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1 Biomedical Physics Laboratory
and 2 Chest Service, We assessed the
effects of sustained weightlessness on chest wall mechanics in five
astronauts who were studied before, during, and after the 10-day
Spacelab D-2 mission (n = 3)
and the 180-day Euromir-95 mission (n = 2). We measured flow and pressure at the mouth and rib cage and
abdominal volumes during resting breathing and during a relaxation
maneuver from midinspiratory capacity to functional residual capacity.
Microgravity produced marked and consistent changes (
spaceflight; zero gravity; abdominal compliance; rib cage
compliance
THE NORMAL CHEST WALL is exquisitely sensitive to
gravity. Experiments performed during parabolic flights, which produce
gravity-free conditions, have shown that abdominal volume and pressure
at end-expiration decrease, whereas abdominal contribution to tidal
volume and abdominal compliance increase (4, 12). The opposite results
were observed during hypergravity in a human centrifuge as head-to-foot
acceleration (+Gz) was increased
(6). Measurements obtained during parabolic flights, however, only
provided information on the short-term response to weightlessness
because periods of microgravity lasted <30 s. It is not known,
therefore, whether prolonged exposure to microgravity, as occurs during
spaceflight, would produce similar alterations in chest wall mechanics.
In this study, we report measurements of chest wall mechanics performed
during sustained microgravity in three subjects on the Spacelab D-2
mission and in two subjects on the Euromir-95 mission.
Subjects and Data-Collection Schedule
ABSTRACT
Top
Abstract
Introduction
Methods
Results
Discussion
References
) in the
contribution of the abdomen to tidal volume [Vab/(Vab + Vrc), where Vab is abdominal volume and Vrc is rib cage
volume], which increased from 30.7 ± 3.5 (SE)% at
1 G head-to-foot acceleration to 58.3 ± 5.7% at 0 G head-to-foot acceleration (P < 0.005). Values of
Vab/(Vab + Vrc) did not change significantly during the 180 days of the Euromir mission, but in the two subjects Vab/(Vab + Vrc) was greater on postflight day
1 than on subsequent postflight days or preflight. In
the two subjects who produced satisfactory relaxation maneuvers, the slope of the Konno-Mead plot decreased in microgravity; this decrease was entirely accounted for by an increase in abdominal compliance because rib cage compliance did not change. These alterations are
similar to those previously reported during short periods of
weightlessness inside aircrafts flying parabolic trajectories. They are
also qualitatively similar to those observed on going from upright to
supine posture; however, in contrast to microgravity, such postural
change reduces rib cage compliance.
INTRODUCTION
Top
Abstract
Introduction
Methods
Results
Discussion
References
METHODS
Top
Abstract
Introduction
Methods
Results
Discussion
References
Experimental Setup
Multiuser facilities developed to perform a wide range of physiological tests were used in Spacelab and Euromir, and functionally identical facilities were used on ground for training, preflight, and postflight data collection. The subsystems utilized for the present study consisted of 1) a respiratory inductive plethysmograph (RIP), which measured variations of inductance of two wires sewed with a zigzag pattern around the rib cage (RC) and the abdomen (AB) in a suit tailored for each astronaut; and 2) a three-way nonrebreathing valve with a pressure transducer incorporated near the mouthpiece [a flow restrictor with a resistance of 10 cmH2O · l
1 · s
measured at 0.2 l/s could be added to the expiratory path; the
inspiratory port could be connected either to room air or to the gas
mixture used for the multiple-breath nitrogen washouts (MBW)]; and
3) an ultrasound flowmeter located
between the mouth and the rotary valve. The instrumental dead space was
85 ml on Spacelab and 181 ml on Euromir. For Spacelab, the sampling
rate was 100 Hz for the RIP signals, and 200 Hz for flow and mouthpiece pressure. Corresponding values for Euromir were 67, 100, and 33 Hz,
respectively.
RIP Calibration
Spacelab experiments. Calibration of the RIP was performed by using the 20-25 one-liter inspirations taken from functional residual capacity (FRC) during the MBW. The calibration method used is a variant of the method described by Stagg et al. (14) for magnetometers. Each breathing sequence is first decomposed into individual breaths beginning and ending with zero flow. For each breath (n) and each acquisition (i) (i takes ~400 values for each breath, with i = 0 corresponding to the beginning of inspiration), the RIP volume-motion coefficients (An and Bn) and a constant Cn are computed such that the following function is minimal
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(1) |
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(2) |
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(3) |
The abdominal contribution to tidal volume for breath n is given by
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(4) |
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(5) |
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(6) |
Vab and Vrc are the changes in abdominal and rib cage volumes,
respectively, ABn,E and
RCn,E are the RIP
amplitudes at the beginning of the expiration of breath n, and
ABn,0 and
RCn,0 are the RIP
amplitudes at the beginning of the inspiration of breath
n.
Euromir experiments. Standard isovolume maneuvers were performed at FRC with a closed glottis. The ratio of volume-motion coefficients (A/B) is equal to the ratio of RC/AB signal amplitude during the isovolume maneuver
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(7) |
Measurements
For the Spacelab mission, all measurements were performed with the subjects sitting on a cycle ergometer (with the trunk approximately vertical) and holding two vertical handgrips, with the arms horizontal and slightly bent. The Euromir subjects were also seated for the pre- and postflight acquisitions, but as there was no seat during the mission, the astronauts were asked to adopt a position approximately similar to that used for the ground experiments. All subjects were instructed to perform relaxation maneuvers, which consisted of several tidal breaths followed by an inspiration to a midinspiratory capacity, at which point the airway was occluded and the subject relaxed; when mouth pressure reached a stable value, the subject was allowed to expire passively through the flow restrictor. A representative maneuver obtained in one astronaut at 0 Gz is illustrated in Fig. 1A, which shows RIP-calibrated RC and AB volumes above FRC and mouth pressure as a function of time. The sharp increase in pressure corresponds to relaxation against the closed valve, and the rapid decrease corresponds to opening of the valve. Figure 1B shows changes in RC and AB volumes during tidal breathing (dashed loops) and relaxation (left continuous curve) on a Konno-Mead plot.
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Data Analysis
For the Spacelab and Euromir experiments, the abdominal contribution to tidal breathing [Vab/(Vab + Vrc)] was computed from the tidal breaths preceding the relaxation maneuver. In addition, for the Euromir experiments, Vab/(Vab + Vrc) was also computed from tidal breaths recorded during a 3-min period of resting breathing with the mouthpiece. Relaxation curves were considered
satisfactory if 1) mouth pressure on
occlusion reached a stable value, and 2) mouth pressure, Vrc, and Vab
showed a smooth and quasi-exponential decrease. On satisfactory
relaxation curves, we computed the slope of the Konno-Mead plot
(Vrc/Vab) in the tidal volume range; in addition, we calculated
the rib cage (Crc) and abdominal (Cab) components of the total
respiratory system compliance as Vrc/P and Vab/P. For the
sake of brevity, Crc and Cab will be referred to as rib cage and
abdominal compliance.
A two-way analysis of variance was used to compare pre- and postflight sessions for the same subject. Scheffé's multiple-comparison procedure was used to test significance between Gz levels for subject's group. Except when stated otherwise, values are means ± SE. Significance was accepted at the P < 0.05 level.
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RESULTS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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All experiments and acquisitions were successfully performed, except data collection on Spacelab postflight day 0, which could not be carried out for logistical reasons.
Abdominal Contribution to Tidal Breathing
Figure 2 shows mean ± SD values ofVab/(Vab + Vrc) recorded in each session for
subjects M1 and
M2. Values obtained immediately before
the relaxation were similar to those obtained during periods of tidal
breathing with the mouthpiece, but the SD was smaller for the latter.
Weightlessness produced a marked increase in Vab/(Vab + Vrc)
in both subjects. Values recorded on ground before the flight and in
space did not change significantly over time, and values recorded
preflight and on postflight days 7, 12, 25 or 26, and
118 were not significantly different.
In contrast, values of Vab/(Vab + Vrc) recorded in the two
subjects on postflight day
1 (31 and 40% for
subjects
M1 and
M2, respectively) were significantly greater than those recorded both preflight and on subsequent
postflight days (on average: 21 and 23% for
subjects
M1 and
M2, respectively). In
subjects
S1-S3,
values of Vab/(Vab + Vrc) recorded 2, 4, and 9 days after
landing were not significantly different from preflight data.
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Figure 3 and Table
1 give average values for Vab/(Vab + Vrc) for all subjects; for subjects
S1-S3, pre- and postflight 1 Gz data have been pooled, but for
subjects M1 and
M2 only preflight 1 Gz data are provided. For
comparison, Fig. 3 also shows values of Vab/(Vab + Vrc)
obtained during two campaigns of parabolic flights (4, 12). Values of
Vab/(Vab + Vrc) were invariably greater in space than on
ground, and changes during spaceflights were qualitatively similar to
those observed during parabolic flights.
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Relaxation Curve
Only two subjects, one in each mission, produced satisfactory relaxation maneuvers. In subject S1, three relaxation maneuvers were satisfactorily performed preflight and on day 9 of the mission;Vrc/Vab was 3.39 ± 0.89 at 1 Gz and 1.57 ± 0.45 at 0 Gz. In subject
M1, 4 and 15 relaxation maneuvers were
valid on ground and in space (at least one adequate maneuver was
obtained on each mission day studied), respectively; Vrc/Vab
averaged 7.47 ± 2.46 at 1 Gz
and 1.07 ± 0.24 at 0 Gz, and there was no significant change during the mission. These values are presented in Fig. 4, together with those obtained in two
subjects during parabolic flights. Changes observed in short and
prolonged exposure to microgravity were qualitatively similar.
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Figure 5 shows individual values (±SD) for Crc and Cab in subjects S1 and M1. Microgravity produced a marked increase in Cab in the two subjects: in subject M1, Cab was 0.016 l/cmH2O at 1 Gz and 0.079 l/cmH2O at 0 Gz; corresponding values for subject S1 were 0.037 l/cmH2O at 1 Gz and 0.089 l/cmH2O at 0 Gz. On the other hand, Crc did not change significantly with changes in Gz.
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DISCUSSION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Up until now, the only reported data of chest wall mechanics in microgravity were obtained in eight normal subjects during three campaigns of parabolic flights. These studies showed that going from 1 to 0 Gz elicited 1) a dramatic increase in the contribution of the abdomen to tidal volume; 2) a reduction in the tidal expansion of the rib cage, particularly in its upper part; 3) a decrease in the slope of the Konno-Mead plot, with an increase in abdominal compliance; and 4) a reduction in FRC, which was entirely accounted for by the inward displacement of the abdomen. There was no consistent effect of 0 Gz on the temporal pattern of breathing, pulmonary resistance, and dynamic pulmonary compliance.
Although safety and logistical reasons did not allow us to repeat all these measurements in space, we found that prolonged exposure to microgravity also increased the abdominal contribution to tidal volume and decreased the slope of the relaxation curve on the Konno-Mead plot. The validity of these results, however, critically depends on the accuracy of the RIP calibration. In the parabolic flight and the Euromir experiments, the calibration procedure was the classic isovolume maneuver. During training of the astronauts for the Spacelab mission, however, it appeared unlikely that each of them could perform the maneuver properly. We decided, therefore, to use a multiple linear-regression technique for the RIP calibration (14). We used the integrated flow signal from the MBW, but, unlike Stagg et al. (14), we included both the inspiratory and expiratory phases of the breathing cycle, rather than the inspiratory phase alone, in the linear regression. We used a standard statistical criterion to exclude the breaths for which the multiple regression yielded volume-motion coefficients that were very large or very small (including negative values). Finally, we designed a method that weighted the mean volume-motion coefficients computed from each MBW by the goodness of the fit of individual breaths (Eqs. 2 and 3).
Measurements in two subjects indicated that the decrease in the slope of the relaxation curve at 0 Gz was entirely accounted for by an increase in Cab because Crc did not change significantly. In contrast, the rib cage becomes less compliant on going from the upright to the supine posture (1, 7, 10). Two mechanisms have been proposed for this observation (7). First, because of the different orientation of gravitational forces with respect to the body, the rib cage at end expiration is more elliptical in the supine than in the upright posture (16), which might hinder the movements of the rib cage joints and cause a decrease in Crc. Second, the distensibility of the cage might decrease in the supine posture because of the development of passive tension in the diaphragm (7, 8). These mechanisms, however, are not expected to play a significant role in microgravity. Compared with 1 Gz, the rib cage at end expiration adopts a more circular, rather than a more elliptical, shape (5) and, although some passive tension develops in the diaphragm at 0 Gz, it is much smaller than that elicited in the supine posture. We have previously reported that the end-expiratory transdiaphragmatic pressure at 0 Gz was ~2 cmH2O (4), whereas Agostoni and Rahn (2) have reported values of ~10 cmH2O in the supine posture. Therefore, on this basis, it is possible to understand why going from 1 Gz to 1 Gx and from 1 Gz to 0 Gz has different effects on Crc.
The increase in Cab observed in space is consistent with our previous observations during parabolic flights (4) and with the effects of a change from upright to supine posture (1, 7, 10). In the present studies, we measured the abdomen component of the total respiratory system compliance and not the actual compliance of the abdominal compartment. The relationships between volume and mouth pressure and volume and abdominal pressure may diverge at low lung volume when the diaphragm is passively stretched (1). As mentioned above, however, transdiaphragmatic pressure at FRC does not exceed ~2 cmH2O at 0 Gz (4), which is probably insufficient to make abdominal pathway compliance different from the actual compliance of the free abdominal wall.
The increase in Cab observed at 0 Gz and in the supine posture (1,
4, 7, 10) results primarily from release of passive tension in the
ventral abdominal wall. Because this tension is determined by abdominal
transmural pressure, changes in the orientation and magnitude
of the hydrostatic gradient should produce immediate changes in Cab
and, with it, in Vab/(Vab + Vrc). This is exactly
what we observed during parabolic flights (4). In contrast,
measurements in subjects
M1 and
M2 showed that Vab/(Vab + Vrc) was greater on postflight day
1 than on either preflight days or
subsequent postflight days.
Because Vab/(Vab + Vrc) is also determined by the distribution
of neural activation between the diaphragm and the rib cage inspiratory
muscles, this observation might indicate a change in the neural control
of respiratory muscles; for example, the reduction in the phasic
inspiratory activity of the scalene and parasternal intercostal muscles
that has been observed at 0 Gz (5)
might persist to some extent after the flight. This possibility, however, appears very unlikely, since previous studies involving postural changes (9) and partial immersion (13) have shown that
adjustments in neural activation between the diaphragm and the rib cage
inspiratory muscles are synchronized with changes in diaphragm length.
Alternatively, the increase in Vab/(Vab + Vrc) on postflight
day 1 might be due to a persistent change in abdominal compliance. Previous
studies in rats (11, 15) have shown that 5-8 days of
weightlessness produced atrophy of the soleus muscle, which has an
important antigravity function in rodents. The abdominal muscles in
humans also have a prominent postural function (3). They might,
therefore, undergo some degree of atrophy in weightlessness, which, in
turn, might increase the distensibility of the ventral abdominal wall.
It should be stressed, however, that there are no data that we are
aware of on the effects of muscle atrophy on abdominal compliance. In
addition, because changes in muscle mass occur gradually over time,
such changes would be expected to produce a progressive increase in Cab
and Vab/(Vab + Vrc) in space and a progressive decrease to
baseline values after landing, which was not readily observed in the
present experiments. Therefore, further studies involving more subjects
and repeated measurements during and after the flight are needed to
assess the effects of sustained weightlessness on the static
pressure-volume characteristics of the abdomen.
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ACKNOWLEDGEMENTS |
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We acknowledge the dedication of the crews of the Spacelab D-2 and Euromir-95 missions, of the members of the Microgravity User Support Center of Cologne, and the support of the European Space Agency.
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FOOTNOTES |
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This study was supported by the Belgian National Fund for Scientific Research, the Federal Office for Scientific Affairs (PRODEX contract), and the National Aeronautics and Space Administration contract NAS 9-17884. M. Wantier is a fellow of the Université Libre de Bruxelles.
Address for reprint requests: M. Paiva, Biomedical Physics Laboratory, CP 613/3, 808 Route de Lennik, B-1070 Brussels, Belgium (E-mail: mpaiva{at}ulb.ac.be).
Received 19 November 1997; accepted in final form 17 February 1998.
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REFERENCES |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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) and
M2 (
) during resting breathing.
Vertical dashed lines indicate launch
(day 0) and end of mission, respectively.
