Background: Peripheral blood clearance of indocyanine green (ICG) has been used as a test of liver function but gives little information on biliary excretion. Hepatic ICG uptake and clearance can be measured directly by near-infrared spectroscopy (NIRS). Direct ICG measurement has not previously been correlated with liver blood flow and function in cirrhosis.
Methods: Two groups of New Zealand white rabbits (n = 12) underwent laparotomy for liver exposure. Cirrhosis was induced by feeding animals (n = 6) with a high-cholesterol (2 per cent) diet for 16 weeks. Hepatic blood flow and microcirculation were measured. Hepatic ICG concentration was measured directly using NIRS probes on the liver surface. From the ICG concentration-time curve, hepatic ICG uptake and excretion rates were calculated by a non-linear least square curve fitting method.
Results: There was a significant reduction in ICG uptake rate (mean(s.d.) 0.300(0.130) versus 2.040(0.420) min-1; P = 0.0001) and ICG excretion rate (0.007(0.009) versus 0.227(0.096) min-1; P = 0. 002) in cirrhotic animals. The hepatic ICG uptake rate correlated with hepatic blood flow and flow in the microcirculation (r = 0.81, P = 0.002; r = 0.92, P < 0.001, respectively). The hepatic ICG excretion rate was significantly associated with indicators of impaired liver function including bilirubin (r = - 0.86, P = 0.0004), aspartate aminotransferase (r = - 0.81, P = 0.001) and lactate dehydrogenase (r = - 0.83, P = 0.0008).
Conclusion: ICG uptake measured directly by NIRS reflects the reduced liver blood flow and perfusion in cirrhosis and its excretion correlates with the degree of liver parenchymal dysfunction. This technique may allow a more accurate method of liver function assessment than peripheral blood ICG clearance.