Galactose clearance measurements and liver blood flow

Galactose clearance, measured during low galactose infusion and calculated as infusion rate divided by peripheral galactose concentration (systemic clearance), has been proposed as a measure of liver blood flow. This requires nearly complete hepatic extraction as well as negligible extrahepatic elimination. The purpose of the study was to examine if these assumptions are fulfilled in subjects with no liver disease, and to compare the galactose clearance measurement with an independent measurement of liver blood flow. Liver blood flow was measured in 6 subjects by means of a constant indocyanine green infusion, indocyanine green concentration measurements in a peripheral artery and a hepatic vein, and calculation according to Fick's principle. The mean (+/- SEM) blood flow rate was 1.2 +/- 0.1 L/min. Galactose was given at a constant infusion rate of 142 +/- 10 mumol/min, and steady-state concentrations were measured in the peripheral artery (A) and the hepatic vein (V). The hepatic extraction fraction [(A - V)/A] was 0.91 +/- 0.03. The hepatic galactose elimination rate [(A - V) X flow] was 101 +/- 12 mumol/min; this is about two-thirds of the total elimination rate (viz., infusion rate). Urinary excretion was negligible. This indicates an extrahepatic galactose elimination of approximately 41 mumol/min. Systemic galactose clearance, calculated as mentioned above, was 1.5 +/- 0.1 L blood/min. It was significantly higher than the liver blood flow in each subject (paired t-test, each p less than 0.02), on average 133% of the flow. Thus the systemic galactose clearance value overestimates liver blood flow, probably due to a small, but in this context quantitatively important, extrahepatic galactose elimination.