THE
ALVEOLAR
LINING
LAYER
A
Review
of Studies
on Its Role
in Pulmonary
Mechanics
and
in the
Pathogenesis
of Atelectasis
Mary Ellen Avery, M.D.
Department of Pediatrics, Jo/ins Hopkins Universit,, ?Ie(lical School and Harriet Lane Home, Jolin.c Hopkins Hospital, Baltimore, Maryland
l’his work was supported in part by U. S. Public Flealth Service Grant H 5429.
Dr. Avery is a John and Mary B. Markle Scholar in Medical Science.
ADDRE55: Johns Hopkins Hospital, Baltimore 5, Maryland.
REVIEW
ARTICLE
324
PE1)LA’IRICS, August 1962
I
T 15 HARDLY SURPRISING tilat the forces ofsurface tension must play a major role in tile distensibility and stability of the
lung, since its internal surface area is esti-mated to be about 70 square meters in the adult, or approximately the size of a bad-minton court.1 Even in tile infant the area is large, about 3 square meters.2
Many puzzling aspects of lung function
may be understood in the bigilt of recent
studies on surface forces. Why does the first breath of the newborn infant inflate some alveoli fully before others begin to inflate? Why are tile lungs of small pre-mature infants and those with hyaline membrane disease airless at autopsy, when it is the rule to find some air in lungs after death in most other circumstances? What
keeps most alveoli air-containing at the
end of expiration? The key to these ques-tions lies in a consideration of the prop-erties of tile alveolar lining layer. It is tile
purpose of tilis review to state the
pninci-pies underlying the operation of surface forces in the lung, methods of study, and
tile relevance of recent observations on the alveolar lining layer to tile first breath of
the newborn infant, atelectasis, and hyaline
membrane disease. Tile studies to be
dis-cussed in tilis review, applicable to tile
lung at all ages of life, have opened a new
chapter in respiratory physiology.3
LUNG GEOMETRY AND SURFACE FORCES
The geometric arrangement of the in-ternal surface of the lung, in multiple tubes
and sacs, enilances the mechanical
advan-tage of surface forces, which operate to
reduce area. The more sharply curved tile
surface, the greater tile effect of surface
tension. This is in accord with the LaPlace expression for a sphere wilich relates sun-face tension, tile curvatures of the surface,
and tile resultant pressure difference across
it, as P = 2T/r. (P = pressure, T surface
tension, r radius of curvature.)0 A single spherical or hemispherical alveolus, with
a radius of 50 micra, and the surface
ten-sion of plasma (50 dynes/cm) would have a pressure difference of 20 cm H2O-op-erating to make it smaller. Hence a pleural
pressure of 20 cm H30 subatmosphenic,
or about five times that which is in fact tile
case at end-expiration, would be necessary
to keep it open.
In order to stay open at bow pleural pres-sures, alveoli must be lined by some
sub-stance with a much bower surface tension
than that of plasma. Pattle first suggested
tilis possibility after observing tile life span
of bubbles expressed from lungs and
sus-pended in saline solution saturated with air. From their dimensions and long life span,
ile reasoned that the surface tension was
very bow, of the order of 0.05 dynes/cm, compared to 30-50 dynes/cm for most bio-logical fluids. If alveoli were lined by the
material in Pattle’s bubbles, the pressure
0 Pressure = gm/cm2 (1 cm3 1-LO = 1 gm);
tension = gm/cm (980 dynes = 1 grn); radius =
1L:J
REVIEW ARTICLE
tending to make them smaller would be
only 0.002 mm Hg.
Another problem posed by the geometry
of tile lung is that of multiple small air spaces in parallel. Such a system is me-chanically unstable if the air spaces are of dissimilar size. The LaPlace expression, P = 2T/r, again makes this point clear. If
tile radius (r) varies, the pressure (P) must
vary inversely witll it. An increase in
pres-sure in a smaller alveolus which freely
com-municates with a larger one will cause gas
to go from the smaller one to the larger one
until tile small alveolus closes completely.
The extent to which this uneven
clistnibu-tion of gas occurs will depend on the prop-erties of the lung itself. It is capable of
being airless; for example, this state can
be achieved by the evacuation of air from
tile lung in a vacuum jar. Portions of lung
are often observed to be airless at operation
OI at autopsy. There are then no structural
components sucil as reticubar fibers or
yes-sels or pleura which in themselves offer
significant opposition to collapse of tile
lung. By their arrangement they may
facili-tate stability, but something else is
re-juirecl for gas to remain in the lung at
atmospheric pressure. The necessary
corn-ponent is tile alveolar lining layer itself.
Tile property of the surface film lining
tile alveoli which promotes alveolar
sta-hility is an increase in surface tension to
40-50 dynes/crn on increase in area and a
decrease in surface tension to 5-10 dynes/
cm on a decrease of area. Again, referring
to the LaPlace expression, if tension
in-creases as radius increases, tile pressure need not change. First demonstrated by
Clements et al., tilis property of a cilange
in tension with area, as well as the ability to achieve low tensions, is essential to lung stability. In Clements’ words, “the alveolar
lining bayer operates as an anti-atelectasis
factor.”
APPROACHES TO STUDIES OF THE
ALVEOLAR LINING LAYER
1\V() different l)ut colnplenlentary’
uletil-ods have been used to study the properties
of tile alveolar lining layer. One involved
the measurement of the surface tension of
material expressed from the lung. The other method permitted study of surface
forces in the intact lung by a comparison of
tile properties of air and saline filled
ex-cised lungs.
Studies on Material Expressed from Lungs
The approaches to the study of the
alveo-lar lining layer by Pattl& and Clements
et al. involved the use of material ex-pressed from the panenchyma of lungs, or washed out from the airway. Despite the apparent significance of these findings, seni-ous question could be raised about whether
tile measurements were made on the
alveo-lan lining layer itself. Pattle’s bubbles were
carefully expressed from peripheral lung,
but would, of course, have been contami-nated with other fluids. Clements’ lung
ex-tract was derived from minced lung. In an
attempt to answer this question, both
work-ens demonstrated that plasma and other biological fluids did not achieve a corn-parable fall in surface tension. Since a sun-face-active substance will by definition seek the surface, Clements’ method of allowing his extract to age in a trough presumably
permitted the most surface-active
com-ponent in his extract to form a surface film. He then changed the area of the surface
film, and measured the surface tension as
a function of surface area, witil a modified
Wilhebmy balance (Fig. 1).
Ftc;. I. ‘Irougli with partly sul)merged platinum
stirrup aiid inoable barrier to Ineasure the surface
tension of t film as a function of surface area.
AIR INFLATION
I
TISSUE VOLUME
10 20 O 40
PRESSURE (cmH2O)
SALINE INFLATION
I
TISSUEVOLIJME
30 40
326
VOLUME
FIG. 2. Static pressure-volume characteristics of
excised dog lung. Curve I is tile first air inflation
of the degassed lung. Curve II is the second air
in-flation immediately following the first. Note that air
enters the lung wllich is partially air containing at
much lower pressures than those required for
the first inflation.
Surface Forces in Intact Lungs
The role of surface forces in the intact lung was investigated in 1929 by von Neengaard,6 and more recently by Mead
et They described the
volume-pres-sure relationships of excised animal lungs
distended with air and with saline. Starting with an airless lung, the stepwise elevation of pressure permits the introduction of a volume of air (Fig. 2). At equilibrium, this describes the elastic properties of the lung. The volume-pressure relationships on the
first filling of the airless lung depend on
an opening pressure phenomenon, which is
a function of surface tension and the radii
of curvature of the airways. In tile degassed lung, the surface tension is that of the moist
surface stretched by the incoming bubble of air. Surface tension is high under these circumstances (from measurements on the film balance, about 40-50 dynes/cm), hence high pressures are required for the introduc-tion of air. Once tile lung is distended, the surface film, also stretched, enhances the elastic recoil of the lung. Stepwise deflation of the air-containing lung describes another
volume-pressure relationship, which is a measure of the ebastance of the lung. The second air filling requires less pressure, since some air has remained in the lung after the first filling. The introduction of air on air does not involve as much stretching of the fluid films, hence less surface forces to overcome. When the airless lung is dis-tended with saline solution, first to rinse
out particulate matter in the airway, and
the second time to describe its volume-pressure relationships in the absence of an air interface (thus in the absence of
sun-face forces), a striking difference is
appar-ent from tile behavior of air inflation of the
lung (Fig. 3). The saline-filled lung is fully
distended at lower pressures; on deflation
the lung empties in nearly the same
man-ner in vhich it filled. Since all the tissues
are stretched to a similar degree, the
differ-ences on air and saline inflation are a
meas-ure of surface forces in the lung. At large
lung volumes, they contribute to the elastic recoil of the lung, and must, therefore, be higher under these circumstances than the
negligible values Pattle assigned from his
VOLU ME
PRESSURE (cmH2O)
FIG. 3. Static pressure-volume characteristics of
excised dog lung. Curve I is the first saline inflation
of the degassed lung. Curve II is the second saline
inflation. The same volume of saline can be intro-duced into a lung as air (Fig. 2). The saline-filled
lung is maximally distended at less than half the
a
Jr
0 Atlas Powder Co., \Vilmington 99, Delaware.
observation of bubbles. At low lung vol-times, tile differences in air and saline dis-tention of the lung are less, and surface tension must be very low indeed.
NATURE OF THE ALVEOLAR
LINING LAYER
The presence of an alveolar layer was first proposed by Mackim,9 who deduced from histochemical evidence tilat it was a mucoprotein film. However, tile light micro-scope can reveal only films of many mole-cules. Tile material at tile alveolar-air inter-face need not be abundant to achieve its important role. Chemical analysis of the material in bubbles expressed from tile lung,
or in films formed on the surface of a minced lung extract, reveals it to be a lipoprotein, predominantly pilospho-lipid.i 1O ii Fat-containing materials such as cholesterol-lecithin films have long been known to achieve low surface tensions.12 The precise composition and amount of the film in human lungs in vivo is not clear. Knowledge of tile nature of tile film in the lung is relevant to considerations of gas ex-cilange, tile discovery of agents wilich might alter it, and disease states in which it may
cilange. Pattlel3 has tested the action of a variety of substances on the alveolar lining layer. He could find no changes with agents
which were not themselves surface active,
including blast injury, cobra venom, pure oxygen, cadmium oxide, and black smoke. On tile other hand, lecithin and Tween 80, powerful surfactants, altered the properties of the alveolar lining and promoted lung collapse.
Tile action of Tweens* (hydrophilic non-ionic surfactants) and Spans* (lipophilic
surfactants) is to lower the opening
pres-sure of tile airless lung previously rinsed
with these agents, and to increase the elastic recoil of the lung at low lung volumes to the point of collapse at 2-3 cm H2O
pres-sure. Such agents replace the normal lung
lining layer with a film of fixed surface ten-sion of about 30 dynes/cm. Radford’s
studieslS with these agents lend furtiler
sup-port to the role of the lung lining layer in the maintenance of alveolar stability. If during life the normal alveolar lining were replaced by an agent with a fixed surface tension, characteristic of most detergents, at end expiration much of the lung would be atelectatic.
ROLE OF PULMONARY SURFACE FORCES IN NEONATAL LIFE
Surface Forces in First Breath of
the Newborn Infant
J
ust as the introduction of air in tile de-gassed lung required higher pressures than the introduction of air into the air-contain-ing lung, so too must the first introduction of air at birth require relatively high ap-plied pressures. The newborn infant’s lung probably contains a small amount of fluid) ‘ 13 The first breath must first movethis column of fluid overcoming the viscous forces of fluid flow.9 In the smaller airways,
tile forces of surface tension oppose the
in-troduction of air. In the bronchioles, where
tile radii of curvature are small, the
La-Place expression tells us that pressure will be high if surface tension is high. The sun-face film approximates a hemispherical
FIG. 4. Opening pressure. In the diagram on the
left, tile incoming air first encounters a fluid film
which can be thought of as an arc of a large circle.
Here little pressure is required to advance the
air column, since the radius of curvature is large
(P = 2T/r). In the diagram in the center the fluid
film has reached maximal curvature, r is small, and
the applied pressure is high. In the diagram on
the right, the radius is again large, hence the
ap-plied pressure need not he high. The bubble (or alveolus) will go on to complete filling before the
next bubble will open. Tissue forces or increasing
shape ahead of the advancing air column, hence the tensions would be those of the stretched film or 40-50 dynes/cm (Fig. 4). Once alveoli open, and their radii increase, the applied pressure need not be so high, so that they proceed to full expansion before the next alveoli “open.” This phenomenon explains the serial opening of terminal air
spaces, or the “pop, pop, pop” of air spaces opening one by one, readily observed when-ever air is introduced slowly into an airless lung. The decrease in surface tension of the material lining the alveoli, when they de-crease slightly in size, normally helps to prevent alveolar closure.
The Alveolar Lining Layer in Atelectasis of
the Newborn Infant
Observations on the role of alterations in the behavior of the alveolar lining layer in
human illness were first made in lungs of
infants with hyaline membrane
Atelectasis is the predominant pathologic al-teration in this condition. The lungs at autopsy are airless, sometimes described as “liver-like.” Foam is absent in the airway, and on installation of saline solution, no typical pulmonary foam is forthcoming. During life the infants appear to apply large pressures to achieve adequate ventila-tion; the severe retractions attest to this. The opening of airless segments or the yen-tilation of only a small volume of the lung in the face of widespread atelectasis would
require large applied pressures.
Measure-ments of the surface tension of extracts of these lungs (by methods of Clements and Brown on a surface film balance) show the films fail to achieve the low tensions requis-ite for alveolar stability. The pressure-vol-time characteristics of lungs of infants with hyaline membrane disease are grossly
al-19 Not only are they poorly
distensi-ble unless extraordinary high pressures are applied (80 cm H2O), but on deflation they
trap little or no air. Gruenwald2o, 30 has dem-onstrated a correlation between pressure-volume characteristics of lungs of infants with hyaline membrane disease and the fail-une of extracts of these lungs to achieve low
tensions on a surface film balance. By both experimental approaches, it appears these
lungs lack the surface-active alveolar lining layer necessary to prevent atelectasis.
The possibility that in fetal development there is a time, or period of time, in which the alveolar lining layer should appear, was first suggested by Pattle, who observed tin-stable bubbles from lungs of fetal guinea
pigs. Later extending his observations to
fetuses of other species he concluded that when cuboidal epithelium attenuates and alveoli appear, a iipoprotein film appears.21 Buckingham and Avery22 found the alveolar lining substance appeared late in gestation
of the fetal mouse. The time of its
appear-ance was coincidental with the appearance of osmophilic granules in alveolar lining cells, previously demonstrated by \Voodside and Dalton2 with an electron microscope. They suggested the alveolar lining cells themselves could be the source of the film, an observation consistent with Pattle’s find-ings.
Whether the absence of the lipoprotein film in hyaline membrane disease is due to its failure to appear or its inactivation is not certain. The argument that the process is one of developmental immaturity de-pends on the observation that an alveolar lining substance with characteristic surface active properties is not present in lungs of most infants of less than 1,000 gm birth weight. In these infants, atelectasis is almost always prominent. Membranes may be found, but are not as common as in slightly
heavier infants. If the membrane depends
argu-REVIEW ARTICLE
ment against a purely developmental de-feet depends chiefly on the observation that the process may occur in infants of 2,000 grn and even in some nearer term. The possibility that afflicted infants undergo an intrauterine insult related to maternal hem-orrhage or maternal diabetes is suggested by statistical studies. Such an insult may in some way denature or prevent the for-mation of the normal alveolar lining layer. The resolution of this problem depends on further experiment.
The probability that the disease may be self-limited after several days of extra-uterine life is consistent with either hy-pothesis for the absence of the alveolar lin-ing layer. Recovery could occur if the pre-cursors for lipoprotein formation became ac-tive in tile first days of life. Likewise if a
toxic agent were detoxified, normal func-tion could reappear.
Demonstration of alteration of the bron-ciliOlar and alveolar cells in hyaline mem-brane disease was reported by Buckingham and Sommers,26 and Barter and Maddison’ with the light microscope; Campiche et al.28
with the electron microscope showed de-struction of alveolar cells and replacement of tile lining with hyaline membrane. The lipoprotein lining of the alveoli may be es-sential to prevent injury to the underlying cells; alternately, the same insult may injure the cells and denature the lining substance. These morphologic changes, and the al-terations in surface tension are consistent findings in hyaline membrane disease, and surely play a role in its pathogenesis. They do not at this time point to any specific therapy.
SUMMARY
The alveoli of the normal lung are lined by a substance which exerts surface tension at the air-liquid interface. In the expanded lung the tension is high and operates to in-crease the elastic recoil of the lung. In the lung at low volumes the surface tension be-comes extremely low. This confers stability on the airspaces and thus prevents atelec-tasis. This lining layer is a lipoprotein film,
which is not found where alveoli are still lined by cuboidal epithelium. Its time of ap-pearance coincides with the appearance of alveolar lining cells. Electron microscopic evidence of secretory activity in alveolar cells suggests that they may be the source of the surface-active film. The normal al-veolar lining layer is not present in lungs of infants who die from profound atelectasis and hyaline membrane disease. Whether its absence is a failure of development or due to inactivation is not established.
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