then calculated for each subject, for each gravity condi-
tion. Descriptive statistics are presented as inter-subject mean values ± SE, and variability
expressed as mean(CV). Asynchrony is the only variable not calculated on a breath-by-
breath basis, thus its variability is expressed differently, as mean(SD) for each class.
Pre-flight µG Post-flight
mean ± SE CV mean ± SE CV mean ± SE CV
Abd. Awake 59.8 ± 4.6 17.2 63.4 ± 3.3 15.4 59.6 ± 6.0 17.6
Cont Light sleep 52.1 ± 5.6 9.5 56.4 ± 5.9 9.9 53.2 ± 6.2 8.2 (%) Deep sleep 45.7 ± 6.5 9.2 52.2 ± 7.0 ** 10.3 47.4 ± 7.5 7.2 REM 64.8 ± 3.7 8.4 70.6 ± 4.4 ** 7.4 64.9 ± 5.7 † 7.9 Async. Awake − 0.13 ± 0.30 8.2 +0.82 ± 0.99 6.5 1.37 ± 0.59 ** 10.5 (%) Light sleep +0.33 ± 1.39 3.6 − 1.51 ± 1.48 2.5 0.53 ± 1.50 4.5
Deep sleep − 0.87 ± 1.69 2.5 − 2.17 ± 2.11 * 1.8 0.26 ± 2.66 *, †† 4.5 REM +3.22 ± 0.73 4.7 +5.21 ± 1.43 ** 4.8 4.55 ± 1.08 * 5.1
Table 7.1: Abdominal contribution and asynchrony, average values ± standard error and variability. Abdominal contribution variability is expressed as mean(CV); asynchrony vari- ability is expressed as mean(SD).
∗ 0.05 < p < 0.1, ∗ ∗ p < 0.05 compared to pre-flight;
† 0.05 < p < 0.1, †† p < 0.05 compared to µG.
Multiple way analyses of variance were performed for each variable to compare pre- flight, in-flight and post-flight data. Prior to post hoc comparison, a Levene’s test for testing the null hypothesis of identical variance was performed. When the null hypothesis was acceptable, post hoc comparison was performed using Tukey’s adjustment. Whenever the null hypothesis was rejected (different variance amongst groups), post hoc comparison was performed using Games-Howell’s adjustment, which does not require equal variances.
Differences were considered significant for p < 0.05. Statistical testing was performed using SPSS 11.0.4 for Macintosh (SPSS Inc., USA).
7.4 Results
7.4.1 Abdominal contribution and thoraco-abdominal asynchrony
For all gravity conditions, abdominal contribution decreased from awake to light sleep, fur- ther decreased in deep sleep, while REM sleep presented the highest abdominal contribution to tidal volume. Transition to µG increased relative abdominal contribution for all sleep classes. Post-flight, abdominal contribution returned to pre-flight values (table 7.1, figure 7.1), for all sleep classes. Average pre-flight - µG difference reached statistical significance (p < 0.05) for REM and Deep sleep. No pre-post-flight difference was observed.
Figure 7.1 presents the time evolution of abdominal contribution in space and upon return to earth. Identical behavior for all sleep classes was observed, with the first recording in space presenting a significantly higher abdominal contribution, followed by a return towards pre-flight values in the subsequent days.
Mean CV of abdominal contribution does not change significantly through the different
gravity conditions, for all sleep stages (table 7.1). Abdominal contribution variability is
higher while the subject is awake. No noteworthy difference in abdominal contribution
variability was observed amongst the remaining classes.
7.4. RESULTS 209
Abdominal contribution
Pre flight
1st 2nd 3rd 4th recording in !G
1st 2nd 3rd recording upon return
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Abdominal Contribution
Figure 7.1: Evolution of abdominal contribution to tidal volume over time, for all sleep classes. All pre-flight values were averaged. ∗ 0.05 < p < 0.1, ∗ ∗ p < 0.05 compared to pre-flight. The grey area indicates the inter-subject variability (SD) for the awake class.
Inter-subject variability for the other sleep classes considered is similar. Black points refer to awake, cyan to light sleep, dark blue to deep sleep and red to REM
Thoraco–abdominal asynchrony prior to flight showed that while the subjects are awake, in light and in deep sleep, the abdomen and rib cage movements are mostly in phase (table 7.1, figure 7.2). Asynchrony for these 3 classes is statistically indistinguishable from syn- chrony (zero phase). This is not true for REM sleep, with the abdomen leading the rib cage by 3.22% of a breath cycle.
Removal of gravitational load resulted in no significant average change for awake and light sleep, an increased tendency of the rib cage to lead in deep sleep and a significant increase in the leading role of the diaphragm during REM sleep (table 7.1).
Upon return to earth, in-phase movements of abdomen and rib cage were observed for light and deep sleep. Asynchrony during REM decreased towards pre-flight values, while asynchrony in the awake class increased, presenting a value that is statistically different from pre-flight.
For all gravity conditions, asynchrony variability is higher while the subject is awake,
decreases in NREM, presenting its lowest value in deep sleep, with REM presenting inter-
mediate values of variability. Variability of asynchrony was generally lower in space, and
was increased post-flight when compared to pre-flight values for awake and NREM sleep.
Thoraco-abdominal asynchrony
Pre flight
1st 2nd 3rd 4th recording in !G
1st 2nd 3rd recording upon return
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Figure 7.2: Evolution of thoraco-abdominal asynchrony over time, for all sleep classes, expressed as % of breath period (T resp ). All pre-flight values were averaged. ∗ 0.05 < p < 0.1,
∗ ∗ p < 0.05 compared to pre-flight. The grey area indicates the inter-subject variability for the awake class. Variability in the remaining classes is lower. Black points refer to Awake, cyan to light sleep, dark blue to deep sleep and red to REM
Percentage of inspiratory time in breath (T I /T T ot )
Pre flight
1st 2nd 3rd 4th recording in !G
1st 2nd 3rd recording upon return
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Figure 7.3: Evolution of duty cycle (T I /T T ot ) in time, for all sleep classes. All pre-flight
values were averaged. ∗ 0.05 < p < 0.1, ∗ ∗ p < 0.05 compared to pre-flight. The grey area
indicates the inter-subject variability (SD) for the awake class. Inter-subject variability for
the other sleep classes is similar or smaller. Black points refer to awake, cyan to light sleep,
dark blue to deep sleep and red to REM
7.4. RESULTS 211
Pre-flight µG Post-flight
mean ± SE CV mean ± SE CV mean ± SE CV
T I /T T ot Awake 40.5 ± 1.5 24.1 38.8 ± 1.4 ** 20.1 41.7 ± 1.3 **, †† 21.4 (%) Light sleep 41.7 ± 2.2 13.0 39.3 ± 1.6 ** 11.5 44.2 ± 1.7 **, †† 11.3 Deep sleep 43.4 ± 1.7 10.6 40.6 ± 1.6 ** 10.8 46.5 ± 1.1 **, †† 8.4
REM 45.0 ± 2.1 13.7 42.1 ± 1.8 ** 12.4 46.5 ± 1.8 **, †† 11.9
V ˙ ins Awake 100.0 39.9 100.0 28.1 100.0 34.4
(%Aw) Light sleep 50.6 ± 3.6 19.3 61.9 ± 2.3 ** 15.3 54.4 ± 2.8 † 16.5 Deep sleep 48.9 ± 4.4 15.9 60.3 ± 2.6 ** 14.2 53.4 ± 4.1 † 12.2 REM 50.4 ± 4.0 23.0 65.7 ± 3.1 ** 16.9 56.6 ± 3.9 **, †† 19.3
Table 7.2: Average values ± standard error and variability (expressed as average coefficient of variation) for duty cycle (T I /T T ot ) and inspired flow ( ˙ V ins ) on earth, in µG and post-flight.
Volume variables are expressed as a % of their awake values. Variability for all variables is expressed as the average coefficient of variation (CV) (%).
∗ 0.05 < p < 0.1, ∗ ∗ p < 0.05 compared to pre-flight;
† 0.05 < p < 0.1, †† p < 0.05 compared to µG.
7.4.2 Drive and timing components of ventilation
Duty cycle (T I /T T ot ) decreased in space for all sleep classes. Post-flight measurements showed an increased T I /T T ot when compared to pre-flight (table 7.2). All average changes reached statistical significance. On earth, T I /T T ot presented the lowest value while awake, increased in light sleep, further increasing in deep sleep, with REM presenting the highest values. This order is maintained in-flight, but not post-flight, for which average deep sleep and REM values were the same. Variability of T I /T T ot was highest during awake periods, and lowest during deep sleep, with light sleep and REM presenting intermediate variability, yet much lower than that observed during awake periods. Variability decreased in µG, and remained lower post-flight, when compared to pre-flight.
Figure 7.3 depicts the time evolution of T I /T T ot . The average T I /T T ot decreased for all sleep classes in µG. Post-flight, average T I /T T ot was higher than pre-flight values (table 7.2).
Early post-flight (day 1 and 3 after return) NREM sleep, light and deep sleep, presented pre-post-flight differences reaching statistical significance (figure 7.3).
Normalized average inspired flow ( ˙ V ins ), expressed as a % of Awake values, increased in µG for NREM and REM sleep (table 7.2). Post-flight, only REM sleep presented a statistical significant increase of ˙ V ins when compared to pre-flight. ˙ V ins is lower during deep sleep in all gravity conditions. Pre-flight light sleep and REM present similar ˙ V ins values, while in µG, REM sleep presented a clearly higher mean inspiratory flow than light sleep.
Variability of ˙ V ins was found to be highest for the awake class, followed by REM, light sleep
and is lowest during deep sleep. Variability decreased in µG, and remained lower than pre-
fligh during the early post-flight period. Inspired flow presented no trend, for none of the
sleep classes considered, with time spent in µG.
Pre-flight µG Post-flight
mean ± SE CV mean ± SE CV mean ± SE CV
T resp Awake 4.25 ± 0.31 25.1 4.48 ± 0.27 ** 21.2 4.04 ± 0.27 **, †† 21.8 (s) Light sleep 4.55 ± 0.43 12.4 4.32 ± 0.31 10.4 4.23 ± 0.29 ** 10.5 Deep sleep 4.34 ± 0.40 9.8 4.13 ± 0.27 ** 10.0 4.13 ± 0.30 ** 7.4
REM 4.35 ± 0.41 15.3 4.16 ± 0.31 13.0 4.03 ± 0.29 ** 13.7 T I Awake 1.61 ± 0.07 27.1 1.65 ± 0.08 23.4 1.63 ± 0.08 26.1 (s) Light sleep 1.83 ± 0.08 14.2 1.67 ± 0.08 ** 13.2 1.84 ± 0.09 †† 13.4 Deep sleep 1.85 ± 0.10 12.4 1.66 ± 0.09 ** 13.3 1.91 ± 0.12 †† 10.1 REM 1.87 ± 0.09 16.3 1.70 ± 0.07 ** 14.8 1.84 ± 0.08 †† 16.1 T E Awake 2.64 ± 0.25 33.8 2.82 ± 0.22 ** 27.9 2.41 ± 0.21 **, †† 29.4 (s) Light sleep 2.72 ± 0.36 17.2 2.65 ± 0.25 14.0 2.39 ± 0.23 **, †† 15.2 Deep sleep 2.49 ± 0.30 13.8 2.48 ± 0.22 13.3 2.22 ± 0.19 **, †† 11.3 REM 2.47 ± 0.33 21.0 2.45 ± 0.26 17.6 2.20 ± 0.23 **, †† 18.6
V T Awake 100.0 37.7 100.0 31.0 100.0 39.7
(%Aw) Light sleep 64.9 ± 4.2 17.3 65.2 ± 1.7 14.0 66.9 ± 3.2 17.0 Deep sleep 63.1 ± 4.1 13.2 63.1 ± 1.8 12.8 67.8 ± 2.7 11.9 REM 66.0 ± 3.6 23.9 71.1 ± 2.5 * 19.0 70.1 ± 4.0 23.5
V ˙ E Awake 100.0 42.5 100.0 31.0 100.0 40.6
(%Aw) Light sleep 50.4 ± 5.6 19.8 59.9 ± 2.8 ** 15.1 56.1 ± 3.5 * 18.3 Deep sleep 51.7 ± 6.0 14.8 61.3 ± 2.2 ** 12.9 59.5 ± 3.9 ** 12.1 REM 57.7 ± 7.3 22.7 71.9 ± 5.0 ** 17.1 64.6 ± 6.5 * 20.8
Table 7.3: Average values ± SE and variability and for timing variables, tidal volume (V T ) and minute ventilation ( ˙ V E ). Tidal volume and minute ventilation were normalized and are expressed as a % of their awake value. Variability for all variables is expressed as the average coefficient of variation (CV) (%).
∗ 0.05 < p < 0.1, ∗ ∗ p < 0.05 compared to pre-flight;
† 0.05 < p < 0.1, †† p < 0.05 compared to µG.
7.4.3 Respiratory timing, tidal volume and minute ventilation
Breathing period (T Resp ) increased in-flight for the awake class (p < 0.05), decreased for the remaining classes, though this decrease only reached statistical significance for deep sleep (table 7.3). These changes in T Resp were attained differently for different sleep classes. T I
decreased in-flight for all sleep classes with the exception of awake, for which a non-significant increase was observed. Post-flight T I was similar to pre-flight values, for all sleep classes. T E
changed differently: no in-flight difference for NREM and REM sleep was observed, while it significantly increased for the awake class. Post-flight, T E was shorter when compared to pre-flight, for all classes with no exception. Average changes in T I , T E and T Resp are represented in figure 7.4. The different directions of change of T I and T E resulted in a consistent behavior of T I /T T ot presented above.
Variability of respiratory timing variables (T resp , T I , T E and T I /T T ot ) decreased in space
for all sleep classes, except for deep sleep (table 7.3). Upon return to earth, variability
results are less consistent. The only consistent change observed post-flight is a systematic
decrease in the variability of most timing variables for deep sleep, when compared to pre-
flight. Variability of timing variables is higher in the awake state, decreases in light sleep,
decreasing even further in deep sleep. REM presents intermediate variability values between
7.4. RESULTS 213
1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00
Awake Light Sleep Deep Sleep REM
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