Ebook Basics of respiratory mechanics and artificial ventilation: Part 2
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Ebook Basics of respiratory mechanics and artificial ventilation: Part 2
Chapter 10Alveolar micromechanicsp.v. RomeroThe mechanical behavior of the air spaces in the periphery of the lung is the result of a delicate balance Ebook Basics of respiratory mechanics and artificial ventilation: Part 2e of forces acting on the tissue scaffold of lung parenchyma. Static and dynamic properties of such a complex system have been an important field of research for many years. Alveolar space micromechanics have important physiological implications in terms of mechanical interdependence, alveolar stabi Ebook Basics of respiratory mechanics and artificial ventilation: Part 2lity, and the maintenance of a gas exchanging surface in constant contact with air. The mechanical behavior of such system has to allow the expansionEbook Basics of respiratory mechanics and artificial ventilation: Part 2
of the alveolar surface at physiological rates at a low energy cost, and without interfering with the exchange process. I will describe how the structChapter 10Alveolar micromechanicsp.v. RomeroThe mechanical behavior of the air spaces in the periphery of the lung is the result of a delicate balance Ebook Basics of respiratory mechanics and artificial ventilation: Part 2is made of a single capillary network interlaced with fibers (mainly collagen and elastine), which form a continuum embedded in the connective matrix, the thin membrane of epithelial cells forming the external boundary of this scaffold. This irregular surface is to some extent smoothed by an extrace Ebook Basics of respiratory mechanics and artificial ventilation: Part 2llular layer of lining fluid that is rather thin over the capillaries but forms small pools in the intercapillary cavities. Alveolar lining consists oEbook Basics of respiratory mechanics and artificial ventilation: Part 2
f an aqueous layer called the hypophase which is of variable thickness and is present mainly in the pools, and a layer of surfactant which forms a filChapter 10Alveolar micromechanicsp.v. RomeroThe mechanical behavior of the air spaces in the periphery of the lung is the result of a delicate balance Ebook Basics of respiratory mechanics and artificial ventilation: Part 2 by the structural disposition but results from the molding effect of the two main forces that have to be kept in balance: tissue tension and surface tension.Structural interaction of tissue fibers and surface liningMany experimental studies agree in the fact that the dimension of the alveolar surfa Ebook Basics of respiratory mechanics and artificial ventilation: Part 2ce is governed by the equilibrium between surface and tissue forces. Surface tension arises at any gas-liquid interface because the forces between theEbook Basics of respiratory mechanics and artificial ventilation: Part 2
molecules of the liquid are much stronger than those between the liquid and the gas. As a result, the liquid surface will tend to become as small as Chapter 10Alveolar micromechanicsp.v. RomeroThe mechanical behavior of the air spaces in the periphery of the lung is the result of a delicate balance Ebook Basics of respiratory mechanics and artificial ventilation: Part 2oefficient g. According to Wilson [1], surface pressure (Ps) can be expressed as a function of the surface-to-vol-ume ratio of the alveolar air space (S/V)a and surface tension (y) by:Ps = (2/3) • y • (S/V)a-1The greater the surface-to-volume ratio, the greater the mean curvature of the surface and Ebook Basics of respiratory mechanics and artificial ventilation: Part 2the greater the surface pressure at any value of y. According to the above equation, the most critical effect of surface tension (y) is that it challeEbook Basics of respiratory mechanics and artificial ventilation: Part 2
nges the stability of airspaces. As a set of connected bubbles, alveoli are intrinsically unstable: since the small ones have a larger curvature than Chapter 10Alveolar micromechanicsp.v. RomeroThe mechanical behavior of the air spaces in the periphery of the lung is the result of a delicate balance Ebook Basics of respiratory mechanics and artificial ventilation: Part 2 mechanisms: the interaction between tissue fibers and surface lining, and the intrinsic properties of surfactant itself.Alveolar walls contain an intricate fiber system. Thus, when an alveolus tends to shrink, the fibers in the wall of the alveoli are stretched and this will prevent the alveolus fr Ebook Basics of respiratory mechanics and artificial ventilation: Part 2om collapsing. This stabilizing phenomenon is known as interdependence [2]. Surfactant lines the complete alveolar suface, and even terminal airways.Ebook Basics of respiratory mechanics and artificial ventilation: Part 2
The surface tension coefficient y of surfactant is variable: it falls as alveolar surface becomes smaller, and rises when alveolar surface expands [3]Chapter 10Alveolar micromechanicsp.v. RomeroThe mechanical behavior of the air spaces in the periphery of the lung is the result of a delicate balance Ebook Basics of respiratory mechanics and artificial ventilation: Part 2the alveolar emptying allows the system to remain stable. If surface tension is modified at the level of the liquid-air interface, the alveolar area will be inversely related to the surface tension at any level of alveolar volume, at least in the range of tidal volumes [4]. This is due to the effect Ebook Basics of respiratory mechanics and artificial ventilation: Part 2 of tissue tensions: as surface tension decreases, the stretching effect of tissue tension is magnified and alveolar area increases, provided that alvEbook Basics of respiratory mechanics and artificial ventilation: Part 2
eolar volume does not vary.Biomechanics of the alveolar lining layerStructure and compositionThe alveolar epithelial cells are covered by a thin liquiChapter 10Alveolar micromechanicsp.v. RomeroThe mechanical behavior of the air spaces in the periphery of the lung is the result of a delicate balance Ebook Basics of respiratory mechanics and artificial ventilation: Part 2ar lining layer has been described as an acellular film that forms a continuous lining over the alveolar epithelial cells and spans the pores of Kohn. It was considered to serve as an anti desiccant to the lungs until, in 1955, Pattie [51 showed that the lung contained surfactant substances capable Ebook Basics of respiratory mechanics and artificial ventilation: Part 2of stabilizing tiny bubbles, and even to decrease air-water surface tension to near zero values. Two morphological regions of the alveolar lining layeEbook Basics of respiratory mechanics and artificial ventilation: Part 2
r (ALL) have to be distinguished: the hypophase, and the hypophaseair boundary or surfactant lining. The hypophase often appears as a homogeneous matrChapter 10Alveolar micromechanicsp.v. RomeroThe mechanical behavior of the air spaces in the periphery of the lung is the result of a delicate balance Ebook Basics of respiratory mechanics and artificial ventilation: Part 2 square tubules. Tubular myelin is a lipoprotein structure of high surface activity that contains dipalmitoyl lecityn, the major component of pulmonary surfactant. Thickness of the hypophase varies, sometimes hardyly visible by electron microscopy in areas where the epithelial cell surface is flat, Ebook Basics of respiratory mechanics and artificial ventilation: Part 2and sometimes appearing as deep pools where there are folds or crevices in the epithelium or between capillaries. The air-hypophase boundary can be diEbook Basics of respiratory mechanics and artificial ventilation: Part 2
stinguished from the hypophase by its osmophilic property. It is provided by a duplex lining layer composed mainly of desaturated phospholipids.BiomecChapter 10Alveolar micromechanicsp.v. RomeroThe mechanical behavior of the air spaces in the periphery of the lung is the result of a delicate balance Ebook Basics of respiratory mechanics and artificial ventilation: Part 2gest portion of the lung’s hysteresis and rheological behavior is attributable to this interface. These effects are well known since, in 1929 von Neergard [6] described the pressure-volume characteristics of the liquid-filled and air-filled lungs (Fig. 1): liquid filling eliminates all air interface Ebook Basics of respiratory mechanics and artificial ventilation: Part 2s between cell walls and their lumina, so that interfacial tensions are negligible, and only the resistance of tissue forces remain. For many years knEbook Basics of respiratory mechanics and artificial ventilation: Part 2
owledge about surface tension in situ was derived from studies based on the difference between air-filled and liquid-filled lungs. In 1977 Hoppin and Chapter 10Alveolar micromechanicsp.v. RomeroThe mechanical behavior of the air spaces in the periphery of the lung is the result of a delicate balance Ebook Basics of respiratory mechanics and artificial ventilation: Part 2-pressure diagrams of isolated lungs inflated from minimal lung volume with air or saline. In saline-filled lung the interfacial tension of the lung lining layer is though to be largely eliminated when air is replaced by saline. The saline curve is typically displaced to the left, and has a lower hy Ebook Basics of respiratory mechanics and artificial ventilation: Part 2steresis than the air-filled curve. A “knee ’ in the inflation arm of the air-filled loop is characteristically seen122 p.v. Romerofilled and liquid-fEbook Basics of respiratory mechanics and artificial ventilation: Part 2
illed status, which indicated clearly that the use of pressurevolume (PV) diagrams for calculation of 7 is unreliable. Between 1976 and 1989 Shiirch eChapter 10Alveolar micromechanicsp.v. RomeroThe mechanical behavior of the air spaces in the periphery of the lung is the result of a delicate balance Ebook Basics of respiratory mechanics and artificial ventilation: Part 2rent 7 deposited on the alveolar surface by means of a micropipette. Surface tension-lung volume and surface tension-recoil pressure relationships have been since then measured in different species. The most important biomechanical features related to surface tension per se can be summarized as foll Ebook Basics of respiratory mechanics and artificial ventilation: Part 2ows.1.The surface tension-lung volume relationship in static conditions is similar for different species, particularly along the deflation limb: surfaEbook Basics of respiratory mechanics and artificial ventilation: Part 2
ce tension decreases quasilinearly with lung volume from total lung capacity (TLC) to functional residual capacity (FRC) level [4J2.Static recoil presChapter 10Alveolar micromechanicsp.v. RomeroThe mechanical behavior of the air spaces in the periphery of the lung is the result of a delicate balance Ebook Basics of respiratory mechanics and artificial ventilation: Part 2alveolar surface to volume ratio and the different participation of lung tissue (tissue component of the recoil pressure, Pt). According to the model proposed by Wilson and Bachofen [101, the component of recoil pressure due to surface tension (py) is directly proportional to y/v1/3, where V is the Ebook Basics of respiratory mechanics and artificial ventilation: Part 2alveolar volume: Py=K»yV1/33.There is a prominent hysteresis in the y-V relationships with values of 7 ranging from near zero at low lung volumes duriEbook Basics of respiratory mechanics and artificial ventilation: Part 2
ng deflation to transiently high tensions near 40 dyn/cm during dynamic inflation. The amplitude of the hysteresis and shapes of y-V relationships difGọi ngay
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