Pathophysiology of Acute Lung Injury Andreas Günther, M.D.,1 Dieter Walmrath, M.D.,1 Friedrich Grimminger, M.D., Ph.D.,1 and Werner Seeger, M.D.1
The acute respiratory distress syndrome (ARDS) is a life-threatening syndrome that may occur in any patient without any predisposition and that is mostly triggered by underlying processes such as sepsis, pneumonia, trauma, multiple transfusions, and pancreatitis. ARDS is defined by (1) acute onset, (2) bilateral infiltrates in chest x-rays, (3) absence of left ventricular failure, and (4) severe arterial hypoxemia with a PaO2/FiO2 ratio less than 200 mmHg. Still, ARDS is feared (mortality 30–40%1) and relatively frequent (incidence between 13.5 per 100,0002 to 75 per 100,0003). Acute lung injury (ALI) describes a similar, but less severe, clinical condition, with PaO2/FiO2 values between 200 and 300mmHg. Despite ongoing and intensive scientific research in this area, the mechanisms underlying ALI/ARDS are still not completely understood, and until recently, there were no studies demonstrating any beneficial effect of a single treatment modality in ARDS. The recent report that a specific approach to ventilatory support can significantly reduce mortality in ARDS underscores the need for better understanding of the pathophysiological events occurring in this syndrome. This review therefore summarizes the current pathophysiological concepts underlying the evolution of acute hypoxemic respiratory failure and focuses on: (1) possible reasons for the development of ALI/ARDS; (2) cellular and humoral mediator responses leading to a sustained and self-perpetuating inflammation of the lung; (3) consequences with regard to fluid balance, pulmonary perfusion, ventilation, and efficiency of gas exchange; and (4) mechanisms underlying the aggravating complications commonly seen in ARDS, especially ventilator-associated lung injury, ventilator-associated pneumonia, and lung fibrosis. KEYWORDS: Surfactant, pathophysiology of ARDS, cytokines, fibrosis, coagulation
Objectives: Upon completion of this article, the reader will be able to differentiate between pulmonary and extrapulmonary trigger mechanisms of ARDS. The reader should understand that various humoral and cellular inflammatory mediator cascades are operative in this disease that result in: (a) increased endothelial and epithelial permeability with development of alveolar edema, (b) pulmonary hypertension due to predominance of vasoconstrictive mediators, (c) alveolar collapse due to alteration of the surfactant system, and (d) severe arterial hypoxemia due to maldistribution of ventilation and perfusion (shunt and ventilation-perfusion mismatch). Finally, we describe the fibroproliferative response in ARDS that is initiated very early in the course and may be enhanced by repetitive lung infections. Accreditation: The University of Michigan is accredited by the Accreditation Council for Continuing Medical Education to sponsor continuing medical education for physicians. Credit: The University of Michigan designates this educational activity for a maximum of 1.0 hours in category one credit toward the AMA Physicians Recognition Award.
Seminars in Respiratory and Critical Care Medicine, volume 22, number 3, 2001. Reprint requests: Andreas Günther, M.D., Zentrum für Innere Medizin, Medizinische Klinik II, Klinikstrasse 36, 35392 Giessen, Germany. Email: [email protected]
1Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, Justus-Liebig-University Giessen, Klinikstrasse 36, D-35392 Giessen, Germany. Copyright © 2001 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 5844662. 1069-3424,p;2001,22,03,247,258,ftx,en;srm00078x.
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REASONS FOR THE DEVELOPMENT OF ALI/ARDS The lung may be injured either from the alveolar side (direct) or via the pulmonary vasculature (indirect, “classical” ARDS, see Table 1). Direct injury of the lung parenchyma may result from either inhaled or aspirated agents that induce an inflammatory response in the epithelial and, subsequently, interstitial and endothelial compartments of the lung. Among these, the most relevant and frequent event is infection of the lung that may spread over both lungs thus fulfilling ARDS criteria. Other mechanisms include aspiration of gastric contents or inhalation of toxic gases (e.g., smoke inhalation). On the other hand, there are numerous extrapulmonary processes that may cause ARDS (see Table 1). In these cases, extrapulmonary inflammatory processes affect the lungs via blood-borne mediators acting through the pulmonary circulation. Under these circumstances, the vascular endothelium is the primary site of inflammation, but other lung compartments may become involved as well. One frequent extrapulmonary trigger event is sepsis, with ARDS developing in ~40% of these patients.4 In addition, ARDS is regularly encountered in the course of massive transfusion, severe trauma, pancreatitis, and other, less common, clinical conditions further detailed in
Table 1 Mechanisms of Direct or Indirect Lung Injury in ALI/ARDS Direct Lung Injury infection of the lung (viral, bacterial, fungal) aspiration of gastric contents aspiration of water (near drowning) lung contusion inhalation of toxic gases (NO2, ozone, smoke inhalation) exposure to high partial pressure of oxygen intoxication with pulmotropic agents (e.g. bleomycin, paraquat, amiodarone) high altitude edema rapid lung re-expansion (e.g. after drainage of pleural effusions)
Table 1 (see also the article by Pelosi, Caironi, and Gattinoni). Regardless of the trigger mechanism (direct vs indirect), an overwhelming and self-perpetuating inflammatory reaction is initiated that rapidly involves the entire delicately structured pulmonary gas exchange unit (Fig. 1). In this early, exudative phase of ARDS, the inflammatory processes induce severe alterations in the different compartments, with increased vasoconstriction, microembolism/microthrombosis and thus increased pulmonary artery pressures in the vascular compartment, edema formation in the interstitial space and, at the alveolar level, plasma protein leakage, cell recruitment, and increase in alveolar surface tension with flooding and collapse of alveoli. This exudative phase may persist for some days (~1 week) and, if survived, is regularly followed by a fibroproliferative, late phase, with resulting pulmonary fibrosis and potentially irreversible restrictive lung function abnormalities.
CELLULAR AND HUMORAL MEDIATOR RESPONSE In ARDS, the activation and self-perpetuation of a complex network of humoral or cellular effector systems represents the key event in the development of lung injury (Fig. 2). A large quantity of circulating inflammatory mediators, including lipids (thromboxane and leukotrienes), complement fragments, compounds of the coagulation and kallikrein-kinin cascade, cytokines, proteases, platelet-activating factor, and endotoxin or other infectious products, are frequently present in both patients with established ARDS and those at risk for
Indirect lung injury sepsis SIRS (systemic inflammatory response syndrome) polytrauma massive transfusion TRALI (transfusion related acute lung injury) DIC (disseminated intravascular coagulation) open heart surgery with prolonged extracorporal circulation pancreatitis severe burns pulmonary embolism narcotic overdose head trauma with increased intracranial pressure severe forms of malaria, sickle cell disease
Figure 1 Schematic illustration of the triggering events in ARDS. For details see text.
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Figure 2 Humoral and cellular mediators involved in ARDS. For details see text.
ARDS.5–9 Also, activation of monocytes/macrophages, granulocytes, lymphocytes, and thrombocytes is regularly seen (Fig. 2). Although ARDS may develop in neutropenic patients,10 and some animal models of ALI are neutrophil-independent, clinical ARDS and most ARDS models are characterized by an accumulation of neutrophils in the lungs, with predominance of this cell type in bronchoalveolar lavage fluids (BALF). Accordingly, it is believed that neutrophilic sequestration products contribute significantly to the pathogenesis of this syndrome (see the following text). It has been suggested that increased expression of the proinflammatory cytokines tumor necrosis factor (TNF-), interleukin-1 (IL-1), and interleukin-8 (IL-8) is a major event in early ARDS.11 These mediators are important for the activation and priming of polymorphonuclear leukocytes (PMN) and their interaction with the activated endothelium. They are important chemoattractants and modify the expression of intercellular adhesion molecules on the microvascular endothelium and their recognition sites on PMNs. TNF-, which is found to be increased in the very early phase of ARDS, may also trigger PMN degranulation and thus protease release and oxygen-radical formation.12–14 These proinflammatory mediators may be regulated by their own naturally occuring inhibitors such as soluble TNF- receptor, soluble IL-1 receptors
(IL-1sRI and IL-1sRII), IL-1 receptor antagonists (IL1Ra), or soluble IL-6 receptor (IL-6sR). In addition, a failure to increase the expression and synthesis of antiinflammatory cytokines such as IL-4, IL-10, and IL-13 may, together with a disturbed balance between the proinflammatory mediators and their inhibitors, determine the extent of inflammation, the recruitment of inflammatory cells, and the severity of the clinical course.15 Increased generation of reactive oxygen species (ROS),16 derived largely from activated PMN, may result in DNA, protein, and lipid oxidation and thus contribute to the degradation of the lung parenchyma. Although the lung is provided with a variety of antioxidant enzymes and substrates [e.g., glutathione (GSH) peroxidase, superoxide dismutase (SOD), catalase, alkenyl-acyl phosphatidylcholine (plasmalogen) in pulmonary surfactant, vitamins E, A, and C] and may even be enriched with these protective species (e.g., 100-fold higher GSH concentration in the alveolar lining layer as compared with blood), these radical scavenger systems may not suffice to neutralize the immense burden of ROS produced under conditions of ARDS. Lipid mediators such as leukotrienes (LTB4, LTC4), eicosanoids (TXA2, PGE2, PGI2), and plateletactivating factor (PAF) are also of major significance for the evolution of lung injury in ARDS.16–18 LTB4 is a potent chemotaxin for neutrophils, increases vascular permeability, and is a potent vasoconstrictor (see the following text). TXA2 also induces vasoconstriction, whereas PGE2 and PGI2 are potent vasodilators. PAF induces bronchospasm and vasoconstriction, the latter primarily via activation of platelets and secondary release of thromboxane. PAF also primes neutrophils for enhanced superoxide production and arachidonic acid release during their adhesion to endothelial cells.19 PGI2 seems to originate mostly from endothelial cells, smooth muscle cells, and fibroblasts. TXA2, LTB4, and 5-HETE are preferentially produced by intravascular, interstitial, and alveolar macrophages. Alveolar type II cells may synthesize great amounts of all prostaglandins. In addition, a cooperative pattern of prostanoid synthesis exists between different cell types. Endothelium-adherent granulocytes may supply the endothelial cell with LTA4 that may then be further processed to the vasoactive LTC4.20,21 Finally, both the vascular endothelium and the alveolar epithelium are comparable to inflammatory cells in their capability to release significant amounts of procoagulant and antifibrinolytic factors under conditions of the systemic inflammatory response syndrome (SIRS) or sepsis. On the endothelial side, tissue factor expression and synthesis are markedly increased, whereas levels of tissue factor pathway remain constant.22–25 Plasminogen activator inhibitor 1 (PAI1) expression and synthesis are also markedly upregu-
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lated.26 Thrombomodulin, which under physiological conditions is widely expressed within the pulmonary circulation,27 is markedly down-regulated during sepsis and ARDS28 (details of the alveolar hemostatic balance will be described in the following text). Thus, local activation of the coagulation system via the extrinsic pathway appears to be common. The ability of the lungs to rapidly clear thrombin (via thrombomodulin) or to cleave fibrin is clearly impaired. Activation of coagulation may thus result in development of microembolism/microthrombosis and thereby add to the functional consequences observed in ARDS (see the following text).
PULMONARY PERFUSION, VENTILATION, AND EFFICIENCY OF GAS EXCHANGE As a result of the overwhelming inflammatory process the following pathophysiological events are encountered:
Loss of Barrier Function—Development of Interstitial and Alveolar Edema Interstitial and alveolar edema formation is a key finding in ARDS. In contrast to the hydrostatic edema (transudate) seen in cardiac failure, lung edema in ARDS is due to an increase in endothelial and epithelial permeability. Again, the underlying reasons for the impaired barrier properties under conditions of ARDS are heterogeneous: Bacterial products may promote edema formation, either directly by affecting the integrity of the cell membrane or indirectly by altering Ca++- or Gprotein-dependent signaling pathways (e.g., pore-forming toxins such as Escherichia coli hemolysin or streptolysin O29) or by inducing cytoskeleton derangement (e.g., endothelial bleb formation induced by loss of factin by Clostridium botulinum C2 toxin30). In addition, by activation of second messenger pathways, inflammatory mediators such as TNF- may influence cytoskeleton structure and permeability and may alter fluid fluxes and ion and protein transport characteristics. Necrosis or (less often) apoptosis of the injured cell results in the denudation of the basement membrane, with a corresponding loss of sieving properties. Under normal conditions the alveolar epithelium is a much tighter barrier than the endothelium.31 It is composed of two cell types, the flat type I pneumocytes that cover almost 90% of the epithelial surface and the cuboidal type II pneumocytes that cover the remaining 10%. The type I pneumocyte is metabolically less active and quite sensitive to injury, while the type II pneumocyte is responsible for ion transport and surfactant production and resists injury. After lung injury the type II cells need to divide and to de-differentiate into type I pneumocytes to cover the denuded basement membrane and to restore the barrier function. This may be im-
proved under the influence of epithelial growth factors such as keratinocyte growth factor.32
Increased Pulmonary Vascular Resistance An increased pulmonary vascular resistance is often observed in patients with ARDS and may have several causes. In septic patients, especially in those with disseminated intravascular coagulation, microembolism may be an important feature, and microthrombi generated in remote sites of the body may be deposited within the pulmonary circulation.22 Persistent microthrombosis of the pulmonary vasculature may represent another facet of ARDS because the pulmonary endothelium responds to inflammatory stimuli such as LPS (lipopolysaccharide) and TNF- with a shift of the hemostatic balance from a profibrinolytic and antithrombotic milieu toward predominance of prothrombotic and antifibrinolytic factors, thus favoring local fibrin deposition (see earlier text). Indeed, microembolism/ microthrombosis is a frequent observation in autopsies of ARDS patients.22 Because the endothelium and platelets respond to the local thrombin activation and fibrin formation by liberating thromboxane33 and serotonin, these coagulation processes may increase pulmonary artery resistance by obliterating vessels on the one hand and vasoconstricting them on the other hand. As already stated, increased pulmonary artery resistance may also result from increased vascular tone. A number of vasoactive mediators are differentially regulated under conditions of ARDS. Of utmost importance in this context are the lipid mediators, especially the cyclo-oxygenase metabolites of arachidonic acid, particularly thromboxane A2 (TXA2), which is a potent inducer of pulmonary vasoconstriction. Other potentially vasodilatory agents, such as PGE2, PGI2, and NO (nitric oxide), may be increased also, but not likely sufficient to counteract the vasoconstrictory effect of prostanoids. The role of endothelins in increased pulmonary vascular resistance in ARDS is currently unclear.
Alveolar Instability and Collapse Due to Surfactant Abnormalities Pulmonary surfactant is a lipoprotein complex covering the alveolar surface.34–37 By profoundly reducing the surface tension at the air-liquid interface, it prevents alveoli from collapse, particularly during expiration. It is composed of ≈ 90% lipids (mostly phospholipids and minor percentages of neutral lipids) and 10% proteins. Phosphatidylcholine (mostly dipalmitoylated, DPPC) accounts for ≈ 80%, phosphatidylglycerol for ≈ 10% of the phospholipids. Four surfactant-specific apoproteins have been described, surfactant-protein (SP)-A, SP-B, SP-C, and SP-D. Inspiratory stretch of the alveolar cell
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layer causes alveolar type II cells to secrete surfactantcontaining lamellar bodies, which are extracellularly reorganized into tubular myelin and large multilamellar vesicles, also summarized as large surfactant aggregates. Adsorption of the phospholipids to the air-liquid interface results in the formation of a stable phospholipid film, which, upon compression, can reduce the surface tension to near zero. DPPC, PG (phosphatidyl glycerol), SP-B and SP-C seem to represent essential constitutents of surfactant to fulfill this function.38 The periodic compression and reexpansion of the interfacial phospholipid film provokes its permanent refinement, with the large surfactant aggregates being converted into unilamellar vesicles (small surfactant aggregates). There are experimental data suggesting a role of an ES2 esterase in this conversion process.39 Additional functions of the alveolar surfactant system include prevention of alveolar edema fluid formation40 and, of unclear significance, an impact on the alveolar host-defense mechanisms.41 Unlike the Infant Respiratory Distress Syndrome (IRDS), where deficiency of surfactant is the primary cause of respiratory failure,42 a broad pattern of biochemical and biophysical abnormalities of the pulmonary surfactant system is observed in ARDS, which in the aggregate favors alveolar collapse with ventilation/ perfusion-mismatch and, most importantly, intrapulmonary shunt.43 Analysis of bronchoalveolar lavage fluids (BALF) from ARDS patients consistently demonstrated a decrease of the surface tension reducing properties, with minimum surface tension values being increased to > 15 to 20 mN/m, instead of near zero in healthy controls (Fig. 3).44,45,46 Several biochemical alterations in these patients have been described: 1. Alteration of the phospholipid profile with a reduction of the relative percentages of phosphatidylcholine and phosphatidylglycerol and an increase in the relative percentages of phosphatidylinositol, phosphatidylethanolamine, and sphingomyelin 2. Alteration of the fatty acid composition with a marked reduction of the relative content of saturated, especially palmitic acid, species among the phospholipid fraction 3. Decreased levels of surfactant apoproteins, as shown for the hydrophilic SP-A in native BALF and for SP-B and SP-C among the large surfactant aggregate fraction 4. Reduced content of large surfactant aggregates, as shown by several studies which suggest that induction of acute lung results in an increase in the ratio of small to large surfactant aggregates 5. Inhibition of surfactant function by leakage of plasma proteins into the alveolar space, a common finding in ARDS. The BALF phospholipid to protein ratio normally ranges close to a value of 0.5 (wt/wt), but
Figure 3 Minimum surface tension in pulmonary surfactant samples from patients with acute respiratory failure. Large surfactant aggregates were isolated from BAL fluids and assessed for the surface-reducing capacity by use of the pulsating bubble surfactometer (at 2mg/mL phospholipid). Single events (circles), mean values (triangles), and median values (boxes) are given for healthy volunteers (Control), ventilated patients with cardiogenic lung edema without any sign of infection (CLE), patients with extra-pulmonary-triggered ARDS (ARDS), patients with severe pneumonia necessitating mechanical ventilation (PNEU), and patients with ARDS and nosocomial infection of the lung (ARDS + PNEU). Significance indicated by *** (p > 0.001).44
reaches mean values as low as 0.05 in ARDS. Strong surfactant inhibitory capacity can be demonstrated ex vivo for the total BALF proteins and in vitro for albumin, hemoglobin, fibrinogen, and especially fibrin,47,48,49 the latter known to be readily generated in the alveolar space of ARDS patients due to increased procoagulant and antifibrinolytic activities.50,51 6. Inhibition by inflammatory mediators, in particular proteases and oxygen radicals released by inflammatory cells, which may attack the surfactant apoproteins52,53 These complex surfactant disturbances, described in patients with ARDS triggered by diseases remote from the lung (e.g., sepsis, polytrauma, pancreatitis) have also been observed under conditions of severe pneumonia demanding mechanical ventilation.44 Given
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these findings, clinical pilot studies have been conducted addressing the safety and efficacy of natural surfactant preparations in ARDS. These studies demonstrated an acute improvement of gas exchange without adverse effects after transbronchial application.54,55 A recombinant SP-C-based surfactant preparation is currently under investigation in two larger, multicenter, randomized, placebo-controlled studies in North America and Europe/South Africa.
Ventilation-Perfusion Mismatch and Arterial Hypoxemia Under physiological conditions pulmonary blood flow is directed to ventilated areas with adjustment of flow via hypoxic pulmonary vasoconstriction (HPV). The underlying mechanism is a reduction of blood flow through poorly ventilated lung regions, thus resulting in an optimum matching of perfusion (Q) to ventilation (V)56,57 (Fig. 4A). Under conditions of ARDS, HPV is largely abolished. In addition, due to marked but undirected vasoconstrictive and vasodilatory events, ventilation and perfusion are mismatched. Figure 4 (B,C)
gives an example of the two principal forms of V/Q mismatch, as observed with the multiple inert gas elimination technique (MIGET).56,57 The major abnormality in patients with ARDS is pulmonary shunt flow (V/Q < 0.01). In this case, a considerable percentage (up to 50%) of the cardiac output circulates through atelectatic or flooded lung regions and does not participate in gas exchange.55,56 In addition, pulmonary blood flow through dystelectatic lung regions (with low ventilation) may occur and contribute to V/Q mismatch and, vice versa, an also relatively high portion of the ventilatory minute volume may be directed to lung regions with a rather limited perfusion. As in the case of shunt flow, this bimodal pattern of V/Q mismatch similarly contributes to severe arterial hypoxemia. In summary, it is evident that disturbances of gas exchange in ARDS are due to (1) loss of gas exchange area due to alveolar collapse and flooding, (2) extension of the distance of gas diffusion by edema formation (in the later phases also fibrotic thickening), and (3) V/Q mismatch. In view of these considerations, it is also clear that the severity of gas exchange abnormality does not necessarily parallel the extent of alveolar and inter-
Figure 4 Ventilation/perfusion (V/Q) mismatch in ARDS. Depicted are the distribution of ventilation and perfusion as characterized by use of the multiple inert gas elimination technique (MIGET) in ventilated patients. As compared with controls (A), two major patterns of V/Q mismatch are encountered, namely predominant shunt flow (B) or a bimodal distribution of ventilation and perfusion (C). In both cases, this V/Q mismatch results in severe arterial hypoxemia.
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stitial edema formation. In some cases, arterial hypoxemia may not be very impressive, although alveolar and interstitial edema formation is prominent. In other cases, there is only moderate fluid accumulation in the lungs, but severe arterial hypoxemia is encountered. Symptomatic treatment options of ARDS aiming to improve gas exchange must therefore either result in a better ventilation of the lung (ventilator strategy, surfactant treatment) or must improve matching of pulmonary perfusion to ventilation (inhaled prostacyclin, inhaled nitric oxide).
COMPLICATIONS IN ARDS Among the numerous possible complications in ARDS patients, nosocomial infection/ventilator-associated pneumonia, ventilator-associated lung injury, and fibroproliferation and lung fibrosis are very important and are discussed in the following.
Nosocomial Infection/Ventilator-Associated Pneumonia Nosocomial infection is very common in ARDS patients (see also the article by Iregui and Kollef ). In four recent clinical studies, the incidence of ventilatorassociated pneumonia (VAP) was found to range between 36.5 and 60% of all patients with ARDS.58,59,60,61 It is unclear why mechanically ventilated ARDS patients have increased susceptibility to lung infection, but certain aspects of this lung injury may underlie this finding (Fig. 5). First, an impaired alveolar host defense system may contribute to increased susceptibility of the lung to infection, especially changes in the innate immune system. The innate immune system consists of cells (macrophages, monocytes, and natural killer cells) and of humoral factors (lysozyme, complement, defensins, collectins). It is unknown whether, in the course
of ARDS, monocytes or macrophages change their modes of transmigration, phagocytosis, or cytokine production. Some clinical studies suggest that over time the cytokine profile of alveolar macrophages changes from a more pro-inflammatory to an anti-inflammatory one in patients with ARDS.15,62 Among the humoral compounds, the two hydrophilic surfactant apoproteins SP-A and SP-D are very potent modulators of the alveolar host defense, with distinct effects on pathogens, their products (e.g., endotoxin) and inflammatory cells41,63 (Fig. 6). It has been shown that the concentration of SP-A in BALF from ARDS patients is reduced44,46 and that it appears to be degraded.53 No significant change in the overall concentration of SRD has been noted.64 Interestingly, SP-D knock-out mice were recently shown to clear bacteria from the bronchoalveolar compartment as effectively as wild-type mice, whereas in SP-A knock-out mice, clearance rate decreased and translocation of bacteria into other organs (e.g., spleen) was markedly increased.65 It is conceivable that downregulation of SP-A in conjunction with proteolytic degradation may open the door for an increased susceptibility of the host to nosocomial infection, although this has yet to be proven. Currently, there are no data as to the regulation of defensins and lysozyme in ARDS. Second, the number of pathogens entering the bronchoalveolar compartment may be substantially increased (see Fig. 5). Underlying reasons for this may be environmental factors (presence of typical nosocomial pathogens such as Pseudomonas aeruginosa) and microaspiration of gastric content, which, under conditions of parenteral feeding and extensive prophylactic therapy for gastric ulcer, may contain increased bacterial counts. The lung may serve as a focus of sepsis under these conditions and may then also trigger multi-organ failure (MOF). Indeed, mortality is dramatically increased once an ARDS patient develops a secondary nosocomial pneumonia.66
Figure 5 Role of secondary lung infection in self-perpetuation of ARDS and the progression into “pneumogenic” sepsis. For details see text.
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Figure 6 Possible interactions between the collectins SP-A and SP-D and microorganisms.41,63
Ventilator-Associated Lung Injury Physicians have long held concerns that mechanical ventilation per se and the use of high fractions of inspired oxygen may injure the lung. Initial evidence for this assumption arose from the observation that, in animal models, ventilation of an otherwise healthy lung with high pressures and volumes may initiate, or in already injured lungs may enhance, edema formation, cell recruitment, protein leakage, and other ARDS-like phenomena. In fact, the changes in permeability, inflammation, and edema formation are indistinguishable from those seen in ALI/ARDS. It is for this reason that, according to the recent Consensus Conference on ventilator-associated lung injury,67 the term ventilatorinduced lung injury (VILI) is reserved for animal models, in which a clear causative relation between mode of ventilation and lung injury can be established, whereas the term ventilator-associated lung injury (VALI) should be reserved for human patients in whom it is impossible to differentiate between ventilator-induced and underlying lung injury. In the past, both increased pressure (barotrauma) and increased volume (volutrauma) were held to be responsible for the induction of VILI. Much evidence now suggests that high lung volume causing overdistension and, possibly, cyclic opening and collapse of alveoli due to inadequately low end-expiratory volume are the primary determinants for development of VALI (see article by Lee and Slutsky). A recent study performed by the ARDS network in the United States has demonstrated the harmful effect of a high (but conventional) tidal volume ventilation strategy in ARDS patients. In this study1 patients ventilated in this conventional mode (tidal volume of ~ 12mL/kg bw) were shown to have a significantly higher mortality (39.8%) and longer period of mechanical ventilation (12d) than patients being ventilated with a low tidal volume strategy (tidal volume ~ 6mL/kg bw; mortality 31%, 10d of mechanical ventilation). Underlying clinical issues, severity of ARDS, positive end-expiratory pressure (PEEP) levels, and gas exchange were all comparable between the groups. Plateau and peak pressures were
lower in the low tidal volume group and respiratory rate was clearly increased. What are the underlying mechanisms for the development of VILI/VALI? In animal studies it has been observed that ventilating the lung with an injurious mode (no or low PEEP, high tidal volume of 12–15 mL/kg bw) may induce damage to the lung. Although the underlying signal transduction pathways are not fully understood, such a ventilatory mode also leads to increased local and systemic cytokine production,68,69 disruption of the endothelial70 and epithelial barrier with increased permeability,71 hyaline membrane formation, recruitment of inflammatory cells into the alveolar space,72 and, finally, impairment of gas exchange and loss of compliance. In addition, an injurious ventilation mode may also induce severe alterations in the surfactant system. In experimental studies, imbalance of the alveolar surfactant subfractions, with predominance of the less active small surfactant aggregates and only low amounts of the otherwise dominating and biophysically highly active large surfactant aggregates, has been encountered upon ventilation with high tidal volumes.73 One possible explanation for this observation is the dependence of the surfactant subtype conversion (from large into small surfactant aggregates) on the magnitude of surface area change. It is thus conceivable that an injurious ventilation mode may promote an accelerated alveolar degradation of biophysically active surfactant subfractions, with a steadily increasing alveolar surface tension and, consequently, a reduction in pulmonary compliance.
Fibroproliferation and Lung Fibrosis As in animal models of acute lung injury (e.g., bleomycin-induced respiratory failure with subsequent lung fibrosis) patients with ARDS may similarly enter a fibroproliferative phase once the acute exudative phase has been survived. Intra-alveolar formation of organization tissue, with mesenchymal cells invading the provisional fibrin matrix and producing new extracellular matrix components such as collagen, may be seen. New vessels are formed and bronchiolization is evident. It is unclear why some ARDS patients may recover from the acute ARDS phase with rapid resolution, whereas others develop severe lung fibrosis. Severity of the acute phase may represent one possible explanation, and, accordingly, McHugh et al74 noticed that a persistent impairment in lung function was most impressive in those patients who had the highest lung injury scores during active disease. However, other explanations are possible and include differences in the underlying diseases and in treatment, a genetically determined overinduction of fibroproliferative response [e.g., mutations in the transforming growth factor (TGF-) gene], and related clinical issues such as recurrent infection. The onset of the fibroproliferative response may be an earlier event than previously thought. Meduri and
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colleagues75 observed increased plasma levels of the procollagen aminoterminal propeptide I (PINP) and III (PIIINP) on day 1 of ARDS and these values increased over time in those patients not improving. Similarly, BALF concentrations of type I procollagen76 and ability of BALF to stimulate fibroblast proliferation ex vivo77 were increased on day 1 of ARDS and were also correlated with outcome. Finally, TGF- concentration in BALF of ARDS patients was shown to be elevated already at day 3 of ARDS.78 The pathophysiological mechanisms underlying such a rapid fibroproliferative response to an acute inflammatory event are poorly understood. Relevant features include a complex scenario of cellular recruitment,79,80 activation of cytokines81,82 and growth factors,83,84,85 and cross-talk between these pro-inflammatory and pro-proliferative pathways. Among all growth factors studied, TGF- represents the most important one. TGF- stimulates angiogenesis, suppresses the activation and proliferation of inflammatory cells, and induces the synthesis and deposition of extracellular matrix components. Although TGF- is essential for regular wound healing, its overexpression promotes formation of scar tissue and fibrosis. Accordingly, overexpression of this growth factor in animals results in fibrosis of the kidney, lung, and liver.86 Further insights into the mechanisms underlying fibroproliferative lung disorders stem from the recent observation that, in addition to the changes in the cytokine and growth factor balance, abnormalities of the pulmonary surfactant system may be involved. As already stated, the surface activity of surfactant is severely impaired in ARDS, and there are profound alterations of its biochemical composition. This is also true for patients with established idiopathic pulmonary fibrosis (IPF), in whom there are similar, although less severe, alterations of the surfactant system.87 In addition, inhibition of surfactant function by plasma-derived proteins has been
shown to occur in vitro,47,48,49 under conditions of severe inflammatory injury with vascular leakage44 and also in diffuse parenchymal lung disease.87 Fibrinogen leakage may be particularly relevant under these conditions: its surfactant inhibitory capacity is further increased by approximately two orders of magnitude upon conversion to fibrin, and a nearly complete “incorporation” of all hydrophobic surfactant compounds into the arising fibrin matrix has been documented as an underlying mechanism47 (Fig. 7). In vitro, surface activity can be restored by induction of fibrinolysis yielding liberation of surface active material from the fibrin lattice with a decrease of surface tension.88 As previously stated, it has been shown that the alveolar hemostatic balance, while being antithrombotic and profibrinolytic under regular conditions, is shifted toward prothrombotic and procoagulant activities with decreased fibrinolytic capacities in the bronchoalveolar compartment under conditions of acute inflammatory50,51 and diffuse parenchymal89 lung diseases. These abnormalities include markedly increased tissue factor and factor VII activities and elevated plasminogen activator inhibitor (PAI)-1 and -2-antiplasmin levels in the alveolar lining layer. Augmented D-dimer concentration in the bronchoalveolar lavage fluid of patients with ARDS and IPF is a direct indicator of overall increased alveolar fibrin turnover. How may surfactant abnormalities and coagulation disorders in the alveolar compartment contribute to lung fibrosis? Loss of surface tension lowering properties at the alveolar fluid-air interface is known to result in alveolar instability. Alveolar fibrin formation, as most potent surfactant inhibitory mechanisms, may thus be operative to cause atelectasis. In addition, histological studies suggest that fibrin-mediated apposition (“glueing”) of alveolar septae is a general feature in lung fibrosis of different etiologies.90 According to this concept of “collapse induration,” such fibrin matrix is a nidus for fibroblast invasion, resulting in scarring and
Figure 7 Influence of surfactant incorporation into fibrin on the fibroproliferative response in ARDS. Under conditions of ARDS, a marked upregulation of procoagulant and antifibrinolytic factors is encountered that overall promotes alveolar fibrin deposition. Polymerization of fibrinogen in presence of surfactant yields almost complete incorporation of all hydrophobic surfactant compounds into the fibrin matrix, with a concomitant loss of surface activity. Timely lysis of these alveolar fibrin deposits may result in liberation of intact surfactant material, with a reopening of the concerned lung regions and a resitutio ad integrum. According to the concept of “collapse induration,” lung fibrosis may be initiated in lung regions with persistent fibrin deposition. In these areas, alveolar recruitment is impossible due to the highly increased surface tension and the fibrin-glued apposition of alveolar walls. Fibroblast invasion into this primary matrix with production of a collagenous matrix and irreversible loss of the concerned lung area may be the consequence.
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thereby irreversible loss of alveoli, with traction of the remaining airspaces (honeycombing). Moreover, direct fibroblast-activating properties have been shown for thrombin91,92 and fibrin(ogen) scission products.93 Taken together, timely dissolution of a provisional alveolar fibrin matrix may be helpful in reestablishing a regular alveolar architecture and avoidance of subsequent fibrosis. In contrast, persistent fibrin deposition may provide the primary matrix into which activated fibroblasts migrate and which may then be replaced by a collagenous, durable matrix. It is in this line of thinking that in recent investigations using the bleomycin model, pulmonary fibrosis was found to be much more pronounced in PAI-1 overexpressing95 or plasminogenactivator-deficient94 mice as compared to wild-type94 or PAI-1 knock-out95 mice. Future studies addressing the efficacy of antithrombotic or fibrinolytic agents in the prevention of fibrotic responses in ARDS patients may thus be warranted.
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