(117a) Development of a Subnormothermic Machine Perfusion System for the Human Liver

The use of livers donated after cardiac death (DCD) of the donor is limited by the increased incidence of primary nonfunction and biliary complications following transplantation of these grafts. While the transplantation field continues to struggle with an ongoing shortage of donor organs, various strategies are being investigated to increase the availability of livers for transplantation, including the exploration of novel techniques to make DCD livers a safer option. Ex vivo machine perfusion holds the potential to strongly increase the quality of a donor liver and improving post-transplantation function and reducing complications. Collaborations with the New England Organ Bank (NEOB), the MGH transplant center and UMass Memorial medical center have provided us with human livers, discarded due to extended warm ischemic time, donor age, excessive steatosis or other contraindications for transplantation. Thus far, we have successfully perfused 8 discarded human livers, showing feasibility of subnormothermic machine perfusion in preserving the human liver ex vivo with active metabolism and demonstrating recovery of viability expressed in significantly improved energy status. Ongoing inclusion of human livers will allow characterization of these livers by broadening the ex vivo liver function testing, to include markers of injury and function.

Isolated (sub)normothermic organ perfusion is challenged by the absence of other homeostatic organs that are involved with maintenance of acid-base physiology (kidney, lung), energy metabolism (pancreas, intestine), fluid and electrolyte balance (kidney). These functions need to be compensated for by exogenous administration of hormones, nutrients, electrolytes and other substrates for metabolism along with a perfusion solution that supports this regulation as an artificial extracellular compartment. Controlling these functions requires a thorough understanding of both hepatic and systemic physiology and a system that supports it by allowing for real-time measurement of various parameters, easy intervention and reliable viability testing.

The subnormothermic system for the perfusion of human organs that we have developed is user friendly, but also enables advanced monitoring of hepatic function and intervention. An artificial cardiopulmonary system, including a set of pumps and hollow fiber oxygenators, drives the flow of an oxygen rich perfusion solution through the portal vein and hepatic artery of the liver. A carbogen mixture of 95% O₂/5% CO₂ creates an inflow pCO₂ of ±75 mmHg, which, in combination with exogenous bicarbonate plays a role in the carbonate buffer system. The pH of the system is periodically monitored and can be controlled by boluses of bicarbonate. Superphysiological partial oxygen pressures of >700 mmHg ensure adequate oxygenation of the tissue in the absence of hemoglobin. A commonly used hepatocyte culture medium is used a base solution for perfusion. Williams medium E is rich in nutrients and antioxidants. Supplemented with hydrocortisone and insulin it has been shown to provide a rich environment supporting hepatocyte viability in ex vivo liver perfusion and increasing survival after transplantation in the rat. Ports integrated into the system allow drawing samples for blood gas analysis, which provides periodical and on demand information about acid/base, oxygenation and electrolyte balance. Transducers proximal to both the portal and arterial inflow of the liver provide live pressure readings that can be used to calculate hepatic resistance. Samples from the perfusion solution can be analyzed for a multitude of metabolites.

Our results show that the human liver can be preserved for a minimum of three hours ex vivo in a subnormothermic setting (±21 °C), with continued metabolic function. A continuous consumption of O₂ and CO₂ output reflects active oxidative metabolism. An initially acidic perfusion solution and high lactate levels are normalized by the end of perfusion and electrolyte levels remain normal. An active biliary system is reflected in the steady production of bile. The portal venous and hepatic arterial circulation decrease in resistance, suggesting an improving circulation.  The recovery of energy status of the graft is demonstrated by an increase in ATP content during perfusion. To our knowledge this is the first report of the application of subnormothermic machine perfusion in human livers and this work will provide a basis for use of this system in a clinical trial for liver transplantation.