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.
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.
The majority of organs come from donation after brainstem death (DBD). 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) form a smaller proportion of the organ donation pool, whereby lack of circulatory output leads to the absence of end organ perfusion. DCD donors are categorised according to the Maastricht criteria (Table 1), the 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, especially for kidney transplants. Extended criteria donors (ECD) are defined as; either donors >60yrs or donors 50-59yrs with two or more comorbidities (hypertension, serum creatinine >150, or cerebrovascular disease).
In such cases, a dual kidney transplantation from an ECD may be performed, in an attempt to provide equivocal renal function.
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.
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.
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 donor retrieval comprises of a warm and a cold phase dissection. Full exposure of the abdomen is obtained and the chest is also opened; the organs to be retrieved are exposed and dissected around, all taking place while the heart is still beating. The cold phase follows the cross-clamping of the aorta and perfusion of the donor with ice-cold solution.
For a retrieval from a DCD donor, a rapid laparotomy and aorta cannulation is performed, before the chest is opened to allow for further topical cooling. The remainder of the procedure is similar to the cold phase dissection of a DBD donor.
The liver is retrieved with the suprahepatic vena cava, a patch of the right hemidiaphragm and the coeliac axis with a patch of aorta. The pancreas is removed with the spleen and duodenum attached. The kidneys are removed with the renal artery with a patch of aorta, the renal vessel (the right one, which is shorter, is removed with a patch 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 and in-situ normothermic regional perfusion.
- 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