Effects of hypoxia due to isovolemic hemodilution on an ex vivo normothermic perfused liver model

Abstract

Background: The liver has a complex hormonal and nervous control mechanism leading to difficulty in the interpretations of its responses to chronic hypoxia. Theoretically an ex vivo perfused model of the liver should, by dissociating the organ from the extrinsic regulatory mechanisms, allow a better and unequivocal analysis of changes obtained.

Materials and methods: Twelve livers were harvested from female pigs and perfused for 6 h. Hypoxia was produced by means of isovolemic hemodilution with hemoglobin and hematocrit reduced to 40% of the baseline value. The extracorporeal circuit included a centrifugal pump, heat exchanger, and oxygenator. Every hour, physiological parameters (arterial/portal venous pressures and flows) were measured and blood samples were collected for the analysis of hemoglobin, red blood cells, hematocrit, lactate, glucose, albumin, alanine aminotransferase, alkaline phosphatase, and total bilirubin, arterial and venous blood gases. The arterio-venous oxygen and carbon dioxide differences, and the hepatic metabolic rate for oxygen, were also calculated. Primary endpoint of the study was the glucose response of the liver to acute hypoxia. Secondary endpoints were eventual changes of markers for hepatic viability and functionality.

Results: Most parameters showed significant variability during the first h of perfusion but subsequently normalized and remained stable at baseline values for the following 5 h. A strong and significant hyperglycemic response was present throughout the experiment (P < 0.001). Lactate rose steadily throughout the study period and after 6 h of perfusion there was a significant deviation from initial values (P < 0.05). Albumin did not change significantly throughout the study although a trend towards decreasing values was observed (Friedman test, P = NS). After an initial rise in levels of alanine transaminase and alkaline phosphatase following perfusion (P < 0.01), values remained constant without any further increase.

Conclusions: Following reperfusion in an ex vivo model, the liver reacts to low oxygen concentrations mobilizing glycogen deposits. This mechanism depends on an intrinsic sensibility of hepatocytes to hypoxia, as demonstrated by the ex vivo liver perfusion. These findings improve our knowledge in organ preservation for liver transplantation.