Methodological problems in the use of indocyanine green to estimate hepatic blood flow and ICG clearance in man

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Abstract
Liver blood flow (Q) is often measured by constant infusion of ICG (i), concentration measurements in an artery (A) and a hepatic vein (V): Q=(A-V)/A. Some authors use ICG clearance, Cl=i/A, as a measure of Q assuming complete hepatic extraction. During the infusion, the ICG concentration often increases. The importance of this for calculated values of Q and Cl was examined, and the use of Cl as a measure of Q was reevaluated. ICG was given as 0.06-0.20 .mu.mol/min to 52 subjects with liver disease, and about 0.20 .mu.mol/min to 86 subjects with no liver disease. ICG concentration increased steeply during the first 90 min after start of the infusion; thereafter the increment was constant as evaluated in successive 40-min periods in eleven 320-min studies (analysis of variance P > 0.05); on average, 6 .+-. 1% per hour (.+-. SD). Q was not time-dependent (P > 0.05). ICG clearance decreased significantly, on average 5 .+-. 2% per hour (.+-. SD). Hepatic extraction fraction, (A-V)/A, (measurement period 90-130 min) was 0.34 .+-. 0.21 in liver patients (.+-. SD) and 0.61 .+-. 0.80 in controls. Cl and Q were positively correlated in both groups but with substantial scatter. Thus, not only is the calculated ICG clearance time-dependent but the extraction fraction is further so low and variable, that any use of ICG clearance as a measure of liver flow is not justified.

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