Introduction
Solid organ transplantation is based on the retrieval of an organ from a donor and its implantation into a recipient.
Transplant organs are predominantly retrieved from cadaveric donors, and less often from living donors. Ongoing organ shortage, due to the exceeding number of patients needing a transplant over the number of available organs, has led to the implementation of new strategies to expand the donor pool.
Deceased Donation
Organs are retrieved from cadaveric donors who expressed their wish to donate in their life or if the donor’s family decides to consent, as long as the donor did not express any previous wishes against organ donation. However, practice often varies among countries.
Deceased organ donation can occur following two broad types of death: donation after brain death (DBD) and donation after circulatory death (DCD). Legality of deceased donation and each type can vary between countries; DBD donors are legally dead (following brainstem testing), but their organs remain functional due to support from mechanical ventilation, hence they are also known as heart-beating donors. Such cases will not survive once support is removed and will inevitably have a cardiac arrest.
Donation after circulatory death (DCD) may proceed when a lack of circulatory output leads to the absence of end organ perfusion. DCD donors are categorised according to the Maastricht criteria (Table 1), the vast majority of which are category III donors.
| Category I: Dead on arrival at hospital; the moment of sudden death must be witnessed and the time documented |
| Category II: Unsuccessful resuscitation, usually in an Accident and Emergency department |
| Category III: Awaiting cardiac arrest; patients in whom cardiac arrest is inevitable, but they do not fulfil criteria for brainstem death testing |
| Category IV: Cardiac arrest in a brainstem dead individual |
| Category V: Unexpected death in a patient in ITU or critical care unit |
Table 1 – The Maastricht criteria for DCD donors; categories I-II are defined as uncontrolled, categories III-V are defined as controlled (i.e. circulatory arrest happens while they are inpatients)
Expanding the Donor Pool for Renal Transplantation
New strategies have been implemented to expand the donor pool. Extended criteria donors (ECD) are defined as; either donors >60yrs or donors 50-59yrs with two or more of the following comorbidities: hypertension, serum creatinine >150, or cerebrovascular disease. The kidneys of extended criteria donors may be used in selected recipients.
For living kidney transplantation, kidney sharing schemes have been started to solve the incompatibility of donor-recipient pairs. One or more pairs that are not directly compatible are matched and paired exchange transplants take place.
In-situ normothermic regional perfusion (NRP) is a technique which is being increasingly used in DCD to mitigate the initial warm ischaemia whilst allowing in-situ functional assessment of organs. This helps to improve the utilisation of organs which may previously have been deemed unsuitable (e.g. DCD hearts and livers from marginal donors).
Live Donation
Live donation is the retrieval of organs from living donors; most commonly performed for kidney transplantation, an increasing numbers of living donor partial liver transplants are also being performed.
Living donors are rigorously assessed, not only to make sure they do not transmit any disease to the recipient, but also to minimise the risks the donation could pose to the donor’s health.
Kidneys from living donors have better long-term functions than from cadaveric donors and are much less likely to suffer from delayed graft function. In many cases, complete-mismatch living-donor kidney transplants have outcomes equivalent to those of zero-mismatch deceased-donor transplants.
Retrieval Operation
The procedure must ensure rapid organ retrieval without damage to the organs and their related anatomy (e.g. vascular supply or drainage systems). The retrieval procedure differs between a DBD and DCD donor; the retrieval operation for each organ is described in more detail in the relevant articles.
A DBD retrieval comprises of a warm and a cold phase dissection. Whilst the donor is still on ventilatory support and the heart is beating, full exposure of the abdomen is obtained and the chest is also opened; the organs to be retrieved are exposed and dissected in order to facilitate rapid organ cooling and retrieval in the cold phase. The cold phase follows the cross-clamping of the aorta, venting of the donor blood volume and perfusion of the donor with cold preservation solution (e.g. University of Wisconsin solution).
Main functions of preservation solution:
- Rapid cooling to minimise warm ischaemia
- Wash out of blood to prevent thrombus formation
- Delivery of nutrients and antioxidants
- Maintaining fluid, ionic, and pH balance of organ parenchyma
For a retrieval from a DCD donor, life support is withdrawn and the donor is often given symptomatic treatment whilst awaiting asystole. After confirmation of cardiac death, a rapid laparotomy, thoracotomy and aortic cannulation is performed, and further topical cooling is provided with ice. Cross-clamping the aorta facilitates effective delivery of preservation solution to the organs which require it. The remainder of the procedure is similar to the cold phase dissection of a DBD donor.
The liver is retrieved with the inferior vena cava, a patch of the right hemidiaphragm and the coeliac axis with a patch of aorta. The pancreas is retrieved with the spleen and duodenum attached. The kidneys are removed with the renal artery with a patch of aorta, the renal vein (the right one, which is shorter, is removed with a portion of the IVC), and the ureter.
Organ Preservation Techniques
The time between the retrieval of an organ, and therefore cessation of its blood supply, to reperfusion in the recipient has a profound impact on the final outcome of the transplant. Reducing warm time ischaemia is of paramount importance.
Standard techniques involve the preservation of the organ by simple cold storage, with the organ placed in sterile plastic bags and surrounded by ice.
More recently, new preservation techniques have been introduced, such as hypothermic or normothermic machine perfusion of organs. This can improve preservation and may allow function assessment of the organ prior to transplantation.
Key Points
- Organ donors can either be live or cadaveric donors, the later being further classified into donation after brainstem death (DBD) and donation after circulatory death (DCD)
- Different techniques for organ retrieval are warranted depending on the type of donor
- Ongoing research allows for new techniques to expand the donor pool and improve the preservation of retrieved organs