Ischemia-reperfusion injury (IRI) is a multifaceted pathological process that complicates the perioperative management of patients undergoing lung transplantation (Fig. 1). The most severe form may lead to primary graft failure and remains a significant cause of morbidity and mortality after lung transplantation. Many efforts have been conducted on the better understanding of IRI to reduce the incidence of the primary graft failure . Organ ischemia begins with an imbalance between metabolic supply and demand and ends with tissue hypoxia, ultimately leading to cellular damage or death. The ultimate treatment is the restoration of adequate organ perfusion, but reestablishing the perfusion of the ischemic lung also carries inflammatory cells/mediators and reactive oxygen species (ROS) that promote further injury . IRI usually presents with the immediate impairment in lung function after transplantation accompanied by rapid development of pulmonary edema, increased pulmonary vascular resistance, and decreased airway compliance. Patients with IRI require prolonged mechanical ventilation with greater hospital stays, and furthermore, IRI becomes a risk factor for late graft failure, such as bronchiolitis obliterans . Autoantibody formation has also been reported after primary graft dysfunction, which is mostly caused by IRI . Thus, it is essential to understand the mechanism of lung IRI for its prevention and treatment, and this article will focus on pulmonary IRI related to lung transplantation.
TEXT OF REVIEW
Scope of the clinical problem
The annual number of lung transplantation worldwide has increased since the first successful operation by the Toronto Group in 1983. To date, more than 45 000 lung transplantations have been performed all over the world. The 5-year survival rate after lung transplantation is more than 50% according to an international survey , being almost satisfactory as a therapeutic option for end-stage respiratory failure with no other options. However, various problems need to be solved, including primary graft failure, acute or chronic rejection, infection due to immunosuppression, surgical complications, and secondary malignant tumors . Among them, primary graft failure occurs in the early postoperative days and represents one of the most frequent causes of early mortality during the first 30 days . In most cases, primary graft failure is caused by IRI, that is, injury due to the interruption (ischemia) and reopening (reperfusion) of the blood flow to the organ. Therefore, a further understanding of the mechanism of IRI and the strategies to reduce IRI will improve the function of the transplanted organ and the outcome of lung transplantation itself.
Effect of ischemic storage
In lung transplantation, hypothermia is essential during organ storage. As hypothermia decreases metabolic rate, biochemical reactions are reduced and the rate of degradation of essential cellular components necessary for organ viability is reduced . However, in hypothermic organ preservation, the function of the Na+ pump (Na–K ATPase) decreases. Na+ and Cl− flow with water from the extracellular space into the intracellular space according to the gradient of ion concentration, resulting in cellular edema. In the field of organ preservation, saccharides are usually considered to prevent this cellular edema by acting as an impermeant and energy source during ischemia. However, the best saccharide to use in organ preservation solutions has not yet been identified, although monosaccharide (glucose), disaccharides (trehalose), and trisaccharides (raffinose) are used in lung preservation . Intracellular calcium overload also occurs during ischemia, which in turn disrupt many intracellular processes causing cellular damage. During ischemic storage, ROS are produced via two important mechanisms, leading to oxidative stress . One mechanism results from the accumulation of hypoxanthine, which is the ATP degradation product due to ischemia, and the other depends on the NADPH oxidase system, which is present mainly on the membrane surface of neutrophils and monocytes/macrophages . Furthermore, cessation of blood flow represents a physical event that is sensed by the pulmonary endothelium leading to a signaling cascade that has been termed ‘mechanotransduction’. Stop of flow is sensed by a mechanosome, which is located in the endothelial cells, leading to activation of NADPH oxidase to generate ROS and to activation of nitric oxide synthase to generate nitric oxide. Increased nitric oxide causes vasodilatation and ROS provides a signal for revascularization; but in lung transplantation overproduction of ROS and nitric oxide can cause oxidative injury and/or activation of proteins that drive inflammation and cell death [9▪].
Consequences of ischemia and reperfusion
Most studies suggest that it is the process of reperfusion of the graft, and that it is not the ischemia per se, that plays a more important role in causing injury . The consequences of IRI are diverse and include generation of ROS, leukocyte activation/recruitment, complement and platelet activation, upregulation of cell surface molecules, increased procoagulant activity, and release of proinflammatory mediators [10▪]. These reactions are increased considerably in the lung after reperfusion. IRI is biphasic, with an acute injury characterized by macrophage activation followed by a neutrophil-dependent injury .
Current strategies to prevent ischemia-reperfusion injury
Various strategies have been clinically conducted to reduce the IRI after lung transplantation. The greatest efforts have been done in the method of lung preservation and reperfusion.
As primary graft failure due to IRI and donor shortage are common in lung transplantation as well as in the transplantation of other organs, the development of a highly effective and reliable organ preservation solution would contribute to improve the function of transplanted organs and to alleviate the shortage of donor organs. Since its first clinical application for renal transplantation in the 1960s, Euro-Collins solution has been used for lung transplantation. Additionally, University of Wisconsin solution has also been utilized in lung transplantation. Both Euro-Collins and University of Wisconsin solutions are intracellular type preservation solutions, whose higher potassium levels might result in the constriction of pulmonary arteries. To produce a more reliable preservation solution, extracellular-type solutions with low potassium levels were developed and have clinically been used worldwide . Furthermore, another solution, whose sodium concentration is 100 mEq/l and potassium level is 43.5 mEq/l, was also developed and has been used in Asian countries with favorable outcomes [7,13▪]. Although various preservation solutions are currently used for lung transplantation, no multiinstitutional randomized controlled studies regarding the comparison of the preservation solutions have been performed to date .
Retrograde flush refers to the administration of the flush solution through the pulmonary veins and drainage through the pulmonary artery. As a retrograde flush was experimentally found to improve lung preservation in comparison with an anterograde flush, this procedure has been performed in addition to the conventional anterograde flush in most lung transplant programs. In some centers, a retrograde flush is conducted in situ at the donor hospital, whereas a late retrograde flush is performed at the recipient hospital in others. Recently, a retrograde flush is reported to be more protective than heparin even in the uncontrolled donation after circulatory death lung donor .
Controlled perfusion and protective ventilation
As the progressive reintroduction of blood flow during the initial period of reperfusion has been shown to reduce lung injury and to improve function of the transplanted lung in several experimental settings , the so-called controlled reperfusion is currently performed in most lung transplant centers. Furthermore, a protective mode of ventilation after reperfusion prevented IRI and ventilator hypercapnia is recommended . According to an international survey comprising a total of 149 individuals from 18 countries [17▪], most respondents reported using lung-protective approaches for mechanical ventilation after lung transplantation. Low tidal volumes based on recipient characteristics were frequently chosen. Donor characteristics often were not considered and frequently were not known by the team managing mechanical ventilation after lung transplantation. To identify ideal ventilator strategies after lung transplantation, a multicenter randomized controlled trial is required .
Recent findings and future possibilities
To date, various studies have been conducted experimentally and introduced to the clinical world , and some of the ones with promising results are described here.
Ex-vivo lung perfusion
To resolve the shortage of brain-dead donors, marginal donor lungs and lungs donated after cardiac death (DCD lungs) have begun to be used . The Toronto group also reported results of a sensational clinical trial that used EVLP to evaluate the function of marginal donor and DCD lungs . Recently, it has been shown that EVLP could even recondition the marginal donor and DCD lungs . As of now, various experimental studies have been performed and several clinical trials are still going on using EVLP, and so the true meaning of the utilization of EVLP in clinical lung transplantation will be clarified in the near future [22,23▪].
Inhaled beta-2 adrenoreceptor agonist
In humans, beta-2 adrenoreceptor agonists injected intravenously or aerosolized and administered through the airway were reported to have protective effects against high-altitude lung edema, acute respiratory distress syndrome, pulmonary edema after lung resection, and pulmonary edema of discarded lungs from brain-dead donors in an ex-vivo model. Inhalation offers a lung-specific route for drug delivery and inhaled beta-2 adrenoreceptor agonists have already been reported to have protective effects against IRI in both ex-vivo lung perfusion and lung transplantation models using small and large animals . The mechanism by which the inhalation provides protection can be inferred from the maintaining the cAMP levels and adenine nucleotide levels and from inactivating inflammatory cells and reducing cytokine production.
Thrombosis of grafted lungs is inevitably associated with high resistance and inadequate perfusion. Heparin is often used when recovering donor lungs, and whereas heparin can prevent new fibrin formation, it does not have a lytic effect on preexisting donor thrombi. Therefore, fibrinolytic treatment of lung grafts using plasminogen activators, such as urokinase and tissue plasminogen activator, has been performed. More recently, plasmin administration during ex-vivo lung perfusion was found to dissolve thrombi in the lungs, resulting in reconditioning of the lungs as assessed by various physiologic parameters and alleviating pulmonary IRI [25,26▪].
Surfactant proteins play important roles in maintaining the microstructural integrity of the lungs by providing low surface tension at the air–liquid interface and preventing alveolar collapse. Surfactant proteins decrease during ischemia, leading to alveolar wall damage [27▪]. Surfactants have been administered in clinical settings to patients with severe primary graft dysfunction after lung transplantation and for donor lungs. Despite various experimental and clinical studies, only a small number of studies has demonstrated a therapeutic role for surfactant therapy in managing lung IRI [28,29]. In the future, larger, randomized controlled studies are required to further elucidate optimal dosing, timing of treatment, and routes of delivery.
In the current protocol, the lung graft is preserved at a temperature of 4 °C for an acceptable ischemic time of 8 h. Theoretically, a lower temperature, namely below 0 °C, has been thought to be desirable for organ preservation because of the lower rate of metabolism. Supercooling is a nonfreezing state of liquid below the freezing point, and the new development of a refrigerator for supercooling has now made it possible to preserve organs at subzero temperatures in a nonfrozen state without cryoprotectants. Lungs stored using this new supercooling method of lung preservation showed better organ function than conventional storage at 4 °C . A novel subzero preservation technique that combines ex-vivo machine perfusion with cryoprotectants to facilitate long-term supercooled preservation of the liver has recently been reported . The combination of supercooling technique and EVLP will be expected for the better lung preservation techniques.
There are several therapeutic gases for the possible prevention and treatment of lung IRI after lung transplantation. First, inhaled nitric oxide, an endogenously derived molecule synthesized by nitric oxide synthase, has been used in most lung transplant programs after lung transplantation. Several clinical studies failed to decrease pulmonary edema formation and resolution after pulmonary reperfusion and so some experts argue that while there does appear to be a role for inhaled nitric oxide in patients with lung IRI, prophylactic use does not appear to change outcomes . When the diagnosis of lung IRI has been made, many clinicians use inhaled nitric oxide for the beneficial effects including reduction of pulmonary vessel resistance, support of right ventricular function, and improvements in ventilation–perfusion mismatch . In addition, lung transplantation causes pulmonary vascular dysfunction via upregulation of inducible nitric oxide expression and inhibition of inducible nitric oxide synthase reverses the posttransplantational pulmonary vascular dysfunction , which supports the importance of controlling nitric oxide levels in lung transplantation.
Secondly, carbon monoxide is famous for toxic gas for its lethal poisoning, but a low concentration of carbon monoxide has proven to be efficacious in several animal models as well as clinical settings involving IRI, shock, and organ transplantation . Endogenous carbon monoxide, which is produced from heme degradation via heme oxygenase enzymes, has various functions, such as antiinflammatory effects through upregulation of potent antiinflammatory cytokines and antiapoptotic effects via upregulation of hypoxia-inducible factor-1alpha (HIF-1α). Beneficial effects of inhaled carbon monoxide to transplanted lungs in a swine model in addition to in rodent models have been reported. In contrast carbon monoxide could protect against IRI through induction of HIF-1α, whose degradation reportedly reduced lung edema and inflammation . More studies are needed to more clearly define the role of therapeutic carbon monoxide.
Thirdly, the antioxidative, antiinflammatory, and antiapoptotic effects of hydrogen therapy in various models, such as sepsis, multiple organ dysfunction syndrome, and organ transplantation have been studied . Now, the potential of hydrogen gas is also expected as a new therapeutic strategy against lung IRI not only by its working as a radical scavenger but also inducing substances against antioxidant stress through Nrf2 signaling . In addition, hydrogen preconditioning during ex-vivo lung perfusion is also reported to improve the quality of lung grafts in rats [37▪,38]. More recently, it has been reported that the combination therapy with nitric oxide and hydrogen provides enhanced therapeutic efficacy for pulmonary IRI in a murine model [39▪].
Lastly, hydrogen sulfide (H2S), which is a known environmental toxin, is also widely known for its significant role in inhibition of mitochondrial oxidation through blockade of cytochrome oxidase. H2S also activates potassium-dependent ATP channels, leading to vasodilation and ischemic preconditioning. Evidence on protective effects of H2S against the lung injury has been accumulated , but more studies are required for the clinical application.
Mesenchymal stromal/stem cells
The application of mesenchymal stromal/stem cells in animal models of IRI shows antiinflammatory and antiapoptotic effects, particularly for damage to the kidneys, heart, and lungs . Infusion of mesenchymal stem cells protected lung transplants from IRI in mice [42▪]. In addition, mesenchymal stromal/stem cells may mediate immunomodulatory effects on the innate and adaptive immune processes triggered during reperfusion and reduce fibrosis. Although few clinical trials have reached completion, adverse effects appear minimal .
Numerous strategies have been conducted to reduce the IRI after lung transplantation both from the experimental and clinical aspects. Despite advances in donor management and graft preservation, the pathophysiology of lung IRI remains incompletely understood. Recently, EVLP has been clinically introduced and new therapeutic modalities are going to be applied to the clinical field. IRI is one of the most critical phenomena in lung transplantation and therefore more studies to control pulmonary IRI are required for improving the outcomes of lung transplantation.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
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