Surfactant and the adult respiratory distress syndrome

Ischemic and endotoxin pre-conditioning reduce lung reperfusion injury-induced surfactant alterations

BACKGROUND

Pulmonary ischemia/reperfusion injury represents a common clinical phenomenon after lung transplantation, pulmonary embolism, and cardiac surgery with extracorporeal circulation. We investigated the influence of ischemic and endotoxin pre-conditioning on gas exchange and surfactant properties in a canine model of ischemia/reperfusion injury.

METHODS

Twenty-six foxhounds underwent 3 hours of warm ischemia of the left lung, followed by 8 hours of reperfusion. Ischemic pre-conditioning was performed for either 5 minutes (IPC-5) or by 2 10-minute ischemic periods (IPC-10), before ischemia. For endotoxin pre-conditioning, dogs were pre-treated by a daily intravenous application of increasing amounts of endotoxin for 6 days. No pre-conditioning was performed in the controls. Bronchoalveolar lavage was performed before ischemia/reperfusion injury (baseline) and after the 8-hour reperfusion period in the non-injured right and in the reperfused left lung. Bronchoalveolar lavage fluids were analyzed for the phospholipid-protein ratio, the content of large surfactant aggregates, the phospholipid and neutral lipid profile, the surfactant protein (SP) content, and for biophysical activity.

RESULTS

Severe surfactant alterations were observed in the ischemia/reperfusion-injured left lung, with increased protein concentrations and depressed concentrations of large surface aggregates, SP-B, dipalmitoylated phosphatidylcholine, and phosphatidylglycerol. Endotoxin pre-conditioning and IPC-5 were both capable of greatly preventing the ischemia/reperfusion injury-related deterioration of surfactant properties. IPC-10 exerted no effects. Endotoxin pre-conditioning and IPC-5, but not IPC-10, also prevented loss of gas exchange.

CONCLUSIONS

Ischemic and endotoxin pre-conditioning may protect against impairment of gas exchange in ischemia/reperfusion injury by restoring physiological surfactant properties.

Cloning and characterization of mouse lung-type acyl-CoA:lysophosphatidylcholine acyltransferase 1 (LPCAT1). Expression in alveolar type II cells and possible involvement in surfactant production

Phosphatidylcholine (1,2-diacyl-sn-glycero-3-phosphocholine, PC), is an important constituent of biological membranes. It is also the major component of serum lipoproteins and pulmonary surfactant. In the remodeling pathway of PC biosynthesis, 1-acyl-sn-glycero-3-phosphocholine (LPC) is converted to PC by acyl-CoA:lysophosphatidylcholine acyltransferase (LPCAT, EC 2.3.1.23). Whereas LPCAT activity has been detected in several tissues, the structure and detailed biochemical information on the enzyme have not yet been reported. Here, we present the cloning and characterization of a cDNA for mouse lung-type LPCAT (LPCAT1). The cDNA encodes an enzyme of 60 kDa, with three putative transmembrane domains. When expressed in Chinese hamster ovary cells, mouse LPCAT1 exhibited Ca(2+)-independent activity with a pH optimum between 7.4 and 10. LPCAT1 demonstrated a clear preference for saturated fatty acyl-CoAs, and 1-myristoyl- or 1-palmitoyl-LPC as acyl donors and acceptors, respectively. Furthermore, the enzyme was predominantly expressed in the lung, in particular in alveolar type II cells. Thus, the enzyme might synthesize phosphatidylcholine in pulmonary surfactant and play a pivotal role in respiratory physiology.

Adjuncts to Mechanical Ventilation in ARDS

Since its first description, acute respiratory distress syndrome has been characterized by abnormal physiologic and gas exchange properties of the lungs. Many adjunctive therapies have been developed to reduce the stresses of mechanical ventilation on already damaged lungs. We examined the mechanism of action and the latest clinical trial information of several adjunctive therapies including prone positioning, nitric oxide, extracorporeal membrane oxygenation, arterial venous carbon dioxide removal, and liquid ventilation. While all of these therapies have demonstrated short-term improvements in arterial blood gases and in the limitation of lung injury, none have shown an evidence-based survival benefit.

Inhaled lysophosphatidylcholine provokes bronchoconstriction in guinea pigs in vivo

Lysophosphatidylcholine is increased in the airway of bronchial asthma, but its role is not clear. We investigated the role of lysophosphatidylcholine in asthma in anaesthetized, mechanically ventilated guinea pigs. Pressure at the airway opening was measured as an index of bronchial response. Increasing doses of lysophosphatidylcholine (1--10 mg/ml) were inhaled and then bronchoalveolar lavage was carried out. 100 and 200 microg/ml methacholine were inhaled 10 min after inhalation of 2.5 mg/ml lysophosphatidylcholine, 10 mg/ml dipalmitoyl phosphatidylcholine and 10 mg/ml glycerophosphocholine, all of which per se did not change the pressure at the airway opening. Effect of 1.0 microg/kg salbutamol, or 60 mg/kg diphenhydramine on the lysophosphatidylcholine-induced increase in the pressure at the airway opening was investigated. Inhalation of lysophosphatidylcholine dose-dependently increased the pressure at the airway opening and increased bronchial responsiveness to methacholine. On the other hand, inhalation of dipalmitoyl phosphatidylcholine decreased the pressure at the airway opening and decreased bronchial responsiveness to methacholine. Intravenously administered salbutamol, but not diphenhydramine, prevented the lysophosphatidylcholine-induced increase in the pressure at the airway opening. The percentage of leukocytes in bronchoalveolar lavage fluid did not change significantly at least within 20 min after the lysophosphatidylcholine inhalation. Lysophosphatidylcholine causes bronchoconstriction and enhances bronchial responsiveness without inducing leukocyte infiltration in the airway, suggesting that lysophosphatidylcholine may be a new bronchoconstrictor mediator in bronchial asthma.

Inhaled nitric oxide alleviates hyperoxia suppressed phosphatidylcholine synthesis in endotoxin-induced injury in mature rat lungs

Background

We investigated efficacy of inhaled nitric oxide (NO) in modulation of metabolism of phosphatidylcholine (PC) of pulmonary surfactant and in anti-inflammatory mechanism of mature lungs with inflammatory injury.

Methods

Healthy adult rats were divided into a group of lung inflammation induced by i.v. lipopolysaccharides (LPS) or a normal control (C) for 24 h, and then exposed to: room air (Air), 95% oxygen (O), NO (20 parts per million, NO), both O and NO (ONO) as subgroups, whereas [³H]-choline was injected i.v. for incorporation into PC of the lungs which were processed subsequently at 10 min, 4, 8, 12 and 24 h, respectively, for measurement of PC synthesis and proinflammatory cytokine production.

Results

LPS-NO subgroup had the lowest level of labeled PC in total phospholipids and disaturated PC in bronchoalveolar lavage fluid and lung tissue (decreased by 46–59%), along with the lowest activity of cytidine triphosphate: phosphocholine cytidylyltransferase (-14–18%) in the lungs, compared to all other subgroups at 4 h (p < 0.01), but not at 8 and 12 h. After 24-h, all LPS-subgroups had lower labeled PC than the corresponding C-subgroups (p < 0.05). LPS-ONO had higher labeled PC in total phospholipids and disaturated PC, activity of cytidylyltransferase, and lower activity of nuclear transcription factor-κB and expression of proinflammatory cytokine mRNA, than that in the LPS-O subgroup (p < 0.05).

Conclusion

In LPS-induced lung inflammation in association with hyperoxia, depressed PC synthesis and enhanced proinflammatory cytokine production may be alleviated by iNO. NO alone only transiently suppressed the PC synthesis as a result of lower activity of cytidylyltransferase.

Interfacial properties of pulmonary surfactant layers

The composition of the pulmonary surfactant and the border conditions of normal human breathing are relevant to characterize the interfacial behavior of pulmonary layers. Based on experimental data methods are reviewed to investigate interfacial properties of artificial pulmonary layers and to explain the behavior and interfacial structures of the main components during compression and expansion of the layers observed by epifluorescence and scanning force microscopy. Terms like over-compression, collapse, and formation of the surfactant reservoir are discussed. Consequences for the viscoelastic surface rheological behavior of such layers are elucidated by surface pressure relaxation and harmonic oscillation experiments. Based on a generalized Volmer isotherm the interfacial phase transition is discussed for the hydrophobic surfactant proteins, SP-B and SP-C, as well as for the mixtures of dipalmitoylphosphatidylcholine (DPPC) with these proteins. The behavior of the layers depends on both the oligomerisation state and the secondary structure of the hydrophobic surfactant proteins, which are controlled by the preparation of the proteins. An example for the surface properties of bronchoalveolar porcine lung washings of uninjured, injured, and Curosurf treated lavage is discussed in the light of surface behavior. An outlook summarizes the present knowledge and the main future development in this field of surface science.

Rekrutierungsmanöver bei Patienten mit Lungenversagen

Recruitment maneuvers have been proposed as an adjunct to mechanical ventilation to re-expand collapsed lung regions. Although, in most patients recruitment maneuvers improve gas exchange a controversial discussion on recruitment maneuvers remains. This article reviews the physiological and patho-physiological backgrounds of recruitment maneuvers. The different recruitment maneuvers and possible monitoring are discussed as well as the influence of recruitment on other organs. Furthermore, we discuss whether recruitment maneuvers are useful if patients with acute lung injury or acute respiratory distress syndrome are ventilated with a lung-protective strategy.

Lung surfactant dysfunction in tuberculosis: effect of mycobacterial tubercular lipids on dipalmitoylphosphatidylcholine surface activity

In pulmonary tuberculosis, Mycobacterium tuberculosis bacteria reside in the alveoli and are in close proximity with the alveolar surfactant. Mycolic acid in its free form and as cord factor, constitute the major lipids of the mycobacterial cell wall. They can detach from the bacteria easily and are known to be moderately surface active. We hypothesize that these surface-active mycobacterial cell wall lipids could interact with the pulmonary surfactant and result in lung surfactant dysfunction. In this study, the major phospholipid of the lung surfactant, dipalmitoylphosphatidylcholine (DPPC) and binary mixtures of DPPC:phosphatidylglycerol (PG) in 9:1 and 7:3 ratios were modelled as lung surfactant monolayers and the inhibitory potential of mycolic acid and cord factor on the surface activity of DPPC and DPPC:PG mixtures was evaluated using Langmuir monolayers. The mycobacterial lipids caused common profile changes in all the isotherms: increase in minimum surface tension, compressibility and percentage area change required for change in surface tension from 30 to 10 mN/m. Higher minimum surface tension values were achieved in the presence of mycolic acid (18.2+/-0.7 mN/m) and cord factor (13.28+/-1.2 mN/m) as compared to 0 mN/m, achieved by pure DPPC film. Similarly higher values of compressibility (0.375+/-0.005 m/mN for mycolic acid:DPPC and 0.197+/-0.003 m/mN for cord factor:DPPC monolayers) were obtained in presence of mycolic acid and cord factor. Thus, mycolic acid and cord factor were said to be inhibitory towards lung surfactant phospholipids. Higher surface tension and compressibility values in presence of tubercular lipids are suggestive of an unstable and fluid surfactant film, which will fail to achieve low surface tensions and can contribute to alveolar collapse in patients suffering from pulmonary tuberculosis. In conclusion a biophysical inhibition of lung surfactant may play a role in the pathogenesis of tuberculosis and may serve as a target for the development of new drug loaded surfactants for this condition.

Ceramide decreases surfactant protein B gene expression via downregulation of TTF-1 DNA binding activity

Ceramide, a sphingolipid, is an important signaling molecule in the inflammatory response. Mediators of acute lung injury such as TNF-alpha, platelet-activating factor, and Fas/Apo ligand stimulate sphingomyelin hydrolysis to increase intracellular ceramide levels. Surfactant protein B (SP-B), a hydrophobic protein of pulmonary surfactant, is essential for surfactant function and lung stability. In this study we investigated the effects of ceramide on SP-B gene expression in H441 lung epithelial cells. Ceramide decreased SP-B mRNA levels in control and dexamethasone-treated cells after 24-h incubation and inhibition of SP-B mRNA was associated with inhibition of immunoreactive SP-B. In transient transfections assays, ceramide inhibited SP-B promoter activity, indicating that the inhibitory effects are exerted at the transcriptional level. Deletion mapping experiments showed that the ceramide-responsive region is located within the -233/-80-bp region of human SP-B promoter. Electrophoretic mobility shift and reporter assays showed that ceramide reduced the DNA binding activity and transactivation capability of thyroid transcription factor 1 (TTF-1/Nkx2.1), a key factor for SP-B promoter activity. Collectively these data showed that ceramide inhibits SP-B gene expression by reducing the DNA biding activity of TTF-1/Nkx2.1 transcription factor. Protein kinase C inhibitor bisindolylmaleimide and the protein tyrosine kinase inhibitor genistein partially reversed ceramide inhibition, indicating that protein kinases play important roles in the ceramide inhibition of SP-B gene expression. Chemical inhibitors of de novo ceramide synthesis and sphingomyelin hydrolysis had no effect on TNF-alpha inhibition of SP-B promoter activity and mRNA levels, suggesting that ceramide does not play a role in the inhibition.

Gene expression changes during the development of acute lung injury: role of transforming growth factor beta

RATIONALE

Acute lung injury can occur from multiple causes, resulting in high mortality. The pathophysiology of nickel-induced acute lung injury in mice is remarkably complex, and the molecular mechanisms are uncertain.

OBJECTIVES

To integrate molecular pathways and investigate the role of transforming growth factor beta (TGF-beta) in acute lung injury in mice.

METHODS

cDNA microarray analyses were used to identify lung gene expression changes after nickel exposure. MAPPFinder analysis of the microarray data was used to determine significantly altered molecular pathways. TGF-beta1 protein in bronchoalveolar lavage fluid, as well as the effect of inhibition of TGF-beta, was assessed in nickel-exposed mice. The effect of TGF-beta on surfactant-associated protein B (Sftpb) promoter activity was measured in mouse lung epithelial cells.

MEASUREMENTS AND MAIN RESULTS

Genes that decreased the most after nickel exposure play important roles in lung fluid absorption or surfactant and phospholipid synthesis, and genes that increased the most were involved in TGF-beta signaling. MAPPFinder analysis further established TGF-beta signaling to be significantly altered. TGF-beta-inducible genes involved in the regulation of extracellular matrix function and fibrinolysis were significantly increased after nickel exposure, and TGF-beta1 protein was also increased in the lavage fluid. Pharmacologic inhibition of TGF-beta attenuated nickel-induced protein in bronchoalveolar lavage. In addition, treatment with TGF-beta1 dose-dependently repressed Sftpb promoter activity in vitro, and a novel TGF-beta-responsive region in the Sftpb promoter was identified.

CONCLUSIONS

These data suggest that TGF-beta acts as a central mediator of acute lung injury through the alteration of several different molecular pathways.

Protective effect of erythropoietin on type II pneumocyte cells after traumatic brain injury in rats

BACKGROUND

The main objective was to evaluate the protective effect of erythropoietin on lung ultrastructure against damage in rats after traumatic brain injury.

METHODS

We used forty Wistar-Albino female rats weighing 170-200 gr. The rats were allocated into five groups. The first group was the control and the second was the craniotomy without trauma. The third group was the trauma group. The fourth and fifth groups were erythropoietin (1000 IU/kg) and vehicle (0.4 mL/rat) groups, respectively. A weight-drop method was used for achieving head trauma. Samples were obtained from pulmonary lobes 24-hour post injury. Lipid peroxidation levels were determined and electron microscopic scoring model was used to reveal the ultrastructural changes.

RESULTS

Ultrastructural evaluation revealed pathologic changes in the trauma group compared with the control group (p < 0.05). Lipid peroxidation levels were found to be higher in the trauma group (p < 0.05). Erythropoietin significantly reduced both the ultrastructural pathologic changes and the lipid peroxidation levels in the treatment group (p < 0.05).

CONCLUSIONS

Erythropoietin protects the ultrastructure of pneumocyte type II cells against damage after traumatic brain injury.

Evolution of treatments for patients with acute lung injury

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are acute life-threatening forms of hypoxemic respiratory failure. ALI/ARDS patients require intensive care with prolonged mechanical ventilation. Despite advances in our understanding of the pathophysiology of ALI/ARDS, mortality rates remain > 30% and survivors suffer significant decrements in their quality of life. The evolving understanding of ALI/ARDS and the complex interactions involved in ALI/ARDS open the door for many potential targets for treatment. The condition is characterised by an acute inflammatory state that leads to increased capillary permeability and accumulation of proteinaceous pulmonary oedema. The changes that occur as a result of this inflammation clinically manifest themselves as hypoxemia, infiltrates on chest radiograph and reduced lung compliance. Many years have been dedicated to analysing the complexities involved in ALI/ARDS in order to improve current and future possibilities for treatment, with the aim of improving patient outcomes. Although some therapies have demonstrated benefits of improved oxygenation, such as surfactant and nitric oxide, these benefits have not translated into reductions in the duration of mechanical ventilation or mortality. Inflammatory mediator-targeted therapies were promising early on; however, larger trials have found therapies such as cytokine modulation, platelet-activating factor inhibition and neutrophil elastase inhibitors to be ineffective in the treatment of ALI/ARDS. Preclinical studies with beta2-agonists and granulocyte macrophage colony-stimulating factor have shown promise for restoring alveolar capillary barrier integrity or reducing pulmonary oedema, and further studies are being conducted to test for true clinical benefit. Despite previous therapeutic failures, newer surfactant formulations have shown promise, particularly in patients with direct forms of lung injury, and are currently in Phase III trials. Anticoagulant therapy with activated protein C has been shown to improve survival in sepsis, the most common risk factor for the development of ALI/ARDS, and is now being studied in ALI/ARDS. Until new data emerge, the focus must remain on supportive care, including optimised mechanical ventilation, nutritional support, manipulation of fluid balance and prevention of intervening medical complications.

Inhalation injury assessed by score does not contribute to the development of acute respiratory distress syndrome in burn victims

OBJECTIVE

To establish the incidence, mortality, and time of onset of acute respiratory distress syndrome (ARDS) in relation to extent of burn and inhalation injury in patients who required mechanical ventilation.

DESIGN

Data about burn and inhalation injury were recorded prospectively whereas ARDS and multiple organ dysfunction were assessed by review of patient charts.

SETTING

National burn intensive care unit at Linkoping University Hospital, Sweden (a tertiary referral hospital).

PATIENTS

Between 1993 and 1999, we studied all patients with thermal injury (n=553) who required mechanical ventilation for more than two days (n=91).

MEASUREMENTS AND RESULTS

Out of the thirty-six burn victims who developed ARDS (40%), 25 (70%) did so early post burn (in less than 6 days). Patients with ARDS had higher multiple organ dysfunction scores (mean 10.5) than those who did not develop ARDS (mean 5.6) (p<0.01). The probable presence of inhalation injury as assessed by an inhalation lung injury score (ILIS) did not contribute to the development of ARDS. Mortality tended to be higher in patients who developed ARDS (14%) compared to those who did not (6%, p=0.2).

CONCLUSIONS

In our burn patients the incidence of ARDS was high whereas mortality was low. We found no association between inhalation injury as assessed using the ILIS and development of ARDS. Our data support a multi-factorial origin of ARDS in burn victims as a part of a multiple organ failure event.

Acute respiratory distress syndrome: update on the latest developments in basic and clinical research

PURPOSE OF REVIEW

Acute lung injury/acute respiratory distress syndrome is a common, serious condition affecting a heterogeneous population of critically ill patients. Other than low tidal volume ventilation, no specific therapy has improved survival. Understanding the epidemiology, pathogenesis, and lessons to be learned from previous clinical trials is necessary for the development of new therapies and the rational design of studies assessing their efficacy.

RECENT FINDINGS

Acute lung injury/acute respiratory distress syndrome occurs in 6-8% of the general intensive care unit population, with a mortality of 32-45%. A recent epidemiologic study found that multi-organ dysfunction, use of tidal volumes higher than 6 ml/kg, and high mean fluid balance were independent risks for mortality. Although high levels of inflammatory mediators are also markers for acute respiratory distress syndrome development and death, short courses of high-dose steroids are not effective in acute cases. The latest theory of biotrauma proposes cellular mechanisms by which mechanical ventilation incites a local and systemic inflammatory response; protective lung ventilation with low tidal volumes can attenuate this inflammation and injury to distal organs. Endogenous surfactant function is clearly impaired, but no commercially available surfactant preparation has been shown to reduce mortality. Results of trials to determine efficacy of steroids in late cases and optimal fluid management are pending.

SUMMARY

The results of recent clinical trials have raised more questions. Further study of the inflammatory response, surfactant regulation, and the cellular impact of mechanical ventilation should help to develop new therapies, target patients most likely to benefit, and identify appropriate timing of intervention.

The Efficacy of Fluorocarbon, Surfactant, and Their Combination for Improving Acute Lung Injury Induced by Intratracheal Acidified Infant Formula

We conducted the current study to compare the efficacy of partial liquid ventilation (PLV), pulmonary surfactant (PSF), and their combination in ameliorating the acidified infant-formula-induced acute lung injury (ALI). In the Part I study, 42 rabbits receiving volume-controlled ventilation with positive end-expiratory pressure 10 cm H(2)O were randomly divided into 6 groups (groups noninjuryI, gas ventilation [GVi], PLVi, PSFi, PLVi-->PSFi, and PSFi-->PLVi). ALI was induced by intratracheal acidified infant formula (2 mL/kg, pH 1.8). Group GVi received neither PLV nor PSF therapy. Groups PLV and PSF received intratracheal fluorocarbon 15 mL/kg or surfactant 100 mg/kg, respectively, 30 min after acidified infant formula. Groups PLVi-->PSFi and PSFi-->PLVi received both treatments at 30-min intervals. In Part II, 42 rabbits (in 6 groups) undergoing pressure-controlled ventilation received the same drug therapies as in Part I. The lungs were excised to assess biochemical and histological damage 150 min after induction of ALI. In Parts I and II, PSF, fluorocarbon, and their combination attenuated lung leukosequestration and edema and superoxide production of neutrophils, consequently improving oxygenation, lung mechanics, and pathological changes. Independent of ventilation mode, PSF followed by fluorocarbon provided the most beneficial effects and fluorocarbon followed by PSF produced the least efficacy.

Lavage administration of dilute surfactant in a piglet model of meconium aspiration

Maldistribution of exogenous surfactant may preclude any clinical response in acute lung injury associated with surfactant dysfunction. Our previous studies have shown the effectiveness of surfactant lavage after homogenous lung injury. The present study utilizes a histologically confirmed non-homogeneous lung injury model induced by saline lung-lavage followed by meconium injected into a mainstem bronchus. Piglets were then treated with Infasurf or Exosurf by lavage (I-LAVAGE, n = 7; E-LAVAGE, n = 5) or bolus (I-BOLUS, n = 8; E-BOLUS, n = 5), or went untreated (CONTROL, n = 4). Lavage administration utilized a dilute surfactant (35 ml/kg; 4 mg phospholipid/ml) instilled into the lung, followed by gravity drainage. The retained doses of the respective surfactant in the lavage and bolus groups were similar. Results showed that the surfactant distribution was more uniform in the lavage groups compared to the bolus groups. Significant and consistent increases in PaO2 were observed in the lavage groups compared to the bolus groups and the controls. PaO2 (mmHg) at 240 min posttreatment: I-LAVAGE = 297 +/- 54, E-LAVAGE = 280 +/- 57; I-BOLUS = 139 +/- 31; E-BOLUS = 152 +/- 29; C = 119 +/- 73 (mean +/- SEM). Other improved pulmonary function parameters favored lavage administration. We conclude that better surfactant distribution achieved by lavage administration can be more effective than bolus administration in this type of non-homogeneous lung injury.

Application of microarray technology in pulmonary diseases

Microarrays are a powerful tool that have multiple applications both in clinical and cell biology arenas of common lung diseases. To exemplify how this tool can be useful, in this review, we will provide an overview of the application of microarray technology in research relevant to common lung diseases and present some of the future perspectives.

Acute lung injury and acute respiratory distress syndrome in pregnancy

Acute respiratory failure can be the result of a variety of clinical conditions, such as congestive heart failure, pneumonia, pulmonary embolism, exacerbation of obstructive lung diseases, and acute respiratory distress syndrome (ARDS). This article focuses on developments related to acute lung injury and ARDS and reviews epidemiology, pathogenesis and therapeutic advances with an emphasis on the obstetric population. A brief discussion of tocolytic-induced pulmonary edema, preeclampsia, venous air embolism, and aspiration-related ARDS is included. Management of pregnant women with ARDS is outlined.

Mechanical interactions between collagen and proteoglycans: implications for the stability of lung tissue

Collagen and elastin are thought to dominate the elasticity of the connective tissue including lung parenchyma. The glycosaminoglycans on the proteoglycans may also play a role because osmolarity of interstitial fluid can alter the repulsive forces on the negatively charged glycosaminoglycans, allowing them to collapse or inflate, which can affect the stretching and folding pattern of the fibers. Hence, we hypothesized that the elasticity of lung tissue arises primarily from 1) the topology of the collagen-elastin network and 2) the mechanical interaction between proteoglycans and fibers. We measured the quasi-static, uniaxial stress-strain curves of lung tissue sheets in hypotonic, normal, and hypertonic solutions. We found that the stress-strain curve was sensitive to osmolarity, but this sensitivity decreased after proteoglycan digestion. Images of immunofluorescently labeled collagen networks showed that the fibers follow the alveolar walls that form a hexagonal-like structure. Despite the large heterogeneity, the aspect ratio of the hexagons at 30% uniaxial strain increased linearly with osmolarity. We developed a two-dimensional hexagonal network model of the alveolar structure incorporating the mechanical properties of the collagen-elastin fibers and their interaction with proteoglycans. The model accounted for the stress-strain curves observed under all experimental conditions. The model also predicted how aspect ratio changed with osmolarity and strain, which allowed us to estimate the Young's modulus of a single alveolar wall and a collagen fiber. We therefore identify a novel and important role for the proteoglycans: they stabilize the collagen-elastin network of connective tissues and contribute to lung elasticity and alveolar stability at low to medium lung volumes.

Lung volumes and alveolar expansion pattern in immature rabbits treated with serum-diluted surfactant

In acute respiratory distress syndrome, mechanical ventilation often induces alveolar overdistension aggravating the primary insult. To examine the mechanism of overdistension, surfactant-deficient immature rabbits were anesthetized with pentobarbital sodium, and their lungs were treated with serum-diluted modified natural surfactant (porcine lung extract; 2 mg/ml, 10 ml/kg). By mechanical ventilation with a peak inspiration pressure of 22.5 cm H2O, the animals had a tidal volume of 14.7 ml/kg (mean), when 2.5 cm H2O positive end-expiratory pressure was added. This volume was similar to that in animals treated with nondiluted modified natural surfactant (24 mg/ml in Ringer solution, 10 ml/kg). However, the lungs fixed at 10 cm H2O on the deflation limbs of the pressure-volume curve had the largest alveolar/alveolar duct profiles (> or =48,000 microm2), accounting for 38% of the terminal air spaces, and the smallest (<6,000 microm2), accounting for 31%. These values were higher than those in animals treated with nondiluted modified natural surfactant (P <0.05). We conclude that administration of serum-diluted surfactant to immature neonatal lungs leads to patchy overdistension of terminal air spaces, similar to the expansion pattern that may be seen after dilution of endogenous surfactant with proteinaceous edema fluid in acute respiratory distress syndrome.

Effect of ventilation rate on instilled surfactant distribution in the pulmonary airways of rats

Liquid can be instilled into the pulmonary airways during medical procedures such as surfactant replacement therapy, partial liquid ventilation, and pulmonary drug delivery. For all cases, understanding the dynamics of liquid distribution in the lung will increase the efficacy of treatment. A recently developed imaging technique for the study of real-time liquid transport dynamics in the pulmonary airways was used to investigate the effect of respiratory rate on the distribution of an instilled liquid, surfactant, in a rat lung. Twelve excised rat lungs were suspended vertically, and a single bolus (0.05 ml) of exogenous surfactant (Survanta, Ross Laboratories, Columbus, OH) mixed with radiopaque tracer was instilled as a plug into the trachea. The lungs were ventilated with a 4-ml tidal volume for 20 breaths at one of two respiratory rates: 20 or 60 breaths/min. The motion of radiodense surfactant was imaged at 30 frames/s with a microfocal X-ray source and an image intensifier. Dynamics of surfactant distribution were quantified for each image by use of distribution statistics and a homogeneity index. We found that the liquid distribution depended on the time to liquid plug rupture, which depends on ventilation rate. At 20 breaths/min, liquid was localized in the gravity-dependent region of the lung. At 60 breaths/min, the liquid coated the airways, providing a more vertically uniform liquid distribution.

Association between surfactant protein B + 1580 polymorphism and the risk of respiratory failure in adults with community-acquired pneumonia

OBJECTIVE

Pulmonary surfactant protein (SP)-B plays a vital role in the formation and function of surfactant in the lung. A genetic polymorphism (SP-B + 1580) is postulated to result in diminished activity of SP-B. The objective was to determine whether the SP-B + 1580 CC genotype is associated with an increased risk of respiratory failure and ARDS in adults with community-acquired pneumonia.

DESIGN

Prospective cohort of adults diagnosed with community-acquired pneumonia.

SETTING

Hospital system.

PATIENTS

We enrolled 402 adults > or = 18 yrs of age with community-acquired pneumonia; 158 were white, 243 were African American, and one was Asian.

INTERVENTIONS

Genotypic analysis was performed on DNA isolated from whole blood using polymerase chain reaction amplification and DdeI restriction enzyme digestion.

MEASUREMENTS AND MAIN RESULTS

We recorded the requirement for mechanical ventilation, the presence of acute respiratory distress syndrome (ARDS) or septic shock, and mortality. Sixty-three patients required mechanical ventilation, 12 patients developed ARDS, and 35 patients developed septic shock. Genotypic frequencies at the SP-B + 1580 site were T/T 183 of 402 (0.45), T/C 160 of 402 (0.40), and C/C 59 of 402 (0.15). Of the 59 patients who were C/C at the SP-B + 1580 site, 21 (0.356) required mechanical ventilation, compared with 26 of 160 patients (0.163) who were T/C and 16 of 183 (0.087) patients who were T/T (p < .001). ARDS developed in five of 59 (0.085) patients with the C/C genotype, compared with six of 160 (.038) patients with T/C and one of 183 patients with T/T (0.005, p < .009). Septic shock occurred in 12 of 59 (0.203) patients with the C/C genotype, compared with 13 of 160 (0.081) patients with T/C and ten of 183 (0.055) patients with T/T (p < .001). Mortality rate was not different between the three genotypes.

CONCLUSION

Carriage of the C allele at the SP-B + 1580 site is associated with ARDS, septic shock, and the need for mechanical ventilation in adults with community-acquired pneumonia.

The use of surfactant in children with acute respiratory distress syndrome: efficacy in terms of oxygenation, ventilation and mortality

PURPOSE

The aim of this prospectively designed study was to investigate the efficacy of surfactant (S) for acute respiratory distress syndrome (ARDS) in children.

MATERIALS AND METHODS

Children with ARDS were included in this study. Surfactant (Survanta, Abbott, USA) was given intratracheally at a dose of 150 mg/kg every 12 h for a total of two doses. During the study period none of the patients received permissive hypercapnia, high frequency ventilation, nitric oxide or ECMO. Peak inspiratory pressure (PIP), positive end expiratory pressure (PEEP), ventilation rate, mean airway pressure, tidal volume (TV), Murray index, PaO2/FiO2, ventilation index (VI), oxygen index (OI) and arterial oxygen tension difference (A-aDO2) were measured before and 48 h after surfactant treatment. Duration of mechanical ventilation therapy, duration in paediatric intensive care unit (PICU) and mortality rate were recorded.

RESULTS

Among the 36 children who met the inclusion criteria, 12 were treated with surfactant. The mean age was 72.5+/-56.2 months; 47% of children were male. Infants were ventilated by pressure-controlled ventilators whereas for older children volume-controlled ventilators were used. Sepsis (42%) was the main predisposing factor followed by pneumonia (25%) and malignancy (17%). The baseline characteristics including age, predisposing factors, gender, PIP, PEEP, A-aDO2, PaO2/FiO2, OI, TV, VI and Murray index were similar in the surfactant and non-surfactant (NS) group (p>0.05). There were significant improvements in PIP, PEEP, A-aDO2, PaO2/FiO2, OI, TV, VI and Murray index in the surfactant group after surfactant treatment compared with NS group (p<0.05). Duration of PICU stay and ventilator treatment was longer in NS group (14+/-3.7, 1.8+/-3.2 days vs. 9.2+/-3.1, 8.6+/-1.9 days), (p<0.05). Mortality rate was 42% in surfactant compared with 63% in the NS group, (p>0.05). Children in the surfactant group lived significantly longer (p<0.05).

CONCLUSIONS

Modified natural surfactant is an effective treatment option in children with ARDS for improving gas exchange, decreasing the use of ventilatory support and increasing survival time.

Deacylated pulmonary surfactant protein SP-C transforms from alpha-helical to amyloid fibril structure via a pH-dependent mechanism: an infrared structural investigation

Bovine pulmonary surfactant protein C (SP-C) is a hydrophobic, alpha-helical membrane-associated lipoprotein in which cysteines C4 and C5 are acylated with palmitoyl chains. Recently, it has been found that the alpha-helix form of SP-C is metastable, and under certain circumstances may transform from an alpha-helix to a beta-strand conformation that resembles amyloid fibrils. This transformation is accelerated when the protein is in its deacylated form (dSP-C). We have used infrared spectroscopy to study the structure of dSP-C in solution and at membrane interfaces. Our results show that dSP-C transforms from an alpha-helical to a beta-type amyloid fibril structure via a pH-dependent mechanism. In solution at low pH, dSP-C is alpha-helical in nature, but converts to an amyloid fibril structure composed of short beta-strands or beta-hairpins at neutral pH. The alpha-helix structure of dSP-C is fully recoverable from the amyloid beta-structure when the pH is once again lowered. Attenuated total reflectance infrared spectroscopy of lipid-protein monomolecular films showed that the fibril beta-form of dSP-C is not surface-associated at the air-water interface. In addition, the lipid-associated alpha-helix form of dSP-C is only retained at the surface at low surface pressures and dissociates from the membrane at higher surface pressures. In situ polarization modulation infrared spectroscopy of protein and lipid-protein monolayers at the air-water interface confirmed that the residual dSP-C helix conformation observed in the attenuated total reflectance infrared spectra of transferred films is randomly or isotropically oriented before exclusion from the membrane interface. This work identifies pH as one of the mechanistic causes of amyloid fibril formation for dSP-C, and a possible contributor to the pathogenesis of pulmonary alveolar proteinosis.

Surfactant lavage with lidocaine improves pulmonary function in piglets after HCl-induced acute lung injury

Acute respiratory distress syndrome (ARDS) is associated with significant morbidity and mortality. The pathophysiology of ARDS includes abnormalities of surfactant function as well as pulmonary inflammation. Immunomodulating drugs, like Lidocaine, have shown some success in decreasing inflammation in ARDS. We attempted to combine surfactant lavage's ability to reverse the surfactant dysfunction, while acting as a vehicle to deliver Lidocaine. Gravity-driven surfactant (Infasurf) lavage (35 ml/kg) was administered alone or mixed with Lidocaine after severe HCl acid injury (0.3 N; 3 cc/kg) in neonatal piglets. Treatment groups included: control (C) ( n = 5), surfactant lavage (SL) (35 ml/kg-diluted Infasurf) ( n = 7) and SL mixed with Lidocaine (SL+L) ( n = 7). About 26-27% of the lavage was retained (phospholipid 73-74 mg/kg; Lidocaine 1.8 mg/kg). Oxygenation progressively increased in the SL and SL+L groups over the 4-hour period (at 240 min: C = 99 +/- 14; SL = 154 +/- 39; SL+L = 230 +/- 40 mmHg) ( p < 0.05). PaCO(2) increased in all groups from 43 +/- 0.3 to 55 +/- 0.7 mmHg. Only SL+L showed a reduction in PaCO(2) (at 240 min: C = 54 +/- 4; SL = 53 +/- 7; SL+L = 49 +/- 2 mmHg) ( p < 0.05). Finally, SL and SL + L had superior characteristics during the quasi-static pressure volume (PV) procedure as compared to Control ( p < 0.05). In our HCl ALI model, SL improved oxygenation and quasi-static lung compliance over C. The pulmonary function effects of SL were further enhanced by the addition of Lidocaine to the surfactant suspension. Combining therapeutic agents with surfactant lavage may be an effective strategy in ALI.

Update on extracorporeal membrane oxygenation

Extracorporeal membrane oxygenation (ECMO) consists of the application of intermediate-term cardiopulmonary bypass for the treatment of potentially reversible heart and/or lung failure in the neonate, child, and adult. Applications in the neonate include congenital diaphragmatic hernia, pulmonary hypertension, meconium aspiration syndrome, and pre- and post-operative congenital heart surgery support. In the older child, myocarditis, infections, and respiratory failure (RSV and ARDS) are the most frequent indications, in addition to peri-operative cardiac surgical support. A review of the institutional experiences at the University of Louisville spanning a 15-year period and comparison international data will be presented, along with a pertinent review of the literature. Technical considerations, complications, and long-term outcomes will be reviewed, and the potential interface between ECMO and other, less invasive technologies, i.e., high-frequency ventilation, replacement surfactant, and nitric oxide, will be discussed.

Intratracheal Application of Recombinant Surfactant Protein-C Surfactant to Rabbits Attenuates Acute Lung Injury Induced by Intratracheal Acidified Infant Formula

Our aim in the current study was to determine whether recombinant surfactant protein-C (rSP-C) surfactant improves acute lung injury (ALI) induced by intratracheal acidified milk products. Twenty-eight rabbits were randomly divided into four groups. ALI was induced with intratracheal acidified infant formula (0.8 mL/kg, pH 1.8) in 3 groups. The control group received intratracheal acidified saline. Therapy groups received 1 of 2 doses of intratracheal rSP-C surfactant (0.5 or 2 SP-C mg/kg) 30 min after the acidified infant formula. The lungs were ventilated with 100% oxygen for 4 h after induction of ALI. Acidified infant formula dramatically reduced oxygenation and lung compliance, and increased resistance. Both doses of rSP-C improved the variables [mean PaO(2) (mm Hg) and compliance (mL/cm H(2)O) at 4 h: 61 and 0.4 for infant formula, 162 and 1.0 for small-dose rSP-C, and 152 and 1.2 for large-dose rSP-C, respectively; P < 0.05]. Pulmonary leukosequestration and edema, and severe morphological changes were attenuated by rSP-C treatment (ALI score: 14, 7, 7 in infant formula, small-dose rSP-C, and large-dose rSP-C; P < 0.05). The efficacy was similar for the two doses of rSP-C. These findings suggest that intratracheal administration of rSP-C ameliorates ALI induced by aspiration of acidified milk products.

IMPLICATIONS

Small or large doses of recombinant surfactant protein-C surfactant given 30 min after intratracheal acidified infant formula attenuated physiological, biochemical, and morphological lung damage.

Pressure–Volume Curve Does Not Predict Steady-State Lung Volume in Canine Lavage Lung Injury

To better understand strategies for recruiting and maintaining lung volume in acute lung injury, we examined relationships between steady-state lung volume and cumulative cyclic recruitment/derecruitment volume history and the quasi-static pressure-volume curve, in an animal saline lavage lung injury model. Small-volume tidal pressure-volume loops performed after inflation from functional residual capacity demonstrated incremental, cyclic recruitment only if the peak pressure achieved exceeded the pressure at which the compliance increased (Pflex) on the pressure-volume curve, whereas loops performed after deflation from total lung capacity remained close to the envelope deflation curve. Recruitment continued to occur up to and beyond a peak inspiratory airway pressure of 40 cm H(2)O, as demonstrated by both the tidal loops and by computed tomography-derived lung volume data. Tidal-specific compliance was relatively constant across positive end-expiratory pressure levels after inflation from functional residual capacity, but peaked at moderate positive end-expiratory pressure after deflation from total lung capacity, further demonstrating the effects of volume history and providing experimental validation of the recruitment models of Hickling (AJRCCM 2001;163:69-78). These results support the interpretation of Pflex as pressure threshold for recruitment, but otherwise do not suggest a role for the pressure-volume curve in predicting steady-state lung volume.

Alveolar recruitment in combination with sufficient positive end-expiratory pressure increases oxygenation and lung aeration in patients with severe chest trauma

OBJECTIVE

Investigation of oxygenation and lung aeration during mechanical ventilation according to the open lung concept in patients with acute lung injury or acute respiratory distress syndrome.

DESIGN

Retrospective analysis.

SETTING

Surgical intensive care unit of a university hospital.

PATIENTS

We retrospectively identified 17 patients with acute lung injury/acute respiratory distress syndrome due to pulmonary contusion who had thoracic helical computed tomography scans before and after ventilation with the open lung concept.

INTERVENTIONS

Baseline ventilation consisted of low tidal volumes (< or =6 mL/kg) and positive end-expiratory pressure (PEEP; 5-17 cm H2O). We briefly applied high inspiratory pressures for opening up collapsed alveoli. External PEEP and intrinsic PEEP were combined to keep recruited lung units open. We generated intrinsic PEEP by pressure-cycled high-frequency inverse ratio ventilation (80 min, inspiratory/expiratory ratio 2:1) and maintained our ventilatory strategy for 24 hrs. Then, after reducing total PEEP by decreasing respiratory rate, Pao2/Fio2 ratio was reevaluated. If it remained >300 mm Hg, weaning was started. If not, previous ventilator settings were resumed for another 24 hrs after recruiting the lungs once again.

MEASUREMENTS AND MAIN RESULTS

Physiologic variables and ventilator settings were obtained from routine charts. Data from computed tomography before and after the open lung concept were analyzed for volumetric quantification of lung aeration and collapse. All results are presented as median and range. During baseline ventilation, PEEP was 10 (range, 5-17) cm H2O and after recruitment 21 (range, 18-26) cm H2O. Opening pressures were 65 (range, 50-80) cm H2O. After recruitment, Pao2/Fio2 ratio was higher in all patients. Total lung volume increased from 2915 (range, 1952-4941) to 4247 (range, 2285-6355) mL and normally aerated volume from 1742 (range, 774-2941) to 2971 (range, 1270-5232) mL. Atelectasis decreased significantly from 604 (range, 147-1538) to 106 (range, 0-736) mL. Hyperinflation increased significantly from 5 (range, 0-188) to 62 (range, 1-424) mL, whereas poor aeration did not change substantially from 649 (range, 302-1292) to 757 (range, 350-1613) mL. No hemodynamic problems occurred.

CONCLUSIONS

Lung recruitment increased arterial oxygenation, normally aerated lung volume, and total lung volume while decreasing the amount of collapsed tissue. These results indicate that the open lung concept is a reasonable mode of ventilation for patients with severe chest trauma.

Imbalances in Regional Lung Ventilation

Imbalances in regional lung ventilation, with gravity-dependent collapse and overdistention of nondependent zones, are likely associated to ventilator-induced lung injury. Electric impedance tomography is a new imaging technique that is potentially capable of monitoring those imbalances. The aim of this study was to validate electrical impedance tomography measurements of ventilation distribution, by comparison with dynamic computerized tomography in a heterogeneous population of critically ill patients under mechanical ventilation. Multiple scans with both devices were collected during slow-inflation breaths. Six repeated breaths were monitored by impedance tomography, showing acceptable reproducibility. We observed acceptable agreement between both technologies in detecting right-left ventilation imbalances (bias = 0% and limits of agreement = -10 to +10%). Relative distribution of ventilation into regions or layers representing one-fourth of the thoracic section could also be assessed with good precision. Depending on electrode positioning, impedance tomography slightly overestimated ventilation imbalances along gravitational axis. Ventilation was gravitationally dependent in all patients, with some transient blockages in dependent regions synchronously detected by both scanning techniques. Among variables derived from computerized tomography, changes in absolute air content best explained the integral of impedance changes inside regions of interest (r(2) > or = 0.92). Impedance tomography can reliably assess ventilation distribution during mechanical ventilation.

Acute respiratory distress syndrome

Several combination therapies have been described throughout this article: in all likelihood, it is combination therapy that will allow improved survival of ARDS patients. As medicine moves into the future, clinical trials evaluating the efficacy of therapies for ARDS will be performed. In human critical care medicine, a large forward step was taken when ALI and ARDS were clearly defined. Unfortunately. as good as the definition is, ALI and ARDS occur secondary to many different underlying pathologic processes,perhaps obscuring the benefits of certain therapies for ARDS based on the underlying condition, for example, trauma versus sepsis. Selection of patients entering any ARDS trial is crucial: not only must those patients meet the strict definition of ARDS but the underlying disease process should be clearly identified. Identification of patients suffering from different disease processes before the onset of ARDS will allow for stratification of outcomes according to the intervention and the underlying pathology--comparing apples to apples and not to oranges. We are in a unique position in veterinary medicine. Although frequently financially limited by our clients, we have the opportunity to achieve several goals. First, we need to clearly define what constitutes ALI and ARDS in veterinary medicine. Do we want to rely on the human definitions? Probably not; however, as a group, we need to determine what we will accept as definitions. For example, we may not be able perform right heart catheterizations on all our patients to meet the wedge pressure requirement of human beings of less than 18 mm Hg. Do we agree that a PAOP of less than 18 mmHg is appropriate for animals, and is it appropriate for all animals? Will we accept another measure, for example, pulmonary artery diameter increases with echocardiographic evidence of acceptable left heart function? What is acceptable left heart function? As veterinarians, what do we consider to be hypoxemia? Is it the same in all species that we work with? What do we define as acute onset? Most human ARDS cases occur while patients are in hospital being treated for other problems, whereas many of our patients present already in respiratory distress. If we are unable to ventilate patients for economic or practical reasons, what do we use as the equivalent of the Pao2/Flo, ratio'? Reliance on the pathologist is not reasonable, because many disease processes can look similar to ARDS under the microscope. If anything, ALI and ARDS are clinical diagnoses. It is time for veterinarians to reach a consensus on the definition for ALI and ARDS in our patients. Only when we have a consensus of definition can rational prospective clinical trials of therapies be designed.

Protective ventilation of patients with acute respiratory distress syndrome

The majority of patients with acute respiratory distress syndrome (ARDS) require mechanical ventilation. This support provides time for the lungs to heal, but the adverse effects of mechanical ventilation significantly influence patient outcome. Traditionally, these were ascribed to mechanical effects, such as haemodynamic compromise from decreased venous return or gross air leaks induced by large transpulmonary pressures. More recently, however, the ARDS Network study has established the clinical importance of lowering the tidal volume to limit overdistension of the lung when ventilating patients with ARDS. This study suggests that ventilator-associated lung injury (VALI) caused by overdistension of the lung contributes to the mortality of patients with ARDS. Moreover, the results from clinical and basic research have revealed more subtle types of VALI, including upregulation of the inflammatory response in the injured and overdistended lung. This not only damages the lung, but the overflow of inflammatory mediators into the systemic circulation may explain why most patients who die with ARDS succumb to multi-organ failure rather than respiratory failure. The results of these studies, the present understanding of the pathophysiology of VALI, and protective ventilatory strategies are reviewed.

Surfactant abnormalities after single lung transplantation in dogs: impact of bronchoscopic surfactant administration

OBJECTIVE

Disturbances of the alveolar surfactant system have been implicated in the pathogenesis of reperfusion injury. The aim of this study was to evaluate the influence of exogenous surfactant administration on surfactant properties in a model of single lung transplantation.

METHODS

We performed heterologous, left lung transplantation (+4 degrees C ischemia; 24 hours, Euro-Collins solution) in 6 foxhounds (untreated) and in 6 animals that received calf lung surfactant extract (Alveofact) prior to explantation (only donor lung; 50 mg/kg body weight) and immediately after onset of reperfusion (both lungs, 200 mg/kg body weight). Separate but synchronized ventilation of each lung was performed, in a volume-controlled, pressure-limited mode, with animals in prone position. Bronchoalveolar lavage fluids were collected in pretransplantation lungs (control), after 24 hours of ischemia prior to transplantation (0 hours) and 6 and 12 hours after reperfusion in both the grafts and the recipient native lungs.

RESULTS

Ischemic storage per se did not provoke any changes of the surfactant system; however, severe alterations occurred within 6 hours of reperfusion, resulting in a severe loss of surface activity, including a decrease in the percentage of the large surfactant aggregate fraction, reduction of the surfactant apoproteins SP-B and SP-C, the dipalmitoyl molecular species of phosphatidylcholine and phosphatidylglycerol within the large surfactant aggregate fraction. These abnormalities were restricted to the graft, with virtually normal surfactant function and composition being found in the recipient native lung. Surfactant administration fully normalized the biochemical and largely improved the biophysical surfactant properties, alongside maintenance of lung gas exchange properties.

CONCLUSIONS

Severe surfactant abnormalities occur exclusively in the graft when performing separate, synchronized ventilation of each lung to attenuate ventilator-induced lung injury. Bronchoscopic surfactant administration provides protection against these abnormalities and may be a therapeutic strategy in lung transplantation.

Phospholipid metabolism in lung surfactant

Pulmonary surfactant is a mixture of lipids, mostly phospholipids, and proteins that allows for breathing with minimal effort. The current chapter discusses the metabolism of the phospholipids of this material. Surfactant phospholipids are synthesized in the type II epithelial cells of the lung. The lipids and surfactant proteins are assembled in intracellular storage organelles, called lamellar bodies, and are subsequently secreted into the alveolar space. Within this extracellular space surfactant undergoes several transformations. First the lamellar bodies unravel to form a highly organized lattice-like lipid:protein structure tubular myelin. Second, the organized structures, in particular tubular myelin, adsorb to form a lipid at the air-liquid interface of the alveoli. It is, in fact, this surface tension reducing film that is responsible for the physiological role of surfactant, to prevent lung collapse and allow ease of inflation. Third, the surface film is converted to a small vesicular form. Finally, these small vesicles are taken-up by the type II cells for recycling and degradation and by alveolar macrophages for degradation.

Elevation of lung surfactant phosphatidylcholine in mouse models of Sandhoff and of Niemann-Pick A disease

Sandhoff disease is caused by the defective activity of the lysosomal enzyme beta-hexosaminidase, resulting in accumulation of the glycolipids, GA2 and GM2. Niemann-Pick A/B disease is caused by the defective activity of lysosomal acid sphingomyelinase resulting in sphingomyelin accumulation. Pulmonary complications have been observed in both diseases. We now demonstrate changes in phospholipid levels in pulmonary surfactant in mouse models of these diseases. In the Hexb mouse, a model of Sandhoff disease, lipid phosphate levels were elevated in surfactant from 3- and 4-month-old mice, which was mainly due to elevated levels of phosphatidylcholine. In the ASM mouse, a model of Niemann-Pick A disease, levels of the primary storage material, sphingomyelin, were elevated as expected, and levels of phosphatidylcholine and two other phospholipids were also significantly elevated in pulmonary surfactant and in lung tissue from 5-, 6- and 7-month-old mice. These results suggest that changes in phospholipid levels and composition in lung surfactant might be a general feature of sphingolipid storage diseases, which may be in part responsible for the increased susceptibility of these patients to respiratory infections and lung pathology, often the main reason for the death of these patients.

Physiological and inflammatory response to instillation of an oxidized surfactant in a rat model of surfactant deficiency

Pulmonary surfactant is a mixture of phospholipids ( approximately 90%) and surfactant-associated proteins (SPs) ( approximately 10%) that stabilize the lung by reducing the surface tension. One proposed mechanism by which surfactant is altered during acute lung injury is via direct oxidative damage to surfactant. In vitro studies have revealed that the surface activity of oxidized surfactant was impaired and that this effect could be overcome by adding SP-A. On the basis of this information, we hypothesized that animals receiving oxidized surfactant preparations would exhibit an inferior physiological and inflammatory response and that the addition of SP-A to the oxidized preparations would ameliorate this response. To test this hypothesis, mechanically ventilated, surfactant-deficient rats were administered either bovine lipid extract surfactant (BLES) or in vitro oxidized BLES of three doses: 10 mg/kg, 50 mg/kg, or 10 mg/kg + SP-A. When instilled with 10 mg/kg normal surfactant, the rats had a significantly superior arterial Po2 responses compared with the rats receiving oxidized surfactant. Interestingly, increasing the dose five times mitigated this physiological effect, and the addition of SP-A to the surfactant preparation had little impact on improving oxygenation. There were no differences in alveolar surfactant pools and the indexes of pulmonary inflammation between the 10 mg/kg dose groups, nor was there any differences observed between either of the groups supplemented with SP-A. However, there was significantly more surfactant and more inflammatory cytokines in the 50 mg/kg oxidized BLES group compared with the 50 mg/kg BLES group. We conclude that instillation of an in vitro oxidized surfactant causes an inferior physiological response in a surfactant-deficient rat.

Distribution dynamics of perfluorocarbon delivery to the lungs: an intact rabbit model

Motivated by the goal of understanding how to most homogeneously fill the lungs with perfluorocarbon for liquid ventilation, we investigate the transport of liquid instilled into the lungs using an intact rabbit model. Perfluorocarbon is instilled into the trachea of the ventilated animal. Radiographic images of the perfluorocarbon distribution are obtained at a rate of 30 frames/s during the filling process. Image analysis is used to quantify the liquid distribution (center of mass, spatial standard deviation, skewness, kurtosis, and indicators of homogeneity) as time progresses. We compare the distribution dynamics in supine animals to those in upright animals for three constant infusion rates of perfluorocarbon: 15, 40, and 60 ml/min. It is found that formation of liquid plugs in large airways, which is affected by posture and infusion rate, can result in a more homogeneous liquid distribution than gravity drainage alone. The supine posture resulted in more homogeneous filling of the lungs than did upright posture, in which the lungs tend to fill in the inferior regions first. Faster instillation of perfluorocarbon results in liquid plugs forming in large airways and, consequently, more uniform distribution of perfluorocarbon than slower instillation rates in the upright animals.

Injury and repair in lung and airways

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are common causes of morbidity and mortality in the intensive care unit. ALI/ARDS occurs as a result of systemic inflammation, usually triggered by a microorganism. Activation of leukocytes and release of proinflammatory mediators from multiple cellular sources result in both local and distant tissue injury. Tumor necrosis factor-alpha and interleukin-1 beta are the best characterized of the proinflammatory cytokines contributing to ALI/ARDS and subsequent fibrosis. The ultimate clinical course of ALI/ARDS often is determined by the ability of the injured lung to repopulate the alveolar epithelium with functional cells. Death may occur when fibrosis predominates the healing response, as it results in worsening lung compliance and oxygenation. The rodent bleomycin model of lung fibrosis allows the use of molecular tools to dissect the cellular and subcellular processes leading to fibrosis. The elements of this response may provide therapeutic targets for the prevention of this devastating complication of ALI/ARDS.

The role of exogenous surfactant in the treatment of acute lung injury

A number of conditions, such as pneumonia, trauma, or systemic sepsis arising from the gut, may result in the acute respiratory distress syndrome (ARDS). Because of its significant morbidity and mortality, ARDS has been the focus of extensive research. One specific area of interest has been the investigation of the role of the surfactant system in the pathophysiology of this disease. Several studies have demonstrated that alterations of surfactant contribute to the lung dysfunction associated with ARDS, which has led to investigations into the use of exogenous surfactant as a therapy for this syndrome. Clinical experience with surfactant therapy has been variable owing to a number of factors including the nature of the injury at the time of treatment, the specific surfactant preparation utilized, the dose and delivery method chosen, the timing of surfactant administration over the course of the disease, and the mode of ventilation used during and after surfactant administration.

Treatment of acute respiratory distress syndrome with recombinant surfactant protein C surfactant

We performed a phase I/II trial in North America of a recombinant surfactant protein C-based surfactant (Venticute) as treatment for the acute respiratory distress syndrome. Patients were prospectively randomized to receive either standard therapy or standard therapy plus one of two doses of exogenous surfactant given four times over 24 hours. Surfactant administration was well tolerated. No significant treatment benefit was associated with surfactant treatment. Bronchoalveolar lavage of treated patients at 48 hours reflected the presence of exogenous surfactant components, did not show evidence of improved surface tension lowering function, and had interleukin-6 concentrations that were significantly lower than control group values, consistent with an antiinflammatory treatment effect. The presence of exogenous surfactant was not detected in lavage fluid obtained at 120 hours. Future studies might rationally employ larger surfactant doses and a more prolonged dosing schedule.

Circadian rhythm of surfactant protein A, B and C mRNA in rats

BACKGROUND

All organisms have developed an internal timing system capable of reacting to and anticipating environmental stimuli with a program of appropriately timed metabolic, physiologic and behavioral events. The alveolar epithelial type II cell of the mammalian lung synthesizes, stores, and secretes a lipoprotein pulmonary surfactant, which functions to stabilize alveoli at low lung volumes.

METHODS

The authors investigated the diurnal variation of surfactant protein A, B and C mRNA accumulation. The diunal variation on gene expression of surfactant protein A, B and C was analysed using filter hybridization at 9 a.m., 4 p.m. and 11 p.m. Lung SP-A protein content was determined by double sandwich ELISA assay using a polyclonal antiserum raised in rabbits against purified rat SP-A.

RESULTS

1. The accumulation of SP-A mRNA at 4 p.m. was significantly decreased by 23.5% compared to the value at 9 a.m. (p < 0.05). 2. The accumulation of SP-B mRNA at 4 p.m. and 11 p.m. was decreased by 15.1% and 5.7%, respectively, compared to the value at 9 a.m. (p = 0.07, p = 0.69). 3. The accumulation of SP-C mRNA at 4 p.m. and 11 p.m. was decreased by 6.8% and 7.7%, respectively, compared to the value at 9 a.m. (p = 0.38, p = 0.57). 4. Total lung SP-A content at 4 p.m. and 11 p.m. was increased by 5.3% and 15.9%, respectively, compared to the value at 9 a.m. (p = 0.64, p = 0.47).

CONCLUSION

These findings represent the diurnal variation of surfactant proteins mRNA expression in vivo. These results indicated that the diurnal variation of significant gene expression is observed in hydrophilic surfactant protein rather than in hydrophobic surfactant proteins.