Individual Vessel Segments Research Articles

A novel method for mapping the heterogeneity in blood supply to normal and malignant tissues in the mouse dorsal window chamber

A novel first-pass imaging method for studying blood flow in tumors and normal tissues in mouse dorsal window chamber preparations is reported in this study. A fluorescence-labeled macromolecule is administered i.v. as a bolus dose, and the signal generated by the bolus is detected as the bolus passes through the vascular network. The sensitivity of the method is sufficiently high that the bolus can be detected in any tumor or normal tissue microvessel. The method is particularly useful for measuring the velocity and direction of blood flow in individual vessel segments of normal and malignant tissues and the blood supply time (BST) of individual tumor vessel segments; i.e., the time required for arterial blood to flow from a supplying tumor artery to a downstream microvessel segment. Since all vessels within a window chamber preparation can be studied simultaneously in a single imaging sequence, a significant benefit of the method is that it can be used to produce maps of velocity of blood flow and BST of the entire microvascular network of xenografted tumors.

Structural adaptation of microvascular networks and development of hypertension.

Blood vessels are capable of dynamic structural adaptation in which their diameters and wall thicknesses change in response to chronic changes in hemodynamic conditions. Such structural changes can have large effects on vascular resistance to blood flow. Structural responses to hemodynamic stresses, i.e., wall shear stress resulting from blood flow and circumferential wall tension resulting from transmural pressure, have been extensively documented. Generally, increased shear stress causes increases in vessel diameter, whereas increased transmural pressure causes opposite effects. Theoretical models have been developed to analyze the consequences of these responses for the behavior of microvascular networks when subjected to changes in systemic circulatory conditions. An initial increase of cardiac output is assumed to increase flow and driving pressure in parallel. According to the models, structural adaptation results in substantially increased overall network flow resistance as flow is increased, and thus amplification of the initially imposed increase in driving pressure. This behavior, resulting from adaptation of individual vessel segments to intravascular pressure, is consistent with data on the development of hypertension, which suggest that an increase in cardiac output precedes the increase in peripheral resistance that is characteristic of established hypertension. Thus, vascular sensitivity to circumferential wall stress may play a crucial role in the development of hypertension.

Fractal branching pattern in the pial vasculature in the cat.

Arborization pattern was studied in pial vascular networks by treating them as fractals. Rather than applying elaborate taxonomy assembled from measures from individual vessel segments and bifurcations arranged in their branching order, the authors' approach captured the structural details at once in high-resolution digital images processed for the skeleton of the networks. The pial networks appear random and at the same time having structural elements similar to each other when viewed at different scales--a property known as self-similarity revealed by the geometry of fractals. Fractal (capacity) dimension, Dcap, was calculated to evaluate the network's spatial complexity by the box counting method (BCM) and its variant, the extended counting method (XCM). Box counting method and XCM were subject to numerical testing on ideal fractals of known D. The authors found that precision of these fractal methods depends on the fractal character (branching, nonbranching) of the structure they evaluate. Dcaps (group mean +/- SD) for the arterial and venous pial networks in the cat (n = 6) are 1.37 +/- 0.04, 1.37 +/- 0.02 by XCM, and 1.30 +/- 0.04, 1.31 +/- 0.03 by BCM, respectively. The arterial and venous systems thus appear to be developed according to the same fractal generation rule in the cat.

Diameter variability and microvascular flow resistance.

Microvessels are known to exhibit irregular shapes, deviating substantially from an idealized cylindrical tube geometry. Such irregularities must be taken into account in calculating microvascular flow resistance and may add to the observation that flow resistance in living microvessels in vivo is about twice that predicted on the basis of tube flow studies in vitro. The present study was aimed at providing a comprehensive database describing the apparent diameter variability for all segments of a complete microvascular network in the rat mesentery and assessing the impact of this variability on segmental flow resistance and the pressure drop across the network. Diameters were estimated by intravital microscopy at axial intervals of 20 microns along the 546 vessel segments of a mesenteric microvessel network, resulting in 6,319 separate diameter measurements. The amplitude of diameter variations in individual vessel segments decreased from approximately 15% of the mean vessel diameter in the smallest segments (approximately 5 microns diam) to approximately 5% in the largest segments (approximately 60 microns diam). Segmental hindrance was estimated to be 10-23% higher than calculated from arithmetic mean diameter, depending on the model used to estimate the hydrodynamically effective segment diameter. The overall pressure drop across the network calculated using a mathematical flow simulation was increased by 7-13.5%. This increase in flow resistance can explain approximately 10% of the observed discrepancy between flow resistance in vivo and in vitro.

Blood flow in microvascular networks. Experiments and simulation.

A theoretical model has been developed to simulate blood flow through large microcirculatory networks. The model takes into account the dependence of apparent viscosity of blood on vessel diameter and hematocrit (the Fahraeus-Lindqvist effect), the reduction of intravascular hematocrit relative to the inflow hematocrit of a vessel (the Fahraeus effect), and the disproportionate distribution of red blood cells and plasma at arteriolar bifurcations (phase separation). The model was used to simulate flow in three microvascular networks in the rat mesentery with 436,583, and 913 vessel segments, respectively, using experimental data (length, diameter, and topological organization) obtained from the same networks. Measurements of hematocrit and flow direction in all vessel segments of these networks tested the validity of model results. These tests demonstrate that the prediction of parameters for individual vessel segments in large networks exhibits a high degree of uncertainty; for example, the squared coefficient of correlation between predicted and measured hematocrit of single vessel segments ranges only between 0.15 and 0.33. In contrast, the simulation of integrated characteristics of the network hemodynamics, such as the mean segment hematocrit or the distribution of blood flow velocities, is very precise. In addition, the following conclusions were derived from the comparison of predicted and measured values: 1) The low capillary hematocrits found in mesenteric microcirculatory networks as well as their heterogeneity can be explained on the basis of the Fahraeus effect and phase-separation phenomena. 2) The apparent viscosity of blood in vessels of the investigated tissue with diameters less than 15 microns is substantially higher than expected compared with measurements in glass tubes with the same diameter.

Generalization of the Fahraeus principle for microvessel networks.

Microvessel hematocrits and diameters were determined in each vessel segment between bifurcations of three complete microvascular networks in rat mesentery. Classification of the segments as arteriolar, venular, or arteriovenular (av) was based on flow direction at branch points. Photographic and videomicroscopic mapping was used to obtain quantitative information on the architecture and topology of the networks. This topological information allowed the analysis of hematocrit distribution within a series of consecutive-flow cross sections, each of which carried the total flow through the network. The observed reduction of mean hematocrit in the more peripheral cross sections is explained by the presence of a "vessel" and a "network" Fahraeus effect. The vessel Fahraeus effect results from velocity difference between red cells and blood within the individual vessel segments due to the existing velocity and cell concentration profiles. The network Fahraeus effect is based on the velocity difference of red cells and blood caused by velocity and hematocrit heterogeneity between the vessels constituting any of the complete-flow cross sections. The network Fahraeus effect is found to account for approximately 20% of the total hematocrit reduction and increases toward the most distal cross sections.