Pulmonary surfactant subfractions in patients with the acute respiratory distress syndrome

Surfactant for pediatric acute lung injury

This article reviews exogenous surfactant therapy and its use in mitigating acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS) in infants, children, and adults. Biophysical and animal research documenting surfactant dysfunction in ALI/ARDS is described, and the scientific rationale for treatment with exogenous surfactant is discussed. Major emphasis is placed on reviewing clinical studies of surfactant therapy in pediatric and adult patients who have ALI/ARDS. Particular advantages from surfactant therapy in direct pulmonary forms of these syndromes are described. Also discussed are additional factors affecting the efficacy of exogenous surfactants in ALI/ARDS.

Pulmonary morphofunctional effects of mechanical ventilation with high inspiratory air flow

OBJECTIVE

Uncertainties about the numerous degrees of freedom in ventilator settings leave many unanswered questions about the biophysical determinants of lung injury. We investigated whether mechanical ventilation with high air flow could yield lung mechanical stress even in normal animals.

DESIGN

Prospective, randomized, controlled experimental study.

SETTING

University research laboratory.

SUBJECTS

Thirty normal male Wistar rats (180-230 g).

INTERVENTIONS

Rats were ventilated for 2 hrs with tidal volume of 10 mL/kg and either with normal inspiratory air flow (V') of 10 mL/s (F10) or high V' of 30 mL/s (F30). In the control group, animals did not undergo mechanical ventilation. Because high flow led to elevated respiratory rate (200 breaths/min) and airway peak inspiratory pressure (PIP,aw = 17 cm H2O), two additional groups were established to rule out the potential contribution of these variables: a) normal respiratory rate = 100 breaths/min and V' = 30 mL/sec; and b) PIP,aw = 17 cm H2O and V' = 10 mL/sec.

MEASUREMENTS AND MAIN RESULTS

Lung mechanics and histology (light and electron microscopy), arterial blood gas analysis, and type III procollagen messenger RNA expression in lung tissue were analyzed. Ultrastructural microscopy was similar in control and F10 groups. High air flow led to increased lung plateau and peak pressures, hypoxemia, alveolar hyperinflation and collapse, pulmonary neutrophilic infiltration, and augmented type III procollagen messenger RNA expression compared with control rats. The reduction of respiratory rate did not modify the morphofunctional behavior observed in the presence of increased air flow. Even though the increase in peak pressure yielded mechanical and histologic changes, type III procollagen messenger RNA expression remained unaltered.

CONCLUSIONS

Ventilation with high inspiratory air flow may lead to high tensile and shear stresses resulting in lung functional and morphologic compromise and elevation of type III procollagen messenger RNA expression.

Protective role of macrophages in noninflammatory lung injury caused by selective ablation of alveolar epithelial type II Cells

Macrophages have a wide variety of activities and it is largely unknown how the diverse phenotypes of macrophages contribute to pathological conditions in the different types of tissue injury in vivo. In this study we established a novel animal model of acute respiratory distress syndrome caused by the dysfunction of alveolar epithelial type II (AE2) cells and examined the roles of alveolar macrophages in the acute lung injury. The human diphtheria toxin (DT) receptor (DTR), heparin-binding epidermal growth factor-like growth factor (HB-EGF), was expressed under the control of the lysozyme M (LysM) gene promoter in the mice. When DT was administrated to the mice they suffered from acute lung injury and died within 4 days. Immunohistochemical examination revealed that AE2 cells as well as alveolar macrophages were deleted via apoptosis in the mice treated with DT. Consistent with the deletion of AE2 cells, the amount of surfactant proteins in bronchoalveolar lavage fluid was greatly reduced in the DT-treated transgenic mice. When bone marrow from wild-type mice was transplanted into irradiated LysM-DTR mice, the alveolar macrophages became resistant to DT but the mice still suffered from acute lung injury by DT administration. Compared with the mice in which both AE2 cells and macrophages were deleted by DT administration, the DT-treated LysM-DTR mice with DT-resistant macrophages showed less severe lung injury with a reduced amount of hepatocyte growth factor in bronchoalveolar lavage fluid. These results indicate that macrophages play a protective role in noninflammatory lung injury caused by the selective ablation of AE2 cells.

Surfactant dysfunction and lung injury due to the E. coli virulence factor hemolysin in a rat pneumonia model

This study tests the hypothesis that the virulence factor hemolysin (Hly) expressed by extraintestinal pathogenic Escherichia coli contributes to surfactant dysfunction and lung injury in a rat model of gram-negative pneumonia. Rats were instilled intratracheally with CP9 (wild type, Hly-positive), CP9hlyA (Hly-minus), CP9/pEK50 (supraphysiological Hly), or purified LPS. At 6 h postinfection, rats given CP9 had a decreased percentage content of large surfactant aggregates in cell-free bronchoalveolar lavage (BAL), decreased large aggregate surface activity, decreased Pa(O2)/FiO2) ratio, increased BAL albumin/protein levels, and increased histological evidence of lung injury compared with rats given CP9hlyA or LPS. In addition, rats given CP9/pEK50 or CP9 had decreased large aggregate surface activity, decreased Pa(O2)/FiO2) ratios, and increased BAL albumin/protein levels at 2 h postinfection compared with rats given CP9hlyA. The severity of permeability lung injury based on albumin/protein levels in BAL at 2 h was ordered as CP9/pEK50 > CP9 > CP9hlyA > normal saline controls. Total lung titers of bacteria were increased at 6 h in rats given CP9 vs. CP9hlyA, but bacterial titers were not significantly different at 2 h, indicating that increased surfactant dysfunction and lung injury were associated with Hly as opposed to bacterial numbers per se. Further studies in vitro showed that CP9 could directly lyse transformed pulmonary epithelial cells (H441 cells) but that indirect lysis of H441 cells secondary to Hly-induced neutrophil lysis did not occur. Together, these data demonstrate that Hly is an important direct mediator of surfactant dysfunction and lung injury in gram-negative pneumonia.

Pneumoproteins as markers of paraquat lung injury: a clinical case

OBJECTIVE

To describe the changes in lung-specific secretory proteins in biological fluids in a fatal case of paraquat ingestion and to present immunostaining data obtained on postmortem lung tissue specimens.

METHODS

A 20-year-old man committed suicide by ingesting 100ml of a 20% paraquat solution. Surfactant associated proteins A (SP-A), B (SP-B) and Clara cell 16kDa protein (CC16) were determined in the serum and on broncho-alveloar lavage performed 18h after admission. Renal failure progressed rapidly and the patient died from refractory hypoxia. Immunostaining studies using antibodies directed against CC16, SP-A and SP-B were performed on postmortem lung tissue specimens.

RESULTS

Serum CC16 seemed to increase gradually with the progression of renal impairment. Serum SP-A and SP-B levels increased before any significant changes in pulmonary gas exchanges. The immunostaining study showed that the labeling for SP-A and SP-B was reduced or absent following paraquat toxicity, while Clara cells were relatively preserved.

CONCLUSIONS

The elevation of serum CC16 with paraquat toxicity is probably mainly related to a reduced renal clearance. The increase of serum SP-A and SP-B could reflect an increased lung to blood leakage, independently of the alteration of the renal function.

Open lung ventilation preserves the response to delayed surfactant treatment in surfactant-deficient newborn piglets

OBJECTIVE

Delayed surfactant treatment (>2 hrs after birth) is less effective than early treatment in conventionally ventilated preterm infants with respiratory distress syndrome. The objective of this study was to evaluate if this time-dependent efficacy of surfactant treatment is also present during open lung ventilation.

DESIGN

Prospective, randomized controlled animal study.

SETTING

University-affiliated research laboratory.

SUBJECTS

Thirty-eight newborn piglets.

INTERVENTIONS

Following repeated whole-lung lavage, animals were randomly allocated to conventional positive pressure ventilation (PPVCON) using a positive end-expiratory pressure (PEEP) of 5 cm H2O and a tidal volume of 7 mL/kg or open lung positive pressure ventilation (PPVOLV). During PPVOLV, collapsed alveoli were actively recruited and thereafter stabilized with sufficient PEEP. Within each ventilation group, animals received surfactant (25 mg/kg) either after 2 hrs (PPVCON-2 and PPVOLV-2) or after 4 hrs (PPVCON-4 and PPVOLV-4) of ventilation. A control group received surfactant immediately after lung lavage. Following surfactant administration, all animals were conventionally ventilated for an additional 2 hrs.

MEASUREMENTS AND MAIN RESULTS

Two hours after surfactant treatment, both oxygenation and lung mechanics showed a clear deterioration in the PPVCON-4 group compared with PPVCON-2 and the control group. However, this deterioration of the surfactant response over time was not observed during PPVOLV. Analysis of the bronchoalveolar lavage fluid obtained at the end of the experiment showed that the protein concentration and the conversion of large to small aggregate surfactant was significantly higher in the PPVCON-4 group compared with the PPVCON-2 group while comparable in both PPVOLV groups. In addition, interleukin-8 and myeloperoxidase levels tended to be higher in the PPVCON-4 group compared with the PPVOLV-4 group.

CONCLUSIONS

In contrast to conventional ventilation, open lung ventilation preserves the response to delayed surfactant treatment in surfactant-deficient newborn piglets. This sustained response is accompanied by an attenuation of secondary lung injury.

Exogenous surfactant therapy for ARDS

Regardless of the cause, a common pathophysiological feature of patients with acute respiratory distress syndrome is a dysfunction of the endogenous surfactant system. Although exogenous surfactant therapy has proven to be an effective treatment for neonatal respiratory distress syndrome, no similar current effective therapy exists for patients with acute respiratory distress syndrome. This is mainly due to the complexity of the lung injury that is involved with this disorder. Results from clinical trials, to date, have failed to show an improvement in patient survival after administration of exogenous surfactant; however, ongoing and future research efforts suggest that this therapy may eventually be feasible.

Physiological effects of oxidized exogenous surfactant in vivo: effects of high tidal volume and surfactant protein A

Oxidative damage to surfactant can decrease lung function in vivo. In the current study, our two objectives were: 1) to examine whether the adverse effects of oxidized surfactant would be accentuated in animals exposed to high tidal volume ventilation, and 2) to test whether supplementation with surfactant protein A (SP-A) could improve the function of oxidized surfactant in vivo. The first objective was addressed by evaluating the response of surfactant-deficient rats administered normal or oxidized surfactant and then subjected to low tidal volume (6 ml/kg) or high tidal volume (12 ml/kg) mechanical ventilation. Under low tidal volume conditions, rats administered oxidized surfactant had impaired lung function, as determined by lung compliance and arterial blood gas analysis, compared with nonoxidized controls. Animals subjected to high tidal volume ventilation had impaired lung function compared with low tidal volume groups, regardless of the oxidative status of the surfactant. The second experiment demonstrated a significantly superior physiological response in surfactant-deficient rats receiving SP-A containing oxidized surfactant compared with oxidized surfactant. Lavage analysis at the end of the in vivo experimentation showed no differences in the recovery of oxidized surfactant compared with nonoxidized surfactant. We conclude that minimizing excessive lung stretch during mechanical ventilation is important in the context of exogenous surfactant supplementation and that SP-A has an important biophysical role in surfactant function in conditions of oxidative stress. Furthermore, the oxidative status of the surfactant does not appear to affect the alveolar metabolism of this material.

Pre-emptive and continuous inhaled NO counteracts the cardiopulmonary consequences of extracorporeal circulation in a pig model

Cardiopulmonary bypass (CPB) activates a systemic inflammatory response characterized clinically by alterations in cardiovascular and pulmonary function. The aim of this study was to measure the cardiopulmonary consequences in sham-operated pigs, and in animals subjected to CPB in the presence or absence of lipopolysaccharide (LPS). We also investigated, if the perioperative administration of inhaled NO exerts significant cardiopulmonary effects in an anaesthetized and mechanically ventilated pig model of extracorporeal circulation. Thirty pigs were randomized into six equal groups (sham; sham+INO; CPB; CPB+INO; CPB+LPS; CPB+LPS+INO) and subjected to anaesthesia with mechanical ventilation for up to 24h. We found that CPB+LPS group has the highest degree of lung injury. We also demonstrated that there was a significant difference on the cardiovascular parameters (heart rate, central venous pressure, stroke volume index, and mean systemic arterial blood pressure) between the CPB groups and the sham groups. The deteriorated lung mechanics was associated with a decrease in active subfraction of surfactant (LA) with time during the procedure (P=0.0003), on which inhaled NO had only an initial beneficial effect. In our model, inhaled NO had no long-term beneficial effect on lung mechanics and surfactant homeostasis despite improving lung haemodynamics, inflammation, and oxygenation. We conclude from this study that the use of pre-emptive and continuous inhaled NO therapy has protective and safe effects against lung ischemia/reperfusion associated with CPB.

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.

Defective surfactant secretion in a mouse model of Hermansky-Pudlak syndrome

Hermansky-Pudlak syndrome (HPS) in humans represents a family of disorders of lysosome-related organelle biogenesis associated with severe, progressive pulmonary disease. Human case reports and a mouse model of HPS, the pale ear/pearl mouse (ep/pe), exhibit giant lamellar bodies (GLB) in type II alveolar epithelial cells. We examined surfactant proteins and phospholipid from ep/pe mice to elucidate the process of GLB formation. The 2.8-fold enrichment of tissue phospholipids in ep/pe mice resulted from accumulation from birth through adulthood. Tissue surfactant protein (SP)-B and -C were increased in adult ep/pe mice compared with wild-type mice (WT), whereas SP-A and -D were not different. Large aggregate surfactant (LA) from adult ep/pe mice had decreased phospholipid, SP-B, and SP-C, with no differences in SP-A and -D compared with WT. Although LA from ep/pe animals exhibited an increased total protein-to-total phospholipid ratio compared with WT, surface tension was not compromised. Phospholipid secretion from isolated type II cells showed that basal and stimulated secretion from ep/pe cells were approximately 50% of WT cells. Together, our data indicate that GLB formation is not associated with abnormal trafficking or recycling of surfactant material. Instead, impaired secretion is an important component of GLB formation in ep/pe mice.

Pseudomonas aeruginosa degrades pulmonary surfactant and increases conversion in vitro

Although it is known that surfactant lipids and proteins are altered in patients with Pseudomonas aeruginosa infections, the mechanisms and implications of these alterations are not clear. In this study, the effects of P. aeruginosa on the surfactant large aggregate fraction were examined using an in vitro surface area cycling model. Large aggregates were isolated from porcine bronchoalveolar lavage fluid and incubated with supernatants from P. aeruginosa cultures (PAO1, parent strain; PAO1-A1, lasA-negative mutant; PAO1-B1, elastase-negative mutant) or purified elastase. Amounts of surfactant protein (SP)-A and SP-B, phospholipid content, and large aggregate conversion were assessed. In addition, lipid degradation was assessed by incubating a mixture of radiolabeled phospholipids with P. aeruginosa supernatants. The results demonstrated that SP-A was degraded by PAO1 and PAO1-A1 supernatants, and by purified elastase. SP-B was degraded by PAO1 and PAO1-B1 supernatants, but not by elastase. P. aeruginosa supernatants degraded phospholipids, a process inhibited by ZnCl(2). P. aeruginosa supernatants and elastase increased conversion. The data suggest that protein degradation facilitates increased conversion, and that phospholipid degradation and conversion enhance degradation of surfactant proteins. In conclusion, P. aeruginosa secretes multiple virulence factors that cooperate to result in degradation of surfactant components and alteration of large aggregate conversion.

Surfactant phospholipid changes after antigen challenge: a role for phosphatidylglycerol in dysfunction

In asthma, inflammation-mediated surfactant dysfunction contributes to increased airway resistance, but the mechanisms for dysfunction are not understood. To test mechanisms that alter surfactant function, atopic asthmatics underwent endobronchial antigen challenge and bronchoalveolar lavage (BAL). BAL fluids were sequentially separated into cells, surfactant, and supernatant, and multiple end points were analyzed. Each end point's unique relationship to surfactant dysfunction was determined. Our results demonstrate that minimum surface tension (gamma(min)) of surfactant after antigen challenge was significantly increased with a spectrum of responses that included dysfunction in 6 of 13 asthmatics. Antigen challenge significantly altered the partitioning of surfactant phospholipid measured as a decreased ratio of large surfactant aggregates (LA) to small surfactant aggregates (SA), LA/SA ratio. Phosphatidylglycerol (PG) was significantly reduced in the LA of the dysfunctional asthmatic BALs. There was a corresponding significant increase in the ratio of phosphatidylcholine to PG, which strongly correlated with both increased gamma(min) and decreased LA/SA. Altered surfactant phospholipid properties correlated with surfactant dysfunction as well or better than either increased eosinophils or protein. Secretory phospholipase activity, measured in vitro, increased after antigen challenge and may explain the decrease in surfactant PG. In summary, alteration of phospholipids, particularly depletion of PG, in the LA of surfactant may be an important mechanism in asthma-associated surfactant dysfunction.

Mechanical ventilation effect on surfactant content, function, and lung compliance in the newborn rat

Studies of ventilator-associated lung injury in adult experimental animal models have documented that high tidal volume (TV) results in lung injury characterized by impaired compliance and dysfunctional surfactant. Yet, there is evidence that, in neonates, ventilation with a higher than physiologic TV leads to improved lung compliance. The purpose of our study was to evaluate how lung compliance and surfactant was altered by high TV ventilation in the neonate. We utilized a new model (mechanically air-ventilated newborn rats, 4-8 d old), and used 40 or 10 mL/kg TV strategies. Age-matched nonventilated animals served as controls. In all animals, dynamic compliance progressively increased after initiation of mechanical ventilation and was significantly greater than basal values after 60 min (p < 0.01). Lung lavage total surfactant with both TV strategies (p < 0.05) and the large aggregate fraction (only in TV = 40 mL/kg; p < 0.01) were significantly increased by 60 min of mechanical ventilation, compared with control animals. Ventilation with 40 mL/kg TV for 60 min adversely affected the lung surfactant surface-tension lowering properties (p < 0.01). After 180 min of ventilation with 40 mL/kg TV, the lung total surfactant content and dynamic compliance values were no longer distinct from the nonventilated animals' values. We conclude that, in the newborn rat, mechanical ventilation with a higher than physiologic TV increases alveolar surfactant content and, over time, alters its biophysical properties, thus promoting an initial but transient improvement in lung compliance.

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.

Carbon dioxide attenuates pulmonary impairment resulting from hyperventilation

OBJECTIVE

Deliberate elevation of PaCO2 (therapeutic hypercapnia) protects against lung injury induced by lung reperfusion and severe lung stretch. Conversely, hypocapnic alkalosis causes lung injury and worsens lung reperfusion injury. Alterations in lung surfactant may contribute to ventilator-associated lung injury. The potential for CO2 to contribute to the pathogenesis of ventilator-associated lung injury at clinically relevant tidal volumes is unknown. We hypothesized that: 1) hypocapnia would worsen ventilator-associated lung injury, 2) therapeutic hypercapnia would attenuate ventilator-associated lung injury; and 3) the mechanisms of impaired compliance would be via alteration of surfactant biochemistry.

DESIGN

Randomized, prospective animal study.

SETTING

Research laboratory of university-affiliated hospital.

SUBJECTS

Anesthetized, male New Zealand Rabbits.

INTERVENTIONS

All animals received the same ventilation strategy (tidal volume, 12 mL/kg; positive end-expiratory pressure, 0 cm H2O; rate, 42 breaths/min) and were randomized to receive FiCO2 of 0.00, 0.05, or 0.12 to produce hypocapnia, normocapnia, and hypercapnia, respectively.

MEASUREMENTS AND MAIN RESULTS

Alveolar-arterial oxygen gradient was significantly lower with therapeutic hypercapnia, and peak airway pressure was significantly higher with hypocapnic alkalosis. However, neither static lung compliance nor surfactant chemistry (total surfactant, aggregates, or composition) differed among the groups.

CONCLUSIONS

At clinically relevant tidal volume, CO2 modulates key physiologic indices of lung injury, including alveolar-arterial oxygen gradient and airway pressure, indicating a potential role in the pathogenesis of ventilator-associated lung injury. These effects are surfactant independent.

Inhibitory effects of surfactant protein A on surfactant phospholipid hydrolysis by secreted phospholipases A2

Hydrolysis of surfactant phospholipids by secreted phospholipases A(2) (sPLA(2)) contributes to surfactant dysfunction in acute respiratory distress syndrome. The present study demonstrates that sPLA(2)-IIA, sPLA(2)-V, and sPLA(2)-X efficiently hydrolyze surfactant phospholipids in vitro. In contrast, sPLA(2)-IIC, -IID, -IIE, and -IIF have no effect. Since purified surfactant protein A (SP-A) has been shown to inhibit sPLA(2)-IIA activity, we investigated the in vitro effect of SP-A on the other active sPLA(2) and the consequences of sPLA(2)-IIA inhibition by SP-A on surfactant phospholipid hydrolysis. SP-A inhibits sPLA(2)-X activity, but fails to interfere with that of sPLA(2)-V. Moreover, in vitro inhibition of sPLA(2)-IIA-induces surfactant phospholipid hydrolysis correlates with the concentration of SP-A in surfactant. Intratracheal administration of sPLA(2)-IIA to mice causes hydrolysis of surfactant phosphatidylglycerol. Interestingly, such hydrolysis is significantly higher for SP-A gene-targeted mice, showing the in vivo inhibitory effect of SP-A on sPLA(2)-IIA activity. Administration of sPLA(2)-IIA also induces respiratory distress, which is more pronounced in SP-A gene-targeted mice than in wild-type mice. We conclude that SP-A inhibits sPLA(2) activity, which may play a protective role by maintaining surfactant integrity during lung injury.

Surfactant biology and clinical application

There is strong evidence that alterations in the pulmonary surfactant system play an important role in the pathophysiology of lung disease, including ARDS . Although it is still unclear whether mortality and morbidity of ARDS will be reduced, surfactant replacement therapy has been shown to improve oxygenation, improve lung compliance, and decrease the need for ventilatory support. The critical need for more standardized studies with one type of intratracheal surfactant and uniform measurements of surfactant proteins and phospholipids by BAL is evident. Further studies will also be needed to elucidate the optimal timing and dosage regimen for different disease processes. Some evidence supports the measurements of surfactant protein levels as markers for predicting the onset and outcome of ARDS and perhaps providing a window for early treatment of patients at risk to develop ARDS. Continued investigation into the role of surfactant in the immune regulation of the lung may also provide additional information to support the efficacy of surfactant replacement in lung disease.

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.

Early surfactant administration protects against lung dysfunction in a mouse model of ARDS

Sepsis can predispose the lung to insults such as mechanical ventilation (MV). It was hypothesized that treating the lung with exogenous surfactant early in the development of sepsis will reduce the lung dysfunction associated with MV 18 h later. Mice underwent sham or cecal ligation and perforation (CLP) surgery. Immediately after surgery, mice were either untreated or given 100 mg/kg of bovine lipid extract surfactant intratracheally. Eighteen hours later, the lungs were removed and analyzed either immediately or following ventilation ex vivo for 2 h by an "injurious" mode of ventilation (20 ml/kg, 0 cm positive end-expiratory pressure). In nonventilated lungs, exogenous surfactant had no impact on compliance or IL-6 concentrations in the lungs. In the ventilated groups, the administered surfactant had a significant protective effect on the lung dysfunction induced by MV, but only in the CLP lungs. We conclude that administration of exogenous surfactant at the time of a systemic insult can protect the lung from the damaging effects of MV 18 h later.

Role of exogenous surfactant in acute lung injury

OBJECTIVES

To outline both the preclinical and clinical data demonstrating surfactant alterations in acute lung injury, which provide the rationale for testing exogenous surfactant administration in this setting. We also review the results of the randomized, controlled clinical trials conducted to date that have evaluated this therapy in patients with acute respiratory distress syndrome, and we review the various factors that may have affected the outcomes of these trials. Future areas for surfactant research will also be addressed.

DATA SYNTHESIS AND EXTRACTION

A review of the literature utilizing a MEDLINE search was performed using the key words: surfactant, surfactant administration, acute respiratory distress syndrome, and lung injury. Personal views are presented and references to unpublished clinical data are made based on the authors' access to this data.

CONCLUSIONS

Exogenous surfactant administration has proven inconsistent as a therapeutic modality for patients with acute respiratory distress syndrome. This is because of the severity of the injury at the time of treatment and because of the variable surfactant preparations, dosing regimes, and delivery methods used in the different trials. Future research efforts will focus on determining the optimal timing of surfactant administration in patients at risk of developing acute respiratory distress syndrome with the aim of preventing progressive lung dysfunction and determining whether surfactant treatments need to be tailored to the specific patient in question. Moreover, with the recognition that surfactant also plays an important role in host defense, the future for surfactant therapy is exciting.

Selective inhibition of large-to-small surfactant aggregate conversion by serine protease inhibitors of the bis-benzamidine type

Conversion of the biophysically active large surfactant aggregate subtype (LA) of alveolar surfactant into the less surface active small surfactant aggregates (SA) occurs in vivo and is reproduced under conditions of cyclic surface area changes in vitro. A serine-active carboxyl esterase has been suggested as the responsible enzymatic activity, although the exact mechanisms underlying the conversion process are presently unclear. We investigated the influence of exogenous serine proteases and synthetic and natural serine protease inhibitors on the conversion kinetics of natural rabbit surfactant, obtained as bronchoalveolar lavage fluid (BALF). In vitro cycling of BALF was performed for various time periods in the absence or presence of increasing amounts of several serine proteases (trypsin, plasmin, thrombin, tryptase), and one natural (aprotinin) and 25 synthetic serine protease inhibitors (including regular benzamidines [group A], 3-amidinophenylalanine derivatives [group B], bis-benzamidines [group C], and analogs of naphthylsulfonyl-glycyl-4-amidinophenylalanine piperidide [group D]). LA were separated from SA by 48,000 x g centrifugation. Surface activity of the LA fraction was measured by means of the pulsating bubble surfactometer. None of the "classical" serine proteases forwarded any acceleration of the LA-to-SA conversion kinetics. Some of the serine protease inhibitors caused moderate retardation of conversion, but at the same dose range inhibited the surface tension-lowering properties of the LA fraction, which per se explained their inhibitory effect. In contrast, specific dose-dependent inhibition of the LA-to-SA transition was observed for four derivatives of the bis-benzamidine group: full blockage of conversion over 240 min of cycling was noted at doses that did not interfere with the surface activity of the LA fraction. In addition, the prototype of these bis-benzamidines, 1,4-bis-[beta-naphthylsulfonyl-(3-aminophenylalanine)]-piperazide, was found to inhibit the activity of the rabbit liver carboxylesterase ES-2 in two different synthetic substrate assays reflecting the amidase and esterase properties of carboxylesterases. These findings support the hypothesis that the LA-to-SA conversion is an enzymatically-driven process with serine-active carboxyl esterase(s) being centrally involved. Synthetic bis-benzamidine-type serine protease inhibitors may offer specific inhibition of this event.

Altered phospholipid composition and aggregate structure of lung surfactant is associated with impaired lung function in young children with respiratory infections

Alterations to pulmonary surfactant structure, composition, and function contribute to the severity of respiratory infections. Analysis of bronchoalveolar lavage fluid (BALF) from children undergoing diagnostic bronchoscopy for structural abnormalities (control group, n = 24), asthma (n = 18), lung infection (n = 30), and cystic fibrosis (CF, n = 15) showed that BALF phospholipid concentration decreased with age for the control group and was elevated in all disease groups. The fractional concentration of the major surface active component, dipalmitoyl phosphatidylcholine (PC16:0/16:0), correlated (r(2) = 0.608, P < 0.01) with airway resistance (FEV(1%) predicted), and decreased PC16:0/16:0 was accompanied by increased concentrations of phospholipid components characteristic of cell membranes (PC16:0/18:1 and PI18:0/20:4). Median minimal surface tension, measured by pulsating bubble surfactometer, was elevated (P < 0.01) in both infection (17.5 mN/m) and CF (17.1 mN/m) compared with the control group (1.5 mN/m). Centrifugation (60,000 x g, 40 min) of BALF indicated that infection was accompanied by accumulation of large aggregate forms of surfactant, in contrast to previous reports of increased conversion to inactive small aggregate surfactant particles in ventilated patients with respiratory failure. This accumulation of surface-inactive, large aggregate forms of surfactant, possibly due to mixing with membrane material from inflammatory cells, may contribute to severity of lung disease in children with respiratory infections.

Total extracellular surfactant is increased but abnormal in a rat model of gram-negative bacterial pneumonia

An in vivo rat model was used to evaluate the effects of Escherichia coli pneumonia on lung function and surfactant in bronchoalveolar lavage (BAL). Total extracellular surfactant was increased in infected rats compared with controls. BAL phospholipid content in infected rats correlated with the severity of alveolar-capillary leak as reflected in lavage protein levels (R(2) = 0.908, P < 0.0001). Western blotting showed that levels of surfactant protein (SP)-A and SP-D in BAL were significantly increased in both large and small aggregate fractions at 2 and 6 h postinstillation of E. coli. SP-B was also increased at these times in the large aggregate fraction of BAL, whereas SP-C levels were increased at 2 h and decreased at 6 h relative to controls. The small-to-large (S/L) aggregate ratio (a marker inversely proportional to surfactant function) was increased in infected rats with >50 mg total BAL protein. There was a significant correlation (R(2) = 0.885, P < 0.0001) between increasing S/L ratio in BAL and pulmonary damage assessed by total protein. Pulmonary volumes, compliance, and oxygen exchange were significantly decreased in infected rats with >50 mg of total BAL protein, consistent with surfactant dysfunction. In vitro surface cycling studies with calf lung surfactant extract suggested that bacterially derived factors may have contributed in part to the surfactant alterations seen in vivo.

In vitro and in vivo intrapulmonary distribution of fluorescently labeled surfactant

OBJECTIVE

To determine the distribution of endotracheally administered surfactant at the alveolar level in an animal model of acute respiratory distress syndrome.

DESIGN

Prospective, randomized animal study.

SETTING

Research laboratory of a university hospital.

SUBJECTS

Seventy-one male Sprague-Dawley rats, weighing 330-370 g.

INTERVENTIONS

To measure surfactant distribution in vitro, a glass trough mimicking dichotomic lung anatomy was used to determine the spreading properties of bovine lung surfactant extract supplemented with fluorescent Bodipy-labeled surfactant protein B. To measure surfactant distribution in vivo, rats were anesthetized, and lipopolysaccharide was aerosolized (12 mg/kg body weight) to induce lung injury resembling acute respiratory distress syndrome; in control rats, buffered saline was aerosolized. Twenty-four hours later rats were anesthetized, tracheotomized, and mechanically ventilated (peak airway pressure = 20 mbar; positive end-expiratory pressure = 6 mbar; inspiration time = expiration time = 0.6 sec; Fio2 = 50%). Surfactant (bovine lung surfactant extract, supplemented with fluorescent Bodipy-labeled surfactant protein B; 50 mg/kg body weight) was applied as a bolus; in control rats, saline was administered as a bolus. Rats were ventilated for 5, 15, 30, or 60 mins (n = 8 or 9 for each group). Then, lungs were excised and sliced. Lung slices, divided into aerated (open), underinflated (dystelectatic), or collapsed (atelectatic) alveolar areas, were examined by both light and fluorescence microscopy.

RESULTS

In vitro experiments revealed that surfactant spread independent of glass trough geometry and lowered the surface tension to equilibrium values (25 mN/m) within a few seconds. In vivo experiments showed that administered surfactant distributed preferentially into underinflated and aerated alveolar areas. Furthermore, surfactant distribution was not affected by length of mechanical ventilation.

CONCLUSIONS

When conventional mechanical ventilation was used in lipopolysaccharide-induced lung injury, surfactant preferentially distributed into underinflated and aerated alveolar areas. Because surfactant rarely reached collapsed alveolar areas, methods aiding in alveolar recruitment (e.g., open lung concept or body positioning) should precede surfactant administration.

Surfactant and corticosteroid effects on lung function in a rat model of acute lung injury

OBJECTIVES

To evaluate pulmonary responses to intratracheal administration of surfactant with and without dexamethasone in rats with paraquat-induced lung injury.

DESIGN

Prospective, randomized, controlled study.

SETTING

University research facility.

SUBJECTS

Adult male Sprague Dawley rats.

INTERVENTIONS

Rats were anesthetized and underwent a tracheostomy and arterial catheter insertion 3 days after intraperitoneal injection of paraquat (35 mg/kg). The rats were ventilated for 90 mins after sequential designation as controls or as recipients of intratracheal surfactant alone (50 or 100 mg/kg) or surfactant (50 or 100 mg/kg) plus dexamethasone (0.5 mg/kg).

MEASUREMENTS AND MAIN RESULTS

Arterial blood gases were determined at 15, 30, 60, and 90 mins. After 90 mins of ventilation, a static pressure-volume curve was performed, and inflammatory cells, total protein content, and cytokines were measured in bronchoalveolar lavage fluid. Postmortem histology was then examined. Treatment with 50 mg/kg dexamethasone/Survanta and 100 mg/kg Survanta with and without dexamethasone significantly increased oxygenation shortly after instillation when compared with the control group, with the response maintained throughout the study period. Static pressure-volume curves showed that the group receiving 100 mg/kg dexamethasone/Survanta had significantly higher lung volumes than the control group. Total cell, neutrophil, and macrophage counts were decreased significantly in the animals treated with 100 mg/kg dexamethasone/Survanta compared with untreated control rats. Total protein recovered from bronchoalveolar lavage fluid in the animals treated with 100 mg/kg Survanta with and without dexamethasone was decreased significantly compared with control animals. The histologic appearance of the lungs was markedly better in the groups treated with surfactant with or without dexamethasone.

CONCLUSIONS

Results suggest that the combined administration of high doses of intratracheal surfactant and dexamethasone improves gas exchange, ameliorates lung inflammation, and alleviates lung damage after paraquat-induced lung injury. Surfactant alone and lower doses of surfactant plus dexamethasone had a lesser effect on these measures.

Surfactant alteration and replacement in acute respiratory distress syndrome

The acute respiratory distress syndrome (ARDS) is a frequent, life-threatening disease in which a marked increase in alveolar surface tension has been repeatedly observed. It is caused by factors including a lack of surface-active compounds, changes in the phospholipid, fatty acid, neutral lipid, and surfactant apoprotein composition, imbalance of the extracellular surfactant subtype distribution, inhibition of surfactant function by plasma protein leakage, incorporation of surfactant phospholipids and apoproteins into polymerizing fibrin, and damage/inhibition of surfactant compounds by inflammatory mediators. There is now good evidence that these surfactant abnormalities promote alveolar instability and collapse and, consequently, loss of compliance and the profound gas exchange abnormalities seen in ARDS. An acute improvement of gas exchange properties together with a far-reaching restoration of surfactant properties was encountered in recently performed pilot studies. Here we summarize what is known about the kind and severity of surfactant changes occurring in ARDS, the contribution of these changes to lung failure, and the role of surfactant administration for therapy of ARDS.

Immunohistochemistry of pulmonary surfactant-associated protein A in acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a fatal complication in severe traumas and diseases. Although the contribution of pulmonary surfactant abnormality to the pathogenesis of ARDS has been clinically fairly well investigated, the histopathological evidence has not been established. The aim of this study was to clarify the immunohistochemical distribution of surfactant-associated protein A (SP-A) for early diagnosis of ARDS with special regard to hyaline membrane (HM) formation. Two-hundred-and-ten autopsy cases of prolonged death from various traumas and diseases were investigated. ARDS were observed in 23 cases, showing speckled SP-A immunostaining. During the early, exudative phase of ARDS, characteristic SP-A distribution showed intense staining in the intra-alveolar massive aggregates and thick 'peeling'-like substances accompanied with a lot of granular staining. During the proliferative phase, localized accumulation of granular SP-A and macrophages containing dense granular SP-A became predominant. During the final fibrotic phase, SP-A staining in HMs became weak, and disseminated granular staining was observed in the alveolar spaces. These findings provide morphological evidence of the increase of SP-A during the early phase of ARDS, including some molecular alteration and its decrease during the late phase. Characteristic SP-A distribution in the exudative phase appeared to be especially useful for early histopathological diagnosis of respiratory distress, even prior to the appearance of typical HMs.

Mechanical ventilation of isolated septic rat lungs: effects on surfactant and inflammatory cytokines

The effects of mechanical ventilation (MV) on the surfactant system and cytokine secretion were studied in isolated septic rat lungs. At 23 h after sham surgery or induction of sepsis by cecal ligation and perforation (CLP), lungs were excised and randomized to one of three groups: 1) a nonventilated group, 2) a group subjected to 1 h of noninjurious MV (tidal volume = 10 ml/kg, positive end-expiratory pressure = 3 cmH(2)O), or 3) a group subjected to 1 h of injurious MV (tidal volume = 20 ml/kg, positive end-expiratory pressure = 0 cmH(2)O). Nonventilated sham and CLP lungs had similar compliance, normal lung morphology, surfactant, and cytokine concentrations. Injurious ventilation decreased compliance, altered surfactant, increased cytokines, and induced morphological changes compared with nonventilation in sham and CLP lungs. In these lungs, the surfactant system was similar in sham and CLP lungs; however, tumor necrosis factor-alpha and interleukin-6 levels were significantly higher in CLP lungs. We conclude that injurious ventilation altered surfactant independent of sepsis and that the CLP lungs were predisposed to the secretion of larger amounts of cytokines because of ventilation.

Effects of high-frequency oscillation on endogenous surfactant in an acute lung injury model

This study evaluated the effects of high-frequency oscillation (HFO) and conventional mechanical ventilation (CMV) on gas exchange and the pulmonary surfactant system in an acute lung injury model. Following induction of lung injury with N-nitroso-n-methylurethane, adult rabbits were anesthetized and randomized to one of the following ventilatory strategies: HFO for 120 min, CMV for 120 min, HFO for 60 min, followed by CMV for 60 min, CMV for 60 min followed by HFO for 60 min or CMV for 60 min. Separate animals were ventilated using CMV with a lower tidal volume and a positive end-expiratory pressure level that was increased throughout the experimental period. Oxygenation was significantly greater in animals ventilated with HFO compared with animals ventilated with CMV. The proportion of surfactant in large aggregate forms was significantly greater following ventilatory support with HFO compared with CMV. Surfactant aggregate conversion was also significantly lower during HFO compared with CMV. We conclude that in our model of acute lung injury, HFO was a superior mode of ventilation and reduced the conversion of alveolar surfactant large aggregates into small aggregate forms, resulting in a greater percentage of large aggregate forms in the alveolar space.

Lung salvage and protection ventilatory techniques

Physicians are in the beginning of an era in intensive care medicine in which they finally are starting to see improved outcomes in patients with AHRF. At the same time, intensivists are presented with a bewildering choice of ventilator options and adjunctive therapies. Trying to sort out which are "cosmetic," that is, improve the blood gases as opposed to influencing the outcome, remains a challenge and will be resolved only with additional RCTs. Principles of ventilator management that are driven by mimicking normal physiology are inappropriate and must be rethought.

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.

Effects of alveolar surfactant aggregates on T-lymphocyte proliferation

The effects of alveolar large aggregate (LA) and small aggregate (SA) surfactant subfractions isolated from healthy adult rats on mitogen-stimulated proliferative responses of human peripheral blood mononuclear cells (PBMC) was examined. Various concentrations of total surfactant suppressed proliferation of stimulated lymphocytes by up to 95% of mitogen-stimulated cells alone. LA subfractions of total surfactant had no effect on proliferation, whereas SA significantly enhanced the lymphocyte proliferation at lower concentrations (7.8 microg/ml) compared to mitogen-stimulated cells alone. Higher concentrations of SA (62.5 microg/ml) inhibited lymphocyte proliferation. This concentration-dependent effect of SA on proliferation of PBMC was also present when cells were stimulated with various lectins including anti-CD3, concanavalin A and phytohemagglutinin. Analysis of the supernatant of mitogen-stimulated cell cultures treated with inhibitory concentrations of SA showed decreased amounts of interleukin (IL)-2, compared to cells alone, which could be reversed by adding exogenous IL-2 to the cell cultures with the SA. These results suggest that alveolar surfactant subfractions have distinct functions within the alveoli, both biophysically and with respect to their effects on the host's immunomodulatory responses.

Effects of mechanical ventilation of isolated mouse lungs on surfactant and inflammatory cytokines

Mechanical ventilation of the lung is an essential but potentially harmful therapeutic intervention for patients with acute respiratory distress syndrome. The objective of the current study was to establish and characterize an isolated mouse lung model to study the harmful effects of mechanical ventilation. Lungs were isolated from BalbC mice and randomized to either a nonventilated group, a conventionally ventilated group (tidal volume 7 mL x kg(-1), 4 cm positive endexpiratory pressure (PEEP)) or an injuriously ventilated group (20 mL x kg(-1), 0 cm PEEP). Lungs were subsequently analysed for lung compliance, morphology, surfactant composition and inflammatory cytokines. Injurious ventilation resulted in significant lung dysfunction, which was associated with a significant increase in pulmonary surfactant, and surfactant small aggregates compared to the other two groups. Injurious ventilation also led to a significantly increased concentration of interleukin-6 and tumour necrosis factor-a in the lavage. It was concluded that the injurious effects of mechanical ventilation can effectively be studied in isolated mouse lung, which offers the potential of studying genetically altered animals. It was also concluded that in this model, the lung injury is, in part, mediated by the surfactant system and the release of inflammatory mediators.

Alteration of fatty acid profiles in different pulmonary surfactant phospholipids in acute respiratory distress syndrome and severe pneumonia

Impairment of alveolar surfactant function has been documented in the acute respiratory distress syndrome (ARDS) and in severe pneumonia (PNEU); however, the underlying mechanisms are not completely understood. In the current report we present a detailed analysis of fatty acid (FA) profiles of different surfactant phospholipid (PL) classes isolated from bronchoalveolar lavage fluids (BALF) and large surfactant aggregates (LSA) from mechanically ventilated patients with ARDS (n = 8), ARDS associated with lung infection (ARDS + PNEU, n = 9), and PNEU (n = 22). Healthy volunteers served as control subjects (n = 8). PLs were isolated by thin-layer chromatography, and the FA profile of each PL class was assessed by gas chromatography. In addition, the minimal surface tension (gamma min) of untreated LSA and of LSA after supplementation with additional dipalmitoylated phosphatidylcholine (DPPC) was analyzed (pulsating bubble surfactometer). As compared with control LSA, the percentage of palmitic acid in phosphatidylcholine (PC) was significantly decreased in all patient groups (ARDS 63.0 +/- 2.0%, ARDS + PNEU 64.6 +/- 4.9%, PNEU 65.6 +/- 1.5%, control subjects 80.1 +/- 1.7%), whereas the relative amount of unsaturated species in PC increased significantly in all groups. Phosphatidylglycerol (PG) and phosphatidylinositol (PI) presented similar FA profiles in control subjects, but differed in the patients. The FA pattern of sphingomyelin (SPH) and phosphatidylethanolamine (PE) displayed only minor changes under conditions of respiratory failure. As compared with control subjects a highly significant increase of gamma min from near zero to approximately 16 mN/m was observed in all patients and was found to be inversely correlated to the percentage of palmitic acid in PC of LSA or BALF. Accordingly, values for gamma min were significantly improved upon secondary supplementation of LSA with DPPC up to control values. We conclude that marked changes in the FA composition of the predominant surfactant PL classes occur, both in ARDS triggered by nonpulmonary events and PNEU. The marked reduction of palmitic acid in the PC fraction may be related to changes in surfactant function under these conditions.

Functional Tests for the Characterization of Surfactant Protein B (SP-B) and a Fluorescent SP-B Analog

Surfactant protein B (SP-B) enhances lipid insertion into the alveolar air/liquid interface upon inhalation. The aim of this study was (i) to apply a palette of tests for a detailed biochemical and biophysical characterization of SP-B and (ii) to use these tests to compare native SP-B with a fluorescent (Bodipy) SP-B analog. The method of labeling was fast and resulted in a covalent fluorophore-protein bond. The ability of both proteins to spread a surfactant film on top of a buffer surface was determined in a spreading tray using the Wilhelmy plate technique to allow detection of alterations in surface tension and calculation of spreading velocities. In a captive bubble surfactometer surface tensions of spread films were measured. Similar biophysical properties were found for both native and Bodipy-labeled SP-B. It is concluded that the combination of tests used allows detection of small differences in structure and activity between the two proteins.

Surfactant replacement therapy

Dysfunction of the surfactant system of the lung in the setting of acute lung injury (ALI) is likely to contribute to the pathophysiology of that syndrome. Multiple mechanisms, including injury to alveolar type II cells and inhibition by plasma proteins contribute to this loss of function. Similar injury occurs in animal models of acute lung injury and, in that setting, treatment with exogenous surfactant causes marked improvement in gas exchange. Clinical studies of surfactant treatment of ALI suggest benefit, and definitive phase III trials are now in progress.

Pulmonary surfactant is altered during mechanical ventilation of isolated rat lung

OBJECTIVE

To test the hypothesis that the lung injury induced by certain mechanical ventilation strategies is associated with changes in the pulmonary surfactant system.

DESIGN

Analysis of the pulmonary surfactant system from isolated rat lungs after one of four different ventilatory strategies.

SETTING

A research laboratory at a university.

SUBJECTS

A total of 45 Sprague-Dawley rats.

INTERVENTIONS

Isolated lungs were randomized to either no ventilation (0-TIME) or to ventilation at 40 breaths/min in a humidified 37 degrees C chamber for either 30 mins or 120 mins with one of the following four strategies: a) control (CON, 7 mL/kg, 3 cm H2O positive end-expiratory pressure); b) medium volume, zero end-expiratory pressure (MVZP, 15 mL/kg, 0 cm H2O end-expiratory pressure); c) medium volume, high positive end-expiratory pressure (MVHP, 15 mL/kg, 9 cm H2O positive end-expiratory pressure); and d) high volume, zero end-expiratory pressure (HVZP, 40 mL/kg, 0 cm H2O end-expiratory pressure).

MEASUREMENTS

Pressure-volume curves were determined before and after the ventilation period, after which the lungs were lavaged for surfactant analysis.

MAIN RESULTS

Compared with 0-TIME, 30 mins of ventilation with the HVZP strategy or 120 mins of ventilation with CON and MVZP strategies caused a significant decrease in compliance. Groups showing a decreased compliance had significant increases in the amount of surfactant, surfactant large aggregates, and total lavage protein compared with 0-TIME.

CONCLUSIONS

A short period of injurious mechanical ventilation can cause a decrease in lung compliance that is associated with a large influx of proteins into the alveolar space and with alterations of the pulmonary surfactant system. The changes of surfactant in these experiments are different from those seen in acute lung injury, indicating that they may represent an initial response to mechanical ventilation.

Surfactant-associated protein A inhibits LPS-induced cytokine and nitric oxide production in vivo

The role of surfactant-associated protein (SP) A in the mediation of pulmonary responses to bacterial lipopolysaccharide (LPS) was assessed in vivo with SP-A gene-targeted [SP-deficient; SP-A(-/-)] and wild-type [SP-A(+/+)] mice. Concentrations of tumor necrosis factor (TNF)-alpha, macrophage inflammatory protein-2, and nitric oxide were determined in recovered bronchoalveolar lavage fluid after intratracheal administration of LPS. SP-A(-/-) mice produced significantly more TNF-alpha and nitric oxide than SP-A(+/+) mice after LPS treatment. Intratracheal administration of human SP-A (1 mg/kg) to SP-A(-/-) mice restored regulation of TNF-alpha, macrophage inflammatory protein-2, and nitric oxide production to that of SP-A(+/+) mice. Other markers of lung injury including bronchoalveolar fluid protein, phospholipid content, and neutrophil numbers were not influenced by SP-A. Data from experiments designed to test possible mechanisms of SP-A-mediated suppression suggest that neither binding of LPS by SP-A nor enhanced LPS clearance are the primary means of inhibition. Our data and others suggest that SP-A acts directly on immune cells to suppress LPS-induced inflammation. These results demonstrate that endogenous or exogenous SP-A inhibits pulmonary LPS-induced cytokine and nitric oxide production in vivo.

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 ventilation on the surfactant system in sepsis-induced lung injury

The present study examined the effects of mechanical ventilation, with or without positive end-expiratory pressure (PEEP), on the alveolar surfactant system in an animal model of sepsis-induced lung injury. Septic animals ventilated without PEEP had a significant deterioration in oxygenation compared with preventilated values (arterial PO(2)/inspired O(2) fraction 316 +/- 16 vs. 151 +/- 14 Torr; P < 0.05). This was associated with a significantly lower percentage of the functional large aggregates (59 +/- 3 vs. 72 +/- 4%) along with a significantly reduced function (minimum surface tension 17.7 +/- 1.8 vs. 11.8 +/- 3.8 mN/m) compared with nonventilated septic animals (P < 0.05). Sham animals similarly ventilated without PEEP maintained oxygenation, percent large aggregates and surfactant function. With the addition of PEEP, the deterioration in oxygenation was not observed in the septic animals and was associated with no alterations in the surfactant system. We conclude that animals with sepsis-induced lung injury are more susceptible to the harmful effects of mechanical ventilation, specifically lung collapse and reopening, and that alterations in alveolar surfactant may contribute to the development of lung dysfunction.

Biphasic regulation of Fc-receptor mediated phagocytosis of rabbit alveolar macrophages by surfactant phospholipids

Dipalmitoyl phosphatidylcholine (DPPC) is a major phospholipid constituent in the pulmonary surfactant, whereas lysophosphatidylcholine (Lyso-PC) is a minor constituent, this membrane phospholipid being produced at inflammatory sites in association with activation of phospholipase A2. To determine the role of these two different forms of phospholipids in the phagocytic function of alveolar macrophages (AM), we examined the effects of DPPC or Lyso-PC on Fc-mediated phagocytosis. We demonstrated a significant decrease of the ingestion activity of AM for anti-sheep erythrocyte immunoglobulin G-coated sheep erythrocytes (EA: IgG) by DPPC. On the other hand, Lyso-PC caused significantly increased ingestion of EA: IgG by AM. These data indicate that increase of Lyso-PC due to the hydrolysis of DPPC through activation of phospholipase A, may up-regulate AM-mediated phagocytic functions in the alveolar milieu associated with infections and inflammation. DPPC may suppress and stabilize the AM-mediated phagocytosis in the normal alveolar environment.

Dissociation of surfactant protein B from canine surfactant large aggregates during formation of small surfactant aggregates by in vitro surface area cycling

Pulmonary surfactant isolated by lavage can be separated into large aggregates (LA) and small aggregates (SA). Pulse labeling experiments have shown that the LA subtype is the precursor of the SA subtype. Conversion of LA to SA can be demonstrated in vitro using the technique of surface area cycling. The precise mechanisms of surfactant subtype conversion remain unknown. We have previously reported a decline in surfactant-associated protein B (SP-B) during in vitro subtype conversion of canine surfactant. This led to the hypothesis that SP-B may be degraded by a serine protease 'convertase' during cycling. The current studies used a quantitative slot-blot assay to investigate the fates of SP-A and SP-B during in vitro cycling. These studies confirmed some SP-A is present in SA, but SP-B is confirmed to LA. Conversion leads to an apparent loss of SP-B during cycling. However, SP-B can be recovered from the walls of polypropylene and Teflon tubes by washing with chloroform:methanol. Recovered SP-B migrated on non-reducing tricine gels as a single band with an apparent molecular weight of 17 kDa, corresponding to intact SP-B dimer. Reconstitution studies demonstrated that the recovered SP-B retained its surface active properties as determined on a pulsating bubble surfactometer. We conclude in vitro surface area cycling of canine LA results in the dissociation of SP-B from surfactant lipids resulting in an apparent decline in SP-B levels.

Ultrastructural alterations in intraalveolar surfactant subtypes after experimental ischemia and reperfusion

Ischemia and reperfusion (I/R) result in surfactant dysfunction. Whether the impairment of surfactant is a consequence or a cause of intraalveolar edema formation is still unknown. The cumulative effects of lung perfusion, ischemic storage, and subsequent reperfusion on surfactant ultrastructure and pulmonary function were studied in a rat isolated perfused lung model. The left lungs were fixed for electron microscopy by vascular perfusion either immediately after excision (control; n = 5) or after perfusion with modified Euro-Collins solution (EC), storage for 2 h at 4 degrees C in EC, and reperfusion for 40 min (n = 5). A stereological approach was chosen to discriminate between intraalveolar surfactant subtypes of edematous regions and regions free of edema. Intraalveolar edema seen after I/R in the EC group occupied 36 +/- 6% (mean +/- SEM) of the gas exchange region as compared with control lungs (1 +/- 1%; p = 0.008). Relative intraalveolar surfactant composition showed a decrease in surface active tubular myelin (3 +/- 1 versus 12 +/- 0%; p = 0.008) and an increase in inactive unilamellar forms (83 +/- 2 versus 64 +/- 5%; p = 0.008) in the EC group. These changes occurred both in edematous (tubular myelin, 3 +/- 1%; unilamellar forms, 88 +/- 6%) and in nonedematous regions (tubular myelin, 4 +/- 3%; unilamellar forms, 77 +/- 5%). The ultrastructural changes in surfactant were associated with an increase in peak inspiratory pressure during reperfusion. In conclusion, surfactant alterations seen after I/R are not directly related to the presence of edema fluid in the alveoli. Disturbances in intraalveolar surfactant after I/R are not merely the result of inactivation due to plasma protein leakage but may instead be responsible for an increased permeability of the blood-air barrier, resulting in a vicious cycle of intraalveolar edema formation and progressing surfactant impairment.

Dosing and delivery of a recombinant surfactant in lung-injured adult sheep

The purpose of this study was to evaluate a surfactant based on a recombinant surfactant protein-C (rSP-C) at three different doses (25, 100, and 200 mg lipid/kg) in the saline lavage adult sheep model of acute lung injury. All three doses resulted in significant improvements in gas exchange, although the 100 and 200 mg/kg doses were superior to the 25 mg/kg dose. There were no significant differences in effect of the 100 and 200 mg/kg doses. In addition, the physiologic efficacy and lobar surfactant distribution patterns were similar when two different surfactant delivery methods were compared. This comparison involved administering the surfactant directly into each lobe under bronchoscopic guidance, versus instilling the surfactant through an endotracheal tube into the lungs. However, the former technique took significantly longer to perform (24.5 +/- 3.3 min versus 11.6 +/- 2.5 min, p < 0.05) and required a skilled bronchoscopist. In conclusion, rSP-C surfactant was effective in improving gas exchange in this model of lung injury, although higher doses were required for optimal responses. The bronchoscopic administration technique produced results similar to those of the tracheal instillation method, but had some disadvantages that may limit the widespread clinical use of this technique in patients with lung injury.

Compositional, structural, and functional alterations in pulmonary surfactant in surgical patients after the early onset of systemic inflammatory response syndrome or sepsis

OBJECTIVES

Sepsis is one of the most important predisposing factors for the development of the acute respiratory distress syndrome (ARDS). Alterations of pulmonary surfactant contribute in the pathogenesis of ARDS. However, little is known about surfactant in patients with less severe grades of lung injury related to sepsis or systemic inflammatory response syndrome (SIRS). Therefore, the purpose of this study was to characterize endogenous surfactant in surgical intensive care patients with sepsis or SIRS.

DESIGN

Prospective, observational study.

SETTING

University-affiliated, interdisciplinary intensive care unit.

PATIENTS

Eleven patients after major surgery with SIRS or sepsis included within 12 hrs of onset and 11 controls without infection or lung disease.

INTERVENTIONS

Operating room and standard intensive care unit management.

MEASUREMENTS AND MAIN RESULTS

Four serial bronchoalveolar lavage samples (BAL) were recovered over 7 days from the patients and single BAL samples were obtained from controls. BAL cells, total protein, surfactant-associated protein A (SP-A), surfactant alveolar transition forms, and surface activity were analyzed. Two of 11 patients met criteria for acute lung injury and six of the 11 patients met ARDS consensus conference criteria but acute lung injury or ARDS was not persistent. The mean Pao2/F(IO)2 for the patients over 7 days was 253.2+/-15.1 (SEM) and Murray's lung injury score was 1.12+/-0.12, indicating mild-to-moderate lung injury. BAL neutrophil counts were increased (p< .01), and the ratio of poorly functioning light aggregate surfactant to superiorly functioning heavy aggregate surfactant was increased compared with controls (0.32+/-0.06 vs. 0.09+/-0.01, p < .05). SP-A was decreased (1.9+/-0.4 vs. 3.5+/-0.6 microg/mL of BAL, p< .05) and there were increases in the ratios of phospholipid to SP-A (p < .05), protein to SP.A (p < .01), and protein to phospholipid (p < .05). The surface tension-lowering ability of purified heavy aggregate surfactant was significantly impaired (15.6+/-1.6 vs. 2.8+/-0.6 milliNewtons/m, p< .05).

CONCLUSIONS

These observations show that surgical patients with SIRS or sepsis who have mild-to-moderate lung injury develop surfactant dysfunction detectable within 7 days of onset. We propose, therefore, that therapeutic strategies to modulate these severe surfactant abnormalities should be considered, as these strategies may have the potential to reduce lung injury, which is associated with a high mortality in sepsis.

Pulmonary surfactant: functions and molecular composition

This review briefly notes recent findings important for understanding the surface mechanical functions of pulmonary surfactant. Currently known surfactant-specific proteins and lipids are discussed, with an eye to their possible functions. Competing models of the alveolar subphase life cycle of surfactant are also presented. It is concluded that, in spite of much effort, we still do not understand the basic molecular mechanisms underlying surfactant's rapid adsorption to the air-water interface.

Structure, processing and properties of surfactant protein A

Surfactant protein A (SP-A) is a highly ordered, oligomeric glycoprotein that is secreted into the airspaces of the lung by the pulmonary epithelium. The in vitro activities of protein suggest diverse roles in pulmonary host defense and surfactant homeostasis, structure and surface activity. Functional mapping of SP-A using directed mutagenesis has identified domains which interact with surfactant phospholipids, alveolar type II cells and microbes. Recently developed genetically manipulated animal models are beginning to clarify the critical physiological roles for SP-A in the normal lung, and in the pathophysiology of pulmonary disease.