Vol. 93, Issue 6, 2044-2052, December 2002
Effect of gravity and posture on lung mechanics
D.
Bettinelli1,
C.
Kays2,
O.
Bailliart3,
A.
Capderou4,
P.
Techoueyres2,
J. L.
Lachaud2,
P.
Vaïda2, and
G.
Miserocchi1
1 Dipartimento di Medicina Sperimentale, Ambientale
e Biotecnologie Mediche, Università di Milano-Bicocca, I-20052
Monza (MI), Italy; 2 Médecine
Aerospatiale, Université de Bordeaux, F-33076 Bordeaux;
4 Centre Chirurgical Marie Lannelougue, UPRES
EA2397, Université Paris XI, F-92350 Le Plessis Robinson; and
3 Hôpital Lariboisière, F-75010 Paris,
France
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ABSTRACT |
The
volume-pressure relationship of the lung was studied in six subjects on
changing the gravity vector during parabolic flights and body posture.
Lung recoil pressure decreased by ~2.7 cmH2O going from 1 to 0 vertical acceleration (Gz), whereas it increased by
~3.5 cmH2O in 30° tilted head-up and supine postures.
No substantial change was found going from 1 to 1.8 Gz.
Matching the changes in volume-pressure relationships of the lung and
chest wall (previous data), results in a decrease in functional
respiratory capacity of ~580 ml at 0 Gz relative to 1 Gz and of ~1,200 ml going to supine posture. Microgravity
causes a decrease in lung and chest wall recoil pressures as it removes
most of the distortion of lung parenchyma and thorax induced by
changing gravity field and/or posture. Hypergravity does not
greatly affect respiratory mechanics, suggesting that mechanical
distortion is close to maximum already at 1 Gz. The
end-expiratory volume during quiet breathing corresponds to the
mechanical functional residual capacity in each condition.
lung compliance; esophageal pressure; functional residual
capacity; interstitial pressure
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INTRODUCTION |
IN A PREVIOUS STUDY
(3), we evaluated how changes in the gravity vector
(Gz) obtained during parabolic flights affect chest wall
mechanics. In the present work, we show data gathered in the same
subjects previously studied to describe how similar changes modify the
elastic properties of the lung. The knowledge of the elastic features
of the chest wall and of the lung allows a full mechanical analysis of
the respiratory system. In particular, coupling the volume-pressure
curve of the lung and of the chest wall allows the mechanical
definition of functional residual capacity (FRC), corresponding to the
resting point of the respiratory system. This volume represents the end
of expiration during quiet breathing at 1 Gz; indications
from previous studies did not allow clarification of how
gravity-dependent changes would influence the end-expiratory point
(10, 11, 18, 30) in relation to changes in elastic properties of the respiratory system and of its two main components: the chest wall and the lung. This study also integrates previous information on how changes in gravity influence other aspects of the
respiratory function, such as regional ventilation and perfusion
(17, 20, 26, 33, 34) and diffusion capacity (32, 35,
37).
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METHODS |
Parabolic flights.
All experiments were done during three European Space Agency
(ESA)-Centre National d'Etudes Spatiales (CNES) campaigns of parabolic
flights in the period between October 1999 and April 2000. Each
campaign included 3 flight days; an Airbus A300 aircraft was used; and
each flight lasted 2.5-3 h and included 30 parabolas (90 parabolas
per campaign). During the parabolic flight, the vertical acceleration
vector Gz changes relative to steady horizontal flight
corresponding to the 1-Gz phase: during pull-up, an
acceleration of 1.8 Gz is reached; subsequently, reducing
engine thrust allows the aircraft to enter a free-falling parabolic
trajectory generating a 0-Gz phase, and, finally, during
pull-out another 1.8-Gz phase is reached. All three phases
lasted ~20 s.
Subjects.
Respiratory variables (lung volume and esophageal pressure) were
obtained during steady horizontal flight and during short periods of
microgravity and hypergravity in four male (age: 53 ± 2 yr;
weight: 74 ± 3 kg; height: 174 ± 1 cm) and two female (age:
42 ± 6 yr; weight: 52 ± 5 kg; height: 165 ± 4 cm)
subjects. The same subjects were also studied in ground experiments in
sitting and supine posture by use of the same equipment. The subjects were members of the experimental team, were nonsmokers, were in good
health, and had no report of pulmonary disease. The subjects were
trained to perform the respiratory maneuvers (see below); furthermore,
they took part in previous parabolic flight campaigns and were well
accustomed to the challenge of abruptly changing Gz several
times during each flight.
Experimental equipment and system.
Subjects were sitting in a body plethysmograph made of wood (empty
volume of 360 liters) equipped with a pneumotachograph and transducers
to measure pressure in the box and at the mouthpiece (Pm); the
mouthpiece was also provided with an electromagnetic shutter. We also
performed some parabolas with subjects off the transducers to evaluate
the dependence of transducer signals from acceleration. To minimize the
effect of changes in aircraft accelerations on both transducers, they
were oriented along the aircraft's transverse axis. Lung volumes were
measured by flow integration. Esophageal pressure (Pes) was derived
from a pressure transducer mounted on a Gaeltec CTO-2 catheter, 2 mm
external diameter (12). Transducer sensitivity was 5 µV · V
1 · mmHg1;
the linear pressure range was ±300 mmHg. Subjects advanced the catheter through the nose until the proper positioning of the esophageal probe was reached on the basis of preliminary experiments aiming at determining the best recording site. On average, the approximate location of the esophageal recording site was ~15 cm
below the jugular notch, which roughly corresponds to the apex of the
lung. The location of the esophageal transducer was chosen on the basis
of a minimal cardiac artifact and a stable pressure signal.
The pneumotachograph response was linear for flow rates compatible with
the respiratory maneuvers performed; the maximum error was ~5% at
high flow rates (~3 l/s).
All signals were acquired through a system made of an analog-to-digital
card (Digimétrie; 50 Hz/channel) and stored in a PC-DOS
"home-made" program allowing on-line plethysmograph pressure conversion into pneumotachograph flow and volume by integration and
displaying on a video screen present lung volume, pressure variables,
and Gz. The software also allowed processing of an off-line
preliminary data analysis.
All the data were also stored on an analog tape recorder as a backup,
as were the live comments from the researchers during the flight.
Calibration.
Before takeoff, calibration of the plethysmograph was done by use of a
2-liter syringe. A syringe volume control was made for each subject
during the flight. Pressure transducer calibration for body box
pressure and Pm was carried out by using a water manometer.
Cabin pressure tends to decrease during the ascending phase relative to
level flight and to increase during the descending phase. In terms of
volume signal, the plethysmograph would overestimate lung volume during
the ascending phase and underestimate lung volume during the descending
phase. Cabin pressure was manually corrected during the parabola, and
over 30 parabolas we checked that the overall change in lung volume
during the 0-Gz phase due to mismatch in pressure
correction averaged 
0.029 ± 0.27 liters, 0.6% of vital
capacity (VC) (a nonsignificant underestimate).
Gaeltec transducers were calibrated by using a special calibration
chamber in which pressure could be set by water manometers; sensitivity
of transducers and zero drift at atmospheric pressure were carefully
noted. Sensitivity of the transducer was independent of temperature,
whereas zero drift was slightly dependent on temperature. Zero value
corresponding to body temperature was obtained on withdrawing the probe
at the end of each experimental session. These zero values were then
used to correct Pes previously recorded.
Protocols for in-flight experiments.
Subjects were sitting inside the plethysmograph and breathing through
the mouthpiece. During 0 Gz exposure, there was a tendency for the subject to float up in the air because of the changing trajectory of the aircraft; to counteract such an inertia-dependent phenomenon, the subject was kept strapped at the thighs and feet; other
loose bands around the arms kept them in a natural position parallel to
the chest. During level flight, a check was performed to ensure regular
recording of all variables. The time frame for data acquisition during
respiratory maneuvers started in the last minute of level flight and
lasted 2 min as follows: level flight (1 Gz, 1 min),
pull-up (1.8 Gz, 20 s), injection (0 Gz,
20 s), pull-out (1.8 Gz, 20 s).
Subjects were instructed to perform different respiratory maneuvers
within each phase, as shown by the experimental record of Fig.
1. After few control breaths, thoracic
gas volume (TGV) was measured close to the end-expiratory volume by
closing the mouthpiece shutter and performing inspiratory and
expiratory efforts against closed airways for 3 s (panting
maneuver, indicated as phase 1 in Fig. 1). In the records of
Pm and Pes, one can easily detect the oscillations referring to the
panting maneuver. Occasionally TGV was measured also at the end of the
2-min time frame. Subsequently, the subjects performed either of these
two maneuvers: 1) volume-pressure curve of the lung. The
subjects inspired to 70% of VC, indicated by phase 2 in
Fig. 1, and expired slowly down to residual volume (RV), indicated by
phase 3. Lung volume and Pes recorded during slow expiration
allowed determination of the volume-pressure relationships of the lung.
During such maneuvers, the alveolar pressure may be considered
atmospheric and, therefore, Pes = PL, where Pes represents an estimate of pleural surface pressure (Ppl) and
PL is the elastic recoil pressure of the lung.
2) VC maneuver (not shown in Fig. 1). The subjects inspired
up to TLC and expired relatively rapidly down to RV.
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Fig. 1.
Experimental records of lung volume, mouth pressure (Pm),
esophageal pressure (Pes), and vertical acceleration (Gz)
during the respiratory maneuvers performed at 1, 1.8, and 0 Gz. Relevant phases are pointed out: 1, panting
maneuver to measure total gas volume; 2, inspiration at
established lung volume; followed by 3, slow expiration
maneuver to determine the volume-pressure curve of the lung.
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Total number of parabolas necessary to gather a complete set of data
ranged, for each subject, from 15 to 20.
Protocol for ground experiments.
Ground experiments were performed on the same subjects adopting the
same general protocols and equipment in sitting, supine posture, and
30° tilted head-up relative to supine. The change in posture was
obtained by leaning the plethysmograph backward; this implied that legs
remained as in the sitting posture.
Data analysis.
TGV, computed from Boyle's law is given by TGV 
Pc · (
V/Pm), where Pc is in-flight cabin
pressure and V/Pm is the ratio of change in thoracic volume to
change in alveolar pressure during panting maneuvers. This ratio was
inferred as the slope of the linear regression between the two
variables. Before the regression was executed, the drift affecting the
volume of the panting maneuvers was subtracted so we generally obtained
regression coefficients near 0.99, ensuring an accurate TGV measurement.
Lung volume recorded throughout the time frame also displayed a drift
due to increasing temperature inside the plethysmograph. Digital
reading of lung volumes was obtained after correction for the volume
drift between two successive TGV values, assuming a linear drift with
time. Pes data were corrected for the zero drift on withdrawal of the
catheter at the end of the session. A "moving average filter" was
employed to reduce high-frequency noise in the pressure records.
At 1.8 Gz, whenever possible, we preferred data gathered
during the pull-up phase because the Gz vector remained
more steady.
At least five lung volume-pressure relationships were obtained in each
subject at 1, 1.8, and 0 Gz, in supine and in 30° tilted head-up position. For each condition, the Pes values corresponding to
the same lung volume were averaged. A statistical approach was adopted
to appropriately characterize the dependence of volume-pressure curves
on gravity vector and posture. We adopted a generalized linear
regression model (19) that represents a powerful tool to
evaluate the dependence of an experimentally measured variable on
multiple factors and therefore allows a simultaneous comparison of
several regressions.
More specifically, we considered Pes (p) as depending on volume (V),
the experimental condition, and an error term (e) as defined by
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(1)
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where 
and 
are the matrixes of slope and
intercept coefficients of the model, determined by the multiple linear
regression method, and 
is the "dummy variable" that takes
into account the gravity and/or posture condition. The program uses a
recursive algorithm based on minimizing the sum-of-squares corrected by the reciprocal of the standard deviation squared of the scatter points
(weighted-least-squared regression, Ref. 19). This
correction was necessary because of statistic heteroscedasticity of the
samples. Statistic tests (Student's t-test) were executed
to verify the significance of the differences in slope (reciprocal of
lung compliance) and/or in position (intercept) of the lung
volume-pressure curves by changing gravity and posture.
A further statistical analysis was carried out to verify the
significance of VC, total lung capacity (TLC), RV, and FRC by ANOVA for repeated measures followed by the Student-Newman-Keuls posttest (95% confident interval).
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RESULTS |
Figure 2 and Table
1 summarize the data of RV, VC and TLC
(= RV + VC). No significant differences were found in RV; VC was significantly decreased in supine and 30° tilted head-up postures (~550 ml, ~10% VC) relative to 1 Gz on ground; the
decrease in VC accounted for a similar decrease in TLC.
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Fig. 2.
Average residual volume (RV), vital capacity (VC), and
total lung capacity (TLC = RV + VC) of the 6 subjects studied
at 1 Gz in flight (1 Gz-fl) and on ground (1 Gz-gr), at 1.8 and 0 Gz, and in supine and
30° tilted head-up postures. Bars denote SE. §Significant
differences relative to 1 Gz on ground (1-way ANOVA for
repeated measures).
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Figure 3 reports the average
volume-pressure curves of the lung from all the subjects in the various
conditions. The relationship was displaced to the right going from 1 Gz (pooling data on ground and in flight) to 0 Gz. No substantial change was observed going from 1 to 1.8 Gz, whereas a leftward shift occurred in supine and 30°
tilted head-up postures. The changes in the position of the
volume-pressure curve are reflected in the corresponding changes in the
Pes values estimated at 40% VC that are reported in Fig. 4A (and Table
2). Relative to 1 Gz
(pooled on-ground and in-flight data), Pes became significantly less
subatmospheric at 0 Gz (by ~2.7
cmH2O), whereas it became significantly more subatmospheric in supine and 30° tilted head-up postures (by ~3.5
cmH2O). No change was observed going from 1 to 1.8 Gz. Lung compliance (Fig. 4B and Table
3), calculated from 20 to 40% VC, was
found to significantly decrease, relative to 1 Gz (pooled
on-ground and in-flight data) only in supine posture.
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Fig. 3.
Average volume-pressure lung curves for the 6 subjects
studied at 1 (solid line), 1.8 (dotted line), 0 Gz (short
dashed line), supine (dash-dotted line), and 30° tilted head-up (long
dashed line). At 1 Gz, pooled data for in-flight and
on-ground experiments are shown. Volume is expressed as % of VC. Bars
are SE. Ppl, pleural surface pressure.
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Fig. 4.
A: average Ppl at 40% VC at 1, 1.8, 0 Gz, and supine and 30° tilted head-up postures.
B: average lung compliance in the volume range 20-40%
VC at 1, 1.8, 0 Gz, and supine and 30° tilted head-up
postures. Bars denote SE. §Significant differences (1-way ANOVA for
repeated measures) relative to 1 Gz (on-ground and
in-flight pooled data).
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Table 2.
Esophageal pressure of each subject measured on volume-pressure curve
at 40% VC and differences relative to 1 Gz
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Figure 5 displays the lung
volume-pressure curves of the subjects obtained in the various
conditions revealing individual differences. In subject F, 0 Gz exposure did not result in a clear rightward shift of
the lung volume-pressure curve. Furthermore, in subject D,
no difference in compliance was observed between 1 Gz
(pooled on-ground and in-flight data) and supine and 30° tilted
head-up postures.
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Fig. 5.
Individual volume-pressure lung curves of the 6 subjects studied at
1 (solid line), 1.8 (dotted line), 0 Gz (short dashed
line), supine (dash-dotted line), and 30° tilted head-up (long dashed
line). Volumes are expressed as % of VC. At 1 Gz, pooled
data for in-flight and on-ground experiments are shown.
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In one subject, we compared in-flight results obtained by the classic
esophageal balloon technique and by the Gaeltec transducer probe. No
differences were found when superimposing the lung volume-pressure curves obtained by the two techniques. This finding confirms that the
two methods provide similar results, as previously found
(31), also considering the intrinsic noise of the Pes due
to heart rate. We also compared the volume-pressure curves at 1 Gz obtained with the catheter at different times, up to
1 h apart, and found no difference, indicating that the response
characteristic of the miniaturized probe remains constant over time.
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DISCUSSION |
This paper provides data on how lung recoil pressure is affected
by varying the gravity vector by means of parabolic flights and body
position. The present results integrate previous data (3)
gathered from the same subjects concerning the effect of changing
gravity on the mechanical properties of the chest wall. Adding the
present to the previous data allows a mechanical analysis of the
respiratory system when exposed to changing posture and gravity, in
particular to the microgravity environment that characterizes spaceflights.
Static lung volumes.
Despite some intra- and interindividual variability, on the average no
significant difference in RV, VC, and TLC was found on comparing 1 Gz in-flight to 1 Gz on-ground data (Fig. 2).
This finding suggests that the mechanical properties of the lung are not altered by repeated exposure to changing gravity vector between 0 and 1.8 Gz. The effects of changing lung volume and body
position are similar to those reported in previous studies, in
particular as far as the decrease in VC observed in supine posture
(1, 11), reflecting a corresponding decrease in TLC (Fig.
2). We also observed no difference in RV after acute switching to
supine posture, in line with previous studies (11, 18,
30). However, our finding of no change in RV after acute
exposure to microgravity contrasts with the significant decrease
(~300 ml) observed in sustained microgravity (11). A
possible explanation of the difference might reside in the progressive
increase in intrathoracic and intrapulmonary blood volume in sustained microgravity.
Volume-pressure curves of the lung.
As shown in Fig. 3, the volume-pressure relationship of the lung seems
to be affected by changing either Gz or posture. In Fig.
6, we attempt an interpretation of the
observed changes based on the following considerations. In the gravity
field, lung distortion results in a vertical gradient of alveolar size,
and the alveoli in the less dependent regions being more inflated
(15); accordingly, a vertical gradient of tissue
recoil pressure also exists, determining a vertical gradient of Ppl
(27). The indirect evidence of such deformation is given
by the shape of the washout curves of inert gases, in particular the
change in slope from phase III to phase IV in head-up (4, 6,
9) and supine postures (17, 21). The elastic
properties of the lung are commonly described by its volume-pressure
relationship; however, one should observe that total lung volume is
plotted as a function of local Ppl measured in the mid portion of the
esophagus: this is the case for the 1 Gz and supine curves
shown in Fig. 6. The deformation undergone by the lung in the gravity
field should be reduced in weightlessness, as suggested by the decrease
in slope of phase III and in height of phase IV of the Ar and
N2 washout curves (17, 26). Therefore, in
microgravity, one may assume that all lung regions should be more
uniformly expanded; as a consequence, regional and total lung volume
are the same percentage of VC. Accordingly, a one-to-one correspondence
between regional volume and regional pleural pressure should define
more accurately the intrinsic elastic properties of the lung (curve
labeled "Regional Lung Volume" in Fig. 6). The differences between
the regional percentage expansion and the overall lung volume for
similar pleural pressure reflect lung distortion.
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Fig. 6.
Effect of lung distortion on the volume-pressure curve.
At 1 Gz and in supine posture (solid and dash-dotted line,
respectively), alveolar size decreases from top to
bottom, and the ordinate refers to %VC of total lung
volume. In microgravity (dashed line), the gravitational unloading
largely abolishes the vertical gradient in alveolar volume, and
therefore the regional and total lung volume are the same percentage of
VC; the end-expiratory (end-exp) point is indicated ( ).
At the end-expiratory pleural pressures in 1 Gz and supine
posture, total lung volume ( and  ,
respectively) corresponds to a smaller percentage of VC relative to
regional volume after gravitational unloading ( and
 , respectively).
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At 0 Gz, the end-expiratory Ppl value is 1.9
cmH2O, corresponding to ~30% regional VC (Fig. 6, ).
At 1 Gz, the end-expiratory Ppl is 6.4 cmH2O,
corresponding to ~40% VC (Fig. 6, ). Note that, at this same
value of Ppl, the regional volume would be 55% VC (Fig. 6, ). In
supine posture, the end-expiratory Ppl value is 3.8
cmH2O, corresponding to only 15% VC (Fig. 6, ); at this
Ppl value, the regional volume would be as high as 35% regional VC
(Fig. 6, ). Switching from 0 to 1.8 Gz would cause changes in percentage total lung volume similar to those found when
going from 0 to 1 Gz because of similarity of the
volume-pressure curve of the lung (Fig. 3) and end-expiratory Ppl
values (Table 4). In particular, at 1 Gz (and also at 1.8 Gz,) the lower total lung
volume compared with regional midthoracic volume (~40 vs. 55% VC,
respectively) suggests that alveolar units below the esophageal pressure recording site (~15 cm below the lung apex) must be less inflated. This finding is in line with the increase in slope of phase
III and IV of the washout curves of inert gases going from 0 to 1 Gz in upright posture (17, 26). Previous data
(26) showed that the hypergravity condition, relative to 1 Gz, induced a further increase in the slope of phases III
and IV of the washout curves for Ar and N2, an increase in
cardiogenic oscillation of O2, CO2, and closing
volume, indicating a further increase in uneven distribution of blood
perfusion and regional lung volume, namely greater expansion of apical
alveoli and greater collapse at the base of the lung. Because
end-expiratory pleural pressure, and therefore regional lung volume,
are similar at 1 and 1.8 Gz, one may hypothesize that the
increase in nonuniformity of regional lung volume involves the most
dependent region of the lung.
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Table 4.
Volume and pressure at the end of expiration and at FRC (% VC)
obtained as crossing point of lung and chest wall volume-pressure
curves of each subject
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In supine posture, the overall end-expiratory volume is about half of
the regional volume in microgravity (15 vs. 35% VC, respectively),
suggesting a greater degree of lung distortion. In particular, this
indicates that in supine posture, the degree of deflation of the
regions below the esophagus (dorsal regions) is greater than that
occurring at 1 and 1.8 Gz in upright posture (basal
regions). This is consistent with a further increase in slopes of phase
III and IV in washout curves (17) and a larger closing
volume (21) when comparing supine to upright at 1 and 1.8 Gz.
The lung compliance in the volume range 20-40% VC was only
significantly decreased in the supine posture compared with 0, 1, and
1.8 Gz, and 30° tilted head-up posture. These findings are in line with previous observations on dynamic lung compliance on
changing Gz (10, 18) and posture from upright
to supine (22, 23) and on static lung compliance (2,
36).
Other factors could be considered to affect differently the
volume-pressure curve of the lung on changing Gz or
posture. Despite a relatively homogeneous lung expansion (17,
26) and blood perfusion (17, 20, 33, 34), an
increase in pulmonary blood volume has been documented (32,
35) in weightlessness and, in turn, this was shown to influence
the alveolar geometry (5). The latter should not impact
greatly on lung recoil, because surface tension is physiologically
relatively low in the volume range 20-40% VC (16).
An increase in blood volume was shown to reduce lung recoil at
lung volume below 25% VC, although changing blood volume has little
effect on static lung recoil pressure (13, 14).
Furthermore, lung recoil pressure also depends on airways smooth
muscles contraction (7, 8, 24, 25); however, it seems
unlikely that changes in bronchomotor tone are so rapid to be in phase
with abrupt changes in Gz.
Volume-pressure curves of the respiratory system.
Because the lung and the chest wall are arranged in parallel, the total
pressure exerted by the respiratory system (Prs) in relaxed conditions
is given by Prs = PL + Pw = Palv, where
PL and Pw are the recoil pressure of the lung and chest
wall, respectively, and Palv is the alveolar pressure. Both lung and
chest wall recoil pressures are derived from Ppl by appropriate
respiratory maneuvers. Respiratory mechanics assume that local Ppl
recording can be representative for the whole structure being analyzed,
either the lung or the chest wall; therefore, one considers the
volume-pressure curves of lung and chest wall as reflecting their
average "functional" mechanical behavior.
Figure 7 presents the average
volume-pressure curves of the lung and of the chest wall obtained for
the same subjects in a previous study (3). The lung and
chest wall volume-pressure curves cross at the resting volume of the
respiratory system (FRC) where one has Pw = PL = Ppl
and therefore Palv = 0.
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Fig. 7.
Average volume-pressure relationships of the lung matched
with the average chest wall curves: at 1 (solid line), 1.8 (dotted
line), 0 Gz (dashed line), and supine (dash-dotted line).
The crossing point of the lung and chest wall curves defines the
equilibrium condition of the respiratory system (FRC). Volume is
expressed as % of VC. At 1 Gz, pooled data for in-flight
and on-ground experiments are shown.
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Going from 1 to 0 Gz results in a strong expiratory
effect on the volume-pressure curve of the chest wall (3)
in the volume range below 40% VC (rightward shift). Because of the
concomitant rightward shift of the volume-pressure curve of the lung,
the resting volume decreases from 41 to 29% VC (~580 ml). Note that FRC would drop to ~17% VC (a further loss of ~500 ml) if the lung volume-pressure curve were not displaced to the right at 0 Gz. Therefore, the reduction in lung recoil pressure at 0 Gz has an inspiratory effect on lung-chest wall
interaction. The decrease in resting volume of the respiratory system
going from 1 to 0 Gz found in the present study, on the
basis of a mechanical analysis of chest wall-lung coupling, compares
well with the average decrease in end-expiratory volume of 390 ± 150 ml reported in previous studies (10, 11, 18, 30).
The data presented in Fig. 7 allow critical reconsideration of the
frequently proposed similarity between supine posture and microgravity
exposure. In fact, although the changes in chest wall mechanical
behavior are qualitatively similar (rightward shift of the curve and
increase in compliance), the changes in overall elastic properties of
the lung are opposite. Going from upright (1 Gz) to supine
posture results in an expiratory effect on the volume-pressure curve
both of the chest wall and of the lung, causing FRC to drop to as low
as ~16% VC (1,200 ml), as previously documented (11, 22,
23).
The volume-pressure curve of the chest wall is minimally affected by
hypergravity as exposure to 1 Gz generates a loading on the
chest wall that already brings its compliance close to its minimum, as
described in the previous study (3). Because the
volume-pressure curve of the lung is not significantly affected by
hypergravity, no significant changes in FRC were observed when in the
shift from 1 to 1.8 Gz. Our conclusions are in line with some data based on flowmeter measurement (18) but not with
others based on flowmeter measurement and inductive plethysmography
revealing an increase of ~200 ml (10, 30).
Lung volumes corresponding to the mechanical FRC are reported in Fig.
8 and in Table 4 to be compared with the
end-expiratory lung volumes measured by the panting maneuver (TGV) on
changing Gz and posture. As one can appreciate, there is a
very good matching between the two estimates, suggesting that
1) despite the limitation concerning the coupling of total
lung volume to esophageal pressure, the mechanical analysis
appears still valid for discussing the interaction between lung and
chest wall, and 2) subjects remain essentially relaxed at
end expiration during quiet breathing despite abrupt changes in either
Gz or posture.
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Fig. 8.
No significant difference was found between the average
mechanical FRC () and the end-expiratory lung volume
during spontaneous respiration () at 1, 1.8, 0 Gz, and supine posture. Bars indicate SE. §Significant
differences relative to 1 Gz (1-way ANOVA for repeated
measures).
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A comment is due concerning the modifications in mechanical properties
as observed during transient changes in gravity environment during
parabolic flights. The immediate change in lung volume, reflecting the mechanical equilibrium point between chest wall and
lung, suggests that major mechanical changes occur with a short time
constant. This is also confirmed by the relative reproducibility of the
measurements throughout the parabolas during the flight. One cannot
exclude that additional gradual changes may occur with a long time
constant, although the similarity in inert gas washout curves in short
and sustained microgravity would suggest that this effect may be negligible.
We used an innovative system to measure esophageal pressure consisting
of a thin catheter mounting miniaturized pressure transducers. The
placement of the probe via nasopharyngeal route was easily tolerated by
the subjects and was found much less uncomfortable than the esophageal
balloon. Therefore we believe that this method might represent a useful
tool to study respiratory mechanics in transient and sustained microgravity.
Interaction between Ppl and pulmonary interstitial pressure.
Some speculation is due concerning pulmonary interstitial fluid
dynamics in microgravity. At end expiration, the hydraulic pressure in
the pulmonary interstitium is physiologically subatmospheric (10
cmH2O), reflecting a complex interaction between
microvascular and/or interstitial fluid exchanges and parenchymal
forces (29). Pulmonary interstitial pressure was
found to become more subatmospheric with decreasing (more negative) Ppl
(28); for this reason, increasing lung volume should lead
to an increase in microvascular filtration. Microgravity is a
potential cause of interstitial edema in the lung because of capillary
recruitment that, in turn, increases microvascular filtration. Because
in microgravity Ppl values are less negative during the respiratory
activity (end-expiratory volume decreases by ~600 ml), this should be
considered a protective factor counteracting the potential edematous
condition due to increased capillary perfusion.
In summary, microgravity causes a decrease in lung recoil pressure
because it removes most of the distortion of lung parenchyma induced by
changing gravity field and/or posture. The rightward shift of the lung
and chest wall volume-pressure curves in microgravity results in a
decrease in FRC (~580 ml). Conversely, lung recoil pressure increased
in supine posture; the leftward shift of the lung volume-pressure curve
combined with the rightward shift of the chest wall curve results in a
much larger decrease in FRC (~1,200 ml) compared with microgravity.
Hypergravity does not greatly affect the volume-pressure curve of the
lung and of the chest wall, indicating that mechanical distortion is
close to maximum already at 1 Gz. The end-expiratory volume
during quiet breathing corresponds to the mechanical FRC in each condition.
| |
ACKNOWLEDGEMENTS |
We are grateful to Suzel Bussières, Isabelle Désormes,
and Daniel Rivière for precious support in the experimental
sessions during parabolic flights. A special thank to Prof. Daniela
Negrini who pioneered respiratory mechanics experiments aboard the
Soyuz spacecraft during the MIR '95 mission. We also thank the
Microgravity Division of the ESA, CNES, and NOVESPACE for the
organization of the parabolic flight campaigns, and in particular
Christophe Mora, local accountable for NOVESPACE.
| |
FOOTNOTES |
Pierre Vaïda was the recipient of grants 793/1999/CNES/7660 and
793/2000/CNES/8147. Giuseppe Miserocchi was the recipient of a research
grant from the Agenzia Spaziale Italiana.
Address for reprint requests and other correspondence: G. Miserocchi, Dipartimento di Medicina Sperimentale, Ambientale e Biotecnologie Mediche, Università di Milano-Bicocca, via Cadore, 48 I-20052 Monza (MI) (E-mail:
giuseppe.miserocchi{at}unimib.it).
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
August 30, 2002;10.1152/japplphysiol.00492.2002
Received 5 June 2002; accepted in final form 26 August 2002.
| |
REFERENCES |
1.
Agostoni, E,
and
Mead J.
Statics of respiratory system.
In: Handbook of Physiology. Respiration. Washington, DC: Am. Physiol. Soc, 1964, vol. I, p. 387-410, sect. 3, chapt. 13.
2.
Baydur, A,
Behrakis PK,
Zin WA,
Jaeger MJ,
Weiner JM,
and
Milic-Emili J.
Effect of posture on ventilation and breathing pattern during room air breathing and rest.
Lung
165:
341-351,
1987.
3.
Bettinelli, D,
Kays C,
Bailliart O,
Capderou A,
Techoueyres P,
Lachaud JL,
Vaïda P,
and
Miserocchi G.
Effect of gravity on chest wall mechanics.
J Appl Physiol
92:
709-716,
2002.
4.
Comroe, JH, Jr,
and
Fowler WS.
Lung function studies. VI. Detection of uneven alveolar ventilation during a single breath of oxygen.
Am J Med
10:
408-413,
1951.
5.
Conforti, E,
Fenoglio C,
Bernocchi G,
Bruschi O,
and
Miserocchi GA.
Morpho-functional analysis of the lung tissue in mild interstitial edema.
Am J Physiol Lung Cell Mol Physiol
282:
L766-L774,
2002.
6.
Craig, DB,
Wahba WM,
Don HF,
Couture JG,
and
Becklake MR.
"Closing volume" and its relationship to gas exchange in seated and supine positions.
J Appl Physiol
31:
717-721,
1971.
7.
Crawford, ABH,
Makowska M,
and
Engel LA.
Effect of bronchomotor tone on static mechanical properties of lung and ventilation distribution.
J Appl Physiol
63:
2278-2285,
1987.
8.
De Troyer, A,
Yernault JC,
and
Rodenstein D.
Effects of vagal blockade on lung mechanics in normal man.
J Appl Physiol
46:
217-226,
1979.
9.
Dollfuss, RE,
Milic-Emili J,
and
Bates DV.
Regional ventilation of the lung, studied with boluses of 133xenon.
Respir Physiol
2:
234-246,
1967.
10.
Edyvean, J,
Estenne M,
Paiva M,
and
Engel LA.
Lung and chest wall mechanics in microgravity.
J Appl Physiol
71:
1956-1966,
1991.
11.
Elliott, AR,
Prisk GK,
Guy HJB,
and
West JB.
Lung volumes during sustained microgravity on Spacelab SLS-1.
J Appl Physiol
77:
2005-2014,
1994.
12.
Evans, SA,
Watson L,
Cowley AJ,
Johnston IDA,
and
Kinnear WJM
Normal range for transdiaphragmatic pressures during sniffs with catheter mounted transducers.
Thorax
48:
750-753,
1993.
13.
Frank, NR.
Influence of acute pulmonary vascular congestion on recoiling force of excised cats' lung.
J Appl Physiol
14:
905-908,
1959.
14.
Frank, NR,
Radford EP, Jr,
and
Whittenberger JL.
Static volume-pressure interrelations of the lungs and pulmonary blood vessels in excised cats' lungs.
J Appl Physiol
14:
167-173,
1959.
15.
Glazier, JB,
Hughes JMB,
Maloney JE,
and
West JB.
Vertical gradient of alveolar size in lungs of dogs frozen intact.
J Appl Physiol
23:
694-705,
1967.
16.
Goerke, J,
and
Schürch S.
Mechanical properties of the alveolar surface.
In: The Lung. New York: Raven, 1991, p. 821-825.
17.
Guy, HJB,
Prisk GK,
Elliott AR,
Deutschman RA, III,
and
West JB.
Inhomogeneity of pulmonary ventilation during sustained microgravity as determined by single-breath washouts.
J Appl Physiol
76:
1719-1729,
1994.
18.
Kays C, Choukroun ML, Techoueyres P, Varenne P, and Vaïda
P. Lung mechanics during parabolic flights (Abstract). Proc
5th Eur Symposium Life Sci Res Space, 26 Sept.-1 Oct. 1993. Arcachon, France, Noordwijk, the Netherlands: European Space Agency,
1993, p. 0-3.
19.
Kleinbaum, DG,
Kupper LL,
and
Muller KE.
Applied Regression Analysis and Other Multivariable Methods. Boston, MA: PWS-KENT, 1988, p. 218-220.
20.
Lauzon, AM,
Elliott AR,
Paiva M,
West JB,
and
Prisk GK.
Cardiogenic oscillation phase relationships during single-breath tests performed in microgravity.
J Appl Physiol
84:
661-668,
1998.
21.
Le Blanc, P,
Ruff F,
and
Milic-Emili J.
Effects of age and body position on "airway closure" in man.
J Appl Physiol
28:
448-451,
1970.
22.
Lim, TPK,
and
Luft UC.
Alterations in lung compliance and functional residual capacity with posture.
J Appl Physiol
14:
164-166,
1959.
23.
Linderholm, H.
Lung mechanics in sitting and horizontal postures studied by body plethysmographic methods.
Am J Physiol
204:
85-91,
1963.
24.
Loring, SH,
Drazen JM,
Smith JC,
and
Hoppin FG, Jr.
Vagal stimulation and aerosol histamine increase hysteresis of lung recoil.
J Appl Physiol
51:
477-484,
1981.
25.
Ludwig, MS,
Dreshaj I,
Solway J,
Munoz A,
and
Ingram RH, Jr.
Partitioning of pulmonary resistance during constriction in the dog: effects of volume history.
J Appl Physiol
62:
807-815,
1987.
26.
Michels, DB,
and
West JB.
Distribution of pulmonary ventilation and perfusion during short periods of weightlessness.
J Appl Physiol
45:
987-998,
1978.
27.
Milic-Emili, J.
Topographical inequality of ventilation.
In: The Lung. New York: Raven, 1991, p. 1415-1422.
28.
Miserocchi, G,
Negrini D,
and
Gonano C.
Parenchymal stress affects interstitial and pleural pressures in in situ lung.
J Appl Physiol
71:
1967-1972,
1991.
29.
Negrini, D,
and
Miserocchi G.
Pulmonary interstitial fluid balance.
In: Gravity and the Lung, edited by Prisk GK,
Paiva M,
and West JB.. New York: Dekker, 2001, p. 255-268.
30.
Paiva, M,
Estenne M,
and
Engel LA.
Lung volume, chest wall configuration, and pattern of breathing in microgravity.
J Appl Physiol
67:
1542-1550,
1989.
31.
Panizza, JA,
and
Finucane KE.
Comparison of balloon and transducer catheters for estimating lung elasticity.
J Appl Physiol
72:
231-235,
1992.
32.
Prisk, GK,
Guy HJB,
Elliott AR,
Deutschmann RA, III,
and
West JB.
Pulmonary diffusing capacity, capillary blood volume, and cardiac output during sustained microgravity.
J Appl Physiol
75:
15-26,
1993.
33.
Prisk, GK,
Elliott AR,
Guy HJB,
Kosonen JM,
and
West JB.
Pulmonary gas exchange and its determinants during sustained microgravity on Spacelabs SLS-1 and SLS-2.
J Appl Physiol
79:
1290-1298,
1995.
34.
Prisk, GK,
Guy HJB,
Elliott AR,
and
West JB.
Inhomogeneity of pulmonary perfusion during sustained microgravity on SLS-1.
J Appl Physiol
76:
1730-1738,
1994.
35.
Vaïda, P,
Kays C,
Rivière D,
Tèchoueyres P,
and
Lachaud JL.
Pulmonary diffusing capacity and pulmonary capillary blood volume during parabolic flights.
J Appl Physiol
82:
1091-1097,
1997.
36.
Venturoli, D,
Semino P,
Negrini D,
and
Miserocchi G.
Respiratory mechanics after 180 days space mission (Euromir '95).
Acta Astonaut
42:
185-204,
1998.
37.
Verbanck, S,
Larsson H,
Linnarsson D,
Prisk GK,
West JB,
and
Paiva M.
Pulmonary tissue volume, cardiac output, and diffusing capacity in sustained microgravity.
J Appl Physiol
83:
810-816,
1997.
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