To the Editor:
- We disagree that the rise in ALT within the first hour of perfusion can be interpreted as ongoing ischemia during that time period. We used a closed perfusion system and concentrations of ALT are a result of cumulative washout of transaminases from the graft after cold preservation, as has been shown in several animal studies . Values obtained in our ex vivo system cannot be interpreted the same way as in vivo values in blood of patients, as the perfusion circuit contains only 2 L compared to an average of 5 L of blood in vivo. Moreover, we strongly disagree that there was ongoing ischemic damage due to hypoperfusion during the first hour. First, as shown in Figure 4 there were no histological signs of zone III injury after 6 h of perfusion. Second, it is a mistake to extrapolate flow data obtained from in vivo studies in cirrhotic patients with portal hypertension such as by Kelly et al. one-to-one to the ex vivo settings used in our system. O2 saturation and pO2 values in both the hepatic artery and the portal vein were much higher in our system than can be obtained in vivo. O2 saturation remained near 100% and pO2 values were >60 kPa (>450 mmHg). Unlike flow data alone, oxygen delivery and extraction rates are much better parameters reflecting tissue oxygenation. In separate experiments we have observed normal oxygen extraction rates by livers in our perfusion system (unpublished data).
- It is true that our system did not contain platelets and leukocytes, because it was our aim to optimally preserve liver grafts and not to cause harm by mimicking reperfusion injury.
- Again we disagree that our system caused “significant ischemic injury” to the mucosa of the bile ducts. As was shown in Figure 5, there was no difference in mucosal loss before and after normothermic perfusion. Similar to what has been described in two independent clinical studies [4, 5], extrahepatic bile duct biopsies taken after cold storage and before machine preservation revealed partial loss of the biliary epithelial cell layer. Importantly, the amount of biliary epithelial cell loss did not increase during 6 h of machine perfusion. Also biliary levels of LDH sharply decreased during machine perfusion which is compatible with an initial washout of LDH from the biliary tree after the cold preservation rather than ongoing ischemic biliary damage. We agree that measuring biliary bile salt-to-phospholipid ratios would be of interest as this has been associated with biliary injury in clinical transplantation , and we are planning to include this parameter in future experiments.
- Thank you for the suggestion to include PAS staining in the histology studies. We will include this in our ongoing experiments.
S. op den Dries1,2, N. Karimian1,2 and R. J. Porte1,*
1Section of Hepatobiliary Surgery and Liver Transplantation, Department of Surgery, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands
2Surgical Research Laboratory, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands
*Corresponding author: Robert J. Porte, firstname.lastname@example.org
The authors of this letter have no conflicts of interest to disclose as described by the American Journal of Transplantation.