J-Hu L , X Xu , Y-F W , H-Y X , J-Yu C , N-Gu D ,Mts Bw , L-M X , Rrt V Pt Rr , H Y , H Z , k ,J-Z Z , k , F Hu , J-Y Y , H-J Y , Hu P , S-Gu L , k ,Y-B Q , J Lu , H-Yu L , Ju-Ju J p , H Yu p , H L , S-J Y ,H W , k , Z-Y Lu , k , L-C Z , k , X-Y Hu p , H Wu p , Y-Q Hu p ,P-Fu T , k , Q-F Y , Su-S Z , , p , ?

a Department of Hepatobiliary and Pancreatic Surgery, Department of Liver Transplantation, Shulan (Hangzhou) Hospital, Zhejiang Shuren University School of Medicine, Hangzhou 310022, China

b Department of Hepatobiliary and Pancreatic Surgery, Hangzhou First People’s Hospital, Zhejiang University School of Medicine, Hangzhou 310 0 0 0, China

c Zhongnan Hospital of Wuhan University, Institute of Hepatobiliary Diseases of Wuhan University, Transplant Center of Wuhan University, Hubei Key

Laboratory of Medical Technology on Transplantation, Wuhan 430062, China

d NHC Key Laboratory of Combined Multi-organ Transplantation, Hangzhou 310 0 03, China

e Wuxi Lung Transplantation Center, Wuxi People’s Hospital Affiliated with Nanjing Medical University, Wuxi 214023, China

f Department of Cardiovascular Surgery, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430022, China

g Peter Munk Cardiac Centre, Toronto General Hospital-University Health Network, Toronto, Canada

h School of Computer Engineering and Science, Shanghai University, Shanghai 2004 4 4, China

i Organ Transplant Center, Shanghai Changzheng Hospital, Shanghai 20 0 0 03, China

j Department of Orthopedics, Chinese PLA General Hospital, Beijing 10 0 039, China

k National Clinical Research Center for Orthopedics, Sports Medicine & Rehabilitation, Beijing 10 0 039, China

l Department of Surgery, General Hospital of Guangzhou Military Command of PLA, Guangzhou 510040, China

m Department of Liver Surgery, Liver Transplantation Center, West China Hospital of Sichuan University, Chengdu 610041, China

n Organ Transplantation Center, Sichuan Provincial People’s Hospital and School of Medicine, University of Electronic Science and Technology of China,Chengdu 610072, China

o Department of Lung Transplantation, The First Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou 310 0 03, China

p Division of Hepatobiliary Pancreatic Surgery, The First Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou 310 0 03, China

Introduction

Organ transplantation increases survival and improves quality of life to many patients with end-stage organ failure. Organ shortage is a worldwide problem that restricts organ transplantation [1] . Organ procurement and preservation as well as ischemia-reperfusion injury (IRI) after transplantation are the important factors affecting prognosis of recipients. Since the development of organ transplantation technology in the 20th century, organ protection technology has been a most promising concept in this field. Organ preservation solutions such as the Collins solution, University of Wisconsin (UW) solution, and histidinetryptophan-ketoglutarate (HTK) solution were developed sequentially [2] , which developed rapidly in static cold storage (SCS) techniques. SCS remains the standard preservation technique for organ transplantation [2] , but it invariably leads to a progressive decline in organ viability and function. It also precludes dynamic assessment of organ function to determine the adequacy for transplantation. As the requirements for the quality and duration of organ preservation have improved, adjustments are needed to address the increasing need for donor organs and to accept organs from extended-criteria donors. Based on specificity of each organ, different preservation solutions have also been developed, such as HTKN [3] , lung perfusion solution (Steen) [4] , and kidney perfusion solution (KPS-1) [5] . Machine perfusion (MP) techniques include normothermic machine perfusion (NMP, 32-37 °C), subnormothermic machine perfusion (SNMP, 20-32 °C), hypothermic oxygenated perfusion (HOPE, 0-12 °C), and hypothermic machine perfusion (HMP,0-12 °C). Additionally, new preservation technologies such as supercooling preservation (-6 to -4 °C) [6] have also been developed.Exsituorgan perfusion systems enable to remove metabolic waste products and provide essential substances that can meet the metabolic needs of the organs, which may improve the organ quality and utilization rate of marginal organs, as well as reduce postoperative complications. Clinical trials for different MP in transplantation have also been published in many prestigious journals,such asNature,NEJM,Lancet, andJAMASurgery[ 1 , 7-11 ].

Table 1 Levels of evidence.

Table 2 GRADE system.

Our working group attempted to provide recommendations based on the Grading of Recommendations Assessment, Development and Evaluation (GRADE) methodology [12] and we concluded that we do not have all the elements to make recommendations in clinical practice. The consensus evidence levels are presented in accordance with “Oxford Centre for Evidence-based Medicine – Levels of Evidence (March 2009)” ( Tables 1 and 2 ).

Abdominal organ procurement and protection

Donor abdominal organ procurement varies from transplant center to center. For donation after brain death (DBD) with relatively stable hemodynamics, the organs can be procured separately,or procured together with multiple organs before separation. For donors with organ donation after circulatory death (DCD) or organ donation after brain and cardiac death (DBCD), the rapid procurement for multiple abdominal organs should be applied to shorten warm ischemia time (WIT). Recently, extracorporeal membrane oxygenation (ECMO), normothermic regional perfusion (NRP), and NMP techniques have been applied in some centers for abdominal organ procurement, which lead to the improvement of prognosis of recipients [13–15] . Living donor organ transplantation andinsituliver splitting are effective ways to alleviate the organ shortage.The quality ofinsitusplitting liver from unstable hemodynamic donor is improved with ECMO support, which also improves the outcomes of recipients [16] . Donor safety is the priority in living organ transplantation. Therefore, grafts must be strictly selected by experienced surgeons [17] .

Donor liver preservation and repair

The transplantation prognosis is linked with graft quality. Evaluation of grafts quality is based on the comprehensive understanding of donor age, body mass index, liver function, duration of stay in the intensive care unit (ICU), warm/cold ischemia time, and histology of liver biopsy [18] . WIT is closely related to donor organ injury. Total donor WIT (tDWIT) is defined as the time from donor withdrawal of treatment to initiation of cold perfusion, and the functional donor WIT (fDWIT) is defined as the time from the mean arterial pressure (MAP) < 60 mmHg or the SpO2(saturation of pulse oxygen) < 80% to the start of cold perfusion [19] . fDWIT of donor liver generally is recommended to be no longer than 30 min [ 15 , 20 ]. Especially in hypernatremia donors, the donor liver ischemia time should be minimized [21] .

The liver graft should be fully perfused with organ preservation solution such as UW or HTK. During this process, the surgeon should carefully protect the liver hilum. Especially for split liver transplantation, the donor liver blood vessels and bile ducts should be protected [22] .

The liver preservation efficiency is linked with liver graft quality. SCS with UW and HTK is most widely used for liver preservation. Other preservation solutions such as Celsior, Institute Georges Lopez-1 (IGL-1), and Leeds solutions have also been applied [ 23 , 24 ]. A meta-analysis of more than 20 0 0 organ preservation showed that different solutions had no significant effect on liver function recovery, primary non-function (PNF), and biliary complications post-transplantation [20] . Cold ischemia injury during SCS can cause biliary complications, PNF, and even recipient death. The optimal cold storage time for donor livers should not exceed 8 h, and clinically, storage time for donor livers should generally not be more than 15 h [ 25 , 26 ].

In contrast to SCS, MP can continuously deliver nutrients and dynamically perfuse the organ to improve organ quality. MP systems have been applied for clinical research and validation stages,which include the Lifeport liver repair system (Organ Recovery Systems, Chicago, IL, USA), the Liver Perfusor system (Lifeperfusor,Hangzhou, China), the NMP OrganOx system (OrganOx Ltd., Oxford,UK), and a multifunctional MP Liver Assist system (Organ Assist BV,Groningen, The Netherlands). Randomized clinical controlled trials have confirmed that MP has a protective effect on different types of donor liver [26–30] and HOPE can reduce the incidence of biliary complications after DCD liver transplantation [1] . However, the repair effect of HOPE on steatosis liver grafts requires confirmation in future studies [31] . Compared to HMP, NMP can restore liver function after transplantation and reduce the donor liver rejection rate [ 7 , 32 ].

With the advancement of technology, MP shows important clinical application prospects [33] . Recommendations for abdominal organ procurement and liver graft protection are presented in

Donor kidney preservation and repair

SCS is the most commonly used transplant organ preservation method due to the advantages of affordability, simplicity, and effectiveness [34] . To compensate for the shortage of donors, the ratio of marginal donors including DCD and expanded criteria donor(ECD) has increased significantly. However, such organs are associated with an increased risk of complications, such as delayed graft function (DGF), PNF and acute rejection reaction. This not only places high demands on donor preservation techniques and pre-transplantation repair, but also greatly drives the enhancement and clinical application of renal MP techniques such as HMP andNMP [35] . The most commonly used preservation solutions for SCS are UW and HTK; others include hyperosmolar citrate (Marshall’s solution, HOC), hypertonic citrate adenine II (HC-A II), Celsior, and IGL-1 solutions [ 2 , 34 ]. Recent studies have shown that IGL-1 fluid can be used in kidney transplantation with comparable efficacy to those of UW and HTK [ 34 , 36 ]. In addition, some studies found that the addition of certain ingredients, such as M101 and antioxidants, to these preservation solutions benefits the preservation of the donor kidney [ 37 , 38 ]. Despite the continuous optimization of preservation solutions, prolonged cold ischemia time is an independent risk factor for DGF after kidney transplantation, especially for donor kidneys that have already experienced warm ischemia.Therefore, the cold storage time should be minimized to improve the outcome of kidney transplantation [ 39 , 40 ].

Table 3 Recommendations for abdominal organ procurement and liver graft protection.

Compared with SCS, HMP has the advantages of reducing vasospasm, supplying energy and oxygen, removing metabolic waste,and facilitating donor kidney evaluation or repair [41] . Studies have confirmed that HMP can significantly reduce the risk of DGF after kidney transplantation [ 42 , 43 ]. At present, commonly used portable HMP systems in the market include LifePort Kidney Transporter (Organ Recovery Systems), RM3 (Waters Medical System,Minneapolis, Minnesota, USA), Kidney Assist (Organ Assist BV) and WAVES (Institut Georges Lopez, Lyon, France). The main parameters of HMP systems include perfusion flow, intrarenal vascular resistance (IRR), pressure, and temperature, among which IRR and flow are important post-transplantation prognostic markers. However, it cannot be used as independent assessment indicators, and a unified standard is lacking. The perfusion pressure should be maintained at 30-40 mmHg to ensure effective perfusion while reducing endothelial injury and the optimal perfusion temperature is 4-10 °C [ 44 , 45]. In addition, although monitoring certain markers [such as glutathione transferase, lactate dehydrogenase,interleukin-18, and flavin mononucleotide (FMN)] in the perfusate during HMP can help assess injury of the donor kidney, the accuracy for determining the prognosis of kidney transplantation (such as DGF, PNF, and graft survival) is still limited, as there are no clinically accepted and reliable biomarkers for the perfusate [ 46 , 47 ].There is no established standard for the optimal perfusion time of HMP; however, clinical evidence suggests that HMP is effective in improving organ quality during cold storage time to some extent and that receiving HMP as early as possible after donor kidney procurement and continuous HMP are more effective in improving donor kidney quality and kidney transplantation prognosis. Considering that excessive cold ischemia time is an independent risk factor for DGF during HMP, the duration of HMP preservation should be controlled [ 4 8 , 4 9 ]. Although there is clinical evidence that HOPE has fewer serious complications and a lower incidence of acute rejection compared with HMP, there is no significant difference in survival, and the incidence rates of DGF and PNF are similar. The effects of high levels of delivered oxygen on the donor kidney remain unclear and need more clinical studies to verify [ 9 , 50 ].

Theoretically, NMP, which is close to the human physiological environment, is a more ideal platform for donor kidney preservation, quality assessment, injury repair, and treatment than HMP or SCS [ 51 , 52 ]. Some studies have preliminarily confirmed that NMP contributes to the recovery of donor kidney function and has some clinical applicability and safety [ 53 , 54 ]. Currently, there is no accepted system for renal NMP instrumentation and perfusate compositions. In existing studies, the perfusate of the NMP basically consists of crystalloid solution (e.g., Ringer’s solution), colloidal solutions (e.g., albumin), red blood cell suspensions, vasodilators, nutrients (e.g., glucose, vitamins), antibiotics, and anti-inflammatory agents (e.g., dexamethasone). A mixture of 95% O 2 and 5% CO2is mostly chosen as the oxygenation gas, and 70-85 mmHg and 37 °C are used as the NMP perfusion pressure and temperature,respectively [ 52 , 55 ]. Currently, the duration ofexvivoNMP is up to 24 h [54] . Although preclinical studies have confirmed that continuous prolonged NMP significantly improves donor kidney quality and short-term prognosis of kidney transplantation compared with SCS or short-duration NMP, the optimal duration of NMP perfusion is not supported by clinical evidence [ 56 , 57 ]. NMP facilitates preoperative quality assessment of the donor kidney as well as targeted interventions to thereby expand the donor pool [ 58 , 59 ].A comprehensive assessment system for NMP donor kidney quality includes perfusion parameters and metabolic markers (renal blood flow, urine volume, oxygen consumption, electrolytes, creatinine clearance), urine biomarkers, histological examinations (e.g.Remuzzi score), and NMP score (exvivonormothermic kidney perfusion score, EVKP score) of the kidney [ 52 , 55 , 60 ]. There is limited research on all aspects of renal NMP and a lack of clinical evidence comparing the advantages and disadvantages with those of other preservation methods [60] .

At this stage, emerging techniques for kidney donation preservation are being developed [e.g. subnormothermic machine perfusion (SNMP) and controlled oxygenated rewarming (COR)]. MP has been developed as a therapeutic platform combined with various interventions (e.g. cellular, gene, etc.), and has shown some potential for clinical application. However, NMP is still at a preclinical study stage and there is a lack of sufficient clinical data. Therefore, NMP and refinement of NMP techniques are required before translation into clinical practice [61–63] . Recommendations for the preservation and repair of kidney are presented in Table 4 .

Donor pancreas preservation and repair

Pancreas transplantation and islet transplantation are currently the main treatments for patients with type I and some type II diabetes [64–66] . Pancreas transplantation includes simultaneous pancreas-kidney transplantation (SPK), pancreas after kidney transplantation (PAK), and pancreas transplant alone (PTA). Islet transplantation has relatively unique advantages for simpler manipulation techniques, smaller surgical trauma and better safety, etc.More than 50% of islet transplant recipients can discontinue insulin therapy in 5 years and islet transplantation is similar to pancreas transplantation on the medium- to long-term prognosis [67] .As donor pancreas become increasingly scarce, ECD pancreases are being increasingly and more widely used [ 68 , 69 ]. The pancreasgraft protection and repair techniques have been reported to reduce the occurrence of early complications (e.g. vascular complications and reperfusion related pancreatitis) after pancreas transplantation [70] .

Table 4 Recommendations for the preservation and repair of kidney.

Currently, pancreatic preservation strategies mainly include SCS,the two-layer method (TLM), and MP. SCS is the main preservation method, by which the cold ischemia time is recommended to be no more than 12 h [71] . TLM is a preservation system that contains UW solution and perfluorochemical, and the isolated pancreas is preserved between two layers of disintegrating solution to provide oxygen to the preserved pancreas, which may protect pancreas during cold ischemia time and increase pancreas utilization [ 72, 73]. As reported, SCS with HTK solution may lead to pancreatic cell edema, which is associated with early graft failure and pancreatitis after transplantation [74] . UW solution is most frequently applied for pancreas and pancreatic islet preservation [75] . Currently, MP for pancreas preservation and repair is still in the experimental stage, and prospective clinical studies are urgently needed. It is difficult to establish the ideal flow and pressure perfusion parameters for the pancreas with low-flow and complex vascular anatomy. Excessively high perfusion pressure induces vascular endothelial injury and thrombosis.However, excessively low perfusion pressure can lead to insuffi-cient perfusion and oxygenation. Suitable low-flow perfusion has some advantages in protecting isolated pancreas [76] . HOPE reportedly is an effective MP strategy to protect pancreas grafts [ 77 , 78 ].HOPE for 6 h improves the quality of discarded DCD pancreas which can be applied to isolate functional islet cells for subsequent transplantation [79] . However, studies related to pancreatic NMP are in the experimental stage, and further research is needed.

Harvest islet cells with carbon monoxide-saturated medium showed a better therapeutic effect on islet auto transplantation patients with chronic pancreatitis for better islet cell quality and higher transplantation success rate [80] . Isolated and purified islet cells were recommended to be incubated in the medium for 24-72 hinvitro. It provides sufficient time for graft quality control and immunological induction and reduces acute rejection for removing infiltrated passenger leukocytes. Moreover, it also reduces inflammatory factors, necrotic and apoptotic islet cells, innate immune response, the damage-associated molecular patterns and instant blood-mediated inflammatory reaction in transplantation. If transport conditions cannot accommodate the islet culture requirements, the islets can undergo cold preservation. Islet cells are suspended and maintained at 37 °C in a 5% CO2atmosphere in CMRL10 6 6 medium containing 10%-15% human albumin(5.5 mmol/L) in serum for 24 h, followed by 22 °C for 24-48 h.However, the culture time for total islet cell is recommended not exceed 72 h [81] .

In conclusion, pancreas and islet transplantation has great potential for clinical applications. More effective pancreatic preservation and repair techniques as well as isolation and culture techniques for islet cells need to be further explored. Recommendations for donor pancreas preservation and repair are presented in Table 5 .

Donor small intestine preservation and repair

Small intestinal transplantation is the most efficacious therapy for intestinal failure [82] . Intestinal grafts are more sensitive than other organs to ischemic injury, which can lead to intestinal mucosal morphological damage and intestinal bacterial translocation to the liver, spleen, and other parenteral organs, and even cause systemic infections. Ischemic injury is a non-specific injury, and it will also increase graft immunogenicity, and aggravate acute and chronic rejection [83] . Thus, organ protection is particularly important in small intestinal transplantation.

SCS and cold perfusion are important methods to reduce intestine injury during intestinal procurement and preservation. In contrast to solid organs, a large number of digestive enzymes, bacteria,and toxins are present in the intestine and thus, intestine preservation requires vessel and luminal double perfusion. An initial vascular flush during intestinal procurement is effective, but a second vascular flush before the end of preservation is not recommended.The WIT should be shortened as much as possible, and the maximum should generally not exceed 60 min [84] .

UW solution is widely used for the vascular flush and preservation solution, and HTK solution is increasingly being used [85] .Intestinal grafts preserved in UW or HTK solution showed no significant difference in graft and patient survival rate, intestinal function, or complication rate [86] . However, HTK is more economical than UW, and has a low viscosity, which means that it is more conducive to microvascular lavage [87] . Studies also found that IGL-1 solution can be a good substitute in the intestine than UW solution [88] for its good short-term effects on clinical preservation [89] . However, it has not been determined that which one is the best for intestine preservation. Cold ischemia time can affect the prognosis of small intestinal transplantation and should be kept within 9 h [90] . Clinical trials and animal experiments confirmed that most preservation solutions for small intestine can effectively preserve the intestine for 6-8 h [91] .

Table 5 Recommendations for pancreas preservation and repair.

Table 6 Recommendations for donor small intestine preservation and repair.

Intestinal luminal intervention during cold storage can alleviate IRI. Luminal preservation solution containing polyethylene glycol (PEG) can be used for intestinal preservation, but it can combine with epithelial cells to alter the functional characteristics of the mucosal barrier [ 92 , 93 ]. S?fteland et al. found that intraluminal low-sodium solutions containing PEG can better preserve the intestine without aggravating cell edema [92] . Although IGL-1 contains PEG, its sodium content is relatively high, which has been shown to be unfavorable for luminal preservation [89] . A study found that the use of HTK solution or the modified HTK solution (HTK-N) as the luminal preservation solution can better protect the intestinal mucosal structure and graft vitality [93] .

MP in intestinal transplantation is still in the preclinical stage.In 2003, Zhu et al. performed the first luminal HOPE and found that MP preservation can better preserve small intestine compared with SCS [94] . In 2016, Yale University reported a new intestinal preservation unit using vascular and luminal doublechannel perfusion for the first time to further reduce intestinal pathology injury [95] . In 2020, Guo et al. found that short-term ECMO support can alleviate IRI in the small intestine and improve the early intestinal absorption function after transplantation [96] . In 2021, Hamed et al. firstly used NMP for intestinal graft preservation. The small intestine exhibited retained intestinal peristalsis, glucose absorption, and glucagon-like peptide-1 secretion function during storage, and intestinal histopathology did not deteriorate [97] . Recommendations for donor small intestine preservation and repair are presented in Table 6 .

Thoracic organ procurement and protection

Donor heart preservation and repair

There are more than 20 million patients with heart failure worldwide, and the 5-year mortality rate is up to 50%. Heart transplantation is the standard of care for eligible patients with endstage heart failure [98] . However, the lack of donor hearts has created a mismatch between the number of available organs and the number of patients in need. There are scarce clinical practice and study of heart transplantation. Consequently, there has been increasing pressure to use higher-risk donor hearts, such as marginal donors and long-distance procurements with extended ischemia time. Thus, the focus of researchers in this field is donor heart preservation and repair techniques.

Currently, DBD donors are the primary donors for heart transplantation. DCD donor hearts are one of the primary alternatives being studied to address organ shortage [98–100] . However, despite reports of over 200 successful cases in the recent era, mainly from the UK and Australia,there are still several issues being studied in the preclinical phase to confirm the safety of this modality [98–101] .

The procurement procedure of the donor heart is directly linked to the quality of the graft. After confirming the brain death of the donor, a median sternotomy is performed followed by an incision in the pericardium. The ascending aorta is cross-clamped and 1-3 L of cold preservation solution (4-8 °C) is rapidly infused into the aortic root to produce rapid cooling and electromechanical arrest of the heart. Simultaneously, the right and left ventricle should be adequately vented to avoid distension and injury.This can be achieved by opening the inferior vena cava and the left atrial appendage, respectively. The perfusion pressure is maintained at 50-70 mmHg for cardioplegic solutions. The donor heart is then covered with a saline slush (between 0-4 °C) for rapid cooling. Followed by an adequate flush of the heart with the preservation solution, the cardiectomy is completed. Care must be taken at this point during the left atrial sectioning to permit enough cuff length for both the heart implant and the lung implant. Various studies have shown that donor heart function is directly correlated to the duration of SCS. Acceptable transplant outcomes have been obtained with SCS of the donor heart for ideally less than 4 h but acceptably up to 8 h [ 98 , 101 ]. For the DCD heart, the acceptable WIT is less than 30 min. Recent studies have shown that the cardiac WIT should be counted from the moment the donor’s systolic blood pressure falls below 50 mmHg until organ flush. It is unclear if longer periods can be tolerated or if pre- and post-conditioning agents can be used to mitigate warm ischemic damage [ 102 , 103 ].

Exsitudonor heart preservation and repair technology consists of SCS, HMP storage, and normothermic beating-heart MP storage.SCS is the most widely used preservation method. The donor heart is excised and placed in an organ bag containing a preservation solution. The organ bag is then sealed and placed in an insulated container packed with ice (between 0-4 °C), in which it is stored until implantation. HMP is a process that involves continuouslow-flow infusion of a preservation solution between 4-8 °C through the coronary circulation. In addition to ensuring a uniform cooling of the heart and a significantly decreased metabolic rate,this procedure also provides the myocardial tissue with an oxygenated and nutrient-enriched solution, protecting the cardiomyocytes and the coronary artery endothelial cells from IRI. The continuous flow also enables metabolic clearance of the organ.Brant et al. showed that both antegrade and retrograde perfusion demonstrated excellent functional preservation, at least equivalent to static hypothermic Celsior solution storage [104] . However, myocardial tissue edema caused by a long duration of HMP has limited its application [98] . Finally, this technique is still under investigation to determine its usefulness for clinical heart transplantation, and there are only a few reports of clinical cases to date.Normothermic continuous perfusion of donor hearts in a beating state mimics the physiological status, possibly prolonging the safe preservation period to 12 h [ 101 , 105 ]. Currently, the Organ Care System (OCS) Heart System has been approved by the Food and Drug Administration (FDA) for extracorporeal perfusion and preservation of donor hearts in the USA [106] . A late prospective randomized controlled trial has shown that heart transplantation using donor hearts adequately preserved with the OCS, a normothermic beating-heart perfusion system, or with standard cold storage yields similar short-term clinical outcomes regarding 30-day survival rate; follow-up of allograft function has shown the safety of this technique with similar outcomes up to two years following transplantation [107] . A recent small-scale clinical trial demonstrates the superiority of NMP over SCS [108] . Althoughexsituheart perfusion has demonstrated its potential in DCD heart resuscitation [109] , further research is needed on selecting the perfusion solution and optimizing the perfusion parameters. Furthermore,exsituheart perfusion provides a way of evaluating donor hearts’ viability prior to transplantation [110] . White et al. [111] , in an elegant set of large animal experiments, have demonstrated the superiority of contractility parameters over metabolic parameters in discriminating between hearts with adequate or reduced function. The optimal assessment tool, however, has yet to be determined.

Table 7 Recommendations for donor heart preservation and repair.

HTK solution, UW solution, and Celsior solution are the most widely used ones, all based on a hyperkalemic cardiac arrest. The UW solution is a widely used myocardial preservation solution with oncotic pressure and viscosity due to the hydroxyethyl starch content. Compared to Celsior solution, UW solution prevents tissue edema during HMP; however, it may lead to abnormal contraction of the vessels of the heart. HTK solution is an intracellular solution with low sodium, low calcium, and slightly high potassium.The basic design of the solution consisting of histidine, a potent buffer, which along with the aforementioned features of the solution, helps prevent the myocardial cells from generating edema.Celsior solution combines the osmosis effect of UW solution with the buffer effect of HTK solution. However, the long duration of donor heart preservation using Celsior solution might lead to myocardial edema. Currently, none of the solutions has been proven to be superior to one another [112] . Recently, novel and modified types of myocardial preservation solutions have emerged, including SOM-TRN-001, an extracellular preservation solution, CRMB, a Celsior-based solution, and Custodiol-N, an HTK-based solution. Although these new solutions theoretically have more advantages regarding myocardial preservation, the applications of these solutions are only in the phase of animal testing, requiring more clinical evidence [113] .

Recommendations for donor heart preservation and repair are presented in Table 7 .

Donor lung preservation and repair

Lung transplantation is the only effective treatment for endstage pulmonary disease. The process of procurement and preservation for donor lung affects the quality directly, which is strongly related to the success or failure of transplantation. Lung transplantation data for 2015-2018 highlight that the rate of donor lung utilization was only 5.5% in China [114] . With the increasing demand for lung transplantation as well as the development of corresponding preservation and repair techniques, marginal donor lungs have been used clinically, and acquired similar effects to DBD donor lungs [115–118].

Appropriate assessment helps to improve the success rate for lung transplantation. Therefore, it’s important to meet the requirements or to utilizeexsiturepair technology before transplantation. The main assessment contents include oxygenation index,age, smoking history, chest imaging, bronchoscopy, etiology, and sputum test. Additionally, organ system should be concurrently supported and evaluated [119] .

Procurement directly affects the quality of donor lungs, and the WIT should be shortened as much as possible during donor lung acquisition. A retrospective study showed that there was no significant effect on recipient survival when WIT of donor lung was less than 60 min [120] . Cold ischemia time affects the prognosis, and some researchers suggested that the cold ischemia time should be less than 8 h [ 121 , 122 ]. Whenexvivolung perfusion (EVLP) is used, the cold ischemia time after EVLP should not exceed 287 min[123] . A preservation temperature of 4-8 °C is recommended for isolated lung preservation. A total ischemia time of less than 4 h could significantly improve the recipients’ overall survival within 30 days after transplantation [124] .

SCS technology is widely used for lung preservation, and a variety of preservation solutions that are suitable for SCS have achieved satisfactory effects. Compared with intracellular preservation solutions, an extracellular preservation solution can avoid pulmonary contraction that is induced by a high potassium concentration and extended cold ischemia time, obtain a better oxygenation index [partial pressure of oxygen (PaO 2 )/fraction of inspired oxygen(FiO2)] value, shorten the mechanical ventilation time, and reducethe postoperative ICU time. Thus, an extracellular solution for lung preservation is preferred clinically, and one of the most commonly used extracellular solutions is the Perfadex solution [125] .

Table 8 Recommendations for donor lung preservation and repair.

Table 9 Recommendations of protection for limb procurement and transplantation.

Sufficient perfusion can greatly protect the donor lungs. Pulmonary artery anterograde perfusion and pulmonary vein retrograde perfusion are convenient and feasible. Under hypothermic perfusion (4-8 °C), parameter settings maintain pulmonary artery perfusion pressure at 10-15 mmHg and perfusion flow at 60 mL/kg.If pulmonary vein retrograde perfusion is selected, each pulmonary vein receives perfusion with 250 mL, and reperfusion occurs after 6 h. For ventilation during perfusion, a FiO 2 of 50%, positive endexpiratory pressure of 5 cmH2O, pressure of < 20 cmH 2 O, and tidal volume of 6-8 mL/kg should be maintained [121] .

MP can repair and improve the donor lung, which has the potential for broad application. Pulmonary MP is mainly used without red blood cells under normal temperatures, and the most important technology of MP is EVLP. EVLP has been shown to improve the quality of high-risk donor lungs, and has similar outcome as DBD lung transplantation [126] . Additionally, the OCS Lung, the first portable extracorporeal lung perfusion and ventilation device, was shown to be a safe and effective equipment in clinical research, and it is already used in some countries and regions [ 10 , 127 ]. Recommendations for donor lung preservation and repair are presented in Table 8 .

Donor limb procurement and protection

Limb replantation is the implantation of one’s limbinsituusing direct or indirect methods after it has been severed. Reperfusion is suggested starting within 6 h to prevent further permanent tissue damage, especially for a severed limb with numerous muscles. The most common preservation method is SCS, but its therapeutic effect seems to be limited. Limb allotransplantation,the transfer of allogeneic limbs between individuals of the same species and different genotypes, has previously been named as vascularized composite tissue allograft. Because allogeneic limb transplantation is not a life-saving treatment for patients with amputation, and the skin is highly immunogenic, limb transplantation is less commonly used in clinical practice compared with the solid organ transplantation [128] . Since Dubernard et al. reported the first upper extremity transplantation in France in 1998, more than ten countries including China, USA, Italy, and Malaysia have performed 113 upper limb transplantation in 76 patients [129] . All patients with unilateral or bilateral hand transplantation recovered a degree of protective sensation after surgery. Tactile sensitivity was present in 90% of patients, and discriminative sensitivity was 82.3%; over 75% patients had an improved quality of life and returned to some part of daily work [130] . Twenty years after limb transplantation, patients recovered sensation and were able to use the grafted hands. The outcome of their physical and mental health has been greatly improved as well as social integration [131] . Unlike solid organ transplantation, limb transplantation is in the preclinical stage, and with the progress of basic research in the field of preservation, transportation, perfusion, and immunosuppression,it has broad prospects in clinical application in the foreseeable future [132] .

The procurement process of the transplanted limb is directly related to the success and prognosis of limb transplantation. In limb procurement process, the transplant surgeon has to be gentle to prevent the mechanical injury and minimize ischemia time.The donor amputation should be at an appropriate level to retain enough of the vascular pedicle, and a small amount of residue from the tissue of skin can protect vascular anastomosis and reduce immune rejection. The donor limb is often perfused before amputation. As is well known, for forearm donor, a 5 cm circular incision through the skin is made above the elbow joint, the brachial artery is divided and catheterized and then perfused with low-temperature UW solution (4 °C). The forearm is then amputated, and the perfusion tube is removed when the venous out flow is clear. The cold perfusion preservation method for donor limbs can effectively simplify surgery, ensuring perfusion quality and maintaining sterility [133] .

SCS is the most commonly used technique for limb preservation, and it can significantly reduce cell metabolism [134] . Limbs with static cold preservation technology must undergo rewarming and reperfusion before transplantation in order to avoid IRI [135] .Data from the International Hand Transplant Registry showed that the cold ischemia time of hand allotransplantation ranged from 30 min to 13.5 h (median 5.5 h), which may explain the difference in function and long-term survival after transplantation [136] .

MP for limb preservation is still in the preclinical investigation stage. Limb preservation time was reported to be successfully expanded byexvivoperfusion with an oxygenated solution, a hemoglobin-based oxygen carrier, and an acellular solution. Haug et al. showed that hypothermicexsituperfusion with an oxygenated acellular solution may extend the isolated preservation time four to six-times compared with SCS [137] . Said et al.reported that muscle contractility was preserved for 10.6 h using a hemoglobin-based oxygen carrier solution [138] . Krezdorn et al. [139] reported that low-potassium dextran perfusate with added albumin could preserve tissue for up to 24 h. Recommendations of protection for limb procurement and transplantation are presented in Table 9 .

Acknowledgments

We thank Li Li, Jiang-Hua Ran, You-Hua Zhu, Jiang-Qiao Zhou,Jin-Zhen Cai, Jun Chen, Zheng Chen, Wei Chai, Zhen-Yu Deng, Jian-Hui Dong, Xiao-Li Fan, Gui-Wen Feng, Hong-Xing Fu, Jie Gao, Liang-Hui Gao, Hua Guo, Lei Geng, Wei-Li Han, San-Yuan Hu, Nan Jiang,Guo-Ping Jiang, Guang-Ping Li, Qi-Yong Li, Guo-Ling Lin, Lian-Xin Liu, Jun Liu, Jun Liu, Zhi-Kun Liu, Cai-De Lu, Guo-Yue Lv, Tong-Yi Men, Zhi-Hai Peng, Xun Ran, Jian-An Ren, Yan Shen, Bing-Yi Shi,Jun Shi, Zhang-Fei Shou, Wei Si, Peng-Hong Song, Xu-Yong Sun, Yu-Ling Sun, Xiao-Yu Tan, Pu-Xun Tian, Zhen-Hua Tu, Kai Wang, Hao Wen, Qiang Wei, Xu-Yong Wei, Jian Wu, Xiang-Wei Wu, Zhong-Jun Wu, Xiao-Tong Wu, Xin-Yu Peng, Qiang Xia, Qin-Fen Xie, Mei-Fang Xu, Wu-Jun Xue, Lv-Nan Yan, Guang-Shun Yang, Yang Yang,Yue Yang, Zhe Yang, Jian-Min Yao, Shao-Jun Ye, Guang-Sheng Yu,Jun Yu, Feng Zhang, Min Zhang, Shui-Jun Zhang, Wu Zhang, Hai-Ge Zhao, Wen-Yu Zhao, Lin Zhong, Zi-Biao Zhong, Lin Zhou, Jian Zhou,Ji-Ye Zhu, Li Zhu, Zhi-Jun Zhu, and Li Zhuang for their contributions to the consensus.

CRediT authorship contribution statement

Jian-Hui Li: Funding acquisition, Methodology, Writing – original draft, Writing – review & editing. Xiao Xu: Writing –original draft, Writing – review & editing. Yan-Feng Wang:Writing – original draft, Writing – review & editing. Hai-Yang Xie: Writing – original draft, Writing – review & editing. Jing-Yu Chen: Writing – original draft, Writing – review & editing.Nian-Guo Dong: Writing – original draft, Writing – review & editing. Mitesh Badiwala: Writing – original draft, Writing – review& editing. Li-Ming Xin: Writing – original draft, Writing – review & editing. Roberto Vanin Pinto Rib eiro: Writing – original draft, Writing – review & editing. Hao Yin: Writing – original draft, Writing – review & editing. Hao Zhang: Writing – original draft, Writing – review & editing. Jian-Zheng Zhang: Writing – original draft, Writing – review & editing. Feng Huo:Writing – original draft, Writing – review & editing. Jia-Yin Yang:Writing – original draft, Writing – review & editing. Hong-Ji Yang:Writing – original draft, Writing – review & editing. Hui Pan:Writing – original draft, Writing – review & editing. Shao-Guang Li: Writing – original draft, Writing – review & editing. Yin-Biao Qiao: Writing – original draft, Writing – review & editing. Jia Luo:Writing – original draft, Writing – review & editing. Hao-Yu Li:Writing – original draft, Writing – review & editing. Jun-Jun Jia:Writing – original draft, Writing – review & editing. Hao Yu:Writing – original draft, Writing – review & editing. Han Liang:Writing – original draft, Writing – review & editing. Si-Jia Yang:Writing – original draft, Writing – review & editing. Hao Wang:Writing – original draft, Writing – review & editing. Zhong-Yang Liu: Writing – original draft, Writing – review & editing. Li-Cheng Zhang: Writing – original draft, Writing – review & editing.Xiao-Yi Hu: Writing – original draft, Writing – review & editing.Hao Wu: Writing – original draft, Writing – review & editing.Yi-Qing Hu: Writing – original draft, Writing – review & editing.Pei-Fu Tang: Conceptualization, Supervision, Writing – review &editing. Qi-Fa Ye: Conceptualization, Supervision, Writing – review& editing. Shu-Sen Zheng: Conceptualization, Funding acquisition,Supervision, Writing – review & editing.

Funding

This study is supported by grants from Major Science and Technology Projects of Hainan Province (ZDKJ2019009), Research Project of Ji’nan Microecological Biomedicine Shandong Laboratory (JNL-2022002A and JNL-2022023C), Public Projects of Zhejiang Province (LGF21H030 0 06), Research Unit Project of Chinese Academy of Medical Sciences (2019-I2M-5-030), the National Natural Science Foundation of China ( 81721091 , 62073211 ), and the National S&T Major Project for Infectious Diseases (2017ZX10203205).

Ethical approval

Not needed.

Competing interest

No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.