- Top of page
- MATERIALS AND METHODS
Early detection of vascular complications following liver surgery is crucial. In the present study, intrahepatic microdialysis was used for continuous monitoring of porcine liver metabolism during occlusion of either the portal vein or the hepatic artery. Our aim was to assess whether microdialysis can be used to detect impaired vascular inflow by metabolic changes in the liver. Changes in metabolite concentrations in the hepatic interstitium were taken as markers for metabolic changes. After laparotomy, microdialysis catheters were introduced directly into the liver, enabling repeated measurements of local metabolism. Glucose, lactate, pyruvate, and glycerol were analyzed at bedside every 20 minutes, and the lactate/pyruvate ratio was calculated. In the arterial clamping group, the glucose, lactate, glycerol, and lactate/pyruvate ratio significantly increased during the 2-hour vessel occlusion and returned to baseline levels during the 3-hour reperfusion. In the portal occlusion group and in the control group, the measured metabolites were stable throughout the experiment. Our findings show that liver metabolism, as reflected by changes in the concentrations of glucose, lactate, and glycerol and in the lactate/pyruvate ratio, is markedly affected by occlusion of the hepatic artery. Surprisingly, portal occlusion resulted in no major metabolic changes. In conclusion, the microdialysis technique can detect and monitor arterial vascular complications of liver surgery, whereas potential metabolic changes in the liver induced by portal occlusion were not seen in the current study. Microdialysis may thus be suitable for use in liver surgery to monitor intrahepatic metabolic changes. Liver Transpl 15:280–286, 2009. © 2009 AASLD.
Vascular complications after liver transplantation are rare but potentially fatal,1-3 and up to 50% of patients with vascular complications require retransplantation.3 Arterial thrombosis is the most common complication after liver transplantation, with a reported incident of up to 12% in adult recipients and up to 40% in pediatric recipients, and it constitutes up to 60% of all posttransplant vascular complications.4 Hepatic arterial thrombosis occurs about as often early after liver transplantation (within 30 days after the operation, 46.7%) as late (after 30 days, 53.3%).3 However, severe allograft dysfunction occurs predominantly after early hepatic arterial thrombosis, which also is associated with increased mortality.3 Late hepatic arterial thrombosis, on the other hand, results in biliary tract complications. Other vascular complications are inferior caval and hepatic vein thrombosis (1%),4 portal thrombosis (4%), and arterial and portal stenosis and aneurysm.5 Acute portal vein thrombosis or stenosis during the early course after liver transplantation may result in graft failure and necessitate retransplantation in up to 15% of the patients.2 Early detection of impaired liver graft function is thus vital for efficient treatment. The microdialysis technique6, 7 may be a suitable monitoring technique for vascular complications in the liver, as it enables continuous bedside monitoring of liver cell metabolism through a microdialysis catheter placed in the liver tissue.8, 9 The catheter has a tubular semipermeable dialysis membrane on its tip and is slowly perfused with Ringer solution, which equilibrates over the membrane with the molecules in the surrounding interstitial fluid (see Rosdahl et al.10). The dialysis fluid is collected, and glucose, lactate, pyruvate, and glycerol are analyzed by a bedside analyzer every 20 minutes.
Fluctuations in glucose may reflect changes in the blood glucose level or cellular glucose uptake,10 glycogenolysis, or changes in blood flow and thus altered glucose delivery to the tissue.11 Glycogenolysis is normally triggered in order to maintain a steady blood glucose level12 but can also be triggered by liver ischemia.13-16 During ischemia, large quantities of lactate are produced. However, increased lactate production can also be the result of hypermetabolism.17 To discriminate ischemia from hypermetabolism, pyruvate, which is the precursor of lactate, is analyzed, and the lactate/pyruvate ratio is calculated. In case of hypermetabolism, both lactate and pyruvate increase, and therefore the ratio remains unchanged, whereas during hypoxia/ischemia, pyruvate decreases and lactate increases; this leads to an increased lactate/pyruvate ratio.18 Glycerol is released primarily from fatty acid and phospholipid metabolism. As phospholipids are the main compounds of the cell membrane, glycerol release may also indicate cell membrane damage.19-22 In the present study, glycerol was considered as a marker for (hepatic) cellular membrane disintegration.
There are several publications based on the clinical use of microdialysis in transplanted livers that report significant correlations between increased interstitial lactate in the liver with sinusoidal endothelial cell injury, initial poor liver function,23 and reperfusion injury24 after liver transplantation. Other studies have described the normal course of glucose, lactate, glycerol, and pyruvate postoperatively after liver transplantation.8 Although clinical investigations are mainly observational, an experimental preclinical setting offers the possibility of inducing and studying liver ischemia under controlled conditions. The present study was undertaken to assess microdialysis as a monitoring technique for detection of vascular complications following liver surgery through the clamping of either the hepatic arteries or the portal vein. Warm liver ischemia in the pig is well tolerated for 120 minutes.25, 26 To ensure successful reperfusion and recovery, the hepatic artery was occluded for only 2 hours. Portal occlusion, on the other hand, is better tolerated by the liver27 and was therefore extended to 3 hours. In this group, portocaval shunts were made for splanchnic decompression.28, 29
- Top of page
- MATERIALS AND METHODS
In the present study, we show that hepatic artery occlusion causes ischemia and cell damage that is detectable with microdialysis, as indicated by increased glucose, lactate, and lactate/pyruvate ratio as ischemic markers and intrahepatic glycerol as a marker of cell damage. Portal occlusion, however, did not result in any significant changes in the measured metabolites.
During the first hour of arterial occlusion, interstitial liver glucose showed a significant increase, and this was followed by a decrease to baseline levels during the second hour of occlusion. The initial increase was most likely due to glycogenolysis triggered by the hypoxia caused by arterial occlusion.13-16 Glucose levels normalized before reperfusion, and this may indicate that the liver starts to adjust to the impaired arterial blood flow, decreasing the rate of glycogenolysis. As the portal vein continuously perfused the liver, it is unlikely that the decrease in glucose levels could be explained by impaired blood glucose delivery.
During arterial occlusion, there was an initial increase in lactate, peaking at 1 hour, and subsequently, there was a slow decrease. This decrease in lactate coincided with the decrease of glucose levels, and furthermore, the lactate/pyruvate ratio increased significantly upon clamping, after which levels stabilized throughout occlusion, indicating a steady state of anaerobic metabolism. After reperfusion, the liver metabolism normalized, and all metabolites returned to baseline levels.
Elevated levels of glycerol are seen during arterial occlusion but not during portal occlusion. This indicates that the liver cell damage is more pronounced by impaired arterial flow than by portal vein blockage. The increased glycerol could also be caused by the increased fat metabolism that has been shown to occur during liver ischemia.33 During reperfusion, glycerol levels normalized, and this indicated that the liver cell injury was not progressive.
Occlusion of the portal vein resulted in no major metabolic changes. We interpret the lack of any major effect on liver metabolism as due to the hepatic arterial buffer response,34 an intrinsic property of the hepatic artery, which dilates upon restricted portal vein flow. The hepatic arterial buffer response has been reported to have the capacity to increase hepatic arterial flow by 25%,34, 35 which may be enough to support the liver with oxygen, because the liver has an extraordinary ability to increase its oxygen extraction.36 The liver is, however, unable to reciprocally compensate for an arterial occlusion by dilating the portal vein.34 One limitation of the current study is that portal occlusion was performed under splanchnic decompression by the performance of an H-shunt between the portal vein and the inferior caval vein. However, for a controlled and stable porcine model, this is necessary in order to avoid cardiac stress or even death because the pig is particularly susceptible to portal venous obstruction.28, 29 The tendency for increased lactate in the liver was similar to that in the subcutaneous reference catheter. This may be the result of the limited liver capacity for lactate uptake during portal occlusion as well as the use of an H-shunt because the lactate from the intestines enters the systemic blood system and then goes back to the liver through the hepatic artery, hence increasing interstitial lactate levels in the liver. The lack of significant changes during portal occlusion may be explained by the use of the H-shunt because physiological changes due to intestinal stasis did not add to the pathophysiological picture. Further studies are needed on other species, shorter ischemia times, partial portal occlusion, and the occlusion of portal vein branches.
Our findings may be relevant in liver transplantation, where reperfusion traditionally is performed through the portal vein first. In a previous study of porcine liver transplantation, in which the portal vein was reperfused before the hepatic artery, we demonstrated that the lactate/pyruvate ratio did not decrease until after arterial reperfusion.9 This indicates that hepatic ischemia continues until arterial reperfusion, and areas such as the biliary tract, mainly supplied by the hepatic artery,37 are exposed to warm ischemia until arterial reperfusion. Although clinical reports have not demonstrated any differences in reperfusion injury, graft function, or outcome between initial arterial reperfusion and initial portal reperfusion38, 39 and no major differences in liver function,40 preclinical studies have shown that initial arterial reperfusion may cause less reperfusion injury to the liver.41, 42 Additionally, the majority of clinical studies are based on plasma levels of serum alanine aminotransferase, serum aspartate aminotransferase, and serum bilirubin and do not focus on biliary complications. The biliary tract depends entirely on an arterial blood supply.37 The importance of early arterial reperfusion is further emphasized by a lower rate of biliary complications when simultaneous arterial and portal revascularization is used in humans.43
The differences between the arterial and portal groups may be explained by different surgical preparations of the liver hilus between the groups. This is, however, not likely because it has been shown that microdialysis data quickly normalize after preparation to the same level within 120 minutes, with or without preparation.44 A limitation of the study may be that only 1 microdialysis catheter was used in the liver, which was assumed to reflect changes in the whole liver; however, according to Kannerup et al.,45 there are no differences in metabolic changes during ischemia between liver lobes, and they emphasized that 1 microdialysis catheter in the liver is enough for postoperative liver monitoring. Our results demonstrate furthermore that measurements from the subcutaneous reference catheters did not differ from the control group liver catheter measurements. Thus, catheters placed subcutaneously may be used as a reference to the liver.
In conclusion, the present study shows that arterial occlusion causes ischemia and cell damage that is detectable with microdialysis, as indicated by increased glucose, lactate, lactate/pyruvate ratio, and glycerol. The present experimental conditions could not reveal whether portal occlusion can be detected by microdialysis, as there were no ischemic changes in the portal occlusion group. The lack of metabolic changes upon portal occlusion may be explained by the arterial buffer response, which increases the hepatic arterial flow and oxygen delivery to compensate for the restricted portal flow. Another possible interpretation of these results is that arterial occlusion is more harmful to the liver than portal occlusion.