Alveolar epithelial glycocalyx degradation mediates surfactant dysfunction and contributes to acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a common cause of respiratory failure yet has few pharmacologic therapies, reflecting the mechanistic heterogeneity of lung injury. We hypothesized that damage to the alveolar epithelial glycocalyx, a layer of glycosaminoglycans interposed between the epithelium and surfactant, contributes to lung injury in patients with ARDS. Using mass spectrometry of airspace fluid noninvasively collected from mechanically ventilated patients, we found that airspace glycosaminoglycan shedding (an index of glycocalyx degradation) occurred predominantly in patients with direct lung injury and was associated with duration of mechanical ventilation. Male patients had increased shedding, which correlated with airspace concentrations of matrix metalloproteinases. Selective epithelial glycocalyx degradation in mice was sufficient to induce surfactant dysfunction, a key characteristic of ARDS, leading to microatelectasis and decreased lung compliance. Rapid colorimetric quantification of airspace glycosaminoglycans was feasible and could provide point-of-care prognostic information to clinicians and/or be used for predictive enrichment in clinical trials.

The effect of maternal protein restriction during perinatal life on the inflammatory response in pediatric rats

Fetal growth restriction can affect health outcomes in postnatal life. This study tested the hypothesis that the response to an inflammatory pulmonary insult is altered in pediatric fetal growth restricted rats. Using a low-protein diet during gestation and postnatal life, growth-restricted male and female rats and healthy control rats were exposed to an inflammatory insult via the intratracheal instillation of heat-killed bacteria. After 6 h, animal lungs were examined for lung inflammation and status of the surfactant system. The results showed that in response to an inflammatory insult, neutrophil infiltration was decreased in both male and female rats in the growth-restricted animals compared with the control rats. The amount of surfactant was increased in the growth-restricted animals compared with the control rats, regardless of the inflammatory insult. It is concluded that fetal growth restriction results in increased surfactant and altered neutrophil responses following pulmonary insult.

The effects of aging and exercise on lung mechanics, surfactant and alveolar macrophages

Purpose: Advancing age leads to changes to the respiratory system associated with increased susceptibility to lung diseases, and exercise may counteract this effect. To explore the underlying processes, we investigated the effects of aging and exercise on lung mechanics, alveolar macrophage function, and surfactant pools and activity, in mice. It was hypothesized that aging would impact lung mechanics, macrophage polarization, and the status of the surfactant system, and that these changes would be mitigated by exercise. Methods: Male C57BL/6 mice were housed from 2-3 to 22 months, for the aged group, or until 4 months of age for young mice. Mice in both groups were randomized to voluntarily running exercise or to non-exercise, for a 2-month period. Mice were euthanized and lung mechanics were analyzed using a flexiVent ventilator. Subsequently, the lungs were lavaged to obtain pulmonary surfactant and alveolar macrophages. Pulmonary surfactant was analyzed for pool sizes and activity whereas alveolar macrophages were examined for response to pro and anti-inflammatory stimuli. Results: Changes in lung mechanics, such as increased compliance and decreased airway resistance, were associated with aging but were not affected by exercise. The quantity as well as the biophysical activity of the pulmonary surfactant system was unaffected by either aging or exercise. More alveolar macrophages were recovered from exercising aged mice compared to both the young and non-exercising groups. Macrophages in this aged exercise group were more responsive to an anti-inflammatory stimulus. Conclusions: Our data supports previous literature that suggest the development of emphysema-like alterations to lung mechanics with aging. This effect was independent of exercise. Our data also indicates that surfactant is unaffected by aging and exercise. Alveolar macrophage properties and numbers were affected by exercise in the aging lung and may represent the main, potentially beneficial, effect of exercise on the pulmonary system.

Maternal protein restriction during perinatal life affects lung mechanics and the surfactant system during early postnatal life in female rats

Limited information is available on how fetal growth retardation (FGR) affects the lung in the neonatal period in males and females. This led us to test the hypothesis that FGR alters lung mechanics and the surfactant system during the neonatal period. To test this hypothesis a model of FGR was utilized in which pregnant rat dams were fed a low protein diet during both the gestation and lactation period. We subsequently analyzed lung mechanics using a FlexiVent ventilator in male and female pups at postnatal day 7 and 21. Lung lavage material was obtained at postnatal day 1, 7 and 21, and was used for analysis of the surfactant system which included measurement of the pool size of surfactant and its subfraction as well as the surface tension reducing ability of the surfactant. The main result of the study was a significantly lower lung compliance and higher tissue elastance which was observed in FGR female offspring at day 21 compared to control offspring. In addition, female LP offspring exhibited lower surfactant pool sizes at postnatal day 1compared to controls. These changes were not observed in the male offspring. It is concluded that FGR has a different impact on pulmonary function and on surfactant in female, as compared to male, offspring.

Impact of ventilation-induced lung injury on the structure and function of lamellar bodies

Alterations to the pulmonary surfactant system have been observed consistently in ventilation-induced lung injury (VILI) including composition changes and impairments in the surface tension reducing ability of the isolated extracellular surfactant. However, there is limited information about the effects of VILI on the intracellular form of surfactant, the lamellar body. It is hypothesized that VILI leads to alterations of lamellar body numbers and function. To test this hypothesis, rats were randomized to one of three groups, nonventilated controls, control ventilation, and high tidal volume ventilation (VILI). Following physiological assessment to confirm lung injury, isolated lamellar bodies were tested for surfactant function on a constrained sessile drop surfactometer. A separate cohort of animals was used to fix the lungs followed by examination of lamellar body numbers and morphology using transmission electron microscopy. The results showed an impaired ability of reducing surface tension for the lamellar bodies isolated from the VILI group as compared with the two other groups. The morphological assessment revealed that the number, and the relative area covered by, lamellar bodies were significantly decreased in animals with VILI animals as compared with the other groups. It is concluded that VILI causes significant alterations to lamellar bodies. It is speculated that increased secretion causes a depletion of lamellar bodies that cannot be compensated by de novo synthesis of surfactant in these injured lungs.

Lack of matrix metalloproteinase 3 in mouse models of lung injury ameliorates the pulmonary inflammatory response in female but not in male mice

BACKGROUND

The acute respiratory distress syndrome (ARDS) is a complex pulmonary disorder in which the local release of cytokines and chemokines appears central to the pathophysiology.

OBJECTIVE

Based on the known role of matrix metalloproteinase-3 (MMP3) in inflammatory processes, the objective was to examine the role of MMP3 in the pathogenesis of ARDS through the modulation of pulmonary inflammation.

MATERIALS AND METHODS

Female and male, wild type (MMP3^+/+) and knock out (MMP3^-/-) mice were exposed to two, clinically relevant models of ARDS including (i) lipopolysaccharide (LPS)-induced lung injury, and (ii) hydrochloric acid-induced lung injury. Parameters of lung injury and inflammation were assessed through measurements in lung lavage including total protein content, inflammatory cell influx, and concentrations of mediators such as TNF-α, IL-6, G-CSF, CXCL1, CXCL2, and CCL2. Lung histology and compliance were also evaluated in the LPS model of injury.

RESULTS

Following intra-tracheal LPS instillation, all mice developed lung injury, as measured by an increase in lavage neutrophils, and decrease in lung compliance, with no overall effect of genotype observed. Increased concentrations of lavage inflammatory cytokines and chemokines were also observed following LPS injury, however, LPS-instilled female MMP3^-/- mice had lower levels of inflammatory mediators compared to LPS-instilled female MMP3^+/+ mice. This effect of the genotype was not observed in male mice. Similar findings, including the MMP3-related sex differences, were also observed after acid-induced lung injury.

CONCLUSION

MMP3 contributes to the pathogenesis of ARDS, by affecting the pulmonary inflammatory response in female mice in relevant models of lung injury.

The effect of matrix metalloproteinase-3 deficiency on pulmonary surfactant in a mouse model of acute lung injury

The acute respiratory distress syndrome (ARDS) is characterized by arterial hypoxemia accompanied by severe inflammation and alterations to the pulmonary surfactant system. Published data has demonstrated a protective effect of matrix metalloproteinase-3 (Mmp3) deficiency against the inflammatory response associated with ARDS; however, the effect of Mmp3 on physiologic parameters and alterations to surfactant have not been previously studied. It was hypothesized that Mmp3 deficient (Mmp3(-/-)) mice would be protected against lung dysfunction associated with ARDS and maintain a functional pulmonary surfactant system. Wild type (WT) and Mmp3(-/-) mice were subjected to acid-aspiration followed by mechanical ventilation. Mmp3(-/-) mice maintained higher arterial oxygenation compared with WT mice at the completion of ventilation. Significant increase in functional large aggregate surfactant forms were observed in Mmp3(-/-) mice compared with WT mice. These findings further support a role of Mmp3 as an attractive therapeutic target for drug development in the setting of ARDS.

The effects of exogenous surfactant administration on ventilation-induced inflammation in mouse models of lung injury

Background

Mechanical ventilation (MV) is an essential supportive therapy for acute lung injury (ALI); however it can also contribute to systemic inflammation. Since pulmonary surfactant has anti-inflammatory properties, the aim of the study was to investigate the effect of exogenous surfactant administration on ventilation-induced systemic inflammation.

Methods

Mice were randomized to receive an intra-tracheal instillation of a natural exogenous surfactant preparation (bLES, 50 mg/kg) or no treatment as a control. MV was then performed using the isolated and perfused mouse lung (IPML) set up. This model allowed for lung perfusion during MV. In experiment 1, mice were exposed to mechanical ventilation only (tidal volume =20 mL/kg, 2 hours). In experiment 2, hydrochloric acid or air was instilled intra-tracheally four hours before applying exogenous surfactant and ventilation (tidal volume =5 mL/kg, 2 hours).

Results

For both experiments, exogenous surfactant administration led to increased total and functional surfactant in the treated groups compared to the controls. Exogenous surfactant administration in mice exposed to MV only did not affect peak inspiratory pressure (PIP), lung IL-6 levels and the development of perfusate inflammation compared to non-treated controls. Acid injured mice exposed to conventional MV showed elevated PIP, lung IL-6 and protein levels and greater perfusate inflammation compared to air instilled controls. Instillation of exogenous surfactant did not influence the development of lung injury. Moreover, exogenous surfactant was not effective in reducing the concentration of inflammatory cytokines in the perfusate.

Conclusions

The data indicates that exogenous surfactant did not mitigate ventilation-induced systemic inflammation in our models. Future studies will focus on altering surfactant composition to improve its immuno-modulating activity.

The biological effects of lung-derived mediators on the liver

Despite the use of lung-protective mechanical ventilation (MV), the mortality of patients with acute lung injury remains at 30 to 40%, predominantly due to multiorgan failure. The objective of this study was to determine the biological significance of lung-derived mediators on peripheral organ inflammation. The authors utilized an isolated perfused mouse lung model of lipopolysaccharide (LPS)-induced lung inflammation and protective MV to collect lung-derived mediators. Aliquots of perfusate from these animals (or appropriate controls) were then injected intravenously into a cohort of normal animals whose livers were subsequently assessed in vivo using intravital video microscopy. Perfusate from LPS-inflamed lungs contained significantly higher concentrations of inflammatory mediators than perfusate from saline-instilled lungs. Assessment of livers in the second cohort of animals 120 minutes after perfusate injection revealed decreased sinusoidal blood flow, leukocytosis, and increased cell death in those receiving perfusate from LPS-inflamed lungs compared to perfusate from saline controls. There were no differences between control animals that received pure perfusate or pure LPS mixed with perfusate. These results showed that lung-derived mediators had a significant biological effect on nonpulmonary organs within a short period of time after administration. Therapies targeting these mediators may prevent multiorgan failure and death in patients with acute lung injury.

Surfactant protein-A reduces translocation of mediators from the lung into the circulation

The objective of this study was to characterize a mouse model of lung inflammation and determine the effect of surfactant protein A (SP-A, or sftpa) on the transfer of inflammatory mediators from these injured lungs into the systemic circulation. Lung inflammation was induced in either sftpa-deficient (-/-) or wild-type (+/+) spontaneously breathing, adult mice via intranasal lipopolysaccharide (LPS). Four hours later, lungs were isolated, perfused, and mechanically ventilated for 2 hours. Perfusate was collected for analysis over the duration of ventilation and lung lavage was obtained in groups of animals immediately before and after mechanical ventilation (MV). Lavage analysis showed an increase in interleukin-6 (IL6) and tumor necrosis factor-alpha (TNFalpha) 4 hours after LPS, with a further increase in IL6 following MV. LPS and MV also caused an increase in total cell and neutrophil numbers as well as total protein in the lavage compared to controls. Perfusate analysis revealed a significant increase in IL6 and TNFalpha after LPS and MV, with significantly greater levels of these mediators in sftpa (-/-) versus (+/+) mice. The authors conclude that LPS followed by MV resulted in lung inflammation and injury, and that SP-A significantly influenced inflammatory mediator release from these inflamed lungs into the perfusate.

Mediators released from LPS-challenged lungs induce inflammatory responses in liver vascular endothelial cells and neutrophilic leukocytes

The systemic inflammatory response plays an important role in the progression of acute lung injury (ALI) to multiple organ dysfunction syndrome (MODS). However, the role of lung-derived inflammatory mediators in induction of the inflammatory response in remote organs is poorly understood. To address the above, we investigated the effects of lung inflammation on induction of inflammatory response(s) in the liver in vitro. Inflammation in mouse lungs was induced by intranasal administration of lipopolysaccharide (LPS; 1 mg/ml) followed by mechanical ventilation using the isolated perfused mouse lung method to obtain and characterize lung perfusate from the pulmonary circulation. LPS administration to mouse lungs resulted in an increased release of inflammation-relevant cytokines and chemokines into the perfusate (Luminex assay) compared with the saline-controls. Subsequently, primary mouse liver vascular endothelial cells (LVEC) or mouse polymorphonuclear leukocytes (PMN) in vitro were stimulated with the perfusate obtained from saline- or LPS-challenged lungs and assessed for various inflammation-relevant end points. The obtained results indicate that stimulation of LVEC with perfusate obtained from LPS-challenged lungs results in 1) reactive oxygen species (ROS) production; 2) activation of NF-kappaB; and 3) expression of E-selectin, ICAM-1, and VCAM-1 and a subsequent increase in PMN rolling and adhesion to LVEC. In addition, perfusate from LPS-challenged lung induced activation of PMN with respect to increased ROS production and upregulation of cell surface levels of adhesion molecules MAC-1 and VLA-4. Heat-inactivation of the perfusate obtained from LPS-challenged lungs was very effective in suppressing increased proadhesive phenotype (i.e., E-selectin and ICAM-1 expression) in LVEC, whereas targeted inhibition (immunoneutralization) of TNF-alpha and/or IL-6 in LPS-lung perfusate had no effect. Taken together, these findings indicate that multiple proinflammatory mediators (proteinaceous in nature) released from inflamed lungs act synergistically to induce systemic activation of circulating PMN and promote inflammatory responses in liver vascular endothelial cells.

PROTEIN INHIBITION OF SURFACTANT DURING MECHANICAL VENTILATION OF ISOLATED RAT LUNGS

This study tested the hypothesis that material leaking into the airspace from the vasculature during ventilation interferes with surfactant function and contributes to decreases in lung compliance. Rats were euthanized and the lungs were isolated either with or without flushing of the vasculature, followed by mechanical ventilation and analysis of lung compliance and lung lavage analysis. Flushed lungs had higher lung compliance compared to the non-flushed lungs. This was associated with lower protein concentrations and improved surfactant activity. It is concluded that during mechanical ventilation, leakage of proteins results in surfactant inhibition and thereby contribute to decreased lung compliance.

LUNG MECHANICS IN THE TIMP3 NULL MOUSE AND ITS RESPONSE TO MECHANICAL VENTILATION

Tissue inhibitor of metalloproteinase-3 (TIMP3) null mice develop emphysema-like airspace enlargement due to an enzymatic imbalance. This study investigates how these abnormalities alter lung mechanics and the response to 2 different mechanical ventilation strategies. Phenotypically, TIMP3 null mice had increased compliance, and decreased resistance, tissue damping, and tissue elastance over wild-type controls. Decreased compliance and increased resistance were observed following the injurious ventilation strategy; however, the TIMP3 null response to both ventilation strategies was similar to wild-type mice. In conclusion, TIMP3 null mice have significant alterations in lung mechanics; however, this does not affect their response to ventilation.

Mechanisms responsible for surfactant changes in sepsis-induced lung injury

Pulmonary surfactant is altered in sepsis, and these changes contribute to the predisposition of septic lungs to subsequent insults, ultimately leading to acute lung injury. Specifically, the total amount of surfactant is lower in sepsis, mainly due to decreased small aggregate (SA) surfactant pools. The amount of large aggregate (LA) surfactant is not altered. To evaluate the mechanisms responsible for these alterations, trace doses of tritium-labelled dipalmitoylphosphatidylcholine (3H-DPPC)-labelled LA were instilled intratracheally into adult rats 20 hrs after caecal ligation and perforation (CLP) or sham surgery. Animals were sacrificed at 0, 1 and 4 h after instillation and recovery of 3H-DPPC in alveolar macrophages (AM), LA and SA was measured. In separate in vitro experiments, AM isolated from CLP/sham rats were incubated with LA or SA isolated from normal animals to evaluate the uptake of these aggregates into the AM. Results showed increased surfactant radioactivity associated with AM of CLP animals compared with sham animals both in vivo and in vitro. In addition, more 3H-DPPC label remained in LA forms in the CLP animals in vivo compared with sham. These findings indicate that differences in surfactant aggregate uptake and large aggregate conversion occur in septic lungs, resulting in changes in surfactant pools.

Differential response of TIMP-3 null mice to the lung insults of sepsis, mechanical ventilation, and hyperoxia

An imbalance in matrix metalloproteinases (MMPs) and the tissue inhibitors of metalloproteinases (TIMPs) leads to excessive or insufficient tissue breakdown, which is associated with many disease processes. The TIMP-3 null mouse is a model of MMP/TIMP imbalance, which develops air space enlargement and decreased lung function. These mice responded differently to cecal ligation and perforation (CLP)-induced septic lung injury than wild-type controls. The current study addresses whether the TIMP-3 knockout lung is susceptible to different types of insults or only those involving sepsis, by examining its response to lipopolysaccharide (LPS)-induced sepsis, mechanical ventilation (MV), and hyperoxia. TIMP-3 null noninjured controls of each insult consistently demonstrated significantly higher compliance vs. wild-type mice. Null mice treated with LPS had a further significantly increased compliance compared with untreated controls. Conversely, MV and hyperoxia did not alter compliance in the null lung. MMP abundance and activity increased in response to LPS but were generally unaltered following MV or hyperoxia, correlating with compliance alterations. All three insults produced inflammatory cytokines; however, the response of the null vs. wild-type lung was dependent on the type of insult. Overall, this study demonstrated that 1) LPS-induced sepsis produced a similar response in null mice to CLP-induced sepsis, 2) the null lung responded differently to various insults, and 3) the null susceptibility to compliance changes correlated with increased MMPs. In conclusion, this study provides insight into the role of TIMP-3 in response to various lung insults, specifically its importance in regulating MMPs to maintain compliance during a sepsis.

Host response to intratracheally instilled bacteria in ventilated and nonventilated rats

OBJECTIVE

Pneumonia occurs in approximately 7% of hospitalized patients. Susceptibility to certain bacteria such as Pseudomonas aeruginosa increases in critically ill patients, particularly those requiring mechanical ventilation. Previous studies investigating this susceptibility have used injurious modes of ventilation. The objective of this study was to evaluate the host's response to intratracheal instillation of P. aeruginosa in the setting of noninjurious mechanical ventilation and compare this with normal, spontaneously breathing animals receiving bacteria.

DESIGN

Randomized, controlled in vivo animal study.

SETTING

Research laboratory at a university-affiliated institution.

SUBJECTS

Adult male Sprague-Dawley rats.

INTERVENTIONS

Rats were randomized into four groups: spontaneously breathing given saline, spontaneously breathing given bacteria, mechanically ventilated given saline, and mechanically ventilated given bacteria. The ventilation strategy used involved low stretch (tidal volume of 8 mL/kg) with a positive end-expiratory pressure of 5 cm H2O.

MEASUREMENTS AND MAIN RESULTS

Lung compliance, bacterial recovery, surfactant, total cells, and cytokine concentrations in the lung lavage were analyzed after 4 hrs. Results showed that neither ventilation nor bacteria alone altered lung function, although the combination of ventilation and Pseudomonas significantly decreased arterial oxygenation and lung compliance. Increases in lavage cell counts, cytokines, and surfactant were observed in both groups administered bacteria compared with animals given saline. However, there were no significant differences in bacterial recovery, cell counts, cytokines, and surfactant measurements in the groups given bacteria.

CONCLUSIONS

These data suggest that bacterial instillation with low-stretch ventilation had a significant effect on lung function but did not alter the inflammatory response to a bacterial challenge over this time course compared with spontaneously breathing animals.

High-frequency oscillation and exogenous surfactant administration in lung-injured adult sheep

OBJECTIVE

To evaluate the effects of high-frequency oscillation on the response to exogenous surfactant in lung-injured adult sheep.

DESIGN

A prospective, controlled, in vivo, animal laboratory study.

SETTING

Animal research facility of a health sciences university.

SUBJECTS

Twenty-eight adult sheep.

INTERVENTIONS

Animals were anesthetized and instrumented with a tracheostomy and vascular catheters. Following whole lung saline lavage, animals were randomized to one of four groups: Group S-CMV received surfactant and was ventilated for 4 hrs using a conventional mechanical ventilation strategy, group S-HFOV/CMV received surfactant and was ventilated with a high-frequency oscillation technique for 2 hrs and a conventional mechanical strategy for 2 hrs, group HFOV/CMV underwent the latter ventilatory strategies without receiving surfactant, and group HFOV was ventilated with high-frequency oscillation only for 4 hrs. At the end of the ventilatory period, the distributions of ventilation and surfactant were evaluated in animals that received surfactant.

MEASUREMENTS AND MAIN RESULTS

Animals in the S-CMV group had a significantly greater mean PaO2 value at the end of the experimental period than animals in the S-HFOV/CMV or HFOV/CMV groups. Evaluation of the distribution of ventilation relative to surfactant demonstrated that animals ventilated with high-frequency oscillation followed by conventional mechanical ventilation had a significantly greater disproportionate distribution of ventilation relative to surfactant compared with the CMV-only group.

CONCLUSIONS

A period of high-frequency oscillation, as used in this study, immediately following exogenous surfactant administration mitigates the host's response to surfactant when subsequently switched to conventional mechanical ventilation.

Evaluation of alveolar surfactant aggregates in vitro and in vivo

In acute lung injury, a decrease in surface-active large aggregates and an increase in the less surface-active small surfactant aggregates are observed. The objective of the current study was to determine if the increase in small aggregates interfered with the function of large aggregates, thereby independently contributing to lung dysfunction. Isolated large aggregates, small aggregates, and large aggregate+small aggregate combinations were analysed for in vitro surface activity utilizing a pulsating bubble surfactometer. Subsequently, large aggregates, small aggregates, and large aggregate+ small aggregate combinations were administered to surfactant-deficient, adult Sprague-Dawley rats. Physiological parameters were measured during 1 h of ventilation. After sacrifice, the whole lung lavage was analysed for protein concentration, and surface activity of the recovered large aggregates. The minimum surface tension of the large aggregate+small aggregate preparations (10 mN x m(-1)) was significantly higher than large aggregates alone (1 mN x m(-1)), but lower than small aggregates alone (21 mN x m(-1) ) after 100 pulsations. In vivo, rats receiving large aggregates+small aggregates showed immediate increases in oxygenation, similar to animals given large aggregates, whereas animals given small aggregates and control animals maintained low oxygenation values. In conclusion, small aggregates interfered with large aggregates function in vitro, but this was not observed in vivo in this experimental model.

Evaluation of exogenous surfactant in HCL-induced lung injury

The efficacy of exogenous surfactant administration is influenced by numerous factors, which has resulted in variable outcomes of clinical trials evaluating this treatment for the acute respiratory distress syndrome (ARDS). We investigated several of these factors in an animal model of acid aspiration including different surfactant preparations, and different delivery methods. In addition, high-frequency oscillation (HFO), a mode of mechanical ventilation known to recruit severely damaged lungs, was utilized. Lung injury was induced in adult rabbits via intratracheal instillation of 0.2 N HCl followed by conventional mechanical ventilation (CMV) until Pa(O2)/FI(O2) values ranged from 220 to 270 mm Hg. Subsequently, animals were given one of three surfactants administered via three different methods and physiological responses were assessed over a 1-h period. Regardless of the surfactant treatment strategy utilized, oxygenation responses were not sustained. In contrast, HFO resulted in a superior response compared with all surfactant treatment strategies involving CMV. The deterioration in physiological parameters after surfactant treatment was likely due to overwhelming protein inhibition of the surfactant. In conclusion, various surfactant treatment strategies were not effective in this model of lung injury, although the lungs of these animals were recruitable with HFO, as reflected by the acute and sustained oxygenation improvements.

Alveolar environment influences the metabolic and biophysical properties of exogenous surfactants

Several factors have been shown to influence the efficacy of exogenous surfactant therapy in the acute respiratory distress syndrome. We investigated the effects of four different alveolar environments (control, saline-lavaged, N-nitroso-N-methylurethane, and hydrochloric acid) on the metabolic and functional properties of two exogenous surfactant preparations: bovine lipid extract surfactant and recombinant surfactant-associated protein (SP) C drug product (rSPC) administered to each of these groups. The main difference between these preparations was the lack of SP-B in the rSPC. Our results demonstrated differences in the large aggregate pool sizes recovered from each of the experimental groups. We also observed differences in SP-A content, surface area cycling characteristics, and biophysical activities of these large aggregate forms after the administration of the two exogenous surfactant preparations. We conclude that the alveolar environment plays a critical role, influencing the overall efficacy of exogenous surfactant therapy. Thus further preclinical studies are warranted to investigate the specific factors within the alveolar environment that lead to the differences observed in this study.

Effects of surfactant distribution and ventilation strategies on efficacy of exogenous surfactant

The effects of both surfactant distribution patterns and ventilation strategies utilized after surfactant administration were assessed in lung-injured adult rabbits. Animals received 50 mg/kg surfactant via intratracheal instillation in volumes of either 4 or 2 ml/kg. A subset of animals from each treatment group was euthanized for evaluation of the exogenous surfactant distribution. The remaining animals were randomized into one of three ventilatory groups: group 1 [tidal volume (VT) of 10 ml/kg with 5 cmH2O positive end-expiratory pressure (PEEP)]; group 2 (VT of 5 ml/kg with 5 cmH2O PEEP); or group 3 (VT of 5 ml/kg with 9 cmH2O PEEP). Animals were ventilated and monitored for 3 h. Distribution of the surfactant was more uniform when it was delivered in the 4 ml/kg volume. When the distribution of surfactant was less uniform, arterial PO2 values were greater in groups 2 and 3 compared with group 1. Oxygenation differences among the different ventilation strategies were less marked in animals with the more uniform distribution pattern of surfactant (4 ml/kg). In both surfactant treatment groups, a high mortality was observed with the ventilation strategy used for group 3. We conclude that the distribution of exogenous surfactant affects the response to different ventilatory strategies in this model of acute lung injury.

Effects of ventilation strategies on the efficacy of exogenous surfactant therapy in a rabbit model of acute lung injury

We evaluated the effects of various ventilation strategies on the efficacy of exogenous surfactant therapy in lung-injured adult rabbits. Lung injury was induced by repetitive whole-lung saline lavage followed by mechanical ventilation. Three hours after the final lavage, 100 mg lipid/kg bovine lipid extract surfactant was instilled. After confirmation of similar responses to exogenous surfactant, animals were then randomized to one of four ventilation groups; (1) Normal tidal volume (VT) (5 cm H2O): VT = 10 ml/kg, respiratory rate (RR) = 30/min, positive end-expiratory pressure (PEEP) = 5 cm H2O; (2) Normal VT (9 cm H2O): VT = 10 ml/kg, RR = 30/min, PEEP = 9 cm H2O; (3) Low VT (5 cm H2O): VT = 5 ml/kg, RR = 60/min, PEEP = 5 cm H2O; (4) Low VT (9 cm H2O): VT = 5 ml/kg, RR = 60/min, PEEP = 9 cm H2O. Animals were ventilated for an additional 3 h and then killed, and lung lavage fluid was analyzed. Animals ventilated with the low-VT modes (Low VT [5 cm H2O] and Low VT [9 cm H2O]) had higher PaO2 values (430 +/- 7 mm Hg and 425 +/- 18 mm Hg versus 328 +/- 13 mm Hg) and higher percentages of surfactant in large aggregate forms (83 +/- 2% and 82 +/- 2% versus 67 +/- 4%) at 3 h after treatment than did the Normal VT (5 cm H2O) group (p < 0.05). Increasing the PEEP level was beneficial for a short period after surfactant administration to maintain oxygenation, but did not affect exogenous surfactant aggregate conversion. We speculate that ventilation strategies resulting in low exogenous surfactant aggregate conversion will result in superior physiologic responses to exogenous surfactant.

Ventilation strategies affect surfactant aggregate conversion in acute lung injury

This study evaluated the effects of varying tidal volumes (VT) and positive end-expiratory pressure (PEEP) levels on surfactant aggregate conversion and lung function in an animal model of lung injury induced by N-nitroso-N-methylurethane. Lung-injured adult rabbits were initially ventilated using a VT of 10 ml/kg (VT10), a respiratory rate of 30 breaths/min (RR30), and a PEEP of 3.5 cm H2O. A trace dose of radiolabeled rabbit large surfactant aggregates was instilled after the onset of ventilation, and animals were then ventilated at different ventilator settings for 1 h. Ventilation strategies involving a lower VT (VT5, RR60) resulted in significantly superior oxygenation and lower surfactant aggregate conversion rates than strategies involving a higher VT ([VT10, RR30], [VT15, RR20], p < 0.05). Increasing the PEEP level to 8.0 cm H2O improved oxygenation, but it was sustained only with a low VT (VT5, RR60), and deteriorated with a high VT (VT10, RR30). Varying VT but not PEEP levels resulted in significant changes in surfactant aggregate conversion. We conclude that increased surfactant aggregate conversion resulting from suboptimal ventilation of injured lungs may play an important role in the pathophysiology of ventilation-induced lung dysfunction in acute lung injury.

Evaluation of exogenous surfactant treatment strategies in an adult model of acute lung injury

Two exogenous surfactant preparations [Survanta and bovine lipid extract surfactant (BLES)] were evaluated in saline lavage-injured adult sheep with two different delivery methods (instillation vs. aerosolization). Instilled BLES resulted in the greatest improvement in lung function, followed by aerosolized Survanta and then instilled Survanta. Aerosolized BLES was ineffective. Total surfactant recovery and distribution patterns were similar for Survanta and BLES for each delivery method tested. There were significant differences, however, in the proportion of surfactant recovered in the alveolar wash relative to the lung tissue between the groups at killing. Moreover, the ratio of poorly functioning small surfactant aggregates to superior functioning large aggregates isolated from alveolar wash samples correlated with the physiological responses. The calculated contribution of secreted endogenous surfactant to the total alveolar phospholipid pool at killing was significantly greater for the aerosolized Survanta group compared with the aerosolized BLES group. This finding suggested that there were differences in the interaction of the exogenous surfactants and their alveolar environments. We conclude that the response to exogenous surfactant in acute lung injury depends not only on the preparation used but also on how the surfactants are delivered to the injured lung.

Pulmonary surfactant subfractions in patients with the acute respiratory distress syndrome

Changes in the surfactant system have been observed in patients with acute respiratory distress syndrome (ARDS). These alterations in surfactant are thought to contribute to lung dysfunction in this disease. In this report we describe the changes in surfactant subfractions in bronchoalveolar wash obtained from five patients with established ARDS compared with five non-ARDS patients. Our results show that, in addition to the changes in surfactant composition and yield reported previously, the ratio of small to large surfactant aggregates is significantly increased in patients with ARDS compared with non-ARDS patients (0.48 +/- 0.09 versus 0.20 +/- 0.05 respectively [p < 0.05]). This increased ratio was associated with a decreased level of SP-A in the large aggregate fraction. We suggest that this increased ratio represents a marker for surfactant alterations in ARDS that is independent of lavage technique and can be measured in a very small surfactant sample.