Clin Chest Med 27 (2006) 579–589 Mechanisms of Acute Lung Injury/Acute Respiratory Distress Syndrome Benjamin T. Suratt, MDa,b,*, Polly E. Parsons, MDa,b a Vermont Lung Center, Burlington, VT 05405, USA b Division of Pulmonary and Critical Care, Fletcher Allen Health Care, Burlington, VT 05405, USA In the 4 decades that have elapsed since changing emphasis on and controversy over the Ashbaugh and colleagues  ﬁrst described acute importance of these many mechanisms. Such lung injury/acute respiratory distress syndrome controversy continues because of the continued (ALI/ARDS), tremendous strides have been lack of clinical advances despite these many made in dissecting the pathophysiology of this discoveries. The inability to ﬁnd a unifying theory disease. This understanding unfortunately has for ARDS pathogenesis almost certainly reﬂects not translated into similar advances in the ability the complexity of a disease with a common end to predict, prevent, or treat this disease that af- point but many causes. fects nearly 200,000 patients a year in the United States alone . The American-European Consen- sus Conference (AECC) on ARDS formally de- Common end point: failure of the alveolar ﬁned ARDS as severe arterial hypoxemia (with capillary membrane a partial pressure arterial oxygen/fraction of in- Decades of clinical and animal research have spired oxygen [PaO2/FIO2] ratio of 200 or less) in established increased permeability edema as the the presence of bilateral alveolar inﬁltrates with- primary physiologic abnormality in the early stages out evidence of elevated left atrial pressure . of ALI/ARDS. Such edema diﬀers from high- This deﬁnition recognizes a clinical syndrome, ir- pressure or hydrostatic edema (eg, congestive heart respective of speciﬁc molecular, immunologic, or failure) in that increased permeability edema is physical events that may precede it. Thus, the def- driven primarily by a failure of the alveolar struc- inition reﬂects a common ﬁnal end point in the tures that normally retain plasma within the alveo- acute phase of this disease: respiratory compro- lar capillaries (the alveolar capillary membrane mise caused by failure of the alveolar capillary [ACM]). Failure of the ACM in ALI/ARDS allows barrier and the resulting development of protein- proteinaceous ﬂuid to ﬂood the alveolar airspaces aceous edema. and contributes directly to the impairment of gas From the formidable body of research exam- exchange and loss of lung compliance that charac- ining the advent and perpetuation of this central terize this disorder. The ACM is formed by two injury, several pathophysiologic themes have separate components: the capillary endothelium and arisen, and the history of this work reﬂects ever- the alveolar epithelium; the function of both is disrupted in ALI/ARDS (Fig. 1). The mechanisms leading to the failure of the ACM are multiple but This work was supported by Grants No. HL084200 can be divided roughly into those aﬀecting the cap- and NCRR P20 RR15557 from the NIH. * Corresponding author. Vermont Lung Center, 149 illary endothelium and those aﬀecting the alveolar Beaumont Avenue, HSRF 230, Burlington, VT 05405. epithelium. E-mail address: email@example.com Damage to the alveolar capillary endothelium (B.T. Suratt). has long been recognized as a key feature of the 0272-5231/06/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.ccm.2006.06.005 chestmed.theclinics.com 580 SURATT & PARSONS Fig. 1. The normal alveolus (left) and the injured alveolus in the acute phase of acute lung injury and the acute respi- ratory distress syndrome (right). In the acute phase of the syndrome there is sloughing of both the bronchial and alveolar epithelial cells with the formation of protein-rich hyaline membranes on the denuded basement membrane. Neutrophils are shown adhering to the injured and activated capillary endothelium and marginating through the interstitium into the air space, which is ﬁlled with protein-rich edema ﬂuid. In the airspace an alveolar macrophage is secreting cytokines, interleukin-1b, -6, -8, and -10, (IL-1b, 6, 8, and 10) and tumor necrosis factor-a (TNF-a), which act locally to stimulate chemotaxis and activate neutrophils. Neutrophils can release oxidants, proteases, leukotrienes, and other proinﬂamma- tory molecules. The inﬂux of protein-rich edema ﬂuid into the alveolus has led to the inactivation of surfactant. (From Ware LB, Matthay MA. The acute respiratory distress syndrome. N Engl J Med 2000;342(18):1339; with permission. Ó 2000, Massachusetts Medical Society.) acute phase of ALI/ARDS. Ultrastructural stud- to localized inﬂammation or injury (eg, bacterial ies demonstrate endothelial cell swelling and pneumonia), this process becomes dysregulated widening of the intercellular junctions , and ra- and uncontrolled in ALI/ARDS . Among the dionuclide studies have conﬁrmed the presence of phenotypic and functional changes that occur, profound capillary leak in these patients . More endothelial cell contraction and deranged vaso- recent work, however, has established that endo- motor response contribute to the development thelial structure and function may be altered in- of capillary leak; the expression of adhesion mol- dependently of cellular injury in a process ecules and cytokines promulgates alveolar injury, called ‘‘endothelial activation’’ that seems to pre- as discussed later . cede and promote such injury. Endothelial acti- As with alveolar endothelial injury, the pres- vation may occur in response to a wide range ence of epithelial damage, characterized by necro- of stimuli implicated in ALI, including cytokines, sis and often profound disruption, has been thrombin, lipopolysaccharide, and other micro- appreciated as a cardinal ﬁnding of ALI/ARDS bial products, and extreme changes in blood for many years . The multiple consequences of pressure. Whereas endothelial activation seems this injury and their central importance to the de- to be part of a limited and reversible response velopment of edema in ALI have been recognized MECHANISMS OF ALI/ARDS 581 only more recently. The alveolar epithelial lining endothelial and epithelial components of the alve- is composed predominantly of ﬂat type I epithelial olar membrane and represents a loss of both bar- cells that provide a thin surface for gas exchange rier function and ﬂuid resorption. and a tight barrier against ﬂuid extravasation into the airspace. This epithelial barrier function Pathophysiologic mechanisms seems to be even more critical than that played by the endothelial surface of the ACM . Type Although increased permeability edema is the II alveolar epithelial cells, although relatively central physiologic event in the development of few in number, provide another crucial protection ALI/ARDS, the events leading to the failure of against edema formation: the resorption of air- the alveolar capillary membrane are numerous space ﬂuid . The degree of loss of this function and complex (Fig. 2). This diversity of pathways in ALI/ARDS has been shown to correlate with invoked by the many inciting injuries leading to poorer prognosis . This same study found ALI/ARDS and inﬂuenced by multiple host fac- that increased rates of alveolar ﬂuid clearance in tors is the basis for much of the heterogeneity of patients who had ALI/ARDS were associated disease manifestations and outcomes. One of the with female sex, nonsmoking status, and ALI/ more intuitive examples of the potential diﬀer- ARDS risk factors other than sepsis, suggesting ences among pathways leading to ALI is reﬂected that some clinical heterogeneity may result from by the AECC categorization of risk factors for the varying rates of alveolar ﬂuid clearance. Taken to- development of ALI/ARDS, which groups these gether, the dual functions of the alveolar epithe- factors into direct and indirect (or pulmonary lium underscore the dynamic nature of edema and extrapulmonary) injuries to the lung . Ap- formation in which increases in ACM permeabil- parent diﬀerences in the radiographic and physio- ity rapidly overtake the diminished resorptive logic manifestations of ALI caused by direct capacity of the alveolus, and alveolar ﬂooding oc- versus indirect insults (and possibly in response curs. Thus, the proteinaceous edema that charac- to some therapies) have led to ongoing contro- terizes ALI/ARDS results from disruption of both versy about whether these groupings should be Fig. 2. Pathophysiologic mechanisms of acute lung injury and the acute respiratory distress syndrome. In this schema- tized view of the pathophysiologic pathways of ALI/ARDS, direct injuries to the lung damage the alveolar capillary membrane and initiate local and subsequently systemic inﬂammatory cascades. Indirect injuries initiate the pathophys- iologic pathways of ALI/ARDS primarily through release of systemic cytokines. Following both direct and indirect ini- tiators of ALI/ARDS, the release of systemic inﬂammatory mediators activates circulating neutrophils and the vascular endothelium of the lung, leading to pulmonary microvascular sequestration of neutrophils and inﬂammatory injury to the ACM. Such injury results in failure of ACM barrier function and ﬂooding of the alveoli with proteinaceous edema ﬂuid. Both ACM injury and alveolar edema cause surfactant loss and dysfunction, which promote alveolar instability and collapse, driving further edema formation and alveolar injury, particularly in the setting of mechanical ventilation. Not depicted is the role of the coagulation cascade, which promotes alveolar collapse, capillary obstruction with micro- thrombi, and further release of inﬂammatory mediators. 582 SURATT & PARSONS considered separate entities for the purposes of indirect causes of ALI/ARDS have conﬁrmed further research and clinical trials [11,12]. the critical role of neutrophil recruitment in the in- It seems logical that a direct injury to the jury to the alveolar membrane , and although ACM, such as aspirated stomach acid, would studies in neutropenic patients have demonstrated diﬀer from an indirect mechanism, such as extra- the occurrence of ALI/ARDS in the apparent ab- thoracic trauma, and in fact ALI/ARDS mortality sence of neutrophils [22,23], it is widely recognized from these two mechanisms diﬀers substantially that such patients worsen dramatically with the . There also, however, are signiﬁcant diﬀerences resolution of neutropenia . The mechanisms in mortality among indirect causes themselves: for governing neutrophil traﬃcking to the lung in instance, sepsis with an extrapulmonary source ALI have been investigated intensely. Neutrophils has a greater risk of mortality than extrathoracic are recruited to the lung through the release of sol- trauma . Furthermore, there seems to be no uble mediators that activate these cells through overall diﬀerence in mortality between direct and cell surface receptors. Some soluble factors, such indirect causes of ALI/ARDS [12,13]. In animal as lipopolysaccharide (as seen in gram-negative models examining direct injuries such as aspiration, sepsis), are triggers of the innate immune system, the direct chemical injury to the ACM is only a small whereas others are endogenous signaling mole- component of the resultant pathophysiologic pro- cules such as complement fragments, lipid factors, cess . Thus, the relationship between inciting in- and, most importantly, cytokines. jury and ALI/ARDS pathophysiology (at least as The role of cytokine signaling in ALI/ARDS is reﬂected by mortality) seems to be more complex. vast and complex , but it can be schematized Recent epidemiologic studies of ALI/ARDS have using the paradigm of the innate immune response suggested that additional factors such as age, sex, . The cascade of cytokine release that occurs and race may inﬂuence both the risk of developing during such a response begins with the activation the disease and subsequent mortality, and these of monocytes and macrophages in the lung (in di- ﬁndings probably also reﬂect diﬀerences in the rect injury) or in blood or other tissues (in indirect pathophysiology of the disease as manifested in injury). This activation stimulates the release of these patients [2,15–17]. Therefore, any apprecia- the ﬁrst wave of cytokine signal: tumor necrosis tion of ALI/ARDS pathophysiology must take factor-a (TNF-a) and interleukin-1b (IL-1b), in to account the numerous underlying pathways also known as ‘‘early-response cytokines.’’ These thus far implicated in this disease and the grow- key pleiotropic cytokines act upon leukocytes ing number of host and other factors that seem and other cell types (such as the alveolar epithe- to inﬂuence their expression. lium and endothelium and lung ﬁbroblasts) to ini- tiate a cascade of secondary cytokines and to release other soluble mediators, diversifying and amplifying the inﬂammatory signal. Critical Leukocytes and soluble mediators among these secondary cytokines are those acting Much of the evolved understanding of ALI/ primarily on neutrophils (CXC cytokines, the ARDS pathophysiology has centered on the role most important of which is IL-8) and monocytes of leukocytes and, most recently, the numerous (CC cytokines, eg, macrophage inﬂammatory soluble mediators that both drive their recruit- protein-1), which serve both to activate and to ment and inﬂuence their behavior (see Fig. 1). As recruit these cells. Recent evidence suggests that with the initial appreciation of alveolar epithelial inﬂammation in ALI/ARDS results not so much and endothelial injury in ALI/ARDS, early histo- from the unopposed actions of these proinﬂam- logic studies also identiﬁed neutrophils as a likely matory cytokines as from the net balance of a mi- key participant in this injury [1,4]. Since this ob- lieu of both pro- and anti-inﬂammatory mediators servation was made, studies have demonstrated (eg, IL-10, soluble TNF-a receptor, and IL-1 re- neutrophils to be the predominant cell type in ceptor antagonist) . Also crucial during this bronchoalveolar lavage of patients who have inﬂammatory cascade are cytokines (including ALI/ARDS, and the persistence of this ﬁnding TNF-a and IL-1b) that activate the vascular seems to correlate with increased mortality endothelium, particularly that of the lung, leading [18,19]. Examination of bronchoalveolar lavage to expression of leukocyte adhesion molecules . has also implicated the release of injurious neutro- Activation of both circulating neutrophils phil contents, such as proteases and oxidants, in and the vascular endothelium leads to rapid the process . Animal models of direct and sequestration of neutrophils to the microvascular MECHANISMS OF ALI/ARDS 583 beds of the lung (Fig. 2). Unique among the tissues converts plasminogen to the ﬁbrinolytic factor, of the body, the lung uses both classic leukocyte plasmin. Plasmin, in turn, cleaves the small adhesion mechanisms in this process (selectin- amount of ﬁbrin that is formed through the co- and integrin-mediated endovascular adhesion and agulation cascade, which is largely quiescent in diapedesis ) and also a ﬁlter-like form of me- the resting lung. In the setting of injury to the al- chanical arrest, in which neutrophils stiﬀen after veolar capillary membrane, this balance is over- activation and can no longer pass through the whelmed by the leakage of coagulation factors small capillaries of the alveoli [29,30]. This mecha- into the interstitium and airspace of the alveolus. nism seems to be accentuated by the release of im- This imbalance, coupled with the expression of mature marrow neutrophils, which are larger and procoagulant molecules (such as tissue factor) less deformable [31,32], and may in part explain by the injured endothelial and epithelial compo- the elevated risk of ALI/ARDS in patients during nents of the ACM and an increase in inhibitors the recovery from neutropenia [24,33]. of ﬁbrinolysis (such as plasminogen activator-1, Given the importance of the innate immune or as plasminogen-activator inhibitor type 1 [PAI- system in the development of ALI/ARDS, it is 1]), leads to unopposed procoagulant activity easy to appreciate how genetic variation involving . The resulting formation of ﬁbrin-platelet cytokines and their receptors might lead to microthrombi in the capillaries and ﬁbrin-rich signiﬁcant variability in the expression of ALI/ proteinaceous casts in the airspaces of the lung ARDS after a given injury. For example, poly- contributes signiﬁcantly to ventilation-perfusion morphisms in the gene for TNF-a have been mismatching in this disease. What has only re- shown to be associated with increased susceptibil- cently been appreciated is the interaction be- ity to and mortality from ALI/ARDS , and tween this imbalance of alveolar coagulation multiple other polymorphisms in both pro- and and ﬁbrinolysis and the inﬂammatory processes anti-inﬂammatory mediators and receptors such of ALI/ARDS. as IL-1 receptor antagonist and toll-like recep- Recent studies have emphasized the multiple tor-4 (the receptor for lipopolysaccharide) have eﬀects of thrombin and ﬁbrin deposition on local been shown to inﬂuence the clinical manifesta- and systemic cellular activation and cytokine tions of sepsis and possibly ALI/ARDS . Fur- production . Fibrin and thrombin seem to ac- thermore, the pivotal role of neutrophils and their tivate both neutrophils and the vascular endothe- cytotoxic contents in ALI/ARDS may explain, in lium directly, leading to augmented expression of part, the epidemiologic observations that chronic adhesion molecules on both cell types and, in the alcoholism, which is associated with a profound case of the endothelium, to increased capillary deﬁciency in the antioxidant glutathione, is associ- permeability . Such activation also drives the ated with increased susceptibility to and mortality further release of TNF-a and IL-1b and of other from ALI/ARDS , whereas diabetic patients, cytokines, such as IL-8, which further propagate who manifest defects in neutrophil activation the inﬂammatory response . Platelets seem to and function, are at a lower risk of developing have similar proinﬂammatory actions in ALI/ ALI/ARDS . ARDS but also may have some ameliorating ef- fects on the capillary leak, making their net eﬀect in this process unclear . Although trials of sev- eral anticoagulant therapies for ALI/ARDS have Coagulation and platelets yielded mixed results , recent success with acti- The appearance of intra-alveolar hyaline mem- vated protein C in the treatment of sepsis has sug- branes and microvascular thrombi has been rec- gested that this therapy may have a role in the ognized as a histologic hallmark of acute ALI/ treatment of ALI/ARDS and has conﬁrmed that ARDS for decades (see Fig. 1) [1,4], but the full such an approach may indeed attenuate both the implications of these ﬁndings for the pathophys- thrombotic and inﬂammatory events of ALI/ iology of ALI/ARDS have been appreciated only ARDS [41,42]. Furthermore, studies of genetic recently. Both ﬁndings reﬂect the exuberant de- polymorphisms of coagulation proteins, such as position of ﬁbrin in the lung and represent a pro- PAI-1 and thrombospondin, have related these found imbalance in production and degradation polymorphisms to variations in both proinﬂam- of this molecule. In the uninjured lung a net ﬁ- matory cytokine levels and clinical outcomes for brinolytic state is maintained through the action patients who have sepsis and ALI/ARDS of urokinase plasminogen activator, which [43,44]. This ﬁnding also suggests that genetic 584 SURATT & PARSONS variation may inﬂuence the clinical expression of mortality in those who had ALI/ARDS caused by ALI/ARDS through polymorphisms of the cyto- indirect injury . Furthermore, genetic studies kines themselves and of the coagulation pathways have suggested that certain polymorphisms in as well . the SP-B protein may confer a greater risk for the development of ALI/ARDS, particularly after direct lung injury . Thus, the importance of surfactant dysfunction may be inﬂuenced by the Surfactant mechanism of injury and by host factors. One of the earliest theories about the patho- physiology of ALI/ARDS concerned lung surfac- tant and its dysfunction during ALI/ARDS . Ventilator-induced lung injury The role of surfactant in this disease has since been appreciated to be complex and, as regards Since the early observations of Webb and possible therapeutic applications, somewhat per- Tierney  that high tidal volume ventilation in plexing. Surfactant is a lipoprotein complex, rats leads to proteinaceous lung edema and composed of phospholipids, neutral lipids, and histologic ﬁndings consistent with ALI/ARDS, surfactant proteins (SP-A, B, C, and D), that is se- mechanical ventilation has been suspected to in- creted by the type II alveolar epithelium and lines ﬂuence the manifestation of ALI/ARDS in hu- the alveolar surfaces. Its functions are both bio- mans. How such an interaction might be physical and immunologic, in that it serves to de- particularly accentuated in ALI/ARDS during crease surface tension at the air/liquid interface even physiologic levels of ventilation was sug- (maintaining alveolar patency), and participates gested subsequently by radiographic studies dem- in critical elements of innate host defense . onstrating the extreme heterogeneity of alveolar Studies of bronchoalveolar lavage in patients patency in this disease . These studies revealed who have ALI/ARDS have demonstrated that that aeration of the lung varies markedly, not only surfactant is altered quite early in the course of by lung region (with widespread patchy consolida- the disease: the composition and form of surfac- tion, particularly in the more dependent areas) but tant changes, and it develops abnormal surface also by the respiratory cycle. Thus, in the course tension properties [47,48]. These changes seem to of cyclic tidal volume delivery, three areas may be driven by the inﬂux of edema ﬂuid and serum be considered to exist : proteins into the alveoli and worsened by injury 1. Fluid-ﬁlled or collapsed areas in which the al- to the type II epithelium (as described previously), veoli never inﬂate during the respiratory cycle ultimately leading to alveolar instability and col- and which lead to a decrease in total lung lapse, as well as to altered immune function (see capacity Fig. 1). Alveolar collapse decreases lung compli- 2. Patent areas in which alveoli may become ance, worsens hypoxemia, and draws additional overdistended during inspiration because of edema ﬂuid in to the alveoli , driving a vicious shunting of delivered breath away from col- cycle of further surfactant dysfunction and alveo- lapsed areas lar edema (see Fig. 2). 3. Atelectatic areas in which the alveoli repeat- Given the importance of surfactant in the edly open and close with the respiratory cycle pathophysiology of ALI/ARDS, the use of re- because of alveolar instability, particularly combinant surfactant to treat ALI/ARDS has at lower levels of positive end-expiration been of great interest [50,51]. Unlike the positive pressure results seen in neonatal respiratory distress syn- drome (nRDS), however, trials of surfactant in Therefore, mechanical ventilation in ALI/ ALI/ARDS have been disappointing. These re- ARDS may lead to recurrent overdistension of sults seem to underscore the relatively greater im- the patent alveoli and to shear injury in the areas portance of impaired surfactant production in exposed to cyclic atelectasis. nRDS versus surfactant function in ALI/ARDS Alveolar overdistension and shear seem to . Post hoc analysis of the most recent trial of perpetuate and augment lung injury in three surfactant replacement for ALI/ARDS suggested important ways: local and systemic inﬂammatory that such therapy reduced mortality in patients signaling, direct disruption of the ACM, and who had ALI/ARDS secondary to direct injury impairment of alveolar ﬂuid clearance (see (pneumonia or aspiration) but may have increased Fig. 2). Animal studies have shown that MECHANISMS OF ALI/ARDS 585 mechanical ventilation, particularly at high tidal as much by these corrective responses as by the volumes, leads to both pulmonary and systemic initiating and ongoing injuries discussed previously. release of inﬂammatory cytokines, including Resolution of ALI/ARDS involves the termination TNF-a, IL-1b, and IL-6, and the recruitment of of the inﬂammatory response, the clearance of both neutrophils to the lung. This process seems to be ﬂuid and debris from the alveoli, and the repair of driven, in part, by cyclical stretching of lung mac- the ACM (see Fig. 1). Clinical and animal studies rophages and type II pneumocytes, which pro- have suggested that an imbalance of corrective motes activation of these cells [56,57]. Clinical and injurious forces or dysregulation of the repair studies conﬁrm that inﬂammatory cytokines, in- process itself may lead to pathophysiology in the cluding IL-6 and IL-8, are released into the lung later phase of ALI/ARDS and to poor outcome and blood during mechanical ventilation of pa- from this disease . tients who have ALI/ARDS, and that this eﬀect Fluid resorption from the alveolar airspace is attenuated by the use of lower tidal volumes and interstitium is dependent on the active trans- (6 mL/kg) than traditionally used (10–12 mL/kg) port of sodium (and with it, water) by the type II [58–60]. In addition to the possible proinﬂamma- epithelium and reconstitution of the ACM barrier. tory eﬀects of mechanical ventilation, direct me- Failure of ﬂuid resorption, even as measured quite chanical injury to the ACM may occur at high early in the disease process, is predictive of tidal volumes as the result of cell membrane dis- mortality from ALI/ARDS . Thus, regenera- ruption [61,62] and mechanically transduced acti- tion of the alveolar epithelium structure and func- vation of the alveolar endothelium . These tion, as described later, is a critical event in the processes seem to be exacerbated by concomitant recovery from ALI/ARDS. The removal of cellu- ALI/ARDS, so that even tidal volumes consid- lar and other debris from the airspace is also im- ered to be physiologic may provoke signiﬁcant di- portant, both in the restoration of gas exchange rect injury to the lung . Last, mechanical and in the resolution of inﬂammation and repair ventilation in ALI/ARDS may reduce airspace of the damaged epithelium. Apoptosis (or con- edema clearance further through alteration of trolled cell death) is the central mechanism of type II pneumocyte function . cell clearance for both injured alveolar epithelium The observation that mechanical ventilation, and recruited inﬂammatory cells. Apoptosis seems even at physiologic tidal volumes, may compound to be a double-edged sword in ALI/ARDS: al- the severity of lung injury in ALI/ARDS led to though the induction of apoptosis may contribute a series of clinical trials examining low tidal to epithelial injury in early ALI/ARDS , it also volume ventilation strategies for this disease. is critical in the removal of neutrophils by airspace The largest of these trials demonstrated that a macrophages . Furthermore, the clearance of 6-mL/kg (ideal body weight) tidal volume strategy apoptotic cells from the airspace seems to alter led to a reduction in ALI/ARDS mortality from the cytokine milieu and attenuate the inﬂamma- 40% to 31% compared with a conventional tidal tory response in the lung . In addition to the volume strategy of 12 mL/kg . The signiﬁcance removal of cellular debris, the clearance of insolu- of this ﬁnding cannot be overemphasized, because ble proteinaceous debris, particularly hyaline to date it represents the only speciﬁc intervention membranes, is essential for gas exchange and for ever shown in a large-scale trial to reduce mortal- restoration of the normal architecture of the ity from this disease signiﬁcantly. It also suggests alveolus. that yet another factor may dramatically inﬂuence Repair of the ACM is a complex process the pathophysiology and outcome of ALI/ARDS: involving local cellular proliferation and, proba- physician practice. bly, the recruitment of circulating progenitor cells. As the clearance of airspace edema and debris progresses, repair of the alveolar structure occurs through the orderly replacement of its cellular Late-phase pathophysiology: dysregulated constituents. The denuded alveolar epithelium is and maladaptive repair replaced through a proliferation of the remaining Even as the multiple pathophysiologic processes type II pneumocytes, which spread to cover the of early ALI/ARDS promulgate alveolar injury and exposed basement membrane and subsequently edema, reparative processes are initiated within the transdiﬀerentiate into type I cells. At the same lung that drive toward resolution of the disease. time, the alveolar interstitium is restored through Thus, the course of ALI/ARDS may be determined the orderly expansion of ﬁbroblasts and 586 SURATT & PARSONS deposition of extracellular matrix proteins, the The frustration of generations of clinicians and alveolar microcirculation is recanalized, and the researchers in developing consistent paradigms for damaged endothelium is replaced. Although re- the understanding and treatment of ALI/ARDS pair to the damaged lung in ALI/ARDS can lead underscores the fundamental variability of mech- to substantial, although incomplete, restoration of anisms that underlie this condition. Recent prog- lung function in survivors , divergence from ress in the ﬁelds of immunology, molecular this choreographed sequence of events may lead biology, genetics, and epidemiology has allowed to severe consequences. The best described of greater appreciation of the multiple factors inﬂu- these consequences is the ‘‘ﬁbroproliferative encing this disease. In the end, each individual phase’’ of ALI/ARDS that may develop approxi- aﬀected by ALI/ARDS must be seen as represent- mately 5 to 7 days into the disease course and is ing a unique combination of genetics, comorbid characterized by an exuberant ﬁbrotic response conditions, inciting events, and factors yet to be  that has been likened to wound healing in the discovered that drive the process of ALI with alveolar airspace . This maladaptive repair re- variable contributions from the many pathophys- sponse seems to be promoted by severe injury to iologic mechanisms elucidated in the past 40 the ACM and extensive hyaline membrane forma- years. tion, which lead to migration of myoﬁbroblasts into the alveoli and subsequent formation of gran- ulation tissue with physiologic compromise . 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