Amy E. DeClue, DVM, MS, DACVIM
Acute lung injury (ALI) is a syndrome of pulmonary inflammation and edema resulting in acute respiratory failure. The clinical presentation varies in severity with the most severe manifestations termed acute respiratory distress syndrome (ARDS). The major difference between ALI and ARDS is the degree of hypoxemia as defined by the ratio of arterial oxygen tension to fractional inspired oxygen concentration. In a patient with appropriate risk factors and clinical findings, a ratio of less than 300 or 200 differentiates ALI from ARDS, respectively.
Risk factors associated with ARDS in veterinary medicine.
Direct pulmonary injury
Indirect pulmonary injury
Lung lobe torsion
Gastric and splenic torsion
Disseminated intravascular coagulation
This syndrome is traditionally described based on morphologic changes separated into phases: exudative, proliferative, fibrotic and resolution. The exudative phase of lung injury begins with pulmonary vascular leakage and inflammatory cell infiltration. Loss of capillary integrity, alveolar epithelial damage, accumulation of protein-rich fluid and development of pulmonary edema are characteristic features of the exudative phase in dogs and cats. Irreversible damage of type I pneumocytes leads to a loss of normal gas exchange mechanisms. To repair the denuded alveolar epithelium, type II pneumocytes abandon their primary responsibility of surfactant production and, instead, proliferate. The combination of type I pneumocyte death and altered type II pneumocyte function leads to formation of hyaline membranes, deficiency of surfactant and collapse of alveoli. Vascular endothelial damage incites local thrombosis further impairing gas exchange during this phase. Then, in the proliferative phase, there is organization of exudates and development of fibrosis. During this time, type II pneumocyte proliferation is amplified. The final phase prior to resolution is the fibrotic phase. Although the fibrotic phase is considered the "last phase", the activation of fibroblastic proliferation begins very early in the course of disease. Fibroblastic proliferation begins in the pulmonary interstitium and progresses to the alveolar lumen leading to narrowing and collapse of the airspaces and pulmonary hypertension. During this phase, collagen is deposited in the alveolar, vascular and interstitial beds. This leads to the development of microcysts in the pulmonary parenchyma.
Regardless of the inciting trigger, the lung has a limited repertoire of response to insult. Therefore, the pathologic findings of ARDS are relatively consistent despite varying etiologies. ARDS develops secondary to an inappropriate, overzealous inflammatory response which may last long after the inciting cause is removed. It is logical that a direct pulmonary insult like bacterial pneumonia would lead to alveolar epithelial damage, pulmonary inflammation, capillary leakage and the development of ARDS. However, in humans, the most common cause of ARDS is sepsis. Understanding how an infectious process initiated outside of the lung could result in such severe pulmonary inflammation has been the focus of a great deal of research in the last few decades. This lecture will focus on how sepsis initiates pulmonary inflammation and pathology.
Although there are substantial similarities in the pathology associated with infection-induced ARDS, there are some differences between direct and indirect insults. Direct pulmonary insults like bacterial pneumonia result in more pronounced alveolar damage and formation of alveolar edema, fibrin, neutrophilic inflammation and consolidation. Indirect insults like sepsis result in microvascular congestion, interstitial edema, alveolar collapse and less severe alveolar damage. Interestingly, in models of bacterial pneumonia, alveolar damage is prevalent in the area instilled with bacteria as would be expected with a direct pulmonary insult. However, areas protected from direct bacterial application develop interstitial edema and vascular congestion consistent with indirect damage. These differences may have important treatment implications, especially when mechanical ventilation is instituted.
Sepsis is defined as the systemic inflammatory response to infection. Sepsis does not necessarily indicate bacteremia. For a localized infection to progress to sepsis, microbial products (e.g., endotoxin from gram-negative bacteria; exotoxins, peptidoglycans and superantigens from gram-positive bacteria; and fungal cell wall material) must induce inflammation on a systemic level. Sepsis is thought to develop due to global immune cell activation resulting in an imbalance between pro-inflammatory mediators (e.g., TNF) and anti-inflammatory mediators (e.g., IL-10). Tumor necrosis factor (TNF)-α, IL-1β, IL-6, IL-8 and leukotrienes are examples of important pro-inflammatory mediators involved in the initiation of sepsis. In some patients, sepsis progresses to multiple organ dysfunction. ARDS is the most common organ involved in sepsis-induced multiple organ dysfunction in humans and has been reported in animals with sepsis. The exact incidence of sepsis-induced ARDS in dogs and cats is not known. There is species variation in the pulmonary sensitivity to microbial products, so it is probable that the incidence of sepsis-induced ARDS will vary between species. Additionally, many human patients with sepsis are managed with mechanical ventilation, an additional risk factor for pulmonary injury, which may explain the high incidence in that species.
Since gram-negative bacterial infection is the most common cause of sepsis, we will focus on the interaction of gram-negative bacteria and the lung. However, other forms of sepsis may result in the development of ARDS as well. During gram-negative sepsis, lipopolysaccharide (LPS), the glycolipid component of the cell wall of gram-negative bacteria, is released. Upon release, the lipid A portion of LPS binds to LPS binding protein. LPS is recognized via macrophage cell surface receptors like CD14. LPS binding protein promotes these interactions by increasing the binding affinity LPS to CD14. The main function of CD14, which lacks a transmembrane domain, is to transfer LPS to toll like receptor (TLR) 4 and MD-2 for subsequent cellular activation. Some forms of LPS may also be able to interact with TLR2. Once LPS binds to these cell surface receptors, the macrophage becomes activated and intracellular signaling is initiated. Intracellular events consist of an early response which is dependent on MyD88 and a late response through TIR-domain-containing adapter-inducing interferon β (TRIF) and TRIF-related adapter molecule (TRAM). The outcome of both the early and late response is activation of NF-kappa B and other transcription factors.
At a molecular level, activation of NF-kappa B is an incipient event for the development of ARDS and increased NF-kappa B activation in alveolar macrophages has been demonstrated in humans with ARDS. NF-kappa B is a transcription factor involved in the activation of a myriad of genes. Activation and nuclear translocation of NF-kappa B results in the transcription of multiple inflammatory mediators that have been implicated in the induction and maintenance of ARDS (see inflammatory mediators below). NF-kappa B is also involved with the induction of apoptosis. Interestingly, many physical and chemical stimuli are capable to NF-kappa B activation. This may be one explanation for why, although there is a plethora of inciting causes for ARDS, the inflammatory outcome is similar.
Apoptosis, or programmed cell death, is a tightly controlled, energy dependent process. Apoptosis plays a role in sepsis-induced pathology in many organ systems. The vascular endothelium is particularly susceptible to sepsis-induced apoptosis since bloodborne microbial products like LPS are able to induce apoptosis independently of inflammatory mediator stimulation. LPS stimulates epithelial cell apoptosis via the NF-kappa B pathway and activation of caspases. Additionally, inflammatory mediators like TNF-α are able to activate NF-kappa B and initiate apoptosis. Nitric oxide and reactive oxygen species have also been implicated in promoting apoptosis during sepsis. Through these pathways and others, endothelial cell apoptosis leads to microvascular injury. Ultimately, vascular leak and breakdown of the alveolocapillary barrier results in movement of inflammatory proteins into the alveolar compartment. Microvascular damage can activate local coagulation resulting in the formation of microthrombi further perpetuating lung injury. Additionally, there is an inactivation of surfactant because of the effects of inflammatory mediators and damage to type II pneumocytes during sepsis. Pulmonary surfactant lowers the surface tension allowing the alveolus to stay open at the end of expiration. Therefore, surfactant depletion leads to alveolar collapse.
Although the idea that pulmonary inflammation is initiated solely by bacterial products may hold true in some situations, the question of how other non-infectious, indirect insults result in ARDS has supported the hypothesis that circulating inflammatory mediators are involved as well. Administration of IV TNF-α, for instance, induces a sepsis-like syndrome and the development of lung injury consistent with ARDS in experimental models. Multiple inflammatory mediators are capable of activating NF-kappa B independently of microbial products. These circulating inflammatory mediators have intimate contact with the pulmonary vascular endothelium. In health, these mediators would not reach the alveolar compartment due to the alveolocapillary barrier. During sepsis, alveolar compartmentalization is lost allowing inflammatory mediators to pass through the capillary endothelium to gain access to the alveolus. Therefore, the combination of circulating bacterial products along with microbial-induced inflammatory mediators released into the circulation lead to the development of ARDS during sepsis.
Primary pulmonary infection will induce inflammation in a similar fashion to sepsis with a few distinct differences. Similarly to sepsis, any form of pulmonary infection may lead to ARDS, regardless of the type of organism. Microbial products and inflammatory mediators work in concert to activate macrophages and induce inflammation and cell damage or death. However, primary pulmonary infections have their greatest impact on the alveolar side of the alveolocapillary barrier. Alveolar epithelial cells are injured or killed in the initial phases of direct pulmonary infection with subsequent movement of inflammatory mediators to the interstitium and vascular endothelium. In this way, more alveolar damage is noted with less prominent vascular lesions early in the disease. Conversely, in sepsis bloodborne microbial products and inflammatory mediators are involved so the vascular endothelium receives the first insult.
Inflammatory Cells and Mediators
Once macrophages are activated, cytokines and chemokines, including tumor necrosis factor (TNF) -α and interleukin (IL)-1β, are produced and reactive oxygen species (ROS) released. Alveolar macrophage activation and elaboration of pro-inflammatory mediators initiates a series of events leading to neutrophil migration into the pulmonary interstitium and alveolus. Pulmonary neutrophil accumulation is seen in the early stages of ARDS histologically and neutrophils predominate in the BAL fluid from dogs and cats with ARDS. Similarly to activated macrophages, activated neutrophils release inflammatory mediators and ROS. Such oxidants in turn lead to dysfunction and death of alveolar epithelial cells and decreased surfactant production. Listed below are several inflammatory mediators that are involved in ARDS, however this list is far from exhaustive. As you will note, this list has striking similarity to lists of inflammatory mediators involved in sepsis.
TNF-α and IL-1β: Numerous studies have confirmed that TNF-α and IL-1β are the earliest soluble mediators in ARDS with increased concentrations 30-90 minutes post injury. In experimental canine models of ARDS, TNF-α and IL-1β are increased in both serum and BALF. They trigger additional production of inflammatory mediators including cytokines, lipid mediators and ROS. TNF-α and IL-1β play an essential role in neutrophil recruitment through upregulation of endothelial-leukocyte adhesion molecules. TNF-α induces microthrombosis by stimulating endothelial procoagulant activity; increases lung capillary permeability; and is directly toxic to endothelial cells. IL-1β stimulates inflammatory and fibroproliferative processes by altering fibroblast gene expression. Interestingly, IL-1β, a potent pro-inflammatory mediator in the initiation of ARDS, may promote repair of injured alveolar epithelium later in the syndrome.
TGB-β: Transforming growth factor beta (TGF-β) is a key mediator of tissue fibrosis and can be produced by virtually every cell type. TGF-β plays a role in promotion of pulmonary edema formation, neutrophil chemotaxis and macrophage activation. TGF-β is also important in the fibroproliferative response.
PAF: Macrophages, neutrophils and endothelial cells produce platelet activating factor (PAF). Much of the vascular endothelial effects of neutrophils are mediated through the production of PAF. PAF induces vasodilation, bronchoconstriction, and vascular leakage along with its traditional role of platelet activation.
IL-6: A wide range of cells can produce the pro-inflammatory cytokine IL-6. IL-6 induces fibroblast activation and proliferation.
CXCL-8: CXC chemokine ligand (CXCL)-8 (aka IL-8) is a chemokine produced by many cells including fibroblasts, macrophages, lymphocytes and endothelial cells. CXCL-8 stimulates neutrophil recruitment and activation. Further, it stimulates the respiratory burst and leukotriene release.
Eicosanoids: Eicosanoids are a group of hormones produced from arachidonic acid that include prostaglandins, thromboxanes and leukotrienes. Products of the arachidonic acid cascade are important for many aspects of homeostasis in health and disease including vascular permeability, vascular tone, bronchial tone and platelet function. Production of thromboxane has been recognized as part of the pathogenesis of ARDS in canine models. Furthermore, modulation of prostacycline via COX-2 inhibition preserves pulmonary gas exchange in canine experimental ARDS. Increased airway resistance, capillary permeability and hypoxemia have been attributed to leukotriene production in cats. However, some arachidonic acid derived mediators may be crucial for resolution of ARDS.
Anti-inflammatory mediators: Although much attention is given to the proinflammatory mediators, the imbalance of pro-inflammatory to anti-inflammatory mediators may be critical to the development of ARDS. Interleukin-10, for example, is an anti-inflammatory cytokine which inhibits the release of pro-inflammatory cytokines. Human patients with lower circulating concentrations of IL-10 are more likely to develop ARDS and an increased ratio of pro-inflammatory to anti-inflammatory mediators corresponds with poor outcome.
ARDS is a common complication associated with sepsis or pneumonia in humans and is becoming more frequently recognized in dogs and cats. The induction of sepsis-induced ARDS is complex and involves multiple inflammatory cells and mediators. Continued research is needed to appropriately characterize ARDS in dogs and cats and to search for novel strategies for prevention and treatment of this devastating syndrome.
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