Equilibrium of force of subpleural alveoli: implications to lung mechanics

Lung Mechanics: A Review of Solid Mechanical Elasticity in Lung Parenchyma

The lung is the main organ of the respiratory system. Its purpose is to facilitate gas exchange (breathing). Mechanically, breathing may be described as the cyclic application of stresses acting upon the lung surface. These forces are offset by prominent stress-bearing components of lung tissue. These components result from the mechanical elastic properties of lung parenchyma. Various studies have been dedicated to understanding the macroscopic behaviour of parenchyma. This has been achieved through pressure-volume analysis, numerical methods, the development of constitutive equations or strain-energy functions, finite element methods, image processing and elastography. Constitutive equations can describe the elastic behaviour exhibited by lung parenchyma through the relationship between the macroscopic stress and strain. The research conducted within lung mechanics around the elastic and resistive properties of the lung has allowed scientists to develop new methods and equipment for evaluating and treating pulmonary pathogens. This paper establishes a review of mathematical studies conducted within lung mechanics, centering on the development and implementation of solid mechanics to the understanding of the mechanical properties of the lung. Under the classical theory of elasticity, the lung is said to behave as an isotropic elastic continuum undergoing small deformations. However, the lung has also been known to display heterogeneous anisotropic behaviour associated with large deformations. Therefore, focus is placed on the assumptions and development of the various models, their mechanical influence on lung physiology, and the development of constitutive equations through the classical and non-classical theory of elasticity. Lastly, we also look at lung blast mechanics. No explicit emphasis is placed on lung pathology.

Lung tissue biomechanics imaged with synchrotron phase contrast microtomography in live rats

The magnitude and distribution of strain imposed on the peripheral airspaces by mechanical ventilation at the microscopic level and the consequent deformations are unknown despite their importance for understanding the mechanisms occurring at the onset of ventilator-induced lung injury. Here a 4-Dimensional (3D + time) image acquisition and processing technique is developed to assess pulmonary acinar biomechanics at microscopic resolution. Synchrotron radiation phase contrast CT with an isotropic voxel size of 6 µm3 is applied in live anesthetized rats under controlled mechanical ventilation. Video animations of regional acinar and vascular strain are acquired in vivo. Maps of strain distribution due to positive-pressure breaths and cardiovascular activity in lung acini and blood vessels are derived based on CT images. Regional strain within the lung peripheral airspaces takes average values of 0.09 ± 0.02. Fitting the expression S = kVn, to the changes in peripheral airspace area (S) and volume (V) during a positive pressure breath yields an exponent n = 0.82 ± 0.03, suggesting predominant alveolar expansion rather than ductal expansion or alveolar recruitment. We conclude that this methodology can be used to assess acinar conformational changes during positive pressure breaths in intact peripheral lung airspaces.

Morphometric changes in the human pulmonary acinus during inflation

Despite decades of research into the mechanisms of lung inflation and deflation, there is little consensus about whether lung inflation occurs due to the recruitment of new alveoli or by changes in the size and/or shape of alveoli and alveolar ducts. In this study we use in vivo (3)He lung morphometry via MRI to measure the average alveolar depth and alveolar duct radius at three levels of inspiration in five healthy human subjects and calculate the average alveolar volume, surface area, and the total number of alveoli at each level of inflation. Our results indicate that during a 143 ± 18% increase in lung gas volume, the average alveolar depth decreases 21 ±5%, the average alveolar duct radius increases 7 ± 3%, and the total number of alveoli increases by 96 ± 9% (results are means ± SD between subjects; P < 0.001, P < 0.01, and P < 0.00001, respectively, via paired t-tests). Thus our results indicate that in healthy human subjects the lung inflates primarily by alveolar recruitment and, to a lesser extent, by anisotropic expansion of alveolar ducts.

Alveolar expansion imaged by optical sectioning microscopy

During lung expansion, the pattern of alveolar perimeter distension is likely to be an important determinant of lung functions as, for example, surfactant secretion. However, the segmental characteristics of alveolar perimeter distension remain unknown. Here, we applied real-time confocal microscopy in the isolated, perfused rat lung to determine the micromechanics of alveolar perimeter distension. To image the alveolar perimeter, we loaded alveolar epithelial cells with a fluorescent dye that we microinjected into the alveolus. Then we viewed single alveoli in a 2-microm-thick optical section at a focal plane 20 mum deep to the pleural surface at baseline. In each alveolus, we identified five to eight segments of the perimeter. For each segment, we determined length (L(seg)) by means of image analysis. At baseline alveolar pressure (P(alv)) of 5 cmH(2)O, L(seg) averaged 46 microm. We hyperinflated the lung to P(alv) of 20 cmH(2)O and identified the same optical section as referenced against morphological landmarks. Hyperinflation increased mean L(seg) by 14%. However, segment distension was heterogeneous, even within the single alveolus. Furthermore, distension was greater in alveolar type 1 than type 2 epithelial cells. These findings indicate that alveoli expand nonuniformly, suggesting that segments that distend the most might be preferred alveolar locations for injury in conditions associated with lung overdistension.

Tidal volume increases do not affect alveolar mechanics in normal lung but cause alveolar overdistension and exacerbate alveolar instability after surfactant deactivation

OBJECTIVE

We utilized microscopy to measure the impact of increasing tidal volume on individual alveolar mechanics (i.e., the dynamic change in alveolar size during tidal ventilation) in the living porcine lung.

DESIGN

In three anesthetized, mechanically ventilated pigs, we observed normal alveoli (n = 27) and alveoli after surfactant deactivation by Tween 20 lavage (n = 26) at three different tidal volumes (6, 12, and 15 mL/kg). Alveolar area was measured at peak inspiration (I) and at end expiration (E) by image analysis and I minus E was calculated as an index of alveolar stability (I-Edelta).

MEASUREMENTS AND MAIN RESULTS

In normal alveoli, increasing tidal volume did not change alveolar area at I (6 mL/kg = 9726 +/- 848 microm; 15 mL/kg = 9,637 +/- 884 microm ), E (6 mL/kg = 9747 +/- 800 microm; 15 mL/kg = 9742 +/- 853 microm ), or I-Edelta (6 mL/kg = -21 +/- 240 microm; 15 mL/kg = -105 +/- 229 microm ). In contrast, with surfactant deactivation, increasing tidal volume significantly increased alveolar area at I (6 mL/kg = 11,413 +/- 1032 microm; 15 mL/kg = 13,917 +/- 1214 microm ), at E (6 mL/kg = 10,462 +/- 906 microm; 15 mL/kg = 12,000 +/- 1066 microm ), and I-Edelta (6 mL/kg = 825 +/- 276 microm; 15 mL/kg = 1917 +/- 363 microm ). Moreover, alveolar instability (increased I-Edelta) was significantly increased at all tidal volumes with altered surface tension when compared with normal alveoli.

CONCLUSIONS

We conclude that high tidal volume ventilation does not alter alveolar mechanics in the normal lung; however, in the surfactant-deactivated lung, it causes alveolar overdistension and exacerbates alveolar instability.

Intraalveolar bubbles and bubble films: III. Vulnerability and preservation in the laboratory

BACKGROUND

Having confirmed (Scarpelli et al. 1996. Anat. Rec. 244:344-357 and 246:245-270) the discovery of intraalveolar bubbles and films as the normal anatomical infrastructure of aerated alveoli at all ages, we now address three questions. Why have these structures been so elusive? Visible in fresh lungs from the in vivo state, can they be preserved by known laboratory methods? Can they be preserved intact for study in tissue sections?

METHODS

Lungs of adult rabbits and pups were examined in thorax directly from the in vivo state to confirm normal bubbles both at functional residual capacity and at maximal volume; other lungs were permitted to deflate naturally to minimal volume. The fate of bubbles in situ (either intact, transected, or diced lung tissue) and of isolated bubbles was assessed (1) during conventional histopreparative processing, (2) during inflation-deflation after degassing, (3) after drying in air, (4) during and after quick freezing in liquid N2, and (5) after preservation in fixed and stained tissue sections prepared by a new double-impregnation procedure in which glutaraldehyde-fixed tissue was preembedded in agar, dehydrated and clarified chemically, embedded in paraffin, sectioned, and stained. Control studies included both blocking of bubble formation by rinsing the air spaces with Tween 20 prior to double impregnation and preparation of normal tissue without preembedding in agar.

RESULTS

(1) Each of the following procedures in conventional processing dislocated and disrupted bubbles and films: osmium tetroxide and glutaraldehyde:formaldehyde:tannic acid mixture fixation; chemical dehydration (70-100% ethanol) and clarification (xylene and acetone); and embedding in paraffin or epoxy resin. Transection and dicing of the tissue aggravated the untoward effects. In contrast, bubbles and films remained stable in either glutaraldehyde or formaldehyde, which, however, did not protect against the other agents. (2) Degassing destroyed all bubbles as expected; however, bubbles and films re-formed immediately with reinflation. (3) Topography of fixed bubbles and films was retained after air drying. The dry polygonal configuration reverted to spherical-oval either in saline solution or in 50% ethanol, whereas vulnerability to upgraded ethanol concentrations was unchanged. (4) Normal topography and shape appeared to be retained during quick freezing and after thawing. (5) Intraalveolar and intraductal bubbles and films were preserved and photographed in sections from tissue prepared by the double-impregnation procedure; they were not seen either when bubble formation had been blocked (double-impregnation procedure) or when preembedding in agar had been omitted.

CONCLUSIONS

(1) Whether or not fixed in glutaraldehyde or formaldehyde, preservation of intraalveolar and intraductal bubbles and films is not to be expected in tissue prepared by conventional histopreparative procedures, whereas product artifacts may be expected from bubble rupture in situ. (2) Degassing cannot be recommended for studies of alveolar structure-function interrelations because all natural bubbles are disrupted in the process, and bubble re-formation may not parallel their "natural history" in vivo. (3) Compared with glutaraldehyde or formaldehyde fixation, air drying offers no added protection against the untoward effects of conventional processing. (4) Quick-frozen tissue is equally at risk. (5) A new double-impregnation procedure does preserve bubbles and films during processing, sectioning, and staining.

Intraalveolar bubbles and bubble films: II. Formation in vivo through adulthood

BACKGROUND

Intraalveolar bubbles and bubble films have been shown to be part of the normal alveolar architecture in vivo from birth through the first 2 days of extrauterine life of rabbit pups (Scarpelli et al., 1996a. Anat. Rec. 244:344-357). The intraluminal boundary between air-way free gas and alveolar bubbles at the level of respiratory bronchioles is established within 1 hour after birth. We now examine the lung through the rest of development, namely, 2 weeks, 1, 2, and 3 months, and adulthood.

METHODS

In quick succession in anesthetized spontaneously breathing rabbits, the abdominal aorta was transected and trachea was occluded either after an end-tidal exhalation at functional residual capacity (FRC) or after volume expansion in vivo by a single inflation from FRC to 20 or 25 cm H2O pressure (V20, V25). Immediately the thorax was opened and lungs were examined (anterior, anterolateral) through a dissecting stereomicroscope while still in the chest, unperturbed (pleural surface temperature 34 degrees C). Heart and lungs were then removed en bloc and re-examined (anterior, lateral, posterior) to confirm that architecture had not changed (22-27 degrees C). After these immediate examinations, lungs were entered into one of the protocols enumerated in Results.

RESULTS

Immediate examination revealed bubbles in all aerated subpleural and deep ("central") alveoli from apex to base at all ages and temperatures. Bubbles were confirmed from two views (top and tangential) and from their individual mobility in response to gentle microprobe pressure. A "common bubble" (> 30 microns to approximately 120 microns inside diameter at FRC) appeared to occupy a single alveolus, sometimes arranged in clusters and collectively accounting for approximately 84% of the total bubble population. Few "large bubbles" appeared to be intraductal. We concluded that "small bubbles" (< or = 30 microns; approximately 16% of the total population) were contracted common bubbles. The free gas-bubble film boundary of the airways was at the level of respiratory bronchioles. Subsequent protocols: (1) Common bubbles moved out of adjoining tissue following subpleural incision. Adjacent bubbles either moved into vacated spaces or into the outside liquid medium. Large bubble(s) followed common bubbles out of the tissue. Small bubbles were less mobile and distal common bubbles did not move. The sequence of bubble movement at V25 was the same. Isolated bubbles had normal surfactant content and surface tension according to "Pattle's stability ratio." Transection revealed analogous conditions in central alveoli. (2) Bubble size increased during inflation from FRC to V25. Airless spaces were aerated with bubbles during inflation. (3) The bubble surface was compressed during deflation to 81% of maximal volume (Vmax) and below, including deflation to minimal volume (Vmin). (4) Bubble/alveolar shape changed from spherical-oval to polygonal when the pleural surface dried at FRC and V25. The original shape was restored when the surface was re-wet. Dry tissue showed but did not emit bubbles when cut; re-wet tissue did. (5) Lung liquid content and volume-pressure were normal at FRC. (6) As expected, conventionally fixed, dehydrated, and embedded sections showed no bubbles.

CONCLUSIONS

Bubbles and bubble films are fundamental to normal architecture of aerated alveoli at all lung volumes from birth through adulthood. As infrastructure, they sustain aeration and resist deformation. With ductal films, they may be expected to form an alveolar surface liquid (foam film) network (Scarpelli, 1988. Surfactants and the Lining of the Lung) that modulates liquid balance principally at Plateau borders. They expand and contract respectively during inflation and deflation, maintaining their closed film integrity. Films are compressed to "film collapse" in situ during deflation from volumes well above FRC to Vmin. At these volumes, intact films sustain aeration; some may disperse into t

Time shift in ventilation-induced density fluctuation of arterial blood

In an artificially ventilated dog, the varying tracheal pressure causes a density fluctuation in the blood sampled from the aorta. We cross-correlated the tracheal pressure with the density to determine the time shift or delay of the latter from the former waveform for a ventilation frequency in the range of 6-30 CPM. The delay time was found to be 29% of the mean transit time (MTT) of the pulmonary vasculature and independent of the ventilation frequency. A comparison of this percentage with the reported arterial-to-capillary-to-venous fractional volumes of the lung suggested that the delay time may be the MTT time for blood flowing through the venous network of the lung and the cross-correlation may serve as an in vivo means to partition the MTT of the pulmonary vasculature at its capillaries. These results and an analysis on the deformation of the viscoelastic, pulmonary capillaries indicated that the tracheal pressure, acting primarily through the viscous part of the viscoelasticity, deforms the capillaries to produce the density fluctuation in blood outflowing from the lung.

Alveolar surface tensions in excised rabbit lungs: effect of temperature

In excised, perfused rabbit lungs the alveolar surface tension was measured in individual alveoli over the entire P-V loop at different temperatures (22 and 37 degrees C), using an improved microdroplet method. Additional in vitro experiments are reassuring that the microdroplets do not affect the properties of the alveolar surface film. The in situ measurements show that the alveolar surface tension and the surface tension to volume relation are essentially the same at 22 and 37 degrees C. A maximal surface tension of about 30 mN X m-1 was measured at TLC, and there is a substantial surface tension to volume hysteresis, which amounts to almost 10 mN X m-1 in the middle volume range of a complete pressure volume cycle of the lung. However, with respect to the absolute values of alveolar surface tension, and the shape and width of the hysteresis, these directly obtained results are different from previous findings.

Functional anatomy and volume-pressure characteristics of immature lungs

The mechanical behavior of immature rabbit fetal lungs in situ was assessed by air and saline volume-pressure diagrams. All lungs were in their natural fetal state, i.e., filled with fetal pulmonary fluid, prior to inflation. Anatomic correlates were determined by continuous stereomicroscopic monitoring of the lungs. We found the following to be characteristic of immature lungs: (1) Tissue retractive forces are similar to adults. (2) Fetal lungs are not 'plastic' above functional residual capacity. (3) Initial aeration is by 'axial filling' in which airways are distended several times their resting size. (4) Invariably, peripheral rather than central saccules are the first to be aerated and saccules are recruited by both pressure- and time-dependent processes. (5) Pressure-dependence is related to surface forces and terminal orifice size, while time-dependent processes include orifice enlargement, liquid flow through terminal conduits, and the formation of very short-lived, labile bubbles. (6) 'Opening pressure' inflection in the VP diagram is not coincidental with, but follows the onset of saccular aeration. (7) Negative compliance at the onset of deflation is due to saccular enlargement and recruitment. (8) Hysteresis is due to tissue conformational characteristics at high pressures and air entrapment at low pressures. (9) Surface tension cannot be measured reliably from the saline and air VP diagrams.

Change pattern of pleural deformation pressure on varying lung height and volume

Simultaneous measurements of pleural surface (Ppl) and liquid (Pliq) pressures were made at various lung heights in lateral and supine dogs on increasing lung volume from FRC to 80% TLC by decreasing abdominal pressure. The difference between Pliq and Ppl, /Pdef liq/, i.e. the pressure elicited by the deformation forces of the contacting pleurae over the areas of intervening liquid, was greater in the superior parts at all lung volumes. Previous data on pleural liquid thickness (9) and the present measurements show that an increase of /Pdef liq/ may occur either with a decrease or an increase of 9. delta Pdef liq/delta Ppl over a given range of Ppl, increases progressively from bottom up. For a given height the greater the initial /Pdef liq/, the greater delta Pdef liq/delta Ppl. When /Pdef liq/ was decreased by injecting liquid into the pleural space at FRC, delta Pdef liq/delta Ppl decreased. Hence, delta Pdef liq/delta Ppl depends on the initial Pdef liq, i.e. upon the initial tightness of fitting of the pleural membranes. A model interpretation of this relationship is provided.