172: Ex Vivo Lung Perfusion

Lung transplantation is a life-saving therapy for patients suffering from end-stage lung diseases. The number of patients waiting for lung transplantation, however, greatly exceeds the number of donor lungs available. Moreover, only about 15% of lungs from multiorgan donors are deemed usable for transplantation.¹ Most potential lungs are considered unsuitable as the result of injury that occurs with brain death and ICU-related complications (i.e., barotrauma or lung edema associated with fluid resuscitation). Since primary graft dysfunction leads to severe early and long-term consequences for lung transplant recipients, transplant teams tend to be very conservative in their selection of donor lungs. As a result, fewer organs are selected and the wait list mortality may climb as high as 30% to 40%.^2,3 While the current cornerstone of clinical lung preservation is to limit the metabolic rate of the donor lung by hypothermia, this strategy best serves lungs meeting ideal acceptance criteria. The current donor organ shortage has prompted most donor programs to use increasing numbers of extended criteria organs, where lung function is not as assured as with an ideal lung. The ex vivo phase of organ preservation, prior transplantation into the recipient, provides a window of opportunity for further evaluation and even resuscitation of these compromised lungs. To take advantage of this opportunity, however, it is necessary to preserve the donor organs under normothermic or near-normothermic conditions. One strategy, termed ex vivo lung perfusion (EVLP), attempts to simulate in vivo conditions through ventilation and perfusion of the donor lung graft. One important advantage of normothermic perfusion is that it maintains the active metabolic functions of the lung, providing an opportunity for continued assessment of the organ during the ex vivo phase of organ preservation, active treatments, and restoration of normal function. This chapter focuses on the resurgence of EVLP, important technical aspects, clinical outcomes, and future perspectives.

Normothermic EVLP—Historical Perspectives

Originally proposed in 1938 by Carrel for organs in general and then in 1970 by Jirsch et al. for the evaluation and preservation of lungs in cases of distant procurement, attempts at EVLP in those eras failed because of an inability to maintain the air/fluid barrier integrity within the lung, which led to the development of edema and increased pulmonary vascular resistance in the donor lung.^4,5 For most of the past century, EVLP systems were used in small animal studies of lung physiology. More recently, normothermic perfusion became a research focus as a preservation alternative in experimental models of lung, liver, and kidney transplantation.^6–11

Experimental Work

The resurgence of EVLP as a potential tool in lung transplantation started with the work of Steen et al.¹² This group described EVLP as a method to reassess lungs after uncontrolled donation from cardiac death donors (DCD),¹³ since these organs cannot be evaluated in vivo. A lung perfusion specific solution (Steen^® solution) with optimal osmolarity and high dextran content was developed by this group.^12,14 After Steen's publications, other groups demonstrated the feasibility of blood-based short-term EVLP (30–90 minutes) to evaluate lung function in animal models of donation after cardiac death and experimentally using injured human lungs deemed unsuitable for transplantation.^15–21 However, even in this most current era of EVLP, attempts to perfuse ex vivo lungs for greater than 2 hours have largely failed, as extended perfusion times lead to the development of pulmonary edema and an increase in the pulmonary vascular resistance meaning that the system, itself, inflicts some degree of injury.²²

With the vision that ex vivo treatment and repair of injured organs would, in general, require an ex vivo period lasting longer than 2 hours, we developed a protective EVLP system and strategy with the goal of maintaining lungs in the EVLP system for at least 12 hours without causing additional injury to the lungs (currently known as the Toronto technique of EVLP, see details below). Several important modifications were made to achieve 12 hours of perfusion stability. These included (1) an acellular perfusate, (2) a closed circuit with protective low perfusion pressure (pulmonary artery pressure [PAP] 10–13 mm Hg) and stable positive left atrial (LA) pressure (5 mm Hg), and (3) a protective mode of mechanical ventilation (tidal volume of 7 mL/kg, rate of 7 breaths per minute [bpm], with a positive end-expiratory airway pressure [PEEP] of 5 cmH₂O). In our early work with normal pig lungs, lung function was stable during 12 hours of normothermic EVLP.²³ This stability during prolonged EVLP translated into excellent posttransplant lung function, absence of edema formation, and preserved lung histology after transplantation. The acellular perfusion assessment of lung function accurately correlated with posttransplant graft function, and the addition of red blood cells did not provide additional functional information compared to acellular perfusate alone.²³ This study provided the proof of concept that EVLP is able to maintain normal donor lungs for a prolonged period of time without damaging the organ. Further examination was then performed to determine the impact of prolonged EVLP using injured ischemic donor lungs.²⁴ Pig donor lungs were cold preserved for 12 hours and subsequently divided into two groups: cold static preservation (CSP) or normothermic EVLP for a further 12 hours (total 24 hours of preservation). EVLP preservation resulted in significantly better lung oxygenation and lower edema formation rates after transplantation when compared to CSP. Alveolar epithelial cell tight junction integrity, evaluated by zona occludens-1 protein staining, was disrupted in the cell membranes after prolonged CSP but not after EVLP.

The components and set up of the circuit are demonstrated in Fig. 172-1. The EVLP perfusion and ventilation strategy is shown in Table 172-1. The preparation of the donor lungs occurs on the back table with the lungs immersed in a CSP solution. Specifically designed funnel-shaped cannulas with built in pressure sensors (XVIVO cannulas, Vitrolife) are attached to the LA and pulmonary artery (PA). The lungs are then transferred from the back table to the XVIVO chamber. The PA cannula is connected to the circuit and anterograde flow (PA–LA) is initiated at 150 mL/min with the perfusate at room temperature. The temperature of the perfusate is then gradually increased to 37°C over the next 30 minutes. Before increasing flow beyond this level, a careful check of the system is made. The PA and LA pressure readings are double-checked. When a temperature of 32 to 34°C is reached (usually over 20 minutes), ventilation is started and the perfusate flow rate is gradually increased to the target flow (40% of estimated donor cardiac output) within 60 minutes. Once ventilation is started, the flow of gas (86%N₂, 6%O₂, 8%CO₂, Praxair) that will deoxygenate and provide carbon dioxide to the inflow perfusate via the gas exchange membrane is initiated (started at 1 L/min) and titrated to maintain inflow perfusate pCO₂ between 35 and 45 mm Hg. Steroids, antibiotics, and heparin are added to the perfusate prior to EVLP initiation. Recruitment lung maneuvers are performed every hour to an airway peak pressure of 25 cmH₂O. Ex vivo bronchoscopic examination and x-rays of the lungs are performed every 2 hours. Every 2 hours, 250 cc of perfusion solution is exchanged for a fresh perfusate.

Assessment of Lungs During EVLP

Current donor lung evaluation is a clinical process greatly dependent on the judgment of the surgeon. While some evaluation does occur prior to retrieval, that is., chest x-rays and ICU bronchoscopy, the majority of the evaluation leading to the decision of utilization occurs at a single time point—the time of organ retrieval. EVLP allows the decision of lung utilization to be made at a later time point in the transplantation process (which is very useful in the case of donors after cardiac death) and allows for the use of more objective parameters. During EVLP, lung functional parameters can be monitored carefully and trends in compliance and airway pressure can be detected over at least a few hours. Injury, as represented by the development of edema during EVLP, is reflected in changes in compliance and airway pressure and this precedes the effect on perfusate pO₂.²⁵ To reduce the effect of atelectasis that might occur during donor lung transport, the baseline time point should be 1 hour after warming the perfusate and after careful recruitment of the lung. All subsequent physiological measurements (compliance, airway pressures, PVR, and perfusate pO₂) can be compared to this time point. A cut-off, or normal value, is often sought for lung evaluation, but compliance and resultant airway pressure is based in part on lung volume and thus can fall within a large range of values. With this strategy, the trend of values becomes more important than the absolute values themselves. In general a P/F >400 mm Hg and stability or improvements of other functional parameters and ex vivo lung x-ray are required to accept the lungs for transplantation (Fig. 172-2). Perfusate biomarkers have now been extensively studied and will certainly assist in the overall organ assessment in the near future.

Clinical Outcomes with EVLP

The first clinical use of an EVLP system was described by Steen et al.¹⁴ in 2001 to briefly assess lung function from a donor after cardiac death. The same group reported their experience with 60 to 90 minutes of blood-based perfusion to assess six high-risk donor lungs prior to transplantation. These case reports showed acceptable outcomes; however, the mean time in the intensive care unit was longer in recipients of perfused lungs compared to conventional transplantation (13 vs. 7 days).^26,27

The first prospective clinical trial using EVLP was recently completed at the University of Toronto and the results were recently published.²⁸ In this study, 20 EVLP lung transplants were performed after 4 hours of EVLP using the acellular protective ventilation/perfusion strategy.^23,24 This trial demonstrated that extended acellular normothermic EVLP is safe for the assessment of high-risk donor lungs, and similar early outcomes were obtained compared to conventionally selected and transplanted donor lungs. This experience has been rapidly expanded and a more recent report at the American Association for Thoracic Surgery annual meeting demonstrated excellent outcomes in over 50 patients transplanted after EVLP assessment/treatment of brain death and cardiac death donor lungs. In this study, incidence of PGD 3 at 72 hours was 2% in EVLP and 8.5% in controls. Thirty-day mortality (4% in EVLP; 3.5% controls, p = 1.00) and 1-year survival (87% in EVLP; 86% controls, p = 1.00) were similar in both groups. Other groups have equally reported their promising experience with the technique, mostly in a form of published abstracts. The Vienna group recently reported very good outcomes after EVLP lung transplantation using initially unsuitable lungs.²⁹

Potentials of EVLP in Lung Transplantation

Normothermic preservation demonstrates great promise for resuscitating injured donor lungs. Given that the majority of potential donor lungs are injured by a variety of mechanisms including brain death, contusion, aspiration, infection, edema, and atelectasis, one can imagine that targeted therapies for each of these injuries could be delivered ex vivo for repair and greatly increase the donor lung pool. The requirements for perfusion for repair compared to perfusion solely for evaluation differs by the time requirements. While the majority of lungs can be evaluated within 2 to 4 hours of perfusion, repair will require longer noninjurious stable perfusion while potential treatments are administered.

Early studies in the use of EVLP for lung repair have been reported, many still only in abstract form. Each of these studies have been targeted at a different form of donor lung injury and it is this breadth of exploration that will ultimately result in an arsenal of ex vivo lung therapy techniques applicable to each uniquely injured donor lung. Pulmonary edema is a common injury in donor lungs due to brain death physiology and/or ICU fluid management prior to retrieval. Unlike the in vivo situation, use of terbutiline was found to accelerate the clearance of alveolar fluid during perfusion.³⁰ Another common mechanism of injury is aspiration. Inci et al.³¹ have attempted to improve porcine lungs injured by acid aspiration. By lavaging the donor lung with surfactant during EVLP, they were able to achieve improved graft function when compared with controls. A significant number of lungs are rejected for suspicion of infection or pneumonia, making the delivery of high doses of antibiotics during EVLP an attractive therapy. Both the Newcastle and Toronto groups have early data showing potential reduction in the burden of infection following EVLP antimicrobial therapy.^32,33

EVLP-based gene and cellular therapy have also been explored. We have shown that ex vivo gene therapy with an adenoviral vector is effective and additionally attractive because of the reduced vector-associated inflammation. Furthermore, this strategy can easily fit into the logistical flow of clinical lung transplantation, simplifying adoption.³⁴ We have demonstrated that EVLP-based IL-10 gene therapy of rejected human donor lungs resulted in improved function and reduced biomarkers of inflammation suggesting that IL-10 gene therapy could possibly increase the resilience of all donor lungs to reperfusion injury.³⁵ Lee et al.³⁶ have shown that the delivery of mesenchymal stem cells to EVLP lungs can restore endothelial barrier permeability and alveolar fluid balance after endotoxin-induced lung injury.

Finally, EVLP may be a strategy to minimize cold ischemia if the lungs are inserted in the circuit immediately after organ retrieval and transported on mobile EVLP systems. A clinical trial is currently under way in the international INSPIRE trial using the Organ Care Systems machine.^37,38 Compelling experimental and clinical data demonstrate that continuous mobile normothermic perfusion is superior to a combination of short intervals of cold ischemic preservation (currently used for transportation of the lungs to the EVLP site), but additional normothermic evaluation and treatment will be needed to justify the conversion to this strategy considering the logistical challenges and the added economic expenses needed for mobile normothermic perfusion of lungs.

Ever since the development of clinical lung transplantation, transplant clinicians and scientists have sought to reduce injury and maximize safe preservation time during the storage and transport of donor lungs. Key advancements in lung preservation in the form of hypothermia, inflated storage, and Perfadex flush have culminated in the maturation of lung transplantation into a standard of care for end-stage lung disease. Currently, the emphasis of lung preservation is shifting from slowing down organ death to that of facilitating organ recovery and regeneration prior to implantation. This has led to the emergence of normothermic EVLP as a strategy for lung preservation. Though EVLP is effective for lung preservation alone, its true potential lies in facilitating lung assessment, recovery, and repair. Development of an ex vivo treatment arsenal ranging in complexity from pharmacologic to gene and cellular therapies may one day allow for a personalized medicine approach to the donor organ. The personalized or targeted repair of donor lung injuries specific to each individual lung may finally allow clinicians to realize the full potential of the donor organ pool.

This chapter provides both historical and practical knowledge about this new technology with enormous potential to profoundly change the practice of clinical lung transplantation in the immediate future.