Microviscometric analysis of microvascular hemodynamics in vivo
Studies of blood flow in the microcirculation have been conducted for over a hundred years. Other than in 5–8 μm-diameter capillaries, where red cells travel in single-file flow, no method has been developed for either quantitatively predicting or accurately measuring the salient dynamic, kinematic, and rheological properties of blood flow in intact microvessels in vivo. A new microviscometric approach is presented, which we have validated through glass-tube studies in vitro (30–80 μm in diameter) and through isovolemic hemodilution studies in vivo, that provides estimates of hemorheological and fluid dynamical distributions over the cross section of microvessels greater than ∼20 μm in diameter with an accuracy and detail unprecedented in microvascular research. Microviscometry depends only upon knowledge of the velocity profile over the cross section of a microvessel, which can readily be extracted using fluorescent micro-particle image velocimetry (μ-PIV) from distributions in the translational speed and radial position of systemically injected sub-micron spheres within the microvessel. Microviscometric analysis solves the equation for conservation of momentum for a general incompressible non-Newtonian fluid, subject to the velocity profile extracted from the μ-PIV data, to provide viscosity, shear-rate, and shear-stress profiles as well as the axial pressure gradient, volume flow, local and apparent blood viscosities, and the tube and discharge hematocrit in intact microvessels in vivo. Using microviscometry, together with a detailed three-dimensional analysis of the local fluid dynamics in the vicinity of the vessel wall, we have been able to infer important hydrodynamic properties associated with the vascular endothelium. In recent years, the interface between blood and the vascular endothelium in microvessels has drawn considerable attention as evidence is uncovered that a macromolecular endothelial surface layer (ESL), strategically located at this interface, may play several important functional roles in microvascular physiology. First visualized in vivo using dye-exclusion methods in capillaries, our microviscometric analysis shows that the ESL exerts a significant effect on near-wall microfluidics in post-capillary venules, and provides the first direct estimate of the hydrodynamically relevant thickness of the layer in vivo. Results from our analysis have implications for broad areas of cardiovascular research including hemodynamics, inflammation, endothelial-cell mechanotransduction, angiogenesis, and tissue engineering.