J Appl Physiol
89: 1239–1248, 2000.
highlighted topics
Physiology of a Microgravity Environment
Selected Contribution: Redistribution of pulmonary
perfusion during weightlessness and increased gravity
Received 28 April 2000; accepted in final form 22 June 2000
Glenny, Robb W., Wayne J. E. Lamm, Susan L. Bernard, Dowon An, Myron Chornuk, Sam L. Pool, Wiltz
W. Wagner, Jr, Michael P. Hlastala, and H. Thomas
Robertson. Selected Contribution: Redistribution of pulmonary perfusion during weightlessness and increased gravity.
J Appl Physiol 89: 1239–1248, 2000.—To compare the relative contributions of gravity and vascular structure to the
distribution of pulmonary blood flow, we flew with pigs on the
National Aeronautics and Space Administration KC-135 aircraft. A series of parabolas created alternating weightlessness and 1.8-G conditions. Fluorescent microspheres of varying colors were injected into the pulmonary circulation to
mark regional blood flow during different postural and gravitational conditions. The lungs were subsequently removed,
air dried, and sectioned into ⬃2 cm3 pieces. Flow to each
piece was determined for the different conditions. Perfusion
heterogeneity did not change significantly during weightlessness compared with normal and increased gravitational
forces. Regional blood flow to each lung piece changed little
despite alterations in posture and gravitational forces. With
the use of multiple stepwise linear regression, the contributions of gravity and vascular structure to regional perfusion
were separated. We conclude that both gravity and the geometry of the pulmonary vascular tree influence regional pulmonary blood flow. However, the structure of the vascular tree is
the primary determinant of regional perfusion in these animals.
gional blood-flow distribution (25). The pulmonary vascular system, however, is not infinitely compliant, and
the geometry of the vascular tree must also play a role
in perfusion distribution. To determine the relative
contributions of gravity and vascular structure to the
distribution of pulmonary blood flow, we flew with pigs
on the National Aeronautics and Space Administration
(NASA) KC-135 aircraft. The flight path was a series of
40 parabolas that created alternating weightlessness
and 1.8-G conditions. After a steady state was reached
in each gravitational condition, fluorescent microspheres of varying colors were injected into the pulmonary circulation to mark regional blood flow. The lungs
were subsequently removed, air dried, and sectioned
into small pieces. By analyzing the dye content in each
piece, we obtained high-resolution measurements of
pulmonary blood-flow distribution during 0-, 1-, and
1.8-G conditions in supine and prone postures.
METHODS
gradient down the lung
that distends dependent vessels and influences re-
The Animal Care Committees at the University of Washington, NASA, and Baylor University approved these animal
studies. Six juvenile pigs weighing between 23 and 25 kg
were studied. They were purpose bred and pathogen free (S
and S Farms, Ranchita, CA). All animals were cared for and
handled in accordance with the guidelines established by the
National Institutes of Health (17).
Experiments were performed on the NASA KC-135 microgravity research aircraft. A series of concave and convex
parabolas were flown to provide alternating weightless and
increased gravitational conditions (Fig. 1). The trajectory of
Address for reprint requests and other correspondence: R. Glenny,
Karolinska Hospital and Institute, Dept. of Anesthesia and Intensive Care, Bldg. F2, 00, SE-171 76 Stockholm, Sweden (E-mail:
[email protected]).
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.
blood flow; microgravity; fluorescent microspheres; supine;
prone
GRAVITY INDUCES A HYDROSTATIC
http://www.jap.org
8750-7587/00 $5.00 Copyright © 2000 the American Physiological Society
1239
Downloaded from http://jap.physiology.org/ by 10.220.32.246 on June 18, 2017
ROBB W. GLENNY,1,2 WAYNE J. E. LAMM,1 SUSAN L. BERNARD,1 DOWON AN,1
MYRON CHORNUK,2 SAM L. POOL,3 WILTZ W. WAGNER, JR,4,5,6
MICHAEL P. HLASTALA,1,2 AND H. THOMAS ROBERTSON1,2
Departments of 1Medicine and 2Physiology and Biophysics, University of Washington School of
Medicine, Seattle, Washington 98195; 3National Aeronautics and Space Administration-Johnson
Space Center, Houston, Texas 77058; Departments of 4Anesthesiology, 5Physiology/Biophysics,
and 6Pediatrics, Indiana University School of Medicine, Indianapolis, Indiana 46202
1240
PULMONARY PERFUSION DURING WEIGHTLESSNESS
the aircraft produced three consecutive gravitational phases
of 0-, ⬃1.5-, and 1.8-G conditions. The duration of weightlessness and 1.8-G conditions varied slightly with each parabola but were at least 23 s long. A series of 10 parabolic
loops were flown with 3–5 min of level flight between each
series. A total of 40 parabolas were flown during each flight
and were completed over ⬃1 h. Normal gravitational conditions existed before and in between the 10-loop parabolic
cycles. The cabin was pressurized to an average of 626 Torr
but varied with changes in altitude.
Physiological monitoring and data recording. Physiological and environmental variables were monitored and recorded continuously with the use of a portable PowerPC
Macintosh (Apple Computer) with a MacLab/8s (ADInstruments, Castle Hill, NSW, Australia). All analog signals were
sampled at 40 signals/s. Airway (Paw) and esophageal pressures (Pes) were measured with Valdyne CD-15 transducers
(Northridge, CA). Intravascular pressures were measured
with Abbott Critical Care Systems transducers (Chicago, IL).
Electrocardiogram, end-tidal CO2 concentrations, and arterial O2 saturation were measured with the use of a Pace-tech
3100 (Clearwater, FL). The distance from the midesophagus
to the back of the animal was continuously measured with a
linear Hall-effect sensor (Allegro, A3515EUA). The Halleffect sensor was located at the tip of the esophageal catheter
and generated a signal proportional to the distance between
it and a magnetic source fixed on the back of the animal. The
sensor and magnet were kindly provided by Lucent Medical
Systems (Seattle, WA). The magnitude of the gravitational
force was continuously measured with a calibrated straingauge force transducer (FT03C, Grass Instruments, West
Warwick, RI). An electronic signal was generated at the
beginning and end of important events to correlate later with
gravitational conditions. Cardiac outputs were measured by
thermal dilution using a Baxter SAT-2 cardiac output monitor (Baxter-Edwards Health Care, Irvine, CA). The direction
of the gravitational vector relative to the floor of the aircraft
Downloaded from http://jap.physiology.org/ by 10.220.32.246 on June 18, 2017
Fig. 1. Flight trajectory and gravitational forces during parabolic
flight on the NASA KC-135. Repeating parabolic loops create alternating weightless and 1.8-G conditions lasting ⬃25 s each. A series
of 10 parabolic loops were flown, with 3–5 min of level flight between
each series; 40 parabolas were flown for each flight (based on similar
schematic shown in Ref. 2a).
was measured with the use of a digital level. The cabin
pressure was measured with an analog altimeter. These last
three measurements were recorded by hand at appropriate
times. Arterial blood-gas samples were collected in glass
syringes during scheduled conditions, placed on ice, and
analyzed with an IRMA blood-analysis system (Diametrics
Medical, St. Paul, MN) after we returned to base.
Fluorescent microspheres. Eleven different colors of polystyrene microspheres were obtained from Molecular Probes
(Eugene, OR) and Bangs Laboratory (Indianapolis, IN). The
colors used were blue, blue-green, green, yellow-green, yellow, orange, orange-red, red, crimson, scarlet, and carmine.
The microspheres had diameters of 15.5 ␮m and a specific
gravity of 1.05. The microspheres were suspended in a 17.3%
dextrose solution with a specific gravity of 1.05 so that they
would not settle during parabolic flights. An antibacterial
agent, Thimerisol, was added to the suspension. Just before
each flight, the microspheres were sonicated, vortexed, and
drawn into labeled syringes. Each syringe contained one
million microspheres of a single color in a 1.25-ml volume.
The order in which the colors were injected was random and
different for each flight except for the last two injections,
which were always blue or carmine (see below).
Preflight. The animals were not fed for 12 h before experimentation but had free access to water. Ninety minutes
before flight time, each pig was sedated with an intramuscular injection of ketamine (20 mg/kg) and xylazine (2 mg/kg).
The animals were anesthetized with a continuous intravenous infusion of thiopental, and the infusion rate was adjusted to suppress spontaneous respirations and maintain a
surgical plane of anesthesia. A tracheotomy was performed,
and the animals were ventilated with 30% O2 using an
Auto-vent 3000 (Life Support Products, St. Louis, MO). Tidal
volumes of 210–240 ml were used, and the rate was set to
hyperventilate the animals with a targeted arterial PCO2 of
⬃33 Torr. Paw were measured from a side port of the tracheal tube. O2 saturation was measured with an optical
sensor clipped to the ear of the animal. Internal carotid,
internal jugular, and pulmonary arterial catheters were
placed, and their positions were verified by pressure tracings.
The internal jugular catheter had three separate ports and
lumens. The most distal lumen, used for microsphere injections, was placed just proximal to the right atrium. An
esophageal balloon catheter with a Hill-effect transistor at
the tip was passed so that the tip was in a retrocardiac
position and the balloon was partially inflated. A magnet was
taped over the region of the thoracic spine that produced the
largest signal from the esophageal Hill-effect transistor. This
signal provided an estimate of the mediastinal shift in the
sagittal plane during changes in the gravitational conditions.
Each pig was secured in a wheeled carrier constructed
with a padded steel sling that completely enclosed the animal. The bed of the carrier could be rotated between supine
and prone postures. Paw and tidal volumes were measured
before and after the animal was secured in place with padding and slings. Padding was adjusted so that the animal did
not slide during prone to supine rotation and so that Paw did
not increase when the top sling and pads were secured. All
pressure transducers were fixed to the outer frame of the
carrier at the level of the left atrium. A digital level was
attached to the frame to provide measurement of the gravitational vector through all measurement conditions. The
anesthetized animal was moved to the aircraft in the wheeled
carrier 30 min before takeoff.
In-flight. Paw, Pes, central venous pressure, pulmonary
artery pressure, and systemic artery pressure were continuously monitored and recorded throughout the flight (Fig. 2).
1241
PULMONARY PERFUSION DURING WEIGHTLESSNESS
The animals were ventilated with 30% O2 and given stacked
breaths after each 1.8-G condition to minimize atelectasis.
Anesthesia was adjusted as necessary.
We injected a different fluorescent color while each animal
was in the supine or prone posture during weightlessness,
1-G conditions, and 1.8-G conditions (Table 1). Three repeat
microsphere injections using different colors were made during predetermined postural and gravitational conditions (Table 1). These repeat injections were separated in time and
always performed with a number of intervening postural and
gravitational changes. The repeat measures were used to test
the reproducibility of the methods and to measure changes in
perfusion over time. Three animals began the protocol in the
prone posture, and the remaining three animals began the
protocol supine. The animals were rotated to the alternate
postures during level flight in between the 10 parabolic
cycles. A total of 32 parabolic loops were required to complete
the protocol. At the conclusion of the protocol, blue- and
carmine-colored microspheres were injected while the animals were in the left lateral posture during weightlessness
Table 1. Microsphere injections for each animal and
the number of pieces obtained from each lung
Number of Injections per Gravitational
Condition and Posture
Supine
Prone
Animal
Number
0G
1G
1.8 G
0G
1G
1.8 G
Number of
Lung Pieces
1
2
3
4
5
6
2
1
2
1
2
1
1
2
1
2
1
2
2
1
2
1
2
1
1
2
1
2
1
2
2
1
2
1
2
1
1
2
1
2
1
2
1,344
1,083
1,474
1,248
1,629
1,319
Downloaded from http://jap.physiology.org/ by 10.220.32.246 on June 18, 2017
Fig. 2. Sample of the digitized signals recorded continuously
throughout the flight. Example shows a full parabolic loop, including
weightlessness, a transition phase, and 1.8 G over 100 s. Airway
(Paw), esophageal (Pes), central venous (CVP), pulmonary arterial
(Ppa), and systemic arterial (Psa) pressures are shown. Temporal
course of G conditions (G-force) is recorded at bottom. Marker tracing
indicates the beginning (A) and end (B) of a microsphere injection
and time when cardiac output was measured (C).
and 1.8-G conditions as part of a separate study. A total of 11
different microsphere colors were injected into each animal.
Because the gravitational states were transient, timing of
the microsphere injections was critical. Injections were performed to allow blood flow to come to a new steady state while
also assuring that the microspheres had sufficient time to
lodge before the end of each gravitational condition. Before
each injection, the syringe containing the appropriate microsphere color was vortexed, sonicated, and attached to the
internal jugular catheter, and the dead space of the catheter
was filled with the microspheres. Five seconds after the onset
of the targeted gravitational state, the microspheres were
injected over the next 5 s and then followed by a rapid 10-ml
saline flush. This routine provided a minimum of 10 s for the
microspheres to lodge in the capillary bed before the end of
each gravitational state. The onset and end of each microsphere injection were marked with electrical signals to confirm timing.
Cardiac outputs and arterial blood-gas samples were obtained in the same postures and gravitational states in which
microspheres were injected but during different parabolas.
Cardiac outputs were measured by injecting 5 ml of iced
saline 5 s after the onset of the targeted gravitational state.
Samples for arterial blood-gas measurements were drawn
10 s after the onset of the targeted gravitational state and
placed on ice.
Five thousand units of heparin and 3 mg/kg of papaverine
were given intravenously at the conclusion of the protocol to
aid in flushing the lungs postmortem.
Postflight. The arterial blood-gas samples were analyzed
shortly after we landed. The animals were deeply anesthetized, exsanguinated, and given an overdose of intravenous
thiopental. A sternotomy was performed, large-bore catheters were placed in the pulmonary artery and left atrium,
and the thoracic aorta was tied off. The lungs were perfused
with 2% Dextran (molecular weight of 74,000) in normal
saline until the effluent was clear of blood, removed from the
chest, and allowed to dry inflated at a Paw of 25 cmH2O.
When dry, the lungs were coated with Kwik foam (DAP,
Dayton, OH), suspended vertically in a plastic-lined, squared
box, and embedded in rapidly setting urethane foam (2 lb of
polyol and isocyanate, International Sales, Seattle, WA) to
create a rigid form to which a three-dimensional coordinate
system was applied. The foam block was sliced and cut into
⬃2-cm3 cubes. Foam adhering to lung pieces was removed,
and each lung piece was weighed and assigned a threedimensional coordinate and lobe designation.
The fluorescent signals for each color were determined by
extracting the fluorescent dyes from each piece with an
organic solvent (Cellosolve, Sigma-Aldrich, St. Louis, MO)
and then measuring the concentration of fluorescence in each
1242
PULMONARY PERFUSION DURING WEIGHTLESSNESS
supine and prone postures separately to see if there are any
differences between them.
In the presence of gravity, regional blood flow is diverted
from nondependent to dependent regions. This effect of gravity is superimposed on the regional flow determined by vascular structure. The amount of blood flow redistributed is
determined by the vertical position of the region in the lung.
Dependent pieces gain flow, and nondependent pieces lose
flow. With the use of standard notation from linear regression, the blood flow to piece in the presence of gravity is
Q̇(i) ⫽ a ⫹ b ⫻ Q̇struct(i) ⫹ c ⫻ height(i) ⫹ e(i)
(1)
where a is the intercept, b is the regression coefficient for
vascular structure, height(i) is the vertical distance from the
most dependent lung region to piece i, c is the regression
coefficient for height up the lung, and e(i) is a residual term
not fit by the linear relationship. The residual term represents local changes in local flow over time and error introduced by the microsphere methods. Again, the effects of
vascular structure and gravity are assumed to be independent and additive.
With the use of stepwise multiple regression, the relative
contributions of each component in Eq. 1 are determined
during different postures and gravitational conditions. The
goodness of fit, r2, is used to quantify the proportion of
variability in Q̇(i), which is explained by vascular structure
[Q̇struct(i)] and height(i).
RESULTS
Physiological measures. All intravascular pressure,
Paw, and Pes varied with the gravitational force. The
time lag between changes in gravity and changes in the
measured pressures (Fig. 2) was small relative to the
time scale of interest. The physiological parameters
reported in Table 2 were averaged over the final 7–9 s
of each postural and gravitational condition across all
parabolas. The esophageal balloon was inadvertently
overinflated in all animals, causing artificially high
Pes measurements. Despite this error, the difference in
Pes between postures and gravitational conditions is
accurate. In all animals and in all postures, central
venous pressure, Paw, and Pes decreased significantly
during the transition from 1.8 to 0 G and increased as
the gravitational force increased (Table 2). Paw decreased by ⬃0.5 cmH2O for each step drop in gravita-
Table 2. Average physiological measurements obtained during the last 7–9 s
of each postural and gravitational condition
Prone
Mean Psa, mmHg*
Mean Ppa, cmH2O*
CVP, cmH2O†
HR, beats/min
Q̇, l/min*
Mean Paw, cmH2O†
Mean Pes, cmH2O*
Supine
1.8 G
1.0 G
0G
0G
1.0 G
1.8 G
113 ⫾ 7
38 ⫾ 4
14 ⫾ 4
107 ⫾ 11
2.92 ⫾ 0.50
4.7 ⫾ 0.9
15.9 ⫾ 5.0
112 ⫾ 10
34 ⫾ 3
9⫾3
105 ⫾ 7
2.86 ⫾ 0.38
3.9 ⫾ 0.6
15.7 ⫾ 5.0
100 ⫾ 6
30 ⫾ 3
1⫾3
111 ⫾ 9
3.15 ⫾ 0.51
3.6 ⫾ 0.7
15.5 ⫾ 4.6
102 ⫾ 7‡
32 ⫾ 3
0⫾5
108 ⫾ 8
3.04 ⫾ 0.43
3.6 ⫾ 0.7
17.2 ⫾ 5.9
107 ⫾ 7‡
31 ⫾ 2‡
9⫾2
104 ⫾ 8
2.96 ⫾ 0.45
3.8 ⫾ 0.5
21.5 ⫾ 8.7‡
106 ⫾ 7‡
31 ⫾ 3‡
12 ⫾ 5
109 ⫾ 6
2.66 ⫾ 0.36‡
4.5 ⫾ 0.7
26.6 ⫾ 9.0‡
Values are means ⫾ SD. Psa, systemic arterial pressure; Ppa, pulmonary arterial pressure; CVP, central venous pressure; HR, heart rate;
Q̇, cardiac output; Paw, airway pressure; Pes, esophageal pressure. * Significant difference between postures, but difference is dependent on
gravitational force (interaction between posture and gravitational force is significant). † Significant change with increasing gravitational
force. The effect of posture is similar within each gravitational condition. ‡ Significant difference between postures at same gravitational
force. (Significance was declared for P ⬍ 0.05.)
Downloaded from http://jap.physiology.org/ by 10.220.32.246 on June 18, 2017
sample (8). Spillover from adjacent colors was corrected with
the use of a matrix inversion method (C. D. Schimmel, D.
Frazer, and R. W. Glenny, unpublished observations). The
spatial coordinates of each lung piece were adjusted to account for the angle of the aircraft during each gravitational
state and any tilt in the lungs during the foaming process.
The data set for each animal consisted of an x, y, and z
coordinate, lobe designation, weight, and relative flow for
each lung piece in each postural and gravitational condition.
The relative flow to each lung piece in each condition was
determined by dividing the fluorescent signal by the weight
of each lung piece and normalizing it to the mean. We refer to
the relative blood flow per piece simply as blood flow.
To minimize observed flow heterogeneity caused by artifact or measurement noise, pieces weighing ⬍80 mg were
excluded, eliminating uncertainty in flow and in weight.
Between 22 and 25 lung pieces were excluded from each data
set, resulting in 1,083–1,629 lung pieces per data set.
Statistics. All data are presented as means ⫾ SD except
where noted. The goodness of the linear fit, r2, between two
variables was used to quantify the strength of the relationship. We used ANOVA for statistical comparisons with a
randomized complete block design of posture and gravitational force. This design allowed us to evaluate the main
effects of posture and gravitational conditions and the interaction effect between them. Statistical significance was declared for P ⬍ 0.05. The coefficient of variation (CV ⫽ SD/
mean) was used to characterize the heterogeneity of
perfusion during different postural and gravitational conditions.
We propose a simplified model to characterize the regional
distribution of pulmonary blood flow. The model has only two
determinants of regional perfusion: vascular geometry and
the hydrostatic gradient imposed by gravity. These two components are independent and additive. In the absence of
gravity, local blood flow is determined by the geometry of the
vascular tree, including arterial, capillary, and venous systems. Although the geometry of the pulmonary vascular tree
is dynamic and influenced by local Paw, intravascular pressures, and the thoracic cage, we consider it a fixed influence
in this model.
We consider that each lung piece i receives a given amount
of blood flow due to the structure of the vascular tree.
Q̇struct(i) is the blood flow to piece i, in the absence of gravity.
We estimate Q̇struct(i) from the flow to piece i obtained during
weightlessness. Repeat measures are averaged within each
piece to provide a best estimate. We could construct a model
incorporating body posture as well, but we chose to analyze
1243
PULMONARY PERFUSION DURING WEIGHTLESSNESS
Table 3. Blood-gas measurements obtained during each postural and gravitational condition
Prone
PO2, Torr
PCO2, Torr
A-aDO2, Torr*
Supine
1.8 G
1.0 G
0G
0G
1.0 G
1.8 G
121.6 ⫾ 18.7
26.4 ⫾ 2.3
21.1 ⫾ 17.0
126.4 ⫾ 13.5
28.4 ⫾ 1.7
13.7 ⫾ 12.1
122.9 ⫾ 18.0
27.6 ⫾ 2.0
13.0 ⫾ 16.7
119.1 ⫾ 13.9
27.7 ⫾ 1.6
16.6 ⫾ 14.5
126.0 ⫾ 13.9
28.1 ⫾ 1.7
14.4 ⫾ 14.5
106.7 ⫾ 19.2
27.1 ⫾ 2.4
35.0 ⫾ 18.6
Values are means ⫾ SD. * Significant change with increasing gravitational force. The effect of posture is similar within each gravitational
condition (P ⬍ 0.05).
to be small, and most of the deviation from a perfect
correlation of 1.0 was due to temporal changes in
perfusion (8).
During weightlessness, regional blood flows between
supine and prone postures were highly correlated, with
an r2 value of 0.84. This correlation was only slightly
less than the average r2 value of 0.89 observed with
repeat measures in the same posture and gravitational
condition. This was expected because there was no
gravitational force influencing blood-flow distribution
in the two postures.
Regional blood flows in supine and prone postures
during 1.8-G conditions were also well correlated, with
an average r2 value of 0.72 (Fig. 4).
The effect of gravity on blood-flow distribution can be
most clearly demonstrated by averaging flows within
isogravitational planes. Averaging the flows nullifies
variability within isogravitational planes and highlights the vertical distribution of flow (Fig. 5). Slopes
were determined with a least squares linear fit of
average blood flow per isogravitational plane as a function of height up the lung. We chose to exclude the
traditional “zone 4” region and fit only points with
Fig. 3. Repeat measures of regional perfusion in 1 animal. Two
different colors of microspheres were injected during similar postures and gravitational conditions, which were separated by 20 min
and 10 parabolic loops. Deviation from a perfect correlation of 1.0 is
due to temporal variability in perfusion and noise introduced by the
microsphere method.
Downloaded from http://jap.physiology.org/ by 10.220.32.246 on June 18, 2017
tional condition. Changes in systemic arterial pressure
and pulmonary arterial pressure differed from this
pattern. Systemic arterial pressure decreased by ⬃5
mmHg during weightlessness in the prone posture.
Pulmonary arterial pressure decreased ⬃4 cmH2O
with each step decrease in the gravitational force when
the animals were prone but did not change significantly when they were supine. Heart rate did not
change significantly. Cardiac output varied significantly with both posture and gravitational condition.
Compared with its position during weightlessness, the
esophagus did not move significantly when the animals
were prone. When supine, the esophagus moved an
average of 0.2 and 0.3 cm toward the spine during 1and 1.8-G conditions, respectively.
Gas exchange. Although alterations in gravitational
forces were too short to allow gas exchange to reach a
steady state, significant differences in gas exchange
were observed between gravitational conditions. Table
3 presents the blood-gas data and the calculated alveolar-arterial O2 difference (A-aDO2) for each posture
and gravitational condition. The A-aDO2 was calculated for each postural and gravitational condition
with the use of the alveolar gas equation (2), a respiratory quotient of 0.8, and the average cabin pressure
during a given gravitational condition. A-aDO2 increased significantly in both postures with increasing
gravitational force. A significant difference was not
observed between postures. The lowest arterial PO2
across all animals in all postures and gravitational
conditions was 83 Torr.
Regional blood flow. On average, the CV of perfusion
was 0.67, 0.64, and 0.66 in the prone posture during 0-,
1-, and 1.8-G conditions, respectively. The CV of perfusion averaged 0.71, 0.71, and 0.77 in the supine
posture during 0-, 1-, and 1.8-G conditions, respectively. There was a significant interaction between
posture and gravitational force. Perfusion heterogeneity was significantly less in the prone vs. supine
posture during 1- and 1.8-G conditions. Perfusion heterogeneity was not significantly different between
postures during weightlessness.
Regional blood flow measurements obtained during
the same posture and gravitational conditions were
highly correlated (Fig. 3), with an average r2 value of
0.89 across all animals. Because the microsphere injections were separated in time, differences in blood flow
to each piece were due to both temporal variability and
method error in the microsphere technique. Prior studies have demonstrated the microsphere method error
1244
PULMONARY PERFUSION DURING WEIGHTLESSNESS
increasing blood flow down the lung (Fig. 5). The vertical gradient of perfusion increased with increasing
gravitational forces in both postures. When prone, the
average slopes were ⫺0.134, ⫺0.138, and ⫺0.158 rela-
DISCUSSION
This is the first study to measure regional pulmonary perfusion with microspheres during weightlessness and increased gravitational force. The microsphere method provides high spatial resolution of
regional blood flow during different postures and gravitational forces within the same animal. The important
findings of this study are that both gravity and the
geometry of the pulmonary vascular tree determine
Fig. 5. Vertical distribution of blood flow during 0-, 1-, and 1.8-G conditions in 1 animal in the supine posture.
Slopes were determined from a least squares linear fit to blood flow as a function of height up the lung. We chose
to exclude the traditional zone 4 region and fit only points with increasing blood flow down the lung. Values are
means ⫾ SE. Dependent and independent variable axes have been interchanged so that the formats are similar to
previously published plots. Vertical gradient of perfusion was steeper at higher gravitational forces. Note that the
general pattern of blood flow increasing and then decreasing down the lung (zones 3 and 4) persists during
weightlessness.
Downloaded from http://jap.physiology.org/ by 10.220.32.246 on June 18, 2017
Fig. 4. Regional perfusion in opposing postures during increased gravitational forces. Regional blood flow was determined in supine and
prone postures during 1.8-G conditions in this animal. Despite changes
in posture and increased gravitational forces, high-flow pieces remain
high flow and low-flow pieces remain low flow. An effect of gravity can
be demonstrated when spatial information is added to the plot. Dorsal
lung pieces are colored blue, and ventral pieces are colored red. Note
that the pieces that have the greatest change in hydrostatic gradient
when rotated between postures have the greatest change in perfusion.
tive flow units/cm during 0-, 1-, and 1.8-G conditions,
respectively. In the supine posture, the average slopes
were ⫺0.120, ⫺0.124, and ⫺0.139 relative flow
units/cm during 0-, 1-, and 1.8-G conditions, respectively. The differences in the slopes, however, were not
statistically significant for either posture. The general
pattern of increasing and then decreasing blood flow
down the lung (Fig. 5) was observed in all animals in
both postures during weightlessness.
The relative influences of vascular structure and the
hydrostatic gradient were characterized using our linear model (Eq. 1). The fits of the blood-flow data to this
model are presented in Tables 4 and 5. Blood-flow data
were fit separately to four postural and gravitational
conditions: prone-1 G, prone-2 G, supine-1 G, and supine-2 G. The simple linear model provided excellent
fits to the data, accounting for roughly 90% of the
variability in regional perfusion. Regardless of posture
or gravitational condition, the structural component of
the model explained the majority of the variability in
regional blood flow. Height up the lung accounted for a
range of 1–4% of the variability in regional perfusion
during 1- and 1.8-G conditions.
1245
PULMONARY PERFUSION DURING WEIGHTLESSNESS
Table 4. Coefficients and goodness of fits to the linear model of Eq. 1
in the prone posture during 1- and 1.8-G conditions
Animal
Number
Intercept (a)
Structure
Coefficient (b)
Height
Coefficient (c)
1
2
3
4
5
6
Mean ⫾ SD
0.29
0.56
0.20
0.24
0.21
0.27
0.30 ⫾ 0.13
0.94
0.77
0.99
1.02
0.95
0.79
0.91 ⫾ 0.11
⫺0.02
⫺0.03
⫺0.19
⫺0.02
⫺0.14
⫺0.01
⫺0.07 ⫾ 0.08
1
2
3
4
5
6
Mean ⫾ SD
0.26
0.55
0.43
0.32
0.42
0.26
0.37 ⫾ 0.11
0.91
0.90
1.04
0.97
1.01
0.98
0.97 ⫾ 0.06
r2
Structure
Gravity
Total
0.91
0.75
0.95
0.90
0.94
0.81
0.87 ⫾ 0.08
0.01
0.06
0.01
0.02
0.00
0.00
0.02 ⫾ 0.02
0.92
0.80
0.96
0.93
0.94
0.81
0.89 ⫾ 0.07
0.91
0.82
0.83
0.93
0.85
0.95
0.88 ⫾ 0.05
0.01
0.08
0.06
0.03
0.03
0.02
0.04 ⫾ 0.03
0.92
0.91
0.89
0.96
0.88
0.97
0.92 ⫾ 0.04
1G
1.8 G
A negative height coefficient value indicates that blood flow decreases with increasing height up the lung.
regional blood flow. The vascular structure, however, is
the primary determinant of local perfusion in these
animals in both supine and prone postures.
The microsphere method for measuring regional
organ perfusion is an established and well-accepted
technique (23). It is based on the fundamental principle that labeled microspheres lodge within capillary beds in proportion to local blood flow. The microsphere method has been validated in the lung
with the use of both radiolabeled red blood cells (3)
and a “molecular microsphere” (15). A basic tenet is
that injected microspheres are inert and do not alter
local flow or resistance. It is argued that microspheres occlude only a fraction of capillaries and do
not, therefore, significantly change local vascular
resistance. However, there must be an upper limit to
the number of microspheres that can be injected
without causing hemodynamic changes. Pulmonary
artery pressures and cardiac outputs were unchanged over the course of this study, suggesting
that pulmonary vascular resistance was not altered
by a cumulative dose of 10 million microspheres.
Unfortunately, there is no literature in large animals to support the assumption that injecting microspheres does not alter vascular resistance. Using a
pump-perfused rat lung with a fully dilated circulation, we have recently shown that pulmonary vascular resistance increases 0.8% for every 100,000 microspheres injected (9). Extrapolating this data from
0.25-kg rats to 25-kg pigs suggests that more than 10
Table 5. Coefficients and goodness of fits to the linear model of Eq. 1
in the supine posture during 1- and 1.8-G conditions
Animal
Number
Intercept (a)
Structure
Coefficient (b)
Height
Coefficient (c)
1
2
3
4
5
6
Mean ⫾ SD
0.20
0.27
0.49
0.16
0.41
⫺0.04
0.25 ⫾ 0.19
0.86
0.94
0.81
0.99
0.89
1.10
0.93 ⫾ 0.10
⫺0.01
⫺0.03
⫺0.03
⫺0.02
⫺0.03
⫺0.01
⫺0.02 ⫾ 0.01
1
2
3
4
5
6
Mean ⫾ SD
0.78
0.10
0.10
0.10
⫺0.02
⫺0.04
0.17 ⫾ 0.31
1.02
1.09
1.01
1.03
1.10
1.13
1.06 ⫾ 0.05
r2
Structure
Gravity
Total
0.70
0.93
0.75
0.96
0.80
0.95
0.85 ⫾ 0.11
0.00
0.03
0.04
0.01
0.08
0.00
0.03 ⫾ 0.03
0.70
0.96
0.79
0.97
0.88
0.95
0.88 ⫾ 0.11
0.97
0.91
0.97
0.96
0.98
0.96
0.96 ⫾ 0.02
0.00
0.02
0.01
0.01
0.00
0.01
0.01 ⫾ 0.00
0.97
0.93
0.98
0.97
0.98
0.97
0.96 ⫾ 0.02
1G
1.8 G
⫺0.01
⫺0.03
⫺0.01
⫺0.01
⫺0.01
0.00
⫺0.01 ⫾ 0.01
A negative height coefficient value indicates that blood flow decreases with increasing height up the lung.
Downloaded from http://jap.physiology.org/ by 10.220.32.246 on June 18, 2017
⫺0.02
⫺0.04
⫺0.05
⫺0.03
⫺0.04
⫺0.02
⫺0.03 ⫾ 0.01
1246
PULMONARY PERFUSION DURING WEIGHTLESSNESS
higher spatial resolution in the next decade, Greenleaf
et al. (12) and Reed and Wood (21) uncovered more
perfusion heterogeneity within isogravitational planes.
These observations were later confirmed by Nicolaysen
and colleagues (18). Even higher spatial resolution
methods demonstrated that the spatial distribution of
perfusion has an underlying pattern within isogravitational planes that was not random (7) and was constant over days (10). Most recently, studies demonstrated that perfusion remains heterogeneous during
prolonged weightlessness (20).
One other study has measured perfusion heterogeneity during transient weightlessness (16). Amplitudes
of cardiac oscillations were measured in exhaled respiratory gases from seated humans during parabolic
flight. The amplitudes of the oscillations were nearly
abolished during weightlessness compared with normal gravity. The authors concluded that virtually all
the topographical inequality of blood flow seen under
1-G conditions was abolished during short periods of 0
G. These conclusions differ significantly from our current findings. Although we used supine and prone
animals instead of upright humans, it is difficult to
ascribe the large discrepancies to postural or species
variation. Another possibility is that the exhaled gas
method underestimated the true heterogeneity of pulmonary perfusion.
Recent studies performed on the NASA Spacelab
Life Sciences-1 (SLS-1) (20) measured the amplitude of
exhaled CO2 from human subjects during prolonged
weightlessness and normal gravity. The amplitudes of
the cardiac oscillations decreased by 40% during
weightlessness compared with at 1 G. The authors
concluded that the decrease in amplitude of the cardiac
oscillation may have been due to the absence of gravity
and that the residual cardiac oscillations indicated
persistent perfusion heterogeneity. These results agree
more with our current findings. The gravitational influence may be larger in upright humans compared
with supine and prone quadrupeds. In addition, measurement of exhaled CO2 oscillations may underestimate perfusion heterogeneity. It is therefore difficult to
compare the two studies quantitatively.
We think that our current data, data from studies
performed in the 1960s, and data from SLS-1 can all be
explained by one concept. In this model, regional pulmonary perfusion is determined primarily by the geometry of the vascular tree and the effects of gravity
are superimposed on this underlying structure. When
the spatial resolution of our current data is combined
into isogravitational planes comparable to those of
previous studies (27) (Fig. 5), they look very similar.
Without the increased resolution of our measurements,
it is not possible to observe the underlying pattern of
perfusion heterogeneity. Data from humans in prolonged weightlessness demonstrated that, although
perfusion heterogeneity decreased compared with normal gravitational conditions, the majority of perfusion
heterogeneity persisted (20). The importance of vascular geometry does not preclude the waterfall effects
first proposed by Riley et al. (22) and later conceptual-
Downloaded from http://jap.physiology.org/ by 10.220.32.246 on June 18, 2017
million microspheres would need to be injected to
raise vascular resistance 1%. In a lung in which the
vasculature is not fully dilated, this increased resistance can be easily offset by vascular recruitment.
Our greatest concern in this study was that the
pulmonary circulation would not have time to establish
a steady state before microspheres were injected. The
lack of significant lag between central vascular pressures and change in gravitational forces shows that the
compartmental shifts of blood volume are remarkably
rapid. Microspheres, however, lodge in the capillary
bed, and we did not know how rapidly the microspheres
would transit to their final wedged positions after
abrupt changes in gravitational conditions. To determine the temporal response of the pulmonary capillary
bed to rapid changes in flow, we used a pump-perfused
lung preparation (14). We found that pulmonary capillary recruitment reached a new steady state within
4 s of a step change in flow. In the present study,
microspheres were injected 5 s after the onset of a new
gravitational condition. Given that steady state is
reached in the peripheral pulmonary capillary bed in
⬍4 s, we are confident that pulmonary blood flow to
small lung pieces can reach a new steady state before
microsphere injections.
A secondary concern was that the transit time from
right atrium to the capillary bed would be too long to
allow all of the microspheres to lodge before the onset
of a new gravitational state. We estimated the mean
transit time in our animals by dividing the pulmonary
arterial blood volume by the cardiac output. Pulmonary blood volume is ⬃10% of total circulating blood
volume, and about one-quarter of this resides in the
pulmonary artery (6). Pigs have a circulating blood
volume of 76 ml/kg (5), and we estimated the volume of
blood in the pulmonary arteries of our pigs to be 50 ml.
With a cardiac output of 3 liter/min, the mean transit
time from the main pulmonary artery to the capillary
bed will be 1 s. If four cardiac cycles are needed to wash
microspheres out of the right ventricle, we estimate
that 4–5 s are required for the majority of microspheres to reach the capillary bed following injection
just proximal to the right atrium. Because of the heterogeneity of transit times in the lung (4), transit times
to low-flow regions may be up to 4–5 s. Hence, 8–9 s
may be required for the last microspheres to lodge in
the lowest flow regions. All of our microsphere injections were completed more than 10 s before the next
change in gravitational force. From this, we conclude
that regional perfusion was determined during a constant gravitational force.
Our data on regional blood flow fit with concepts of
pulmonary perfusion that have been evolving since
Harvey demonstrated that the entire output of the
right ventricle flowed through the lungs and Malpighi
described the pulmonary microcirculation in 1661.
Only 40 years ago, lung function was thought to be
uniformly distributed. The first group to measure regional lung function in the early 1960s was understandably “surprised” to find a vertical distribution of
ventilation and perfusion (26). Using methods with
PULMONARY PERFUSION DURING WEIGHTLESSNESS
We thank Dr. Amado Ruiz-Razura, Dr. Aladin Boriek, Dr. Joe
Rodarte, Noel Skinner, Bob Williams, Judy Rickard, and Christine
Murchinson for invaluable assistance and experience.
REFERENCES
1. Albert RK. Turnabout may be more than fair play. Crit Care
Med 28: 571–572, 2000.
2. Anthonisen NR and Fleetham JA. Ventilation: total, alveolar,
and dead space. In: Handbook of Physiology. The Respiratory
System. Gas Exchange. Bethesda. MD: Am. Physiol. Soc., 1987,
sect. 3, vol. IV, chapt. 7, p. 113–130.
2a.Bailliart O, Capderou A, Cholley BP, Kays C, Rivière D,
Téchoueyres P, Lachaud J-L, and Vaı̈da P. Changes in lower
limb volume in humans during parabolic flight. J Appl Physiol
85: 2100–2105, 1998.
3. Beck KC and Rehder K. Differences in regional vascular
conductances in isolated dog lungs. J Appl Physiol 61: 530–538,
1986.
4. Clough AV, Haworth ST, Hanger CC, Wang J, Roerig DL,
Linehan JH, and Dawson CA. Transit time dispersion in the
pulmonary arterial tree. J Appl Physiol 85: 565–574, 1998.
5. Dingley J, Foex BA, Swart M, Findlay G, DeSouza PR,
Wardrop C, Willis N, Smithies M, and Little RA. Blood
volume determination by the carbon monoxide method using a
new delivery system: accuracy in critically ill humans and precision in an animal model. Crit Care Med 27: 2435–2441, 1999.
6. Fishman AP. Pulmonary circulation. In: Handbook of Physiology. The Respiratory System. Circulation and Nonrespiratory
Functions. Bethesda, MD: Am. Physiol. Soc., 1985, sect. 3, vol. I,
chapt. 3, p. 93–166.
7. Glenny RW. Spatial correlation of regional pulmonary perfusion. J Appl Physiol 72: 2378–2386, 1992.
8. Glenny RW, Bernard S, and Brinkley M. Validation of fluorescent-labeled microspheres for measurement of regional organ
perfusion. J Appl Physiol 74: 2585–2597, 1993.
9. Glenny RW, Bernard SL, and Lamm WJ. Hemodynamic
effects of 15-␮m diameter microspheres on the rat pulmonary
circulation. J Appl Physiol 89: 499–504, 2000.
10. Glenny RW, McKinney S, and Robertson HT. Spatial pattern of pulmonary blood flow distribution is stable over days.
J Appl Physiol 82: 902–907, 1997.
11. Glenny RW and Robertson HT. Regional differences in the
lung: a changing perspective on blood flow distribution. In:
Complexity in Structure and Function of the Lung, edited by
Hlastala M and Robertson H. New York: Decker, 1998, p. 461–
481.
12. Greenleaf JF, Ritman EL, Sass DJ, and Wood EH. Spatial
distribution of pulmonary blood flow in dogs in left decubitus
position. Am J Physiol 227: 230–244, 1974.
13. Hughes JMB, Glazier JB, Maloney JE, and West JB. Effect
of lung volume on the distribution of pulmonary blood flow in
man. Respir Physiol 4: 58–72, 1968.
14. Jaryszak EM, Baumgartner WA Jr, Peterson AJ, Presson
RG Jr, Glenny RW, and Wagner WW Jr. Measuring the
response time of pulmonary capillary recruitment to sudden flow
changes. J Appl Physiol 89: 1233–1238, 2000.
15. Melsom MN, Flatebo T, Kramer-Johansen J, Aulie A,
Sjaastad OV, Iversen PO, and Nicolaysen G. Both gravity
and non-gravity dependent factors determine regional blood flow
within the goat lung. Acta Physiol Scand 153: 343–353, 1995.
16. Michels DB and West JB. Distribution of pulmonary ventilation and perfusion during short periods of weightlessness. J Appl
Physiol 45: 987–998, 1978.
17. National Research Council. Institute for Laboratory Animal Research. Guide for the Care and Use of Laboratory Animals. Washington, DC: Natl. Acad. Press, 1996. [http://oacu.
od.nih.gov/regs/guide/guidex.htm]
18. Nicolaysen G, Shepard J, Onizuka M, Tanita T, Hattner
RS, and Staub NC. No gravity-independent gradient of blood
flow in the lung. J Appl Physiol 63: 540–545, 1987.
19. Prisk GK, Elliot 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.
20. Prisk GK, Guy HJB, Elliot AR, and West JB. Inhomogeneity
of pulmonary perfusion during sustained microgravity on SLS-1.
J Appl Physiol 76: 1730–1738, 1994.
Downloaded from http://jap.physiology.org/ by 10.220.32.246 on June 18, 2017
ized as zonal conditions by West and colleagues (28).
We think that heterogeneities in vascular resistance
can produce a range of zonal conditions within isogravitational planes (11) rather than being vertically
stacked in the lung.
When averaged within isogravitational planes, blood
flow increases and then decreases down the lung (Fig.
5). This pattern persists regardless of posture or gravitational force. Traditionally, the increase in perfusion
is attributed to the hydrostatic gradient down the lung
that distends dependent vessels, and increased interstitial pressures cause vessels to collapse in dependent
regions (13). The observation that this pattern of increasing and then decreasing perfusion persists during
weightlessness suggests that vertical pressure gradients may not explain this phenomenon in normal gravitational conditions. The relatively large extent of zone
4 conditions that we observed (Fig. 5) is probably related to the thin dorsal caudal lung in the pig, which is
consistently compressed in the supine position under
anesthesia. Although we hyperinflated the lungs before each microsphere injection, it may be that this
lung region remained underinflated relative to its state
in an awake, prone animal.
Our simple model of pulmonary blood flow uses only
two determinants to explain perfusion heterogeneity:
vascular structure and an imposed gravitational force.
This model accounts for ⬃90% of the observed heterogeneity (Tables 4 and 5). The remaining 10% of perfusion heterogeneity not captured by the model may be
caused by other factors, such as hypoxic pulmonary
vasoconstriction and dependent lung compression. We
attempted to minimize the effect of hypoxic pulmonary
vasoconstriction by using a higher fraction of inspired
O2 and overventilating the animals to maintain a
slight alkalosis. Transient lung compression due to
pleural pressure gradients could not be avoided during
microsphere injections. Atelectasis, which persisted
across all gravitational states, would be erroneously
considered vascular structure. We minimized this complication as much as possible by using recruitment
maneuvers after each high-gravitational parabola.
Acknowledging the combined importance of vascular
geometry and gravity on the regional distribution of
pulmonary blood flow is an important advancement in
our understanding of lung physiology. The shared effect of gravity can no longer be considered the mechanism matching regional ventilation and perfusion (19).
Heterogeneous perfusion must somehow be matched
by local ventilation within isogravitational planes. The
mechanism by which postural changes improve gas
exchange in patients with lung injury may be more
complicated than we suspected (1). Studies exploring
determinants of regional lung injury must consider the
geometry of the pulmonary vascular tree in addition to
hydrostatic gradients.
1247
1248
PULMONARY PERFUSION DURING WEIGHTLESSNESS
21. Reed JH and Wood EH. Effect of body position on vertical
distribution of pulmonary blood flow. J Appl Physiol 28: 303–
311, 1970.
22. Riley RL, Permutt S, Said S, Godfrey M, Cheng TO, Howell
JBL, and Shepard RH. Effect of posture on pulmonary deadspace in man. J Appl Physiol 14: 339–344, 1959.
23. Rudolph AM and Heymann MA. The circulation of the fetus
in utero. Methods for studying distribution of blood flow, cardiac
output and organ blood flow. Circ Res 21: 163–184, 1967.
25. West JB. Regional differences in the lung. Chest 74: 426–437,
1978.
26. West JB. Respiratory Physiology: People and Ideas. New York:
Oxford University Press, 1996.
27. West JB and Dollery CT. Distribution of blood flow and ventilation-perfusion in the lung, measured with radioactive CO2.
J Appl Physiol 15: 405–410, 1960.
28. West JB, Dollery CT, and Naimark A. Distribution of blood
flow in isolated lung: relation to vascular and alveolar pressures.
J Appl Physiol 55: 1341–1348, 1964.
Downloaded from http://jap.physiology.org/ by 10.220.32.246 on June 18, 2017