Indocyanine green R15 ratio depends directly on liver perfusion flow rate

Indocyanine green (ICG) is a synthetic dye that is widely used to evaluate liver function in critically ill patients, before liver resection or after liver transplantation. Controversy still exists about the impact exerted on the ICG ratio after 15 min (ICG R15) by differences in liver perfusion rates, hyperdynamic states, or patient cardiac output. We studied the role of different liver perfusion rates on the ICG R15 ratio in a normothermic extracorporeal liver perfusion system under standardized conditions.

Livers from landrace pigs (40-50 kg) were perfused with fresh porcine blood. Normal and high perfusion rates were defined as 1 ml and 2 ml/g liver/min, respectively. Perfusate pressure of the hepatic artery and portal vein were within the physiological range in both groups. According to manufacturer's instructions, 0.5 mg of ICG per kg was applied and the ICG R15 was calculated. Calculations were based on fifteen experiments in five liver perfusions. Bile production, liver function and histology were analyzed.

All perfusions were characterized by physiological bile production, lack of hepatocellular damage and normal histology. ICG R15 ratio in group I, perfused with 1 ml/g liver, was 18.9 +/- 6%. In group II, perfused with 2 ml/g liver, the ICG R15 ratio was 7.2 +/- 3%. The difference between groups 1 and 2 was statistically significant (p < 0.05).

ICG R15 is reliable within one group at defined perfusion rates. Doubled perfusion rates contribute to higher ICG clearance. For clinical application we would like to suggest considering cardiac output of the patient for interpretation of ICG ratios.