EDITORIAL FOCUS
Rapid alveolar liquid removal by a novel convective
mechanism
P. M.
Wang,
Y.
Ashino,
H.
Ichimura, and
J.
Bhattacharya
Departments of Medicine and Physiology and Cellular
Biophysics, College of Physicians and Surgeons and St.
Luke's-Roosevelt Hospital Center, Columbia University, New York,
New York 10019
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ABSTRACT |
Although alveoli clear liquid by active transport, the presence
of surface-active material on the alveolar surface suggests that
convective mechanisms for rapid liquid removal may exist. To determine
such mechanisms, we held the isolated blood-perfused rat lung at a
constant alveolar pressure (PA). Under videomicroscopy, we
micropunctured a single alveolus to infuse saline or Ringer solution in
~10 adjacent alveoli. Infused alveoli were lost from view. However,
as the infused liquid cleared, the alveoli reappeared and their
diameters could be quantified. Hence the time-dependent determination
of alveolar diameter provided a means for quantifying the time to
complete liquid removal (Ct) in single alveoli. All determinations were obtained at an PA of 5 cmH2O. Ct, which related inversely
to alveolar diameter, averaged 4.5 s in alveoli with the fastest
liquid removal. Injections of dye-stained liquid revealed that the
liquid flowed from the injected alveoli to adjacent air-filled alveoli.
Lung hyperinflations instituted by cycling PA between 5 and
15 cmH2O decreased Ct by 50%.
Chelation of intracellular Ca2+ prolonged
Ct and abolished the inflation-induced
enhancement of liquid removal. We conclude that when liquid is injected
in a few alveoli, it rapidly flows to adjacent air-filled alveoli. The
removal mechanisms are dependent on alveolar size, inflation, and
intracellular Ca2+. We speculate that removal of liquid
from the alveolar surface is determined by the curvature and
surface-active properties of the air-liquid interface.
surfactant; pulmonary edema; cell calcium; Laplace equation; inflation
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INTRODUCTION |
IN PULMONARY EDEMA,
the failure of gas exchange results from alveolar liquid accumulation.
Alveoli are capable of removing the liquid, although the only known
removal mechanism is by active salt transport across the alveolar
membrane (4). However, active transport mechanisms are
slow, effecting liquid transport with a time course of hours. Faster
mechanisms of alveolar liquid removal that are protective in the early
stages of pulmonary edema may coexist but remain unidentified.
We considered the possibility that surface-active forces in the
alveolus effect rapid convective removal of alveolar liquid. Although
airway continuity provides a potential route for convective liquid
removal from the alveoli to the distal airways, potential flow-supportive forces remain unidentified. Nevertheless, classic evidence for the formation of edema foam in the airway
(18) and nonuniform alveolar liquid accumulation
(20) suggests that alveolar-airway liquid flow exists in
early pulmonary edema. However, such flows have not been directly confirmed.
Classically, the distribution of alveolar liquid forces is understood
on the basis of the Laplace equation for curved surfaces (3, 12,
19). However, it is not clear that air-liquid interface curvatures undergo sufficient alterations in alveolar liquid filling as
to generate convective forces for alveolar liquid removal. We
considered that if convective mechanisms exist, then liquid should
drain rapidly from a filled alveolus. On the other hand, lack of
convection should be evident in the retention of alveolar liquid to the
extent that it is slowly removed by active transport. We tested this
hypothesis by microinjecting liquid into small groups of alveoli on the
surface of the constantly inflated lung. Our findings indicate that
alveoli thus filled eject the liquid in seconds, thereby indicating the
existence of a novel convective mechanism for alveolar liquid removal.
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METHODS |
Lung microscopy.
The isolated blood-perfused rat lung was prepared as previously
described (2). Briefly, lungs excised from anesthetized Sprague-Dawley rats were pump perfused with autologous rat blood at 14 ml/min at 37°C. Pulmonary arterial and left atrial pressures were
held at 10 and 5 cmH2O, respectively. At baseline, the
lungs were constantly inflated at an alveolar pressure (PA)
of 5 cmH2O. The lungs were positioned on a vibration-free
table and superfused with saline at 37°C to prevent drying. The lung
surface was viewed through a stereomicroscope (SZH, Olympus) as
previously described (2), and images taken with a
microscope-mounted video camera (T123A CDD camera, NEC) connected to a
video timer (VTG-33, FOR.A Company) were displayed on a video monitor
(PM-127 REK.B, Ikegami). All images were recorded with a videocassette
recorder (BR-S378U, JVC) at 30 frames/min.
Alveolar microinjection.
For each imaged alveolus, the microscope focus was set at the maximum
alveolar diameter. For alveolar micropuncture and microinfusion, we
used previously described methods (2). Briefly, we
prepared beveled glass micropipettes with a tip diameter of 5 µm. We
backfilled each micropipette with the injection solution, and then,
using a micropipette manipulator (Leitz), we micropunctured single
alveoli that we viewed with a stereomicroscope and microinfused it with saline (0.9% sodium chloride injection USP) or Ringer solution (pH
7.4; lactated Ringer injection USP, B.BRAUN Medical) for 1-2 s to
fill ~10 adjacent alveoli. After microinfusion, the micropipette was
withdrawn, and the alveoli were imaged. No alveolus was punctured more
than once.
Quantification of liquid removal.
Before alveolar microinjection, we quantified diameters of several
alveoli in the imaged field as the means of the longest and the
shortest diameters in each alveolus. After microinfusion, loss of
visible alveolar margins denoted alveolar liquid filling. As the
microinfused liquid drained away from each alveolus, the alveolar
margins became evident. Hence we quantified the liquid removal time
(Ct) as the time elapsed from the end of alveolar filling to the appearance of 90% of the alveolar diameter. Ct was quantified first under baseline
conditions at an PA of 5 cmH2O and then after a
1-min hyperinflation period in which the lungs were given 10 inflations
by cycling PA between 5 and 15 cmH2O.
In separate groups, we quantified Ct in control
alveoli that received liquid injections in the absence of pretreatment and in alveoli given a 15-min pretreatment with an infusion
of 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic
acid tetra(acetoxymethyl) ester (BAPTA-AM; 40 µM; Calbiochem, San
Diego, CA) in Ca2+-free HEPES solution or one of the
inhibitors of active transport, ouabain (1 mM, pH 7.4) or amiloride (10 µM, pH 7.4; Sigma, St. Louis, MO). Ca2+-free
HEPES solution (pH 7.4, osmolarity of 295 mosM) contained (in mM) 20 HEPES, 150 NaCl, 5 KCl, and 10 glucose. The vehicle was lactated Ringer solution.
Data are means ± SE shown in plots or means ± SD shown in
text; n is the number of lungs. Significance was accepted at
P < 0.05.
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RESULTS |
Rate of liquid removal.
Figure 1 exemplifies our alveolar
microinfusion procedure. It shows that the light-reflecting air-liquid
interface, which defined the alveolar margin under air-filled
conditions (baseline), was lost after liquid instillation (0 s) in the
marked region. Hence alveoli were not visible while liquid filled.
However, as depicted over time in Fig. 1, as the liquid drained away,
the alveolar margins became progressively evident.

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Fig. 1.
Removal of liquid in microinjected alveoli. Shown is the sequence
of events after microinfusion of Ringer solution in a group of 9 alveoli (nos. in circles). Objects evident in each video image
(left) are highlighted in the corresponding sketch
(right). Baseline, a single alveolus is micropunctured;
0 s, immediately after infusion, alveoli are no longer visible in
the microinfused region (dashed line); 3-34 s, alveoli appear in
the microinfused region as the injected liquid drains away.
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Because the light reflections at the air-liquid interface clearly
denoted the perimeter of each alveolus, we quantified alveolar diameter
as the equatorial distance across an alveolus using either microcalipers placed on the alveolar image or the line scan tool of our
image analysis program (LSM Image Examiner, Zeiss). Light reflections
at the alveolar wall provided sufficient edge contrast as to mark the
alveolar perimeter (Figs. 2, A
and B). The distance between edge contrasts at opposite
poles of the alveolar equator denoted alveolar diameter (Fig.
2B). Loss of edge contrast signified alveolar liquid
filling. Progressive reappearance of the edge contrasts indicated
progressive removal of the alveolar liquid. Alveolar diameter
quantifications obtained by these image analysis methods did not differ
from those with microcalipers (APPENDIX B); hence we have
not distinguished between the determinations.

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Fig. 2.
Quantification of alveolar diameter. A: single alveolus
before (baseline) and at different time points after (postinjection)
alveolar liquid instillation. Dotted line, alveolar perimeter; solid
line, equator. B: tracings are computer-generated line scan
profiles obtained by placing an image analysis line tool on the
alveolar equator. Arrowheads, edge artifacts caused by light
reflections at the perimeter of the air-filled alveolus. Note that the
distance (double-headed arrow) between the edge artifacts gives the
alveolar diameter. Edge artifacts are not detectable 0-3 s after
alveolar liquid filling. Subsequent tracings denote return of edge
artifacts as the liquid is removed. C: time course of
diameter (d) changes in a single liquid-filled alveolus. Dashed line,
alveolar diameter before microinjection (baseline). After
microinjection (arrow), the alveolar diameter could not be recorded for
a short period because the alveolus was not visible. However, as liquid
drained and the alveolus became air filled, the diameter rapidly
increased. As shown for this alveolus, the baseline diameter was
reestablished within 7-8 s of the end of microinfusion. We
quantified the liquid removal time (Ct) as the
time elapsed from the end of microinfusion to the reestablishment of
the baseline alveolar diameter.
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As liquid was removed from an instilled alveolus, alveolar diameter
increased abruptly, with an S-shaped time course (Fig. 2C).
In some instillations, the loss of alveolar margin was incomplete, leaving a residual margin that probably reflected the alveolar opening
into the respiratory duct. The punctured alveolus was neither the first
nor the last liquid-clearing alveolus. To determine the rate of liquid
removal, we quantified time-dependent changes of alveolar diameter in
the postinfusion period. As shown in Fig. 2C, we quantified
Ct as the duration from the end of the
microinjection to the appearance of the baseline alveolar diameter.
On average, the fastest liquid removal occurred within 4.5 s (Fig.
3). All determinations were obtained
at a PA of 5 cmH2O, before (baseline)
and after (hyperinflation) lung expansions established by cyclically
varying the PA from 5 to 15 cmH2O 10 times in 1 min. Hyperinflation markedly enhanced Ct
(P < 0.03; Fig. 3). For alveoli with the fastest
liquid removal, mean Ct decreased from 4.5 s at baseline to 1.9 s in the postinflation period, indicating
that the intervening hyperinflation period enhanced liquid removal by
>50%. These rapid removal rates indicate that the alveolar liquid was
removed convectively.

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Fig. 3.
Ct from the 3 fastest
liquid-clearing alveoli. Baseline determinations were made at alveolar
pressure (PA) of 5 cmH2O. Postinflation
determinations were made at PA of 5 cmH2O after
the lungs were subjected to 10 hyperinflations. N, no. of
lungs. * P < 0.01 compared with bar on
left.
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Route of removal.
To determine the fate of the instilled liquid, we included fluorescent
latex beads (diameter 1 µm) in the alveolar microinfusion. Figure
4 shows that despite a constant
PA of 5 cmH2O, single beads showed
translational displacement out of the injected alveolus within 3-5
s and then continued along a trajectory of spontaneous movement across
alveoli for up to 80 s. To the extent that we could determine,
beads that exited a given alveolus moved in approximately the same
direction, indicating that their movements were not random but were
probably actuated by bulk flow out of the instilled alveolus.

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Fig. 4.
Spontaneous bead movement from liquid-infused alveolus.
A: images taken at times indicated after injection of
fluorescent microbead (diameter 1 µm). Alveolar margins are sketched.
Arrows: left, fluorescent bead in an alveolus;
middle, 25 s later, the bead has moved to the adjoining
alveolus; right, 90 s later, the bead has traveled
further along to the next alveolus. B: detailed trajectory
of bead movement. Trajectory was drawn from bead positions determined
in multiple images obtained during bead movement. Experiment was
replicated 3 times.
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In a single experiment, we instilled an Evans blue-saline solution (0.7 mg/ml). As shown in Fig. 5, the perimeter
of the dye-instilled region increased progressively, suggesting that
convective flow occurred out of the injected alveoli toward the
surrounding alveoli. Consistent with liquid removal from the alveoli,
dye intensity progressively decreased. The dye could be removed by
intra-alveolar microinjections of dye-free saline but not by
superfusing the area with saline, indicating that alveolar
microinjections did not leak across the alveolar membrane.

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Fig. 5.
Disappearance of injected Evans blue dye from alveoli. Images were
recorded at indicated times after Evans blue (0.67 mg/ml) injection in
a single alveolus. Dotted lines, perimeter of dye-stained zone.
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Role of Ca2+.
To determine the role of intracellular Ca2+ as a
determinant of Ct, we treated alveoli with
20-min microinfusions of BAPTA-AM in Ca2+-free HEPES.
BAPTA-AM chelates intracellular Ca2+, and as Ashino et al.
(2) have previously shown, it blocks the
Ca2+ increases and surfactant secretion in type II cells of
intact alveoli. BAPTA-AM markedly prolonged Ct
under baseline conditions (P < 0.05), and it abrogated
the posthyperinflation reduction of Ct (Fig.
6). When we pretreated alveoli with
Ca2+-free HEPES solution alone, Ct
was not different from baseline. However, similar to the BAPTA-AM
experiments, Ca2+-free HEPES abrogated the
hyperinflation-induced decrease of Ct, indicating that the inflation effect was also dependent on external Ca2+. These findings indicate that
Ca2+-regulated processes determined
Ct, although the nonspecific effects of BAPTA-AM
may also have contributed to these results.

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Fig. 6.
Ct during Ca2+
depletion. N, number of lungs; Ca2+(+) and
Ca2+( ), Ca2+-containing and
Ca2+-depleted solutions, respectively; BAP,
1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic
acid tetra(acetoxymethyl) ester (BAPTA-AM; 40 µM).
* P < 0.05 compared with baseline.
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Ct and alveolar diameter.
Although all determinations were made at a constant PA of 5 cmH2O, Ct differed in different
alveoli. Under baseline conditions, Ct
correlated inversely with alveolar diameter (Fig.
7A), indicating a strong
dependence of the liquid removal rate on alveolar size. However, after
a period of cyclic hyperinflations in which PA was
increased to 15 cmH2O 10 times in 1 min, the correlation was lost (Fig. 7B), suggesting the involvement of
surfactant-dependent surface forces in the liquid removal (see
DISCUSSION). Similar to control, the inverse
Ct-alveolar diameter relationship was also
evident in BAPTA-AM-treated alveoli (Fig. 7C). However, different from control, hyperinflation failed to abolish the
relationship (Fig. 7D), indicating that the possible
inhibition of surfactant secretion by BAPTA-AM rescued the
Ct-diameter relationship.

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Fig. 7.
Alveolar diameter determined Ct at
baseline (A and C) and hyperinflation
(B and D) before (A and B)
and after (C and D) 15-min alveolar infusion of
BAPTA-AM. AD, alveolar diameter. ,
Determinations at PA of 5 cmH2O;
, determinations at PA of 5 cmH2O after lungs were subjected to 10 hyperinflations.
Lines were drawn by exponential regression.
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Active transport.
Figure 8 summarizes the experiments in
which we determined the extent to which active transport plays a role
in the observed liquid removal. As shown, the active transport blockers
ouabain and amiloride failed to affect Ct at
either baseline or postinflation conditions, indicating that alveolar
liquid removal was not determined by active transport mechanisms across
the alveolar wall. In two experiments, we confirmed that the presence
of albumin (5% human albumin solution) in the injected liquid did not
affect Ct (data not shown), indicating that
protein osmotic pressures were not a factor in the removal.

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Fig. 8.
Effect of active transport inhibitors on
Ct. N, no. of lungs; Baseline,
determinations at PA of 5 cmH2O; postinflation,
determinations at PA of 5 cmH2O after lungs
were subjected to 10 hyperinflations. Inhibitors were given as 15-min
alveolar infusions before Ct determinations.
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DISCUSSION |
We show here for the first time that in an air-filled lung, when a
few alveoli become liquid filled, as might occur during the initiation
of pulmonary edema (20), the alveoli rapidly eject the
liquid. This was evident in our experiments in which we modeled the
pattern of alveolar filling in early edema by microinjecting a single
alveolus with Ringer lactate to fill 6-10 adjoining alveoli. In
the immediate postinstillation period, the alveoli were lost from view
because each liquid-filled alveolus lost its light-reflecting air-liquid interface that defines the alveolar margin and allows the
alveolus to be visualized by light microscopy. However, although in the
liquid-filled condition the alveoli were not discernible, each
liquid-filled alveolus ejected its liquid load and became visible as it
refilled with air.
The delay to reappearance of a liquid-filled alveolus provided a
measure of the Ct that, although varying over a 10-fold range, occurred as rapidly as 5 s. In every case, alveolar reappearance was a rapid event such that each alveolus seemed to
"pop" open as air replaced the instilled liquid. This was evident in the rapid rate at which the alveolar diameter regained its preinfusion value (Fig. 2C). In an overwhelming majority of
experiments, the micropunctured alveolus was neither the first nor the
last to clear liquid. In alveoli microinjected with Evans blue
solution, no liquid leakage was evident (Fig. 5). These findings rule
out the possibility that liquid leaked into the interstitium at the micropuncture site or elsewhere. Ouabain and amiloride, which are
blockers of NaCl transport, failed to affect Ct, thereby ruling out active transport as the major underlying mechanism. Furthermore, in one experiment, Ct was not
modified by the inclusion of albumin in the instilled Ringer lactate
(5% human albumin solution), indicating that colloid osmotic forces
did not determine removal. The dependence of Ct
on alveolar size, shown in Fig. 7, rules out a gravitational effect on
the removal. Hence, based on these considerations as well as on the rapidity of the removal, we interpret that the instilled liquid was
removed convectively.
Although the mechanisms underlying this rapid removal of alveolar
liquid remain unclear, alveolar surfactant is implicated by three
findings. First, hyperinflation, which stimulates alveolar surfactant
secretion (9, 14) decreased Ct.
Ashino et al. (2) recently showed that even a single brief
period of lung expansion induced alveolar surfactant secretion for a prolonged period. Hence it is possible that in the posthyperinflation period, alveoli were surfactant rich and that the faster liquid removal
was attributable to enhancement of surface forces by
hyperinflation-induced surfactant secretion.
Second, the intracellular Ca2+ chelator BAPTA-AM markedly
prolonged Ct at baseline and also blocked the
posthyperinflation decrease of Ct. Surfactant
secretion is a Ca2+-dependent process (6, 8).
Alveolar stretch due to lung expansion increases alveolar cell
Ca2+ and thereby causes surfactant secretion (2,
8, 21). BAPTA-AM chelates intracellular free Ca2+
and prevents both the stretch-induced Ca2+ increase as well
as the subsequent enhancement of surfactant secretion (2).
If surfactant were responsible for the rapid liquid removal at
baseline, then inhibition of surfactant secretion should delay the
removal. Furthermore, the inhibition of surfactant secretion should
block the hyperinflation effect of increasing surfactant in the
alveolus and thereby further delaying liquid removal. Both predictions
were supported by our findings that in BAPTA-AM-treated alveoli,
Ct was almost two times higher than the control
value and that hyperinflation failed to decrease
Ct, indicating that the enhancement of liquid removal was blocked.
Third, depletion of extracellular Ca2+ also inhibited the
posthyperinflation reduction of Ct. Because
Ca2+ determines the organization and maintenance of tubular
myelin (11), extracellular Ca2+ is an
essential requirement for surfactant function. It is likely that the
depletion of extracellular Ca2+ caused disruption of the
lining and therefore loss of surface-active forces that may have driven
the liquid flow.
In brief, we implicate surfactant in these findings because
hyperinflation, a surfactant stimulant, speeded removal, whereas BAPTA-AM, an inhibitor of surfactant secretion, slowed removal. In
addition, another important determining factor for removal was alveolar
diameter. As predicted by the Laplace equation (see APPENDIX
A), if the interfacial tension (T) and the air pressure
(Pair) are constants, then the alveolar liquid pressure (Pliq) must increase as alveolar radius (r)
increases. If Pliq is the force that convectively clears
the liquid from the alveolus (see APPENDIX A), then we may
expect a higher Pliq, hence faster removal in larger
alveoli. The inverse Ct-alveolar diameter
relationship in Fig. 7 supports this expectation.
Despite these considerations, other interpretations may apply to our
data. For example, a different form of the Laplace equation may be
valid (10). Also, the pores of Kohn that form potential interconnecting pathways between adjoining alveoli (5)
could provide a route of liquid flow. Furthermore, a potential
explanation may lie in a liquid equivalent of the phenomenon of air
"pendulluft" in which out-of-phase airflow occurs between adjacent
lung regions because of imbalances in airway impedance (13, 15,
17). Differences in impedance to liquid flow attributable to
differences in alveolar diameter may account for the present patterns
of liquid removal from instilled alveoli. Although we interpret the
BAPTA-AM effects as being due to inhibition of surfactant secretion,
other processes inhibited by intracellular Ca2+ chelation
may have played a role. These issues require further investigation.
To determine the direction in which the alveolar liquid was removed, we
instilled fluorescent latex beads. After instillation, single beads
were evident as fluorescent spots and could be followed by time-based
fluorescence microscopy. The fluorescent beads were expelled within
3-5 s from the instilled alveolus and, subsequently, continued to
move spontaneously across four alveolar diameters for up to 90 s.
The evidence for such spontaneous translation is consistent with the
notion that the beads were convectively transported in the
postinstillation period. A similar interpretation may be drawn from our
experiments in which we instilled dye-labeled albumin solution. Here as
well, convective flow was evident in that spread of the dye was evident
as a spot of increasing perimeter on the lung surface. To the extent
that the beads and the albumin solution marked the pattern of alveolar
liquid removal, we interpret that the instilled alveolar liquid flowed
out of the filled alveolus and then traversed several adjacent alveoli,
possibly en route to the respiratory duct.
Using a micropuncture technique, Schurch et al. (16)
deposited liquid droplets of different surface tensions on the alveolar surface. Droplets of low surface tension rapidly spread on the surface,
whereas those of high surface tension tended to remain rounded,
indicating that the balance between the alveolar surface pressure and
the surface tension of the deposited liquid droplet was the critical
determinant of surface wetting, hence droplet stability
(7). In our experiments, the situation was somewhat different because we deposited not a droplet but a volume of liquid that filled the entire lumen of the injected alveolus. If the considerations of the droplet experiment of Schurch et al.
(16) were applicable, then given the high surface tension
of water, we would expect the injected liquid to remain as a stabilized drop. Because the liquid flowed away, we interpret that subinterfacial pressures generated by the Laplace considerations were the likely factor in this liquid removal.
In conclusion, our findings are the first evidence that alveoli are
capable of rapidly ejecting instilled liquid, which indicates the
presence of a novel liquid removal mechanism in the lung and suggests
that in addition to active transport, alveoli possess the potential for
convective removal. Staub microinjected a relatively large volume
(1-2 µl) of dyed saline into subpleural alveoli and concluded
that the liquid was removed by interstitial drainage (19).
Our findings are different because, as we discuss above, the present
liquid removal occurred by an intra-alveolar and not an interstitial
route. Although the exact mechanisms need to be clarified, our findings
indirectly implicate surface-active alveolar forces in the liquid
removal. The physiological significance bears on the mechanisms by
which alveoli may be protected from liquid filling in early stages of
pulmonary edema. As alveoli begin to fill with liquid, rapid convection
of the liquid away from the alveoli may be beneficial not only for
maintaining liquid-free alveoli to protect gas exchange but also as a
provision for washing out cells or particles that may have been
deposited on the alveolar surface. The extent to which such a removal
mechanism applies in later stages of pulmonary edema when a large
number of alveoli become liquid filled remains uncertain. However, we
believe that in early pulmonary edema when alveolar liquid entry is
incipient and the alveoli are largely still air-filled, the present
convective mechanisms may provide a crucial liquid elimination process
from the gas exchange area.
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APPENDIX A |
Laplacian considerations.
Because convective flow occurs down a pressure gradient, we
considered the possibility that a flow-inducing pressure gradient develops between two adjacent alveoli when one of them becomes liquid
filled. This situation is illustrated in Fig.
9 in which a fully liquid-filled alveolus
(alveolus A) lies adjacent to an air-filled alveolus
(alveolus B). According to the Laplace equation as
classically applied to pressure distribution across the alveolar air-liquid interface (3, 12)
where Pair = PA.

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Fig. 9.
Pressure distribution between 2 adjacent alveoli when 1 becomes liquid filled. Pair, air pressure; r, alveolar
radius; Pliq, alveolar liquid pressure.
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In alveolus A, the air-liquid interface that occurs
at the alveolar mouth has a curvature that is almost flat, resulting in a large increase in r. According to the Laplace equation,
the increase in r decreases the transinterfacial pressure
drop. Therefore, because Pair is constant, Pliq
increases. In fact, in the absence of interfacial curvature, there is
no transinterfacial pressure drop and Pair = Pliq as shown. In contrast, the adjacent air-filled alveolus B maintains the normal curvature of the
air-liquid interface such that Pliq is less than
Pair as classically predicted by the Laplace equation. Thus
Pliq in alveolus A is greater that that in
alveolus B and thereby establishes the pressure
gradient for driving liquid flow, alveolus A
alveolus B.
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APPENDIX B |
Because the manual determination of alveolar diameter by
microcalipers involved the subjective assessment of alveolar edge detection, we constructed a kappa statistic according to standard methods (1). Two observers determined whether alveoli were air or liquid filled before and after, respectively, alveolar microinjection. Table 1 shows the data
that generated a kappa of 0.934, indicating excellent
agreement between observers. Hence microcaliper measurements provided
dependable quantification of the state of alveolar liquid filling.
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ACKNOWLEDGEMENTS |
This research was supported by National Heart, Lung, and Blood
Institute Grants HL-36024, HL-64896, and HL-57556 (to J. Bhattacharya), and HL-10142 (to P. M. Wang).
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FOOTNOTES |
Address for reprint requests and other correspondence: J. Bhattacharya, St. Luke's-Roosevelt Hospital Center, 1000 10th Ave., New York, NY 10019 (E-mail: jb39{at}columbia.edu).
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.
Received 4 May 2001; accepted in final form 30 July 2001.
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