Normothermic Acellular Machine Perfusion and Bile Duct Injury in Pig Livers Retrieved After Cardiac Death

Abstract

To the Editor: We have read with interest the article by Boehnert et al (1Boehnert MU Yeung JC Bazerbachi F et al.Normothermic acellular ex vivo liver perfusion reduces liver and bile duct injury of pig livers retrieved after cardiac death.Am J Transplant. 2013; 13: 1441-1449Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar), describing normothermic acellular machine perfusion of pig livers donated after cardiac death (DCD). The authors should be commended for performing this important study which adds to the evidence that normothermic machine perfusion (NMP) is a promising method to reduce biliary injury after transplantation of DCD livers. Using an erythrocyte-based perfusion fluid, our group previously reported the feasibility of NMP of human DCD livers, emphasizing that this preservation method is about to enter the clinical arena (2Op den Dries S Karimian N Sutton ME et al.Ex vivo normothermic machine perfusion and viability testing of discarded human donor livers.Am J Transplant. 2013; 13: 1327-1335Abstract Full Text Full Text PDF PubMed Scopus (216) Google Scholar). Nevertheless, there are still some unanswered questions that are not fully addressed by Boehnert et al (1Boehnert MU Yeung JC Bazerbachi F et al.Normothermic acellular ex vivo liver perfusion reduces liver and bile duct injury of pig livers retrieved after cardiac death.Am J Transplant. 2013; 13: 1441-1449Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar).–In contrast to previous studies on NMP, Boehnert et al did not use erythrocytes as oxygen carrier during NMP. The amount of oxygen that can be dissolved in aqueous fluids at 38°C is very low and it can be questioned whether sufficient amounts of oxygen are provided to the liver using an acellular perfusion fluid at 38°C. The authors present a method for calculating oxygen extraction that uses Henry’s constant for O2 in water at 25°C and does not correct for liver mass. Contrary to the described method, oxygen consumption is inappropriately expressed in mmHg (Results section and Figure 4) instead of a unit that takes flow and liver mass into account such as mL O2/g liver/min (3Tolboom H Pouw RE Izamis ML et al.Recovery of warm ischemic rat liver grafts by normothermic extracorporeal perfusion.Transplantation. 2009; 87: 170-177Crossref PubMed Scopus (71) Google Scholar). Because of this discrepancy between method and results, we question whether calculations of oxygen extraction were performed correctly.–Another aspect that raises questions is the preservation of bile ducts. Boehnert et al reported 100% necrosis of bile duct mucosa after reperfusion of DCD livers subjected to 60 min of donor warm ischemia and 4 h of static cold storage (CS). However, only one out of five DCD livers subjected to 60 min donor warm ischemia and 4 h CS plus 8 h NMP was reported to have minor bile duct injury after blood reperfusion. The other four bile ducts in this group demonstrated 100% intact mucosal lining without any necrosis. First, it remains unclear why biopsies of only five bile ducts were analyzed, whereas six livers were included in each group. Second, the results suggest that extension of the preservation time by 8 h of NMP, using oxygenated Steen solution, can almost completely protect bile ducts against universal cell death due to 60 min warm ischemia plus 4 h CS, followed by blood reperfusion. Such an impressive difference is difficult to understand.–Although bile flow has been indicated as the most important parameter of liver viability during NMP (4Imber CJ St Peter SD Lopez de Cenarruzabeitia I et al.Advantages of normothermic perfusion over cold storage in liver preservation.Transplantation. 2002; 73: 701-709Crossref PubMed Scopus (198) Google Scholar), bile production after either ex vivo reperfusion or transplantation was very low (1–2 mL/h) and no significant differences in bile flow were noted between NMP and CS. This raises the question how effective hepatic function was preserved by the presented NMP system.–Finally, it remains unclear why only bile flow and serum AST, and not bile duct histology or any other marker of function or injury, were presented as outcome parameters in the transplant model. The authors of this manuscript have no conflicts of interest to disclose as described by the American Journal of Transplantation.