**6.1. Machine perfusion**

Machine (*ex vivo*) perfusion (MP) entails cannulation and mechanical perfusion of the pancreas via its inflow vessels; perfusion fluid is re-circulated through the circuit for the duration of perfusion. Broadly, MP can be hypothermic, subnormothermic or normothermic, pulsatile or non-pulsatile, and continuous or for a limited proportion of the preservation/transport phase (e.g. pre-implantation). Current pancreatic MP work is lacking in the sphere of clinical transplantation, and is limited to pre-clinical animal and discarded human pancreas studies; only the latter will be the focus of this section, with experimental animal work summarized in detail elsewhere [118–120].

There are certain pancreas-specific issues that need to be considered with respect to MP that do not apply to other organs such as the kidney. Most importantly, the pancreas is a low-flow organ, and even relatively low pressures in a MP setup can result in significant graft edema and weight gain [121]. Furthermore, higher perfusion pressures can contribute to vascular thrombosis secondary to endothelial damage [120]. However, especially if MP is undertaken at normal body temperature (normothermic), such risks must then be balanced against the need for adequate perfusion to sustain normal aerobic metabolism. An additional challenge during pancreatic MP is the need to adequately and safely account for the organ's exocrine output, which is stimulated during normothermic perfusion [122].

As a result of these issues, most pancreatic MP studies have been conducted in the field of islet cell transplantation rather than the whole pancreas [120, 123]. Graft edema, is disadvantageous for both whole organ and cellular transplantation. However some groups have studied its use as it theoretically facilitates the enzymatic digestion of pancreatic acinar tissue [124]. Hypothermic MP can potentially be employed to increase human islet yield, viability, and insulin secretion despite an extended CIT (> 12 hours), possibly increasing the number of pancreata that can be used for successful islet isolation [125]. Cases of human islet transplantation following MP are yet to be published, however. Whole organ pancreas MP has been investigated in the context of extended criteria organs that were not utilized for human transplantation. Some authors have shown 6 hours of oxygenated hypothermic MP using UW machine perfusion solution increases the ATP content of DCD pancreata to reach a level that is similar to DBD pancreata at baseline [126]. Graft edema can be kept to a minimum if low pressure hypothermic MP is utilized, even for as long as 24 hours [127]. Subsequent *ex vivo* normothermic perfusion can be used to simulate reperfusion at transplantation after initial hypothermic MP, and has been shown to demonstrate adequate insulin secretion by such pancreata [128].

Normothermic MP is an attractive alternative for whole pancreas preservation, and likely provides better graft viability assessment than hypothermic perfusion. Both endocrine and exocrine graft function can be assessed during perfusion by measuring C-peptide and/or insulin secretion and stimulation in response to glucose, and amylase and lipase release, respectively [122, 129]. Blood flow and resistance parameters can also be assessed using this technique, although this is also possible with hypothermic MP. However it is important to note that no defined cut-offs or validated protocols for human transplantation have been developed, and will require significantly more pre-clinical work.
