LAUSR

Expansion of donor criteria for transplantation using increasingly marginal grafts and the development of machine perfusion technology to improve the function of donor livers have the potential to expand the donor pool of organs available albeit at the risk of increased morbidity and mortality. Concerns include primary nonfunction and ischemic cholangiopathy. There is a pressing need to develop technologies that allow greater understanding or the physiology of organs either in situ or during ex situ machine perfusion. Ideally, these should be real time, allowing an immediate assessment and prediction of organ performance following transplantation. Sidestream dark field (SDF) imaging is a videomicroscopic technique that can be used in vivo. It uses concentric green light emitting diodes of a wavelength that makes red blood cells appear dark. The optics allow direct visualization of blood vessels, blood vessel density, and blood flow. It is also a handheld device, which can be placed on or near the liver. As well as visualization of the blood vessels, there are numerical outputs that are believed to be important in measuring microcirculation. The vessel density can be seen by counting. The proportion of perfused vessels can also be calculated by counting. The microvascular flow index (MFI) is calculated based on the appearance of blood flow in vessels (absent, intermittent, sluggish, or normal). The heterogeneity index is derived from measuring the MFI in a number of areas of the liver and calculating the difference between the highest and lowest MFI scores and dividing by the mean flow velocity. Thus, SDF potentially allows measurement and visualization of blood flow and can also take into account patchy or variable blood flow in different regions of the liver, which may be an issue in some grafts. In this issue of Liver Transplantation, Pulitano et al. from the Australian National Liver Transplant Unit in Sydney, Australia, present their experience of using SDF to assess the performance of transplanted livers from deceased donors. Measurements were only taken after full reperfusion and were stratified based on the arterial and portal blood flow measured using flow probes. They defined early allograft dysfunction (EAD) as bilirubin> 10mg/dL on day 7 or international normalized ratio >1.6 on day 7 or alanine aminotransferase (ALT)> 2000 IU/L at any time in the first 7 days. Using this definition, a quarter of their patients had EAD, but there was no statistically significant difference between donor or recipient criteria in patients with or without EAD. This may in part be related to the relatively small sample size because there was certainly a higher incidence of reduced arterial and portal flow rates in the patients with EAD. Interestingly, there was an inverse correlation between flow parameters with increasing time difference between portal and arterial perfusion. Furthermore, the longer the portal-arterial perfusion time, the greater the flow heterogeneity in the graft. In a similar way, long cold ischemia times were also associated with more heterogeneous sinusoidal perfusion. The use of inotropes such as noradrenaline or vasopressin was associated with lower functional sinusoidal density (FSD; analogous to proportion of perfused vessels). Steatosis, fat content of >30%, was also associated with reduced FSD. Regardless of the cause of flow heterogeneity, this parameter was associated significantly with peak postoperative ALT and inversely correlated with bilirubin Abbreviations: ALT, alanine aminotransferase; EAD, early allograft dysfunction; FSD, functional sinusoidal density; MFI, microvascular flow index; SDF, sidestream dark field.