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: (i) to establish that liver blood flow is homogeneously distributed and (ii) to assess 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 approximately 13% (n = 7, P < 0.001) and approximately 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 units.