Measurement of Liver Blood Flow

Methods for measuring hepatic blood flow fit into three broad categories: clearance techniques, indicator dilution techniques, and direct measurements.

Clearance Techniques

Hepatic clearance depends on both hepatic blood flow and the liver's intrinsic ability to eliminate the molecule. Certain substances are selectively and extensively extracted from the circulation by the liver; their total clearances are nearly equal to the hepatic blood flow. When using such molecules, the indirect Fick principle can be used to compute hepatic blood flow. Substances with a very high intrinsic clearance include propranolol, lidocaine, ICG, and colloidal particles. ICG, for example, is taken up almost exclusively by hepatocytes and excreted unchanged in the bile. Despite the limited stability of this dye, a constant infusion of ICG may be the most reliable extraction method for determining liver blood flow. Kupffer cells avidly phagocytose radiolabeled colloidal particles such as gold-198. The area under the initial curve of radioactivity versus time provides a useful measure of hepatic blood flow when the reticuloendothelial system is functioning normally. ^[224]

Clearance techniques, however, do not reliably reflect hepatic blood flow in the presence of liver disease. With severe liver diseases, decreased clearances may reflect a combination of factors, including the loss of hepatocyte mass, a reduction in the capacity of hepatocytes to eliminate substances, and a decrease in hepatic blood flow.^[225]

Indicator Dilution Techniques

Unlike with clearance techniques, liver blood flow measurements by indicator dilution methods are unaffected by hepatic function. A radiolabeled marker (e.g., iodinated albumin) is injected into the spleen, and hepatic flow is determined from indicator dilution curves; these curves are obtained by sampling continuously from one of the hepatic veins or by external γ-scintillation counting. For this technique to be valid, the indicator must be resistant to clearance by the liver and uniformly mixed upon injection.^[226]

Direct Measurements

Electromagnetic flow probes provide direct measurements of blood flow through the hepatic artery or portal vein.^[227] Procedures for implanting the probes can themselves alter hepatic blood flow. If the probes are left in place after the implantation procedure, flow can be measured telemetrically, either in the absence or presence of anesthesia.