Liver perfusion of liver graft
During liver transplantation two forms of liver ischemia occurs: cold- and rewarming ischemia.
Cold ischemia occurs during organ retrieval and preservation, and it affects mainly the sinusoidal endothelial cells and causes cell swelling. The extent of endothelial cell detachment and injury correlates to the duration of cold ischemia. During organ implantation, rewarming ischemia occurs, intracellular energy stores decrease, and free radicals accumulate. Reperfusion of the liver after ischemic insults triggers a cascade of pathological events including disintegration of the sinusoidal endothelial cells, sinusoidal constriction, inflammatory reaction, stagnation of leukocytes and aggregation of platelets with formation of microthrombi. Ischemia injury followed by reperfusion injury, provokes two distinctive manifestations of microvascular damage known as capillary “no‑reflow” and the “reflow‑paradox”. The “no‑reflow” phenomenon is characterized by lack of blood flow upon onset of reperfusion, most likely due to swelling of the microvascular endothelium. The “reflow‑paradox” refers to leukocyte stasis and adherence to the lining endothelium and occurs after reperfusion. Therefore, the idea of continuous organ perfusion during preservation has been intensively investigated during last decade.
So far, ischemia‑reperfusion injury is unavoidable and liver graft injury occurs in every case and only the degree of injury varies. Liver grafts of good quality tolerate up to 12 hours of preservation time.
However, due to increasing shortage of donors, the threshold for extended criteria donors (ECD) is pushed further. Liver graft from ECD have higher risk for dysfunction or even non-function related to a combination of risk factors such as advanced donor age, hepatic steatosis, donors after cardiac death (DCD), long stay at intensive care unit before procurement, elevated liver enzymes and prolonged cold ischemia time. In fact, more than 50% of donors utilized in Sweden are classified as ECD.
Analyses of all donors during the last decades indicate that the percentage of ECD will increase and discrepancy between the needs and available donors will remain if not to increase further. Therefore, many countries allow the use of liver grafts from DCDs. DCD donation may increase organ donation rate (in case of kidney donation in the UK up to 30%) but it is associated with higher complication rates due to additional warm ischemia injury before organ cold perfusion caused by stop of blood circulation/heart death. As a standard preservation method, so called hypothermic static preservation is used, where the organ is perfused with a cold (at 4℃) preservation solution and left “on ice” with no further manipulation until the implantation. Use of ECD and DCD generates acute need for preservation methods that will not only minimize injury related to ischemia and reperfusion but that will give us opportunity to improve organ function before implantation (organ regeneration, Figure 1).
An attractive alternative for standard static hypothermic organ preservation is continuous machine perfusion.
Machine perfusion (dynamic organ preservation, DOP) consists of a pump creating a flow of blood or preservation solution through the organ during preservation. There is an increasing number of evidence that continuous organ perfusion provides better preservation by minimizing the effects of ischemic injuries while replenishing adenosine triphosphate (ATP) stores and removal of potentially toxic metabolites. Another advantage of using DOP would be to have the possibility to monitor the performance of the graft (improvement, deterioration, status quo) during perfusion time and to provide adjuvant substances during perfusion (active pharmacological treatment of the perfused graft; organ reconditioning).
Perfusion machine systems can be divided into three groups based on the temperature of preservation solution: hypothermic at 4‑10℃; subnormothermic at 20‑25℃, and normothermic at 37℃. Other parameters subgrouping perfusion machine systems include different flow regimes and pressures (pulsatile vs non-pulsatile), single (portal) vs dual perfusion (arterial and portal perfusion), and oxygenated vs non-oxygenated solution.
During the past decade promising data regarding liver perfusion machines has been reported on two major techniques, a normothermic and a hypothermic oxygenated perfusion. Most of the studies using liver perfusion machine are focused on reconditioning of livers from DCD donors. An alternative approach, dual vessel (portal and arterial) hypothermic oxygenated perfusion protocol (D‑HOPE), could be performed immediately after liver procurement or after transport of the organ to the transplant center (at the end of preservation but before implantation). Already in 2010, the safety of hypothermic perfusion of liver grafts was confirmed in a non-randomized trial, using hypothermic perfusion after cold static preservation. The results showed 60% less of peak ALT and 20% less of graft dysfunction in the machine perfusion group. Current results, even from the first randomized studies, showed improvement in short- and long‑term liver graft functions by using DOP. Similar improvement in organ quality can be seen when a perfusion machine is used in kidney, heart or lung transplantation.
It is important to stress that liver transplantation is a lifesaving procedure with no alternatives such dialysis treatment in patients with end stage renal failure. Every “extra” liver means one more saved life! Despite greater experience and technical advances in transplantation, the number of patients awaiting liver transplantation continues to grow and every year a significant number of patients die while on the waiting list. Experience from other countries showed that by better utilization of available organs thanks to organ reconditioning and use of liver perfusion machines, it is possible to significantly improve the number of transplanted organs.
We have previously introduced microdialysis to monitor liver grafts and recently introduced “Rapid Analyses of Liver Function” (RALF) in liver monitoring. We hope to be able to work out an algorithm for the “decision-making process” when evaluating liver graft for transplantation using these systems and obtain results within an acceptable time during organ preservation.
Elements of the RALF system can be used separately in any other research to understand pathophysiology of the liver as an organ. We aim for the status of a core facility for liver analyses technology with focus on innovation and automatization of the analyses.