Static and dynamic properties of excised cat lung in relation to temperature

Quasi-dynamic breathing model of the lung incorporating viscoelasticity of the lung tissue

We advanced a novel model to calculate viscoelastic lung compliance and airflow resistance in presence of mucus, accounting for the quasi-linear viscoelastic stress-strain response of the parenchyma (alveoli) tissue. We adapted a continuum-based numerical modeling approach for the lung, integrating the fluid mechanics of the airflow within individual generations of the bronchi and alveoli. The model accounts for elasticity of the deformable bronchioles, resistance to airflow due to the presence of mucus within the bronchioles, and subsequent mucus flow. Simulated quasi-dynamic inhalation and expiration cycles were used to characterize the net compliance and resistance of the lung, considering the rheology of the mucus and viscoelastic properties of the parenchyma tissue. The structure and material properties of the lung were identified to have an important contribution to the lung compliance and airflow resistance. The secondary objective of this work was to assess whether a higher frequency and smaller volume of harmonic air flow rate compared to a normal ventilator breathing cycle enhanced mucus outflow. Results predict, lower mucus viscosity and higher excitation frequency of breathing are favorable for the flow of mucus up the bronchi tree, towards the trachea.

The highly packed and dehydrated structure of pre-formed unexposed human pulmonary surfactant isolated from amniotic fluid

By coating the alveolar air-liquid interface, lung surfactant overwhelms surface tension forces that, otherwise, would hinder the lifetime effort of breathing. Years of research have provided a picture of how highly hydrophobic and specialized proteins in surfactant promote rapid and efficient formation of phospholipid-based complex three-dimensional films at the respiratory surface, highly stable under the demanding breathing mechanics. However, recent evidence suggest that the structure and performance of surfactant typically isolated from bronchoalveolar lung lavages may be far from that of nascent, still unused, surfactant as freshly secreted by type II pneumocytes into the alveolar airspaces. In the present work, we report the isolation of lung surfactant from human amniotic fluid (amniotic fluid surfactant, AFS) and a detailed description of its composition, structure and surface activity in comparison to a natural surfactant (NS) purified from porcine bronchoalveolar lavages. We observe that the lipid/protein complexes in AFS exhibit a substantially higher lipid packing and dehydration than in NS. AFS shows melting transitions at higher temperatures than NS and a conspicuous presence of non-lamellar phases. The surface activity of AFS is not only comparable to that of NS under physiologically-meaningful conditions, but displays significantly higher resistance to inhibition by serum or meconium, agents that inactivate surfactant in the context of severe respiratory pathologies. We propose that AFS may be the optimal model to study the molecular mechanisms sustaining pulmonary surfactant performance in health and disease, and the reference material to develop improved therapeutic surfactant preparations to treat yet unresolved respiratory pathologies.

Structural Changes in Films of Pulmonary Surfactant Induced by Surfactant Vesicles

When compressed by the shrinking alveolar surface area during exhalation, films of pulmonary surfactant in situ reduce surface tension to levels at which surfactant monolayers collapse from the surface in vitro. Vesicles of pulmonary surfactant added below these monolayers slow collapse. X-ray scattering here determined the structural changes induced by the added vesicles. Grazing incidence X-ray diffraction on monolayers of extracted calf surfactant detected an ordered phase. Mixtures of dipalmitoyl phosphatidylcholine and cholesterol, but not the phospholipid alone, mimic that structure. At concentrations that stabilize the monolayers, vesicles in the subphase had no effect on the unit cell, and X-ray reflection showed that the film remained monomolecular. The added vesicles, however, produced a concentration-dependent increase in the diffracted intensity. These results suggest that the enhanced resistance to collapse results from enlargement by the additional material of the ordered phase.

Calf Lung Surfactant Recovers Surface Functionality After Exposure to Aerosols Containing Polymeric Particles

BACKGROUND

Recent studies have shown that colloidal particles can disrupt the interfacial properties of lung surfactant and thus key functional abilities of lung surfactant. However, the mechanisms underlying the interactions between aerosols and surfactant films remain poorly understood, as our ability to expose films to particles via the aerosol route has been limited. The aim of this study was to develop a method to reproducibly apply aerosols with a quantifiable particle dose on lung surfactant films and investigate particle-induced changes to the interfacial properties of the surfactant under conditions that more closely mimic those in vivo.

METHODS

Films of DPPC and Infasurf^® were exposed to aerosols containing polystyrene particles generated using a Dry Powder Insufflator^™. The dose of particles deposited on surfactant films was determined via light absorbance. The interfacial properties of the surfactant were studied using a Langmuir-Wilhelmy balance during surfactant compression to film collapse and cycles of surface compression and expansion at a fast cycling rate within a small surface area range.

RESULTS

Exposure of surfactant films to aerosols led to reproducible dosing of particles on the films. In film collapse experiments, particle deposition led to slight changes in collapse surface pressure and surface area of both surfactants. However, longer interaction times between particles and Infasurf^® films resulted in time-dependent inhibition of surfactant function. When limited to lung relevant surface pressures, particles reduced the maximum surface pressure that could be achieved. This inhibitory effect persisted for all compression-expansion cycles in DPPC, but normal surfactant behavior was restored in Infasurf^® films after five cycles.

CONCLUSIONS

The observation that Infasurf^® was able to quickly restore its function after exposure to aerosols under conditions that better mimicked those in vivo suggests that particle-induced surfactant inhibition is unlikely to occur in vivo due to an aerosol exposure.

Validating excised rodent lungs for functional hyperpolarized xenon-129 MRI

Ex vivo rodent lung models are explored for physiological measurements of respiratory function with hyperpolarized (hp) (129)Xe MRI. It is shown that excised lung models allow for simplification of the technical challenges involved and provide valuable physiological insights that are not feasible using in vivo MRI protocols. A custom designed breathing apparatus enables MR images of gas distribution on increasing ventilation volumes of actively inhaled hp (129)Xe. Straightforward hp (129)Xe MRI protocols provide residual lung volume (RV) data and permit for spatially resolved tracking of small hp (129)Xe probe volumes during the inhalation cycle. Hp (129)Xe MRI of lung function in the excised organ demonstrates the persistence of post mortem airway responsiveness to intravenous methacholine challenges. The presented methodology enables physiology of lung function in health and disease without additional regulatory approval requirements and reduces the technical and logistical challenges with hp gas MRI experiments. The post mortem lung functional data can augment histological measurements and should be of interest for drug development studies.

The accelerated late adsorption of pulmonary surfactant

Adsorption of pulmonary surfactant to an air-water interface lowers surface tension (γ) at rates that initially decrease progressively, but which then accelerate close to the equilibrium γ. The studies here tested a series of hypotheses concerning mechanisms that might cause the late accelerated drop in γ. Experiments used captive bubbles and a Wilhelmy plate to measure γ during adsorption of vesicles containing constituents from extracted calf surfactant. The faster fall in γ reflects faster adsorption rather than any feature of the equation of state that relates γ to surface concentration (Γ). Adsorption accelerates when γ reaches a critical value rather than after an interval required to reach that γ. The hydrophobic surfactant proteins (SPs) represent key constituents, both for reaching the γ at which the acceleration occurs and for producing the acceleration itself. The γ at which rates of adsorption increase, however, is unaffected by the Γ of protein in the films. In the absence of the proteins, a phosphatidylethanolamine, which, like the SPs, induces fusion of the vesicles with the interfacial film, also causes adsorption to accelerate. Our results suggest that the late acceleration is characteristic of adsorption by fusion of vesicles with the nascent film, which proceeds more favorably when the Γ of the lipids exceeds a critical value.

Lung tissue mechanics as an emergent phenomenon

The mechanical properties of lung parenchymal tissue are both elastic and dissipative, as well as being highly nonlinear. These properties cannot be fully understood, however, in terms of the individual constituents of the tissue. Rather, the mechanical behavior of lung tissue emerges as a macroscopic phenomenon from the interactions of its microscopic components in a way that is neither intuitive nor easily understood. In this review, we first consider the quasi-static mechanical behavior of lung tissue and discuss computational models that show how smooth nonlinear stress-strain behavior can arise through a percolation-like process in which the sequential recruitment of collagen fibers with increasing strain causes them to progressively take over the load-bearing role from elastin. We also show how the concept of percolation can be used to link the pathologic progression of parenchymal disease at the micro scale to physiological symptoms at the macro scale. We then examine the dynamic mechanical behavior of lung tissue, which invokes the notion of tissue resistance. Although usually modeled phenomenologically in terms of collections of springs and dashpots, lung tissue viscoelasticity again can be seen to reflect various types of complex dynamic interactions at the molecular level. Finally, we discuss the inevitability of why lung tissue mechanics need to be complex.

The biophysical function of pulmonary surfactant

Pulmonary surfactant lowers surface tension in the lungs. Physiological studies indicate two key aspects of this function: that the surfactant film forms rapidly; and that when compressed by the shrinking alveolar area during exhalation, the film reduces surface tension to very low values. These observations suggest that surfactant vesicles adsorb quickly, and that during compression, the adsorbed film resists the tendency to collapse from the interface to form a 3D bulk phase. Available evidence suggests that adsorption occurs by way of a rate-limiting structure that bridges the gap between the vesicle and the interface, and that the adsorbed film avoids collapse by undergoing a process of solidification. Current models, although incomplete, suggest mechanisms that would partially explain both rapid adsorption and resistance to collapse as well as how different constituents of pulmonary surfactant might affect its behavior.

The melting of pulmonary surfactant monolayers

Monomolecular films of phospholipids in the liquid-expanded (LE) phase after supercompression to high surface pressures (pi), well above the equilibrium surface pressure (pi(e)) at which fluid films collapse from the interface to form a three-dimensional bulk phase, and in the tilted-condensed (TC) phase both replicate the resistance to collapse that is characteristic of alveolar films in the lungs. To provide the basis for determining which film is present in the alveolus, we measured the melting characteristics of monolayers containing TC dipalmitoyl phosphatidylcholine (DPPC), as well as supercompressed 1-palmitoyl-2-oleoyl phosphatidylcholine and calf lung surfactant extract (CLSE). Films generated by appropriate manipulations on a captive bubble were heated from < or =27 degrees C to > or =60 degrees C at different constant pi above pi(e). DPPC showed the abrupt expansion expected for the TC-LE phase transition, followed by the contraction produced by collapse. Supercompressed CLSE showed no evidence of the TC-LE expansion, arguing that supercompression did not simply convert the mixed lipid film to TC DPPC. For both DPPC and CLSE, the melting point, taken as the temperature at which collapse began, increased at higher pi, in contrast to 1-palmitoyl-2-oleoyl phosphatidylcholine, for which higher pi produced collapse at lower temperatures. For pi between 50 and 65 mN/m, DPPC melted at 48-55 degrees C, well above the main transition for bilayers at 41 degrees C. At each pi, CLSE melted at temperatures >10 degrees C lower. The distinct melting points for TC DPPC and supercompressed CLSE provide the basis by which the nature of the alveolar film might be determined from the temperature-dependence of pulmonary mechanics.

Ventilatory responses to changes in temperature in mammals and other vertebrates

This article reviews the relationship between pulmonary ventilation (VE) and metabolic rate (oxygen consumption) during changes in ambient temperature. The main focus is on mammals, although for comparative purposes the VE responses of ectothermic vertebrates are also discussed. First, the effects of temperature on pulmonary mechanics, chemoreceptors, and airway receptors are summarized. Then we review the main VE responses to cold and warm stimuli and their interaction with exercise, hypoxia, or hypercapnia. In these cases, mammals attempt to maintain both oxygenation and body temperature, although conflicts can arise because of the respiratory heat loss associated with the increase in ventilation. Finally, we consider the VE responses of mammals when body temperature changes, as during torpor, fever, sleep, and hypothermia. In ectotherms, during changes in temperature, VE control becomes part of a general strategy to maintain constant relative alkalinity and ensure a constancy of pH-dependent protein functions (alphastat regulation). In mammals on the other hand, VE control is aimed to balance metabolic needs with homeothermy. Therefore, alphastat regulation in mammals seems to have a low priority, and it may be adopted only in exceptional cases.

An alternative view of the role(s) of surfactant and the alveolar model

Currently, the study of surfactant proteins is much in vogue, but, in the early days, the physics underlying surfactant function was treated somewhat superficially, leaving assumptions that have become culturally embedded, such as the "bubble" model of the alveolus. This review selectively reexamines these assumptions, comparing each combination of alveolar model and role of surfactant for compatibility with the major features of pulmonary mechanics and alveolar stability, morphology, and fluid balance.

Effect of temperature on pressure-volume hysteresis of excised lungs

The objective of this study was to determine if the effect of temperature on excised lung pressure-volume (P-V) hysteresis during various P-V maneuvers would be consistent with predicted effects based on the recruitment-derecruitment (R-D) model of lung P-V hysteresis. Three sets of P-V curves were recorded for excised rat lungs at (1) 24 degrees C, (2) either 42 degrees or 45 degrees C, and (3) 24 degrees C. After full inflation of the lung, deflation-inflation (D-I) cycles were performed between total lung capacity (30 cmH2O) and successively decreasing end-expiratory pressures (EEPs). Normalized hysteresis (K) was plotted vs EEP. K remained relatively constant at EEPs > or = +5 cmH2O at 24 degrees C and 42 degrees C and > +5 cmH2O at 45 degrees C. Large increases in K occurred as the EEP was further reduced, with the relationship of K vs EEP being shifted to the right at 42 degrees C and 45 degrees C relative to 24 degrees C, with the greater shift occurring at 45 degrees C. Previous work has shown that the R-D of lung units contributes to P-V hysteresis and is EEP-dependent, increasing at EEPs < or = +4 +/- 1 cmH2O at room temperature (Cheng et al., 1995). This study suggests that at increased temperatures, R-D of lung units is initiated at higher EEPs and is more extensive than at room temperature.

Properties of lavage material from excised lungs ventilated at different temperatures

We studied the phospholipid (PL) and protein contents, the PL composition, and some of the surface properties of lavage materials obtained from freshly excised rat lungs and excised lungs which had been ventilated at different temperatures (22, 37, and 42 degrees C). Ventilation (60 breaths/min) was carried out at constant tidal volume with periodic sighs for one hour. Although there is slightly more lavageable PL and protein in lungs ventilated at 22 degrees C than in freshly excised lungs, there is no difference in the PL composition or surface properties of lavage materials from these lungs. However, as the temperature at which lungs are ventilated is increased to 37 degrees and 42 degrees C, there is(are): 1) a reduction in lavage fluid PL, 2) a reduction in the relative amounts of total phosphatidylcholines (PC) and disaturated PC (DSPC), the major surface active component of pulmonary surfactant, 3) an increase in unsaturated PC, and 4) increases in total protein and nonsedimentable protein (100,000 g; 2 hr) in the lavage materials. There are also differences in the surface properties of the lavage materials from lungs ventilated at higher temperatures when compared with freshly excised lungs or lungs ventilated at 22 degrees C, probably as a result of the changes in composition. Maximal surface tension is greater for lavage materials from lungs ventilated at 37 degrees C. For lungs ventilated at 42 degrees C, maximal and minimal surface tension values are increased. These results demonstrate that there are differences in the composition and surface properties of alveolar lavage materials from excised lungs ventilated at different temperatures.

Fluid dynamics during initial aeration of mature fetal lung and effect of temperature

Air volume-pressure (VP) curves were recorded simultaneously on pairs of mature rabbit fetuses from the same litter with one member of the pair at 37 degrees C and the other at 22 degrees C. Intrasaccular bubbles, formed primarily during inflation, were assessed for stability and surface tension (gamma). Average air flow rates (dV/dt) were calculated from the VP data. In separate experiments, liquid VP curves were recorded at 37 degrees and 22 degrees C: maximal liquid V was matched to maximal air V at 37 degrees and 22 degrees C, respectively. Fetal pulmonary liquid (FPL) viscosity (eta) and density (rho) were determined by standard methods. Both the effect of temperature on lung mechanics as reported previously, and the reliability of the rabbit model were confirmed in the paired fetuses. Analysis of fluid dynamics revealed that of the six parameters relevant to initial inflation-deflation of FPL-filled lungs, liquid rho, distensibility (recoil), and gamma were not altered significantly by temperature increase from 22 degrees to 37 degrees C. Enhanced lung mechanics at 37 degrees C (including enhanced inflation at lower P, higher maximal V, increased production of intrasaccular bubbles, and higher V at end-deflation) was primarily due to lowering of FPL eta at the higher temperature which appears to have an effect by augmenting bulk liquid flow and liquid drainage. Lower eta increases bulk flow through airways directly. Consequent recruitment and distention of these conducting units effectively increases radius (r) and further enhances flow. (The ultimate "brake" to airways flow at both temperatures is counter P from gamma at air/liquid menisci.).(ABSTRACT TRUNCATED AT 250 WORDS)

Ventilatory recovery from hypothermia in anesthetized cats

Ventilation and breathing pattern were recorded in a group of seven anesthetized cats during rewarming from 24 to 38 degrees C of esophageal temperature. It was found that at 24 degrees C, ventilation was very much depressed accounting for an alveolar hypoventilation resulting in hypoxia and hypercapnia. During rewarming, ventilation increased steadily; this was caused by sequential changes in central inspiratory activity (VT/Ti) and Ti/Tt ratio reflecting breath timing. Changes in VT/Ti have been initially attributed to an improvement in chemoresponsiveness and subsequently, to an involvement of supra-pontine thermoregulatory control areas during rewarming. Marked changes in breath timing, especially observed between 28 and 34 degrees C, have been attributed to a direct effect of rewarming upon the brain stem respiratory network. It has the result, that during hypothermia, several components of the respiratory control system are differently affected causing marked changes in breathing pattern and ventilation. They are accompanied by modifications in arterial blood pressure and heart rate.

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.

The effect of temperature on gas trapping in excised lungs

In this study the effect of temperature on gas trapping in excised lungs was examined with two types of experiments in rats. In the first, changes in gas trapping following ten successive inflation-deflation cycles at the same constant ventilation rate were examined at 17, 27, 37 and 42 degrees C. In the second, the effects of five different ventilation rates at temperatures of 17, 27 and 37 degrees C were determined. The fraction of gas trapped in lungs repeatedly ventilated for ten inflation-deflation cycles at constant ventilation rates remained nearly constant with time at 17 and 27 degrees C but decreased with time at 37 and 42 degrees C. The amount of gas trapped in the lung at 27 degrees C fell with the logarithm of increasing ventilation rate. Lowering the temperature shifted this relationship toward lower ventilation rates while increasing the temperatures caused an apparent shift toward higher ventilation rates.

Hysteresis in saturated phospholipid films and its potential relevance for lung surfactant function in vivo

The surface pressure-area (pi-A) hysteresis in films of dipalmitoyl phosphatidylcholine (DPPC) is investigated under dynamic cycling at 22 degrees C. The dynamic pi-a hysteresis of such films increases with extent of film compression, becoming maximal when compression is carried out past the point of generalized monolayer collapse based on fatty acid chain limiting areas. Several characteristics of the pi-A hysteresis found in the dynamic cycling of pure DPPC films are shown to be present in mixed surface films containing this saturated phospholipid, including binary DPPC:dioleoyl phosphatidylcholine (DOPC) films and multicomponent lung extract films at body temperature. Specific hysteresis characteristics, particularly a rapid surface pressure fall over a small area change at the beginning of film expansion, may have direct importance for lung surfactant function in vivo, and a possible model for this is presented. A major consequence stressed in terms of pulmonary mechanics concerns the role of lung surfactant pi-A hysteresis characteristics upon alveolar recruitment and increased inflation uniformity during inspiration. An attempt is made to reconcile lung surfactant surface behavior with recent concepts of alveolar area and shape changes during breathing, as well as with known clinical findings in states of lung surfactant deficiency. Some possible numerical parameters that may be useful in evaluating surfactant "activity" are presented.

Quantitative comparisons of mammalian lung pressure volume curves

The deflation pressure-volume curves of the lungs of a wide range of mammalian species were studied to compare their mechanical properties. A monoexponential mathematical function of the form V = Vmax - (Vmax - Vo)e - kp was fitted to the deflation data. It was found that the bulk stiffness index k (approximately 0.12 cm H2O-1) varied little over the 10(5) fold range of animal body weight. This range of k was far smaller than found in man in the presence of pulmonary parenchymal disease. It was concluded that the intrinsic stiffness characteristics of most mammalian lungs are similar.

Evaluation of some alternative mechanisms for interface-related stress relaxation in lung

Lung surfactant might exchange reversibly between the alveolar gas-liquid interface and the liquid lining layer. This solubilization process could then contribute to time-dependent pulmonary mechanical phenomena. Theoretical analysis of a model alveolus shows how solubilization can produce recoil pressure relaxation of the lung. The analysis shows that for most experimental observations reported the solubilization kinetics probably could not have been determined by the actual fitting in, or squeezing out, of surfactant in the surface film. Most data are consistent with a solubilization rate controlled by diffusion in the liquid sublayer. However, in order for a diffusion-controlled mechanism to be significant the diffusing substance should be large, such as a micelle, and the alveolar liquid layer should be at least 1 micrometer thick. These conditions are quite realistic for slightly edematous lungs and thus might apply to many experimental, or pathological, situations.

Temperature and hydration: factor affecting increased recoil of excised rabbit lung

We have examined two factors which strongly affect the rate and amount by which excised rabbit lungs become stiffer during mechanical hyperventilation, Pressure-volume (PV) curves were compared before and after 3 hr ventilation (tidal volume 50% TLC, frequency 8/min, end-expiratory pressure of zero) in the temperature range 4 degrees C to 37 degrees C. Lungs were separated into three groups according to method of suspension: (A) floated on saline, (B) floated on mineral oil, and (C) suspended from the trachea. Lung wet/dry weight ratios (W/D) were obtained from groups A and C. In all lungs marked stiffening occurred below 22 degrees C, but the effect diminished sharply between 22 and 28 degrees C. Lungs not in contact with saline (Groups B and C) revealed little change in recoil above 28 degrees C, whereas those in Group A once again tended to become stiff near 37 degrees C. Lung water content in Group A at 37 degrees C was found to be more than double that in Group C. We conclude that two factors can contribute to the stiffening induced by ventilation: (a) temperature maintained below 27-28 degrees C, or (b) excess lung water in combination with temperature above 28 degrees C.

pH, temperature, humidity and the dynamic force-area curve of dipalmitoyl lecithin

Both high pH at 25 degrees C and humidity at 37 degrees C prevent DPL films from attaining zero surface tension, whereas humidity at 25 degrees C and high pH at 37 degrees C do not. At 37 degrees C DPL lowered surface tension to zero when spread from organic solvent or when absorbed from aqueous 0.15 M NaCl in the surface balance in which the surface film was exposed to the room air (dry film). Upon saturation of the atmosphere with water vapor in equilibrium with the aqueous phase at 37 degrees C in a closed chamber, DPL lost the ability to produce zero surface tension, and the gamma min of the DPL film increased from zero to 22 dyne/cm. Addition of DPL in chloroform to distilled water before dispersion by sonication did not prevent the effect of the humidity. However, when the chloroform solution of DPL was added to 0.15 M NaCl before sonication, the adsorbed film produced immediately a stable gamma min of zero in a saturated atmosphere, 37 degrees C. In the absence of chloroform, with DPL adsorbed from either distilled water or 0.15 M NaCl, the effect of humidity was reversed either by removing the chamber and returning the wet film to room air or by introducing small quantities of dispersing agents such as cholesteryl palmitate. However, whereas the effect of humidifying the air was reversible indefinitely, the effect of cholesteryl palmitate (zero surface tension, wet or dry film) was irreversible. This means that there are substances or conditions that can assist DPL films in maintaining zero surface tension when such films are exposed to humidity-saturated air at 37 degrees C.