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Department of Anesthesiology, University of Texas Medical Branch, Galveston, Texas 77555-0591
| Abstract |
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| Introduction |
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We will review the pathophysiological aspects of smoke inhalation injury and note the various treatment strategies being currently investigated. Because of space limitations, this review will not discuss the pathophysiology of toxic gas inhalation or oropharyngeal and/or tracheobronchial thermal injury. These topics are important issues to be considered elsewhere.
| Bronchial blood flow |
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There are several mediators involved in the regulation of bronchial circulation, including nitric oxide (NO), a potent vasodilator. It has been reported that NO synthase (NOS) inhibitors reduce the increase in bronchial blood flow. There may be other factors, such as neurotransmitters, involved in this phenomenon, but they are still under investigation.
| Role of NO |
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NO + L-citrulline by the presence of NOS. There are three known NOS isoforms: NOS-1 (neuronal NOS or nNOS), NOS-2 (inducible NOS or iNOS), and NOS-3 (endothelial NOS or eNOS). nNOS and eNOS are calcium dependent and constitutively expressed. iNOS is calcium independent and induced in various cells, such as macrophages, by inflammatory cytokine stimulation. As we discussed above, NO is a vasodilator and regulates the microcirculation. In addition, NO is a potent inhibitor of platelet aggregation, neutrophil adhesion, and cytokine production. These effects are important for maintaining microcirculatory blood flow. However, once the production of NO gets extremely high, NO begins to function as a free radical and becomes involved in inflammation. When NO reacts as a free radical, it is highly reactive and interacts with various substances, such as oxygen free radicals. Under certain conditions, NO reacts with superoxide
to form peroxynitrite (ONOO-), an extremely potent oxidant that contributes to cellular injury, including lipid peroxidation, nitrosylation of different molecules, sodium channel interaction, and interaction with different transitional metals. Under normal conditions, NO is rapidly scavenged by the heme group of hemoglobin and metabolized to nitrate
and nitrite
. However, under inflammatory conditions, such as those caused by inhalation injury, leukocytes are activated, which results in the expression of adhesion molecules on the surface of leukocytes and endothelial cells. When leukocytes adhere to the endothelial cells by way of adhesion molecules, hemoglobin is no longer accessible to the small gap between endothelial cells and neutrophils. Because activated neutrophils and monocytes produce
radicals, NO easily reacts with
to form ONOO- in this gap. This reaction may also happen in the extracellular space or in the alveolar space, where hemoglobin does not exist. As a result, ONOO- formation causes an increase in pulmonary vascular permeability and lung edema and a decrease in diffusing capacity.
Our ovine smoke inhalation model has shown levels of NO2 and NO3 (NOx) after the injury that are two to three times that of baseline levels. Since the lung is the major physiological structure affected by inhalation injury, we have measured lung Arg metabolism in this model by using a stable isotope ([15N]Arg) as a tracer. We have found that, 24 h after a combination injury of smoke inhalation and burns, the lung Arg metabolism is threefold higher than that of baseline levels (Fig. 1
). We have shown that this increase is significantly reversed by administering NG-nitro-L-arginine methyl ester (L-NAME), a nonspecific NOS inhibitor (Fig. 1
). These data thus suggest that NOS may be responsible for the increased Arg metabolism.
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There is also a possibility that nNOS plays a role in the pathophysiology of inhalation injury. Recently, we have reported that 7-nitroindazol, an nNOS inhibitor, attenuated ALI after smoke inhalation and pneumonia in sheep. Although at present it is still not known which cells express nNOS, we think that nNOS-derived NO plays a role in the inhalation injury. There have been published reports showing that human neutrophils contain nNOS (6). If this is the case, inhibition of nNOS may be another effective treatment strategy in ALI following smoke inhalation injury.
Nonspecific NOS inhibitors such as L-NAME or NG-monomethyl-L-arginine (L-NMMA) attenuate the gas exchange dramatically by improving the ventilation-perfusion mismatch, but they can have adverse effects on cardiac functions. When L-NAME or L-NMMA are given to the animals after the evident lung injury, in which the oxygen tension of the lung has fallen (P/F ratio < 200), pulmonary arterial pressure rises and cardiac output decreases significantly. Thus the onset of tissue hypoperfusion or ischemia is possible, a fact that prompts careful consideration when these nonspecific NOS inhibitors are given. Another concern regarding treatment strategies using nonspecific NOS inhibitors is that plasma NOx levels decrease to lower-than-normal levels in those animals receiving treatment. The control of the synthesis of NO (formation of mRNA for iNOS) is through nuclear factor (NF)-
B. The activity of NF-
B is inhibited by NO, a process called negative feedback. When NOS is inhibited by these compounds, NO is not formed; thus NF-
B activity is not inhibited; consequently, iNOS mRNA and protein levels could be much higher in NOS inhibitor-treated animals than in vehicle-treated animals (12). This suggests that the rebound reaction might happen when the administration of NOS inhibitors is discontinued. This could cause adverse reactions in patients.
Our animal model has shown that NO induction is not as marked in smoke inhalation injury alone. However, if the inhalation injury is combined with burn injury or pneumonia, NO production has been shown to be significantly higher than with either injury alone. We have found that the inhibition of iNOS is beneficial in both a smoke plus burn model and a smoke plus pneumonia model but that the inhibition of nNOS is beneficial only in a smoke plus pneumonia model. It is important to know in which ways NOS isoforms are involved in the pathophysiology of smoke inhalation injury, especially when considering possible treatment strategies.
NO also plays an interesting role in ONOO--induced DNA damage. When a single DNA strand is broken, the nuclear enzyme poly(ADP-ribose) polymerase (PARP) is activated. Although PARP is known to be involved in the DNA-repairing mechanism, it requires a large amount chemical energy in the form of ATP and NAD. As a consequence, the intracellular depletion of these two substances leads to necrotic cell death (8). The release of intracellular contents into the extracellular space in the necrotic process amplifies the inflammatory reaction. Our recent studies have shown that potent PARP inhibitor INO-1001 (Inotek, Beverly, MA) significantly attenuated ALI after burn plus smoke inhalation in sheep. We believe that activation of PARP by ONOO- is involved in the pathophysiology of ALI after smoke inhalation injury (Fig. 3
).
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| Airway obstruction and ventilator-induced lung injury |
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Activated neutrophils adhere to the activated endothelial cells and injure them, resulting in an increase in pulmonary vascular permeability. The exuded plasma contains coagulation factors such as fibrinogen and/or prothrombin. In addition to the exudation, pulmonary epithelial cells and alveolar macrophages express tissue factor. Tissue factor is an initiator of the extrinsic pathway of coagulation and is known to cause fibrin deposition (clots) in the alveolar space. Fibrin formation in the alveolar space is considered to be a hallmark of acute and chronic lung injury. Fibrin is also known to inhibit surfactant activity. In comparing our various animal injury models, we have found that the obstructing materials contain more fibrin clots in a combination of smoke inhalation plus pneumonia than in smoke inhalation plus burns, suggesting that the airway coagulopathy is more severe if pneumonia is combined with smoke inhalation. Therefore, heparin nebulization has been shown to be effective in the smoke plus pneumonia model but not in the smoke plus burns model.
Ventilator-induced baro/volutrauma is another mechanism of injury after smoke inhalation (Fig. 4
). When some parts of the lung are obstructed by cast formation, other parts of the lung will be overstretched by a ventilator. Generally, low tidal volume ventilation is recommended in the treatment of acute respiratory distress syndrome (13) (6 ml/kg); one of the reasons for this is to prevent ventilator-induced baro/volutrauma. Mechanically overstretching lung tissue induces chemokines (compounds that cause leukocytes to be attracted to inflamed or injured areas) from epithelial cells, another proinflammatory reaction.
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There are several ways to prevent cast formation. As we have described above, heparin nebulization is a practical treatment. However, heparin itself does not have an anticoagulant property. Because of this, it is necessary for heparin to have a high enough level of antithrombin. In most smoke inhalation cases, the antithrombin level is maintained in the normal range, but in patients with inhalation injury with burns or severe infection, the antithrombin level significantly decreases. In our smoke plus pneumonia model, in which plasma antithrombin activity decreases <50%, supplementation of antithrombin reduces the airway obstruction and attenuates ALI. Since neutrophils are the major component of cast formation, inhibition of neutrophil accumulation is also effective in reducing the airway obstruction. The selectin family of adherence molecules are involved in the early processes of leukocyte adherence to the microcirculation. Among these molecules we studied L-selectin, which is on the neutrophil surface, and P-selectin, expressed on vascular endothelial cells. Our smoke inhalation model in sheep showed that anti-L-selectin antibody significantly attenuated the airway obstruction. However, the anti-P-selectin antibody did not attenuate lung injury, suggesting that the inhibition of the early phase of interaction between neutrophils and endothelial cells is not beneficial. Mercaptoethyl guanidine, an iNOS inhibitor, also inhibits airway obstruction. The inhibition of ONOO- formation by iNOS inhibitor will also reduce the endothelial damage and improve the vascular permeability. NOS inhibitors may also inhibit the activation of PARP and the following necrotic changes in epithelial cells, because ONOO- is a potent activator of PARP.
The activity and number of mucus gland cells (goblet cells) seems markedly reduced by smoke inhalation. We have investigated the effect of aerosolized acetylcysteine, which lyses mucus plugging, in the smoke inhalation plus burn model in sheep and in patients. We have found that the airway obstruction and gas exchange are not attenuated by acetylcysteine nebulization; however, if the acetylcysteine is combined with heparin it is effective in raising arterial blood oxygenation and reducing other markers of lung injury.
| Activated leukocyte-mediated tissue injury |
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formation by inhibiting xanthine oxidase, was not shown to be effective in our inhalation injury model (2). Because xanthine oxidase is activated by ischemia-reperfusion in the tissue, oxygen radicals from injured tissues do not seem to be important in the pathophysiology of smoke inhalation injury.
Vitamins C and E are known to scavenge oxygen radicals. As a result, the effects of both of these on burn and inhalation injuries are currently being investigated in clinical trials. There are several isoforms of vitamin E (tocopherol), designated
, ß,
, etc. We have found that plasma levels and lung tissue content of
-tocopherol are decreased 48 h after smoke inhalation injury. Lung levels of
-tocopherol were reduced by 50%. We have also noted that, when sheep suffer a combination injury of smoke inhalation and burns, the drop is more severe. In addition to its oxygen-scavenging capacity,
-tocopherol also has a potent binding capacity for NO. Evidently, the lung is exposed to the oxidative stress as well as excess NO and its metabolites.
Neutrophil elastase is a protease contained in the granules of neutrophils. Neutrophil elastase digests various proteins because of its low enzymatic specificity. When neutrophils are strongly activated, neutrophil elastase is released in the extracellular space. Although there is a mechanism through which the
1 protease inhibitor (
1-PI) immediately binds to elastase and inactivates it, reactive oxygen species, which are also released from activated neutrophils, break
1-PI. Therefore, elastase injures tissue without being inactivated by
1-PI when oxygen radicals coexist.
Gabexate mesilate is a synthetic serine protease inhibitor that attenuates coagulation abnormalities or pancreatitis in animals and humans. We have previously reported that this protease inhibitor attenuates smoke inhalation injury in sheep (10). More recently, it has been noted that gabexate mesilate inhibits elastase release and oxygen radical production (9). We believe that this is one of the mechanisms of action of the drug in reducing smoke inhalation injury. Recently, specific neutrophil elastase inhibitor ONO-5046 (Ono Pharmaceutical, Osaka, Japan) was approved for the treatment of acute respiratory distress syndrome in Japan. Because neutrophil elastase is involved in inhalation injury, this compound would be one possible candidate for the treatment of smoke inhalation.
| Other factors |
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has been hard to detect, but an increase in IL-8 has been shown. In addition, there are some reports demonstrating that the anti-IL-8 antibody attenuates inhalation injury, suggesting that IL-8 plays a role in the pathophysiology. Various neurotransmitters (e.g., bradykinin, neurokinin, tachykinin, etc.) are released by smoke inhalation injury. We are currently studying these neurotransmitters in our ovine model to ascertain what, if any, role these play in the pathophysiology of smoke inhalation injury.
Eicosanoids are additional inflammatory mediators considered to be involved in smoke inhalation injury. Nonspecific cyclooxygenase (COX) inhibitors, specific COX-2 (inducible form) inhibitors, and thromboxane antagonists may attenuate the inhalation injury, but there are reports that both support and reject the involvement of thromboxane. Thus for the time being, this hypothesis is still hotly debated. Hopefully, the involvement of thromboxane in smoke inhalation injury can be confirmed in the near future.
| Conclusions |
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The quest to develop effective therapeutic interventions for the treatment of smoke inhalation-injured patients is an ongoing task. From studies conducted in our laboratory using our ovine model, we have gained several important insights into the pathophysiology of smoke inhalation injury and are currently investigating a number of possible treatment options, including the use of specific inhibitors of iNOS and the application of aerosolized anticoagulants, for use in sheep challenged with smoke inhalation plus burn injuries or pneumonia. We hope that these studies will yield further advances in the general understanding of the pathophysiology of smoke inhalation injury and lead to more efficacious treatments for this condition.
| References |
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