INTERPRETATION OF THE LASER-DOPPLER FLOW SIGNAL FROM THE LIVER OF THE RAT

It has been proposed that the laser Doppler flow (LDF) signal from the surface of the rat liver is almost exclusively a measure of hepatic arterial and not of total liver blood flow and therefore that LDF is not a suitable technique for the measurement of blood flow in the hepatic microcirculation. The objective of the present study was twofold: (1) to establish that liver blood flow is homogeneously distributed and (ii) to asess the behavior of the LDF signal during changes in hepatic perfusion. When 51Cr-labeled microspheres were injected into the portal vein (n = 12), no significant differences in the relative flow (cpm/lobe to cpm/liver) to each of the liver lobes were found nor was there any difference in the ratio of flow to the outer 1-2 mm of lobe as compared to that to the “core” of the liver. Temporary occlusion of the hepatic artery and the portal vein caused ~13% (n = 7, P < 0.001) and ~74% (n = 7, P < 0.001) fall in LDF signal, respectively. Diversion of flow from the anterior to the posterior lobes (n = 5) caused a 97.9 ± 21.1% (SD, P < 0.001) rise in LDF signal in the posterior lobes. Zero-flow LDF signal was found to represent 13.0 ± 4.1% of maximum. Hemorrhage (in 1.5-ml aliquots) was associated with a fall in mean arterial pressure (MAP) and LDF signals. A linear relationship between MAP and the LDF signal (r > 0.9) was found. Reinfusion of blood caused both MAP and the LDF signal to return to normal. We conclude that (i) blood flow in rat liver is homogeneously distributed; (ii) the LDF signal from the liver surface responds in a manner predicted by conventional theories of hepatic hemodynamics during alteration, either independent or combined, in hepatic arterial and portal venous blood flow; and (iii) LDF may be used to measure relative changes in hepatic perfusion but problems associated with zero-flow signal and intersite variability preclude its quantification in absolute flow unit. © 1993 Academic Press Inc.