Effects of Blood Pressure and Blood Viscosity on Fluorescein Transit Time in the Cerebral Microcirculation in the Mouse

Fluorescein transit time from pial arterioles to pial venules was measured in the mouse by analyzing motion pictures taken at 40 frames/sec. Increased blood pressure reduced the transit time. Increased blood viscosity lengthened the transit time. Viscosity was increased by rendering the animals polycythemic, or by infusions of high molecular weight dextran. Two different dextrans were used. The one with the higher molecular weight (500,000) produced a higher viscosity and a more prolonged transit time than the one with the lower molecular weight (250,000). These dextrans produced a prolongation of the transit time equal to that, or greater than that, observed in the polycythemic animals, even though whole blood viscosity in polycythemia exceeded that of the dextran treated mice over a wide range of physiologically pertinent shear rates applied in vitro in a rotational viscometer. Results indicate that increases of blood viscosity produce decreases in the velocity of the plasma, and that viscosity increments brought about by raising the hematocrit, may be less effective than viscosity increments brought about by raising the viscosity of the plasma.