Assessment of liver allograft viability during ex vivo normothermic machine perfusion using the MS² score

Introduction Utilization of extended criteria donor organs becomes more and more frequent in order to counteract waitlist mortality. Normothermic machine perfusion (NMP) facilitates checking liver allografts for their viability and has the scope to improve graft quality. A variety of perfusion criteria have been reported previously to assure graft viability. After establishing NMP at University Hospital Muenster we aim to critically evaluate and simplify existing assessment scores and would like to introduce the MS²score (Münster machine score).

Methods In total 22 NMP of liver grafts were performed from 10/2019 until 10/2020. During graft perfusion portal- and arterial flow, perfusate pH, lactate and glucose levels as well as production of bile, bile viscosity and pH were monitored hourly. A minimum of 4 hours of NMP was local protocol to allow the graft to recover from static cold storage. Perfusate transaminases were monitored at 1, 4, 8, 12 and 16 hours of NMP. Following transplantation, serum lactate levels, clotting factors and transaminases of the recipient were monitored daily.

Results Average DRI was 1.7 despite rather short average cold ischemia time of 6,5 hours. Average NMP lasted 12 hours. Median recipient MELD was 26. 21 transplants were successful with 95,5 % primary function. There was no EAD beyond postoperative day 3, except for 3 cases (13 %) with elevated bilirubin levels for  > 7 days. Using previously reported viability variables such as transaminases  > 6000 U/l and perfusate pH < 7.2 we would have excluded 9 livers. Applying the Cambridge glucose metabolism criteria 7 additional grafts would have been excluded. Relying on perfusate lactate level ( < 3 mmol/l @ 4 h), arterial flow ( > 150 ml/min or  > 20 % of total perfusion volume) and production of viscous bile was sufficient for viability assessment. The presented simplified MS2score seems to be sufficient for viability testing livers on NMP including a significant proportion of grafts which elsewhere would have been excluded.

Conclusion The MS² score provides a simple and therefore ideal tool for viability assessment of livers on NMP. Previously reported viability criteria might have been chosen too carefully and would have excluded significant portion of functioning liver allografts.

Publication History

Article published online:
04 January 2021