Conservation of Flow Demonstrated Using the Two-Slit Velocimeter and Cross Correlator in Epiilluminated Surface Microvessels of the Mouse Brain

Objective: To validate the use of two-slit velocimetry in an epiilluminated microvascular bed by demonstrating conservation of flow and to compare this validation with that provided by the literature for transilluminated beds. Methods: The brain surface vessels of mice were epiilluminated with a mercury lamp and observed with a Leitz Ultrapak objective (22X) and dipping cone. The internal diameters were measured with an ocular micrometer. The Instrumentation for Physiology and Medicine (IPM) velocimeter and cross correlator were used to measure red blood cell (RBC) velocity at the centerline of 17 different microvascular branch points. The velocity in the feeding arteriole and each branch or in the tributaries and draining venule were used, together with the respective diameters, to calculate flow in each vessel. Findings in the arterioles and venules were combined, because the results were the same in either set of vessels. Results: Flow in the main vessel (nl/sec) = 0.97 (sum of flow in branches or tributaries) +2. The correlation coefficient was 0.95. The relationship was not significantly different from an ideal slope of unity. However, in individual bifurcations the sum of flows in branches or tributaries deviated by as much as 40% from predicted values. Conclusions: On the average, conservation of flow was present. The deviations from conservation at individual bifurcations were similar to those reported by others who utilized transilluminated beds. Thus, when conservation of flow is the validating criterion, the two-slit technique used with epiillumination is not less valid than when used with transillumination.