Lung transplants are increasing, with 2,562 performed in the United States in 2018 – a 31% increase over the preceding 5 years. With this increased demand for donor lungs, waitlist mortality in the United States is 9.4 deaths per 100 waitlist-years for obstructive lung diseases and as high as 29.7 deaths per 100 waitlist-years for restrictive lung diseases (Valapour M, et al. Lung.). Conversely, lungs are utilized from eligible multiorgan donors only 15% to 20% of the time, usually due to concerns over donor history or organ quality ( ). In light of this imbalance of supply and demand, lung transplant specialists are making significant efforts to expand the donor pool of available organs. Three of these strategies include: (1) applications of ex-vivo lung perfusion (EVLP) technology; (2) use of lungs from hepatitis C-positive donors for hep-C negative recipients; and (3) increasing utilization of donation after cardiac death.
Normothermic ex-vivo lung perfusion is a technology which allows donor lungs to be perfused and ventilated after removal from the donor but before transplant into the recipient. This is in contrast to the traditional method of cold static preservation. The proposed advantage of using this technology is to allow time for a more thorough assessment of graft quality and to improve function of grafts not meeting established criteria for transplant, all-the-while decreasing organ ischemia despite an increased cross-clamp time. There are currently four commercial systems available capable of EVLP. Broadly speaking, three EVLP management protocols exist (Toronto, Lund, and OCS), which differ in perfusate composition, target flow, pulmonary arterial pressure, left atrial pressure, and ventilatory settings. Notably, the Toronto protocol uses a closed left atrium, whereas the Lund and OCS protocol use an open left atrium. There are excellent published reviews of the different systems (Possoz J, et al.). EVLP has now been studied for two different goals: (1) to allow an extended evaluation of lungs of questionable quality before transplant; or (2) for routine use in all lung transplantations in place of cold static preservation.
In most studies concerning the use of EVLP for reconditioning of donor lungs, “high risk” or “extended criteria” refers to one or more of the following: P/F ratios < 300 on arterial blood gas, macroscopic abnormalities (eg, pulmonary edema, poor lung compliance), donation after circulatory death, or high-risk history (eg, aspiration). The largest cohort with the longest follow-up addressing the role of EVLP for donation of lungs with extended criteria was published from the Toronto Lung Transplant Group. Their results have demonstrated equivalent graft survival and rates of chronic lung allograft dysfunction (CLAD) up to 9 years posttransplant compared with standard criteria donor lungs, despite utilizing lower quality lungs and having a longer median preservation (Divithotawela C, et al.). The group’s subsequent lung transplant rates have increased over the past decade.
A separate study addressed the same question but differed in that it was a single-arm, multicenter, international trial that tracked the outcomes of 93 extended criteria lungs placed on EVLP (including a large proportion acquired via donation after circulatory death) (Loor G, et al.). Among these, 87% of eligible lungs were transplanted, and outcomes were excellent, albeit shorter in follow-up compared with the Toronto cohort (eg, primary graft dysfunction grade 3 (PGD3) within 72 hours was 44% and 1-year survival was 91%). Based on these trials and many other retrospective reports, it has been concluded by many experts in the field that EVLP-treated extended criteria donor lungs perform equally well to standard criteria donor lungs.
Two RCTs have been conducted to evaluate whether EVLP is noninferior to static cold storage with donor lungs meeting “standard criteria” for transplant. The first was a single center study at the Medical University of Vienna, that looked at 80 recipient/donor pairs. Lungs in the EVLP arm underwent 4 hours of perfusion with frequent reassessment of quality before transplant, whereas the lungs in the control arm went directly to transplant. This study met noninferiority criteria looking at primary outcomes of PGD grade >1 and 30-day survival (Slama A, et al.). The second study was a phase 3, multicenter, international trial that included 320 recipient/donor pairs randomized to either EVLP (without a prespecified time on the EVLP system) or static cold storage. This trial met noninferiority for safety endpoints (lung graft-related adverse events within 30 days) and a composite primary outcome of PGD grade 3 incidence within 72 hours and 30-day survival (Warnecke G, et al. ). The authors also tested and found superiority of EVLP in lower PGD grade 3 frequency compared with control. While these RCTs may suggest a role for EVLP in the procurement process of standard criteria organs in addition to extended criteria organs in the future, major criticisms for these trials include the lack of a demonstrable clinical benefit over cold storage beyond the lower PGD3 rates.
In the era of direct-acting antiviral agents available to treat HCV infection, there has been efforts to study the early use of anti-HCV medications to prevent infection as a result of heart or lung transplant from HCV viremic donors to HCV-negative recipients. In one major trial on efficacy, it was found that 4 weeks of sofosbuvir and velpatasvir, when started within a few hours of transplant, was sufficient to achieve a sustained (undetectable) virologic response at 12 weeks after completion of the antiviral regimen (Woolley AE, et al.). Therefore, many transplant centers have adopted protocols to increase the donor pool (by CDC estimates about 4% of solid organ donors are HCV-positive) by accepting HCV nucleic acid amplification test (NAT)-positive donors for HCV-negative recipients, after appropriate informed consent.
Donation after cardiac death (DCD), which is alternatively known as donation after circulatory determination of death (DCDD), generally refers to organ procurement taking place after cessation of circulation, often after inpatient withdrawal of support. This is in contrast to the much more common practice of donation after brain death (DBD). Addressing concerns over the quality of lungs donated in the context of DCD compared with DBD, analyses of ISHLT registry data have demonstrated no differences in hospital length of stay or survival at 1 or 5 years (Van Raemdonck D, et al.). Outcomes comparing specific mechanisms of donor death in DCD remain relatively unknown, such as outcomes from donors withdrawn from life support vs donors who had an uncontrolled cardiac death.
These methods for expanding the donor pool are not mutually exclusive, and, in fact, application of EVLP for lungs obtained in the context of DCD seems to be increasingly common. Optimization of protocols with collaboration between lung transplant centers will be paramount as we move forward in advancing this field. As we do so, efforts to successfully increase the donor pool will serve to provide a life-saving therapy to an ever-growing number of patients with end-stage lung disease.
Dr. Sala and Dr. Tomic are with the Division of Pulmonary and Critical Care Medicine, Northwestern University, Chicago, Illinois.