Lung transplantation is the only viable option for patients with an end-stage lung disease such as chronic obstructive pulmonary disease (COPD), idiopathic pulmonary fibrosis (IPF), cystic fibrosis, and primary pulmonary hypertension. Although the survival rate could be increased after lung transplantation, many patients die while waiting for transplant due to lack of organ availability. It is estimated that lungs used for transplant are recover in only 15–25% of organ donors.1 The demand for lungs far outweighs those obtained from donors. From all the possible strategies to increase the available lungs for transplant, ex vivo lung perfusion (EVLP) is considered a technique with a high potential for clinical application in lung transplantation.
Ex vivo lung perfusion is a novel approach in which nonviable lungs from a donor are perfused and evaluated. It has been proposed as a transitional step to transplantation, to improve the use of lungs that were marginal or otherwise not recovered for transplant. This review will focus on the history, rationale, common techniques, and clinical trials of EVLP. Additionally, we will explore the potential future of EVLP in the field of lung transplantation.
The First EVLP Transplant
In 2000, the pioneering team led by Dr. Steen, at Lund University Hospital in Sweden, successfully performed the first EVLP transplantation. A 19-year-old man suffered from severe head and thoracic injuries after a motor accident. The patient arrived at the hospital in a coma and was later declared a brain-dead donor. His right lung was severely injured with intraparenchymal hematoma. His left lung had limited hemorrhagic areas on the lower lobe.
The left lung was placed on the EVLP circuit for reconditioning, using what is known as the “Lund protocol,” which is the original setting for all the subsequent EVLP protocols. As a consequence of the perfusion, pulmonary artery pressure decreased from 12 to 7 mm Hg. The gas mixture used was adjusted to 7% carbon dioxide (CO2) and 93% nitrogen (N2). After evaluation on the circuit, the lung was found to have good oxygenation and compliance and was deemed acceptable for transplantation.
The recipient was a 70-year-old man with COPD who had been on 24-hour oxygen therapy for 3 years. Six weeks after transplant, the patient left the hospital in good condition. The recipient had no primary graft dysfunction and showed no signs of acute rejection. Eight months later, the patient was diagnosed with Epstein-Barr virus (EBV) infection, which led to sepsis and his death 11 months later. Given that EBV infection is a common posttransplant complication, it was concluded that his death was unrelated to the use of EVLP in the procedure.2
Rationale for EVLP
Ex vivo lung perfusion is a potential strategy to aid transplant by potentially expanding the donor pool. More than 80% of donor lungs are potentially injured or infected and are therefore considered unsuitable for transplantation. With the use of EVLP, the retrieved donor lung can be perfused, offering a further opportunity to reassess its function before transplantation.3 The downstream effect of this is giving the clinician improved confidence in the transplantation of a lung or lungs that otherwise may have been rejected. Ex vivo lung perfusion allows lungs undergoing perfusion to be kept in physiologic status before transplantation. It also offers the possibility of studying preconditioning methods and protection of the pulmonary graft to reduce the subsequent inflammatory and immune insults after transplantation1 and may function as a preclinical model for lung disease.
The EVLP Circuit
The basic design of the EVLP evolved from the original Lund protocol developed at the University of Lund by Dr. Steen’s group. In this system, by contrast with what is used in other systems, the pulmonary venous return is collected in a sterile reservoir either through an open left atrium and then recirculated.
The EVLP circuit has several components that help perfuse, evaluate, and potentially recondition lungs of questionable quality (Figure 1). The circuit consists of a centrifugal/roller pump (1) that circulates the perfusate while passing through the membrane gas exchanger (2). It has a filtered gas line (4) for the gas exchange membrane connected to a tank that contains oxygen (6%), carbon dioxide (8%), nitrogen (86%), and a leukocyte depletion filter (3). This is all in place before the perfusate enters the pulmonary artery. The system also has a temperature probe (8) and a heater/cooler (5), which warms the perfusate to the desired temperature. The outflow perfusate returns through the left atrial cannula (or open atrium) to the reservoir (7). The very last component of the circuit is the ventilator (9). The overall goal of this circuit is to mimic conditions in the human respiratory system, giving the lungs an opportunity to be evaluated and allowing physicians time to further evaluate the lung condition.
Although the concept of EVLP is straightforward, there are a number of techniques used for EVLP, all derived from the Lund protocol. In addition to creating the basis for the setting of the EVLP, Dr. Steen’s group developed the Steen solution, which is used on different EVLP designs. The Steen solution is a buffered solution with high concentrations of albumin, which creates an osmotic gradient that allows the flow to be maintained with a low risk of developing pulmonary edema.
The Toronto technique, developed at Toronto General Hospital, is well known and widely used. This technique aims for a gradual increase in temperature within the first 30 minutes on the circuit, before ventilation is started. The lungs are completely submerged in a cold preservation solution before being placed on the circuit. Funnel-shaped cannulas are attached to the left atrium and pulmonary artery, and the lungs are transferred into the EVLP chamber. Flow is started at 150 ml/min, which is 40% of cardiac output in a closed system.5 The temperature is gradually increased from room temperature to 37°C over the next 30 minutes, after which ventilation is started. Steroids, antibiotics, and heparin are added to the perfusate before EVLP initiation. Recruitment lung maneuvers are performed every hour to an airway peak pressure of 25 cm H2O, along with assessment of the plateau pressure and both static/dynamic compliance. Ex vivo bronchoscopic examination and radiographs of lungs are performed every 2 hours, and partial change (250 ml) of fresh perfusate solution is performed every 2 hours.6 The lungs undergo functional assessment after 4 hours for transplantation into the recipient.
The Normothermic organ care system is another EVLP technique used. This portable system provides perfusion, ventilation, and monitoring to maintain the organ’s near-physiologic state. It also allows the organ to be perfused in transit to its recipient. This technique is different from the others in that the lungs are placed on the perfusion device directly after harvesting, whereas in other methods, the lungs are placed on ice before being connected to the EVLP. Another distinctive characteristic of this technique is the makeup of the perfusate solution, whereas other systems use human albumin, the normothermic EVLP system has a low potassium dextron-40–based solution without albumin. Red blood cells up to a hematocrit of ±15–20% (either packed or full blood) are also added to the solution.1,6
Current Trials and Studies
There are several current and past trials evaluating EVLP because of its promising future in aiding lung transplantation. According to the Clinicaltrials.gov, 18 institutions in the United States are involved in clinical trials (Table1), and there are 15 registered clinical trials of which eight are actively recruiting patients. The normothermic ex vivo lung perfusion, HELP trial is a nonrandomized controlled trial conducted by the Toronto group.5 In this trial, 16 recipients were transplanted using EVLP, along with a control group of 86 patients undergoing standard transplantation. One year after the transplants, the survival rates between the EVLP group and the control group were considered comparable. Furthermore, they showed similar survival rates at 3 years. Because of the success of the trials, the field has been energized to move forward with international acceptance of the technique. Furthermore, the concept of organ repair centers, where EVLP lungs can be treated and recovered, has been discussed. Additionally, the technique has been used for pioneering research aimed at understanding how we can improve and better analyze EVLP lung function for transplantation.
Another EVLP trial underway is the normothermic ex vivo lung perfusion (Evlp) (NOVEL) lung trial being conducted with the goal of acquiring Food and Drug Administration (FDA) approval for EVLP clinical use. It is a two-arm, nonrandomized, open-label study. The purpose of the study is to evaluate the 30 day mortality rate of patients undergoing lung transplant with lungs treated with EVLP from marginal or extended donors and compare these findings to normal lung transplants. This trial proposes using the EVLP technique to improve donor lung assessment before transplant and safely increase the number of available lungs for transplant. For the donor to be considered for the trial, his or her the partial pressure of oxygen (PaO2)/ fraction of inspired oxygen (FiO2) needs to be less than or equal to 300 mm Hg, or they must have one of the following risk factors: multiple blood transfusions, pulmonary edema, donation after cardiac death (DCD), or poor lung quality. There were important differences between the lungs accepted for this study. For example, the average partial pressure of oxygen (PO2) for the EVLP lungs was 350 mm Hg, whereas the average PO2 for the standard lungs was 435 mm Hg. In the end, the study found that there were no significant outcome differences between regular lung transplants and transplants done using EVLP (Figure 2).
The donor ex vivo lung perfusion-UK (DEVELOP-UK) is a nonrandomized trial currently taking place in the United Kingdom. The findings for this trial have not yet been published, but its purpose is to investigate the safety of EVLP in the hope of proving its noninferiority. It will also examine the cost–effectiveness of EVLP (UK Clinical Research Network).
DCD Lungs: The Perfect Application of EVLP
As mentioned earlier, a large obstacle in the way of transplant is the number of acceptable donor lungs. Cardiac death is a significant contribution to the number of DCD lungs per year. One approach in helping to increase the donor pool has been to use lungs retrieved from DCD, otherwise known as DCD lungs. Reports suggest short-term survival rates with these patients due to insufficient evaluation time for the retrieval team to make a full assessment. Ex vivo lung perfusion is the optimal solution to improve this situation.8
Treatment Interventions and the True Potential of EVLP
The use of specific perfusate solutions is important for the resolution of pulmonary edema. Pharmacologic agents are added to remove pulmonary edema during EVLP by enhancing alveolar fluid clearance. This exerts antiedemic affects by stimulating sodium channel activity and increasing the abundance of the β- and γ-epithelial sodium channel in the plasma membrane of alveolar epithelial cells.
Gastric aspiration can also potentially be improved with EVLP. Several attempts have been made through EVLP to investigate further methods to decrease lung injury, including gastric aspiration. It has been shown that 2 hours of EVLP can be used on lungs damaged by gastric acid.9 Additionally, EVLP can be used to detect the features of gastric aspiration in the lungs.10
Ex vivo lung perfusion provides a platform for the treatment of infected donor lungs with the potential result of decreasing post lung transplant pneumonia. Several groups have studied the use of antibiotics during EVLP and demonstrated a significant reduction in the bacterial load of donor lungs after a high dose of antibiotics.11–13
Ex vivo lung perfusion can also help reduce lung inflammation. This can be done by adding anti-inflammatory drugs, such as corticosteroids, β-1-antitrypsin, inhibition of nuclear factor κB, and activated protein to the perfusate. Lung inflammation is measured during perfusion at several time points by means of analysis of the bronchoalveolar lavage (BAL) fluid and bronchial biopsies.5
Donor lungs are rejected because of poor function, and sometimes because of unrecognized pulmonary embolism. In the swine model, the use of fibrinolytics is a strategy that has been proven to help with pulmonary emboli, and it is very important in DCD donors, where donor heparinization might not be possible. Adding urokinase to the perfusate can improve graft function, with reduced pulmonary vascular resistance and improved oxygenation after 3 hours.14
Organ Care Recovery Centers: The Future of Organ Transplantation?
Lung Bioengineering is the first and only commercial provider of clinical EVLP services in the world. The facility consists of a control center, a lab area, and procedure rooms, where they utilize the Toronto protocol to perform EVLP. Currently, they are operational and performing EVLP as part of a phase II clinical trial. As regards potential future use, the lungs may be removed at the donor hospital and then sent to a center, such as Lung Bioengineering, for perfusion and later transported to the transplant hospital for transplant (Lung Bioengineering).
Other Applications of EVLP
Ex vivo lung perfusion is not only a way to improve unacceptable lungs for transplantation but also a research tool and a preclinical model for new therapies.
Stem cell therapy with mesenchymal stem cells (MSCs) has been a recent focus of study for EVLP because it has proven valuable for cell-based therapies by presenting a unique therapeutic opportunity to administer MSCs to lung grafts before transplantation. A recent study demonstrated that treatment with allogeneic human MSCs or its conditioned medium, administered 1 hour after endotoxin-induced lung injury, reduced extravascular lung water, improved lung endothelial barrier permeability, and restored alveolar fluid clearance.15 Another study, conducted by Mordant et al.,16 sought to determine the optimal route and dose of viable human umbilical cord MSCs to be delivered into ex vivo perfused damaged swine lungs. An intravascular delivery of 150 × 106 MSCs showed a preferred outcome compared with intrabronchial delivery. Dose-escalation strategies revealed that the optimal-tolerated dose was 5 × 106 human MSCs per kilogram of animal weight. Furthermore, the effect on the concentration of growth factors and inflammatory mediators was evaluated in the study. An increase in human vascular endothelial growth factor (VEGF) concentration in the lung tissue and a decrease in interleukin (IL)-8 levels in the perfusate were observed after the cell treatment.16
Lobar EVLP may be another possible use for EVLP in the near future. Along with lung shortage, lung size is another key reason for rejecting an organ for a particular recipient. Often, lungs are rejected because the size mismatch to the recipient. Ex vivo lung perfusion may offer the opportunity to tailor in advance the lungs to the size of chest of the recipient which could result in more successful transplants.
Biomarkers could potentially refine diagnostic accuracy by objectively confirming good prognostic lungs or identifying suboptimal lungs during the early stages of EVLP.17 This could prevent high-risk transplants and identify/single out the lungs that will require further ex vivo injury-specific treatment before transplant. A study conducted by Machuca et al.17 reveals changes in cytokines, chemokines, and growth factors at the protein level in high-risk EVLP donor lungs intended for transplant. The specific analytes found, with biological plausibility to be used as biomarkers, include IL-8, CXCL1, macrophage inflammatory protein (MIP)-1α, MIP-1β, and granulocyte-colony stimulating factor (G-CSF). In this study, IL-8, a cytokine associated with the pathogenesis of acute lung injury, was found to be the most discriminative biomarker. Interleukin-8 was the only biomarker significantly elevated at both 1 and 4 hours of EVLP and offered the best discrimination from control cases. These findings suggest that the addition of specific biomarkers to the EVLP assessment could improve the precision of donor lung selection and, furthermore, provide the ability to safely increase the utilization of injured lungs for transplant in future therapeutic studies.
Ex vivo lung perfusion has an exciting future in aiding lung transplantation by means of expanding the donor pool, thus allowing more lungs to be transplanted. Ex vivo lung perfusion could provide a solution to one of the major challenges in organ transplantation and continues to be a major subject of international research. Ex vivo lung perfusion has the potential to become “a benchmark in health-care” in transplantation in the near future because of its possible ability to turn unacceptable lungs into acceptable lungs ready for transplantation.
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