The Fahraeus effect in narrow capillaries (i.d. 3.3 to 11.0 micron)

Blood flow dynamics in the arterial and venous parts of the capillary

Although there is currently sufficient information on various parameters of capillary blood flow, including the average values of blood velocity, there is no data on the dynamics of velocity and the mechanisms of its modulation in various parts of the capillary. The main idea of this work is to develop a tool and image data processing to study the characteristics of the capillary blood flow dynamics. In this study, using the developed method of high-speed videocapillaroscopy, the red blood cells (RBC) velocities in the arterial and venous parts of the nailfold capillaries were compared and a time-frequency analysis of the dynamics of the velocity signals with the calculation of phase coherence was performed. We indicated that the velocity in the arterial part is twice as high and that the ratio of velocities in the arterial and venous parts is stable regardless of the local velocity. This study also empirically confirms the similarity between the oscillations of blood flow in different parts of the capillary and the synchronization of the velocity phases. We believe that the determination of the absolute velocity characteristics of blood flow, together with the mechanisms of its regulation and the ratio of velocities in the arterial and venous parts, can act as a diagnostic approach.

Hemodynamic Characteristics of a Tortuous Microvessel Using High-Fidelity Red Blood Cell Resolved Simulations

OBJECTIVE

Tortuous microvessels are characteristic of microvascular remodeling associated with numerous physiological and pathological scenarios. Three-dimensional (3D) hemodynamics in tortuous microvessels influenced by red blood cells (RBCs), however, are largely unknown, and important questions remain. Is blood viscosity influenced by vessel tortuosity? How do RBC dynamics affect wall shear stress (WSS) patterns and the near-wall cell-free layer (CFL) over a range of conditions? The objective of this work was to parameterize hemodynamic characteristics unique to a tortuous microvessel.

METHODS

RBC-resolved simulations were performed using an immersed boundary method-based 3D fluid dynamics solver. A representative tortuous microvessel was selected from a stimulated angiogenic network obtained from imaging of the rat mesentery and digitally reconstructed for the simulations. The representative microvessel was a venule with a diameter of approximately 20 μm. The model assumes a constant diameter along the vessel length and does not consider variations due to endothelial cell shapes or the endothelial surface layer.

RESULTS

Microvessel tortuosity was observed to increase blood apparent viscosity compared to a straight tube by up to 26%. WSS spatial variations in high curvature regions reached 23.6 dyne/cm2 over the vessel cross-section. The magnitudes of WSS and CFL thickness variations due to tortuosity were strongly influenced by shear rate and negligibly influenced by tube hematocrit levels.

CONCLUSIONS

New findings from this work reveal unique tortuosity-dependent hemodynamic characteristics over a range of conditions. The results provide new thought-provoking information to better understand the contribution of tortuous vessels in physiological and pathological processes and help improve reduced-order models.

A computational framework for investigating bacteria transport in microvasculature

Blood-borne bacteria disseminate in tissue through microvasculature or capillaries. Capillary size, presence of red blood cells (RBCs), and bacteria motility affect bacteria intracapillary transport, an important yet largely unexplored phenomenon. Computational description of the system comprising interactions between plasma, RBCs, and motile bacteria in 5-10 μm diameter capillaries pose several challenges. The Immersed Boundary Method (IBM) was used to resolve the capillary, deformed RBCs, and bacteria. The challenge of disparate coupled time scales of flow and bacteria motion are reconciled by a temporal multiscale simulation method. Bacterium-wall and bacterium-RBC collisions were detected using a hierarchical contact- detection algorithm. Motile bacteria showed a net outward radial velocity of 2.8 µm/s compared to -0.5 µm/s inward for non-motile bacteria; thus, exhibiting a greater propensity to escape the bolus flow region between RBCs and marginate for potential extravasation, suggesting motility enhances extravasation of bacteria from capillaries.

Chain-Like Structures of Biogenic and Nonbiogenic Magnetic Nanoparticles in Vascular Tissues

In this paper, slices of organs from various organisms (animals, plants, fungi) were investigated by using atomic force microscopy and magnetic force microscopy to identify common features of localization of both biogenic and nonbiogenic magnetic nanoparticles. It was revealed that both biogenic and nonbiogenic magnetic nanoparticles are localized in the form of chains of separate nanoparticles or chains of conglomerates of nanoparticles in the walls of the capillaries of animals and the walls of the conducting tissue of plants and fungi. Both biogenic and nonbiogenic magnetic nanoparticles are embedded as a part of the transport system in multicellular organisms. In connection with this, a new idea of the function of biogenic magnetic nanoparticles is discussed, that the chains of biogenic magnetic nanoparticles and chains of conglomerates of biogenic magnetic nanoparticles represent ferrimagnetic organelles of a specific purpose. Besides, magnetic dipole-dipole interaction of biogenic magnetic nanoparticles with magnetically labeled drugs or contrast agents for magnetic resonance imaging should be considered when designing the drug delivery and other medical systems because biogenic magnetic nanoparticles in capillary walls will serve as the trapping centers for the artificial magnetic nanoparticles. The aggregates of both artificial and biogenic magnetic nanoparticles can be formed, contributing to the risk of vascular occlusion. Bioelectromagnetics. 43:119-143, 2022. © 2021 Bioelectromagnetics Society.

Red blood cells under flow show maximal ATP release for specific hematocrit

ATP release by red blood cells (RBCs) under shear stress (SS) plays a pivotal role in endothelial biochemical signaling cascades. The aim of this study is to investigate through numerical simulation how RBC spatiotemporal organization depends on flow and geometrical conditions to generate ATP patterns. Numerical simulations were conducted in a straight channel by considering both plasma and explicit presence of RBCs, their shape deformation and cell-cell interaction, and ATP release by RBCs. Two ATP release pathways through cell membrane are taken into account: pannexin 1 channel, sensitive to SS, and cystic fibrosis transmembrane conductance regulator, which responds to cell deformation. Several flow and hematocrit conditions are explored. The problem is solved by the lattice Boltzmann method. Application of SS to the RBC suspension triggers a nontrivial spatial RBC organization and ATP patterns. ATP localizes preferentially in the vicinity of the cell-free layer close to channel wall. Conditions for maximal ATP release per cell are identified, which depend on vessel size and hematocrit Ht. Increasing further Ht beyond optimum enhances the total ATP release but should degrade oxygen transport capacity, a compromise between an efficient ATP release and minimal blood dissipation. Moreover, ATP is boosted in capillaries, suggesting a vasomotor activity coordination throughout the resistance network.

FEM based simulation of magnetic drug targeting in a multibranched vessel model

BACKGROUND AND OBJECTIVE

Magnetic drug targeting (MDT) is a promising technology to improve cancer therapy. MDT describes the accumulation of drug loaded superparamagnetic iron oxide nanoparticles (SPIONs) at a desired location, e. g. a tumor, by application of a magnetic field. Here, we evaluate the effectivity of MDT for an endoscopic placement of two different configurations of magnet arrays, i. e. six magnets with same poles facing each other and a Halbach array. Compared to conventional magnet setups outside the body, this endoscopic placement gives the possibility to achieve higher magnetic field gradients inside the tumor.

METHODS

For such a MDT concept, we present FEM based simulations of MDT tracing single SPIONs in a 3D geometry of eight multibranched vessels with sizes in the range of capillaries. In these simulations, the effect of the magnetic field gradient as well as of magnet distance to the vessel geometry, magnetic flux density of the magnets, SPIONs hydrodynamic diameter and magnetic moment on the MDT effectivity is calculated. The blood flow is modelled as an incompressible Newtonian fluid and the SPIONs are suspended in the blood flow. Statistical significance of the targeting effectivity results is tested with the Mann-Whitney-U-Test.

RESULTS

The results show that the magnetic targeting effectivity is up to 32 % higher than the one calculated without the presence of a magnetic field. In the investigated vessel network, this effect on the targeting effectivity is dependent on the number of local magnetic field maxima that are approached with a high gradient and is noticeable up to 200 µm distance of the magnet to the vessel geometry.

CONCLUSIONS

We conclude that for an effective application of MDT, the magnet configuration needs to be placed close to the tumor and should yield a large number of magnetic field maxima that are approached with a high gradient.

Emergent cell-free layer asymmetry and biased haematocrit partition in a biomimetic vascular network of successive bifurcations

Blood is a vital soft matter, and its normal circulation in the human body relies on the distribution of red blood cells (RBCs) at successive bifurcations. Understanding how RBCs are partitioned at bifurcations is key for the optimisation of microfluidic devices as well as for devising novel strategies for diagnosis and treatment of blood-related diseases. We report the dynamics of RBC suspensions flowing through a biomimetic vascular network incorporating three generations of microchannels and two classical types of bifurcations at the arteriole level. Our microfluidic experiments with dilute and semidilute RBC suspensions demonstrate the emergence of excessive heterogeneity of RBC concentration in downstream generations upon altering the network's outflow rates. Through parallel simulations using the immersed-boundary-lattice-Boltzmann method, we reveal that the heterogeneity is attributed to upstream perturbations in the cell-free layer (CFL) and lack of its recovery between consecutive bifurcations owing to suppressed hydrodynamic lift under reduced flow conditions. In the dilute/semidilute regime, this perturbation dominates over the effect of local fractional flow at the bifurcation and can lead to inherently unfavourable child branches that are deprived of RBCs even for equal flow split. Our work highlights the importance of CFL asymmetry cascading down a vascular network, which leads to biased phase separation that deviates from established empirical predictions.

Model-based evaluation of the microhemodynamic effects of PEGylated HBOC molecules in the rat brain cortex: a laser speckle imaging study

Hemoglobin-based oxygen carriers (HBOCs) were developed with the aim of substituting transfusions in emergency events. However, they exhibit adverse events, such as nitric oxide (NO) scavenging, vasoactivity, enhanced platelet aggregation, presently hampering their clinical application. The impact of two prototypical PEGylated HBOCs, Euro-PEG-Hb and PEG-HbO₂, endowed by different oxygen affinities and hydrodynamic volumes, was assessed on the cerebrocortical parenchymal microhemodynamics, and extravasation through the blood-brain-barrier (BBB) by laser speckle contrast imaging (LSCI) method and near-infrared (NIR) imaging, respectively. By evaluating voxel-wise cerebrocortical red blood cell velocity, non-invasively for its mean kernel-wise value ( v ¯ RBC ), and model-derived kernel-wise predictions for microregional tissue hematocrit, ^TH_t, and fractional change in hematocrit-corrected vascular resistance, R', as measures of potential adverse effects (enhanced platelet aggregation and vasoactivity, respectively) we found i) no significant difference between tested HBOCs in the systemic and microregional parameters, and in the relative spatial dispersion of ^TH_t, and R' as additional measures of HBOC-related adverse effects, and ii) no extravasation through BBB by Euro-PEG-Hb. We conclude that Euro-PEG-Hb does not exhibit adverse effects in the brain microcirculation that could be directly attributed to NO scavenging.

Spatiotemporal Dynamics of Dilute Red Blood Cell Suspensions in Low-Inertia Microchannel Flow

Microfluidic technologies are commonly used for the manipulation of red blood cell (RBC) suspensions and analyses of flow-mediated biomechanics. To enhance the performance of microfluidic devices, understanding the dynamics of the suspensions processed within is crucial. We report novel, to our knowledge, aspects of the spatiotemporal dynamics of RBC suspensions flowing through a typical microchannel at low Reynolds number. Through experiments with dilute RBC suspensions, we find an off-center two-peak (OCTP) profile of cells contrary to the centralized distribution commonly reported for low-inertia flows. This is reminiscent of the well-known "tubular pinch effect," which arises from inertial effects. However, given the conditions of negligible inertia in our experiments, an alternative explanation is needed for this OCTP profile. Our massively parallel simulations of RBC flow in real-size microfluidic dimensions using the immersed-boundary-lattice-Boltzmann method confirm the experimental findings and elucidate the underlying mechanism for the counterintuitive RBC pattern. By analyzing the RBC migration and cell-free layer development within a high-aspect-ratio channel, we show that such a distribution is co-determined by the spatial decay of hydrodynamic lift and the global deficiency of cell dispersion in dilute suspensions. We find a cell-free layer development length greater than 46 and 28 hydraulic diameters in the experiment and simulation, respectively, exceeding typical lengths of microfluidic designs. Our work highlights the key role of transient cell distribution in dilute suspensions, which may negatively affect the reliability of experimental results if not taken into account.

Chemotaxing neutrophils enter alternate branches at capillary bifurcations

Upon tissue injury or microbial invasion, a large number of neutrophils converge from blood to the sites of injury or infection in a short time. The migration through a limited number of paths through tissues and capillary networks seems efficient and 'traffic jams' are generally avoided. However, the mechanisms that guide efficient trafficking of large numbers of neutrophils through capillary networks are not well understood. Here we show that pairs of neutrophils arriving closely one after another at capillary bifurcations migrate to alternating branches in vivo and in vitro. Perturbation of chemoattractant gradients and the increased hydraulic resistance induced by the first neutrophil in one branch biases the migration of the following neutrophil towards the other branch. These mechanisms guide neutrophils to efficiently navigate through capillary networks and outline the effect of inter-neutrophil interactions during migration on overall lymphocyte trafficking patterns in confined environments.

Krogh’s capillary recruitment hypothesis, 100 years on: Is the opening of previously closed capillaries necessary to ensure muscle oxygenation during exercise?

In 1919, August Krogh published his seminal work on skeletal muscle oxygenation. Krogh’s observations indicated that muscle capillary diameter is actively regulated, rather than a passive result of arterial blood flow regulation. Indeed, combining a mathematical model with the number of ink-filled capillaries he observed in muscle cross sections taken at different workloads, Krogh was able to account for muscle tissue’s remarkably efficient oxygen extraction during exercise in terms of passive diffusion from nearby capillaries. Krogh was awarded the 1920 Nobel Prize for his account of muscle oxygenation. Today, his observations are engrained in the notion of capillary recruitment: the opening of previously closed capillaries. While the binary distinction between “closed” and “open” was key to Krogh’s model argument, he did in fact report a continuum of capillary diameters, degrees of erythrocyte deformation, and perfusion states. Indeed, modern observations question the presence of closed muscle capillaries. We therefore examined whether changes in capillary flow patterns and hematocrit among open capillaries can account for oxygen extraction in muscle across orders-of-magnitude changes in blood flow. Our four-compartment model of oxygen extraction in muscle confirms this notion and provides a framework for quantifying the impact of changes in microvascular function on muscle oxygenation in health and disease. Our results underscore the importance of capillary function for oxygen extraction in muscle tissue as first proposed by Krogh. While Krogh’s model calculations still hold, our model predictions support that capillary recruitment can be viewed in the context of continuous, rather than binary, erythrocyte distributions among capillaries.

NEW & NOTEWORTHY Oxygen extraction in working muscle is extremely efficient in view of single capillaries properties. The underlying mechanisms have been widely debated. Here, we develop a four-compartment model to quantify the influence of each of the hypothesized mechanisms on muscle oxygenation. Our results show that changes in capillary flow pattern and hematocrit can account for the high oxygen extraction observed in working muscle, while capillary recruitment is not required to account for these extraction properties.

Topographic Distribution of Contractile Protein in the Human Macular Microvasculature

Purpose

We studied the topographic distribution of contractile protein in different orders of the human macular microvasculature to further understanding of the sites for capillary blood flow regulation.

Methods

Nine donor eyes from eight donors were cannulated at the central retinal artery and perfusion labeled for alpha smooth muscle actin (αSMA) and filamentous actin (F-actin). Confocal images were collected from the macula region, viewed, projected, and converted to a 255 grayscale for measurements. The mean intensity was measured for macular arterioles, venules, and capillary segments. The diameter of each vessel segment measured was recorded. The normalized mean intensity values from all images were ranked according to vessel types and size with a total of nine categories.

Results

F-actin was present throughout the macular microvasculature whereas αSMA labeling showed variations. Overall, αSMA has a more prominent presence in the macular arterioles than in the macular capillaries and venules, and αSMA strongly labeled the smaller macular arterioles. Some capillaries also labeled positive for αSMA, including some of the capillaries in the innermost capillary ring surrounding the foveola. It was weakly present in the capillaries on the venous side and larger venules. In the larger macular arterioles closer to 100 μm in diameter, αSMA labeling was weakly present and not as ubiquitous as in the smaller arterioles.

Conclusions

Nonuniform distribution of contractile proteins in the different types, orders, and sizes of macular microvasculature indicates that these vessels may have different contractile capability and roles in macular flow regulation.

Experimental and Numerical Study of Blood Flow in μ-vessels: Influence of the Fahraeus–Lindqvist Effect

The study of hemodynamics is particularly important in medicine and biomedical engineering as it is crucial for the design of new implantable devices and for understanding the mechanism of various diseases related to blood flow. In this study, we experimentally identify the cell free layer (CFL) width, which is the result of the Fahraeus–Lindqvist effect, as well as the axial velocity distribution of blood flow in microvessels. The CFL extent was determined using microscopic photography, while the blood velocity was measured by micro-particle image velocimetry (μ-PIV). Based on the experimental results, we formulated a correlation for the prediction of the CFL width in small caliber (D < 300 μm) vessels as a function of a modified Reynolds number (Re∞) and the hematocrit (Hct). This correlation along with the lateral distribution of blood viscosity were used as input to a “two-regions” computational model. The reliability of the code was checked by comparing the experimentally obtained axial velocity profiles with those calculated by the computational fluid dynamics (CFD) simulations. We propose a methodology for calculating the friction loses during blood flow in μ-vessels, where the Fahraeus–Lindqvist effect plays a prominent role, and show that the pressure drop may be overestimated by 80% to 150% if the CFL is neglected.

Retinal capillary perfusion: Spatial and temporal heterogeneity

The central role of the cardiovascular system is to maintain adequate capillary perfusion. The spatially and temporally heterogeneous nature of capillary perfusion has been reported in some organs. However, such heterogeneous perfusion properties have not been sufficiently explored in the retina. Arguably, spatial and temporal heterogeneity of capillary perfusion could be more predominant in the retina than that in other organs. This is because the retina is one of the highest metabolic demand neural tissues yet it has a limited blood supply due to optical requirements. In addition, the unique heterogeneous distribution of retinal neural cells within different layers and regions, and the significant heterogeneity of intraretinal oxygen distribution and consumption add to the complexity. Retinal blood flow distribution must match consumption of nutrients such as oxygen and glucose within the retina at the cellular level in order to effectively maintain cell survival and function. Sophisticated local blood flow control in the microcirculation is likely required to control the retinal capillary perfusion to supply local retinal tissue and accommodate temporal and spatial variations in metabolic supply and demand. The authors would like to update the knowledge of the retinal microvessel and capillary network and retinal oxidative metabolism from their own studies and the work of others. The coupling between blood supply and energy demands in the retina is particularly interesting. We will mostly describe information regarding the retinal microvessel network and retinal oxidative metabolism relevant to the spatial and temporal heterogeneity of capillary perfusion. We believe that there is significant and necessary spatial and temporal heterogeneity and active regulation of retinal blood flow in the retina, particularly in the macular region. Recently, retinal optical coherence tomography angiography (OCTA) has been widely used in ophthalmology, both experimentally and clinically. OCTA could be a valuable tool for examining retinal microvessel and capillary network structurally and has potential for determining retinal capillary perfusion and its control. We have demonstrated spatial and temporal heterogeneity of capillary perfusion in the retina both experimentally and clinically. We have also found close relationships between the smallest arterioles and capillaries within paired arterioles and venules and determined the distribution of smooth muscle cell contraction proteins in these vessels. Spatial and temporal heterogeneity of retinal capillary perfusion could be a useful parameter to determine retinal microvessel regulatory capability as an early assay for retinal vascular diseases. This topic will be of great interest, not only for the eye but also other organs. The retina could be the best model for such investigations. Unlike cerebral vessels, retinal vessels can be seen even at the capillary level. The purpose of this manuscript is to share our current understanding with the readers and encourage more researchers and clinicians to investigate this field. We begin by reviewing the general principles of microcirculation properties and the spatial and temporal heterogeneity of the capillary perfusion in other organs, before considering the special requirements of the retina. The local heterogeneity of oxygen supply and demand in the retina and the need to have a limited and well-regulated retinal circulation to preserve the transparency of the retina is discussed. We then consider how such a delicate balance of metabolic supply and consumption is achieved. Finally we discuss how new imaging methodologies such as optical coherence tomography angiography may be able to detect the presence of spatial and temporal heterogeneity of capillary perfusion in a clinical setting. We also provide some new information of the control role of very small arterioles in the modulation of retinal capillary perfusion which could be an interesting topic for further investigation.

Microvascular hemodynamics: System properties1

The hemodynamics of the microcirculation reflect system properties of the involved components. The blood itself is a complex suspension of water, small and large molecules and different cell types. Under most conditions, its rheologic properties are dominated by the different behaviour of fluid and cellular compartments. When perfused through small-bore tubes or vessels, the suspension exhibits specific emergent properties. The Fahraeus-effect and the Fahreaeus-Lindqvist-effect result from the interaction of cellular particles with each other and with the vessel wall. Additional phenomena occur in vascular networks due to the uneven distribution of blood cells and blood plasma at divergent microvascular bifurcations. In order to understand microvascular hemodynamics in vivo but also in artificial microfluidic geometries it is thus necessary to recognize the pertinent system properties on the level of the blood, the microvessels and the microvascular networks or perfused structures.

Hydrodynamic mobility of confined polymeric particles, vesicles, and cancer cells in a square microchannel

The transport of deformable objects, including polymer particles, vesicles, and cells, has been a subject of interest for several decades where the majority of experimental and theoretical studies have been focused on circular tubes. Due to advances in microfluidics, there is a need to study the transport of individual deformable particles in rectangular microchannels where corner flows can be important. In this study, we report measurements of hydrodynamic mobility of confined polymeric particles, vesicles, and cancer cells in a linear microchannel with a square cross-section. Our operating conditions are such that the mobility is measured as a function of geometric confinement over the range 0.3 < λ < 1.5 and at specified particle Reynolds numbers that are within 0.1 < Rep < 2.5. The experimental mobility data of each of these systems is compared with the circular-tube theory of Hestroni, Haber, and Wacholder [J. Fluid Mech. 41, 689-705 (1970)] with modifications made for a square cross-section. For polymeric particles, we find that the mobility data agrees well over a large confinement range with the theory but under predicts for vesicles. The mobility of vesicles is higher in a square channel than in a circular tube, and does not depend significantly on membrane mechanical properties. The mobility of cancer cells is in good agreement with the theory up to λ ≈ 0.8, after which it deviates. Comparison of the mobility data of the three systems reveals that cancer cells have higher mobility than rigid particles but lower than vesicles, suggesting that the cell membrane frictional properties are in between a solid-like interface and a fluid bilayer. We explain further the differences in the mobility of the three systems by considering their shape deformation and surface flow on the interface. The results of this study may find potential applications in drug delivery and biomedical diagnostics.

Optimal occlusion uniformly partitions red blood cells fluxes within a microvascular network

In animals, gas exchange between blood and tissues occurs in narrow vessels, whose diameter is comparable to that of a red blood cell. Red blood cells must deform to squeeze through these narrow vessels, transiently blocking or occluding the vessels they pass through. Although the dynamics of vessel occlusion have been studied extensively, it remains an open question why microvessels need to be so narrow. We study occlusive dynamics within a model microvascular network: the embryonic zebrafish trunk. We show that pressure feedbacks created when red blood cells enter the finest vessels of the trunk act together to uniformly partition red blood cells through the microvasculature. Using mathematical models as well as direct observation, we show that these occlusive feedbacks are tuned throughout the trunk network to prevent the vessels closest to the heart from short-circuiting the network. Thus occlusion is linked with another open question of microvascular function: how are red blood cells delivered at the same rate to each micro-vessel? Our analysis shows that tuning of occlusive feedbacks increase the total dissipation within the network by a factor of 11, showing that uniformity of flows rather than minimization of transport costs may be prioritized by the microvascular network.

Arterial trees shuttle red blood cells from the heart to billions of capillaries distributed throughout the body. These trees have long been thought to be organized to minimize transport costs. Yet red blood cells are tightly squeezed within the finest vessels, meaning that these vessels account for as much as half of the total transport costs within the arterial network. It is unclear why vessel diameters and red blood cell diameters are so closely matched in a network that is presumed to optimize transport. Here, we use mathematical modeling and direct observations of red blood cell movements in embryonic zebrafish to show that occlusive feedbacks—the pressure feedbacks that alter the flows into a vessel when it is nearly blocked by a red blood cell—can optimally distribute red blood cells through microvessels. In addition to revealing an adaptive function for the matching of vessel and red blood cell diameters, this work shows that uniformity of red blood cell fluxes can be a unifying principle for understanding the elegant hydraulic organization of microvascular networks.

Margination of Fluorescent Polylactic Acid-Polyaspartamide based Nanoparticles in Microcapillaries In Vitro: the Effect of Hematocrit and Pressure

The last decade has seen the emergence of vascular-targeted drug delivery systems as a promising approach for the treatment of many diseases, such as cardiovascular diseases and cancer. In this field, one of the major challenges is carrier margination propensity (i.e., particle migration from blood flow to vessel walls); indeed, binding of these particles to targeted cells and tissues is only possible if there is direct carrier–wall interaction. Here, a microfluidic system mimicking the hydrodynamic conditions of human microcirculation in vitro is used to investigate the effect of red blood cells (RBCs) on a carrier margination in relation to RBC concentration (hematocrit) and pressure drop. As model drug carriers, fluorescent polymeric nanoparticles (FNPs) were chosen, which were obtained by using as starting material a pegylated polylactic acid–polyaspartamide copolymer. The latter was synthesized by derivatization of α,β-poly(N-2-hydroxyethyl)-d,l-aspartamide (PHEA) with Rhodamine (RhB), polylactic acid (PLA) and then poly(ethyleneglycol) (PEG) chains. It was found that the carrier concentration near the wall increases with increasing pressure drop, independently of RBC concentration, and that the tendency for FNP margination decreases with increasing hematocrit. This work highlights the importance of taking into account RBC–drug carrier interactions and physiological conditions in microcirculation when planning a drug delivery strategy based on systemically administered carriers.

The optimum hematocrit

The hematocrit (Hct) determines the oxygen carrying capacity of blood, but also increases blood viscosity and thus flow resistance. From this dual role the concept of an optimum Hct for tissue oxygenation has been derived. Viscometric studies using the ratio Hct/blood viscosity at high shear rate showed an optimum Hct of 50-60% for red blood cell (RBC) suspensions in plasma. For the perfusion of an artificial microvascular network with 5-70μm channels the optimum Hct was 60-70% for high driving pressures. With lower shear rates or driving pressures the optimum Hct shifted towards lower values. In healthy, well trained athletes an increase of the Hct to supra-normal levels can increase exercise performance. These data with healthy individuals suggest that the optimum Hct for oxygen transport may be higher than the physiological range (35-40% in women, 39-50% in men). This is in contrast to clinical observations. Large clinical studies have repeatedly shown that a correction of anemia in a variety of disorders such as chronic kidney disease, heart failure, coronary syndrome, oncology, acute gastrointestinal bleeding, critical care, or surgery have better clinical outcomes when restrictive transfusion strategies are applied. Actual guidelines, therefore, recommend a transfusion threshold of 7-8 g/dL hemoglobin (Hct 20-24%) in stable, hospitalized patients. The discrepancy between the optimum Hct in health and disease may be due to factors such as decreased perfusion pressures (low cardiac output, vascular stenoses, change in vascular tone), endothelial cell dysfunction, leukocyte adhesion and others.

Transverse water relaxation in whole blood and erythrocytes at 3T, 7T, 9.4T, 11.7T and 16.4T; determination of intracellular hemoglobin and extracellular albumin relaxivities

Blood is a physiological substance with multiple water compartments, which contain water-binding proteins such as hemoglobin in erythrocytes and albumin in plasma. Knowing the water transverse (R₂) relaxation rates from these different blood compartments is a prerequisite for quantifying the blood oxygenation level-dependent (BOLD) effect. Here, we report the Carr-Purcell-Meiboom-Gill (CPMG) based transverse (R_2CPMG) relaxation rates of water in bovine blood samples circulated in a perfusion system at physiological temperature in order to mimic blood perfusion in humans. R_2CPMG values of blood plasma, lysed packed erythrocytes, lysed plasma/erythrocyte mixtures, and whole blood at 3 T, 7 T, 9.4 T, 11.7 T and 16.4 T were measured as a function of hematocrit or hemoglobin concentration, oxygenation, and CPMG inter-echo spacing (τ_cp). R_2CPMG in lysed cells showed a small τ_cp dependence, attributed to the water exchange rate between free and hemoglobin-bound water to be much faster than τ_cp. This was contrary to the tangential dependence in whole blood, where a much slower exchange between cells and blood plasma applies. Whole blood data were fitted as a function of τ_cp using a general tangential correlation time model applicable for exchange as well as diffusion contributions to R_2CPMG, and the intercept R_20blood at infinitely short τ_cp was determined. The R_20blood values at different hematocrit and the R_2CPMG values of lysed erythrocyte/plasma mixtures at different hemoglobin concentration were used to determine the relaxivity of hemoglobin inside the erythrocyte (r_2Hb) and albumin (r_2Alb) in plasma. The r_2Hb values obtained from lysed erythrocytes and whole blood were comparable at full oxygenation. However, while r_2Hb determined from lysed cells showed a linear dependence on oxygenation, this dependence became quadratic in whole blood. This possibly suggests an additional relaxation effect inside intact cells, perhaps due to hemoglobin proximity to the erythrocyte membrane. However, we cannot exclude that this is a consequence of the simple tangential model used to remove relaxation contributions from exchange and diffusion. The extensive data set presented should be useful for future theory development for the transverse relaxation of blood.

Label free measurement of retinal blood cell flux, velocity, hematocrit and capillary width in the living mouse eye

Measuring blood cell dynamics within the capillaries of the living eye provides crucial information regarding the health of the microvascular network. To date, the study of single blood cell movement in this network has been obscured by optical aberrations, hindered by weak optical contrast, and often required injection of exogenous fluorescent dyes to perform measurements. Here we present a new strategy to non-invasively image single blood cells in the living mouse eye without contrast agents. Eye aberrations were corrected with an adaptive optics camera coupled with a fast, 15 kHz scanned beam orthogonal to a capillary of interest. Blood cells were imaged as they flowed past a near infrared imaging beam to which the eye is relatively insensitive. Optical contrast of cells was optimized using differential scatter of blood cells in the split-detector imaging configuration. Combined, these strategies provide label-free, non-invasive imaging of blood cells in the retina as they travel in single file in capillaries, enabling determination of cell flux, morphology, class, velocity, and rheology at the single cell level.

Evaluation of erythrocyte flow at a bearing gap in a hydrodynamically levitated centrifugal blood pump

We have developed a hydrodynamically levitated centrifugal blood pump for extracorporeal circulatory support. In the blood pump, a spiral groove bearing was adopted for a thrust bearing. In the spiral groove bearing, separation of erythrocytes and plasma by plasma skimming has been postulated to occur. However, it is not clarified that plasma skimming occurs in a spiral groove bearing. The purpose of this study is to verify whether plasma skimming occurs in the spiral groove bearing of a hydrodynamically levitated centrifugal blood pump. For evaluation of plasma skimming in the spiral groove bearing, an impeller levitation performance test using a laser focus displacement meter and a microscopic visualization test of erythrocyte flow using a high-speed microscope were conducted. Bovine blood diluted with autologous plasma to adjust hematocrit to 1.0% was used as a working fluid. Hematocrit on the ridge region in the spiral groove bearing was estimated using image analysis. As a result, hematocrits on the ridge region with gaps of 45 μm, 31 μm, and 25 μm were calculated as 1.0%, 0.6%, and 0.3%, respectively. Maximum skimming efficiency in this study was calculated as 70% with a gap of 25 μm. We confirmed that separation of erythrocyte and plasma occurred in the spiral groove bearing with decrease in bearing gap in a hydrodynamically levitated centrifugal blood pump.

Red blood cells affect the margination of microparticles in synthetic microcapillaries and intravital microcirculation as a function of their size and shape

A key step in particle-based drug delivery throughmicrocirculation is particlemigration from blood flow to vesselwalls, also known as “margination”,which promotes particle contact and adhesion to the vesselwall. Margination and adhesion should be independently addressed as two distinct phenomena, considering that the former is a fundamental prerequisite to achieve particle adhesion and subsequent extravasation. Although margination has beenmodeled by numerical simulations and investigated inmodel systems in vitro, experimental studies including red blood cells (RBCs) are lacking. Here, we evaluate the effect of RBCs on margination through microfluidic studies in vitro and by intravital microscopy in vivo.We showthatmargination,which is almost absent when particles are suspended in a cell-free medium, is drastically enhanced by RBCs. This effect is size- and shape-dependent, larger spherical/discoid particles being more effectively marginated both in vitro and in vivo. Our findings can be explained by the collision of particles with RBCs that induces the drifting of the particles towards the vessel walls where they become trapped in the cell-free layer. These results are relevant for the design of drug delivery strategies based on systemically administered carriers.

Shear stress is normalized in glomerular capillaries following ⅚ nephrectomy

Loss of significant functional renal mass results in compensatory structural and hemodynamic adaptations in the nephron. While these changes have been characterized in several injury models, how they affect hemodynamic forces at the glomerular capillary wall has not been adequately characterized, despite their potential physiological significance. Therefore, we used intravital multiphoton microscopy to measure the velocity of red blood cells in individual glomerular capillaries of normal rats and rats subjected to ⅚ nephrectomy. Glomerular capillary blood flow rate and wall shear stress were then estimated using previously established experimental and mathematical models to account for changes in hematocrit and blood rheology in small vessels. We found little change in the hemodynamic parameters in glomerular capillaries immediately following injury. At 2 wk postnephrectomy, significant changes in individual capillary blood flow velocity and volume flow rate were present. Despite these changes, estimated capillary wall shear stress was unchanged. This was a result of an increase in capillary diameter and changes in capillary blood rheology in nephrectomized rats.

Comparison of blood flow models and acquisitions for quantitative myocardial perfusion estimation from dynamic CT

Myocardial blood flow (MBF) can be estimated from dynamic contrast enhanced (DCE) cardiac CT acquisitions, leading to quantitative assessment of regional perfusion. The need for low radiation dose and the lack of consensus on MBF estimation methods motivates this study to refine the selection of acquisition protocols and models for CT-derived MBF. DCE cardiac CT acquisitions were simulated for a range of flow states (MBF = 0.5, 1, 2, 3 ml (min g)(-1), cardiac output = 3, 5, 8 L min(-1)). Patient kinetics were generated by a mathematical model of iodine exchange incorporating numerous physiological features including heterogenenous microvascular flow, permeability and capillary contrast gradients. CT acquisitions were simulated for multiple realizations of realistic x-ray flux levels. CT acquisitions that reduce radiation exposure were implemented by varying both temporal sampling (1, 2, and 3 s sampling intervals) and tube currents (140, 70, and 25 mAs). For all acquisitions, we compared three quantitative MBF estimation methods (two-compartment model, an axially-distributed model, and the adiabatic approximation to the tissue homogeneous model) and a qualitative slope-based method. In total, over 11 000 time attenuation curves were used to evaluate MBF estimation in multiple patient and imaging scenarios. After iodine-based beam hardening correction, the slope method consistently underestimated flow by on average 47.5% and the quantitative models provided estimates with less than 6.5% average bias and increasing variance with increasing dose reductions. The three quantitative models performed equally well, offering estimates with essentially identical root mean squared error (RMSE) for matched acquisitions. MBF estimates using the qualitative slope method were inferior in terms of bias and RMSE compared to the quantitative methods. MBF estimate error was equal at matched dose reductions for all quantitative methods and range of techniques evaluated. This suggests that there is no particular advantage between quantitative estimation methods nor to performing dose reduction via tube current reduction compared to temporal sampling reduction. These data are important for optimizing implementation of cardiac dynamic CT in clinical practice and in prospective CT MBF trials.

Red blood cell dynamics in polymer brush-coated microcapillaries: A model of endothelial glycocalyx in vitro

The confined flow of red blood cells (RBCs) in microvasculature is essential for oxygen delivery to body tissues and has been extensively investigated in the literature, both in vivo and in vitro. One of the main problems still open in microcirculation is that flow resistance in microcapillaries in vivo is higher than that in vitro. This discrepancy has been attributed to the glycocalyx, a macromolecular layer lining the inner walls of vessels in vivo, but no direct experimental evidence of this hypothesis has been provided so far. Here, we investigate the flow behavior of RBCs in glass microcapillaries coated with a polymer brush (referred to as "hairy" microcapillaries as opposed to "bare" ones with no coating), an experimental model system of the glycocalyx. By high-speed microscopy imaging and image analysis, a velocity reduction of RBCs flowing in hairy microcapillaries as compared to bare ones is indeed found at the same pressure drop. Interestingly, such slowing down is larger than expected from lumen reduction due to the polymer brush and displays an on-off trend with a threshold around 70 nm of polymer brush dry thickness. Above this threshold, the presence of the polymer brush is associated with an increased RBC deformation, and RBC velocity is independent on polymer brush thickness (at the same pressure drop). In conclusion, this work provides direct support to the hypothesis that the glycocalyx is the main factor responsible of the higher flow resistance found in microcapillaries in vivo.

A first principles calculation of the oxygen uptake in the human pulmonary acinus at maximal exercise

It has recently been shown that the acinus can have a reduced efficiency due to a "screening effect" governed by the ratio of oxygen diffusivity to membrane permeability, the gas flow velocity, as well as the size and configuration of the acinus. We present here a top to bottom calculation of the functioning of a machine acinus at exercise that takes this screening effect into account. It shows that, given the geometry and the breathing dynamics of real acini, respiration can be correlated to a single equivalent parameter that we call the integrative permeability. In particular we find that both V(O(2,max)) and PA(O(2)) depend on this permeability in a non-linear manner. Numerical solutions of dynamic convection-diffusion equations indicate that only a narrow range of permeability values is compatible with the experimental measurements of PA(O(2)) and V(O(2,max)). These permeability values are significantly smaller than those found in the literature. In a second step, we present a new type of evaluation of the diffusive permeability, yielding values compatible with the top to bottom approach, but smaller than the usual morphometric value.

Blood flow in microvascular networks: a study in nonlinear biology

Plasma skimming and the Fahraeus-Lindqvist effect are well-known phenomena in blood rheology. By combining these peculiarities of blood flow in the microcirculation with simple topological models of microvascular networks, we have uncovered interesting nonlinear behavior regarding blood flow in networks. Nonlinearity manifests itself in the existence of multiple steady states. This is due to the nonlinear dependence of viscosity on blood cell concentration. Nonlinearity also appears in the form of spontaneous oscillations in limit cycles. These limit cycles arise from the fact that the physics of blood flow can be modeled in terms of state dependent delay equations with multiple interacting delay times. In this paper we extend our previous work on blood flow in a simple two node network and begin to explore how topological complexity influences the dynamics of network blood flow. In addition we present initial evidence that the nonlinear phenomena predicted by our model are observed experimentally.

Red blood cell distribution in simplified capillary networks

A detailed model of red blood cell (RBC) transport in a capillary network is an indispensable element of a comprehensive model for the supply of the human organism with oxygen and nutrients. In this paper, we introduce a two-phase model for the perfusion of a capillary network. This model accounts for the special role of RBCs, which have a strong influence on network dynamics. Analytical results and numerical simulations with a discrete model and a generic network topology indicate that there exists a local self-regulation mechanism for the flow rates and a global de-mixing process that leads to an inhomogeneous haematocrit distribution. Based on the results from the discrete model, we formulate an efficient algorithm suitable for computing the pressure and flow field as well as a continuous haematocrit distribution in large capillary networks at steady state.

Distribution of capillary transit times in isolated lungs of oxygen-tolerant rats

Rats pre-exposed to 85% O₂ for 5-7 days tolerate the otherwise lethal effects of 100% O₂. The objective was to evaluate the effect of rat exposure to 85% O₂ for 7 days on lung capillary mean transit time t(c) and distribution of capillary transit times (h(c)(t)). This information is important for subsequent evaluation of the effect of this hyperoxia model on the redox metabolic functions of the pulmonary capillary endothelium. The venous concentration vs. time outflow curves of fluorescein isothiocyanate labeled dextran (FITC-dex), an intravascular indicator, and coenzyme Q₁ hydroquinone (CoQ₁H₂), a compound which rapidly equilibrates between blood and tissue on passage through the pulmonary circulation, were measured following their bolus injection into the pulmonary artery of isolated perfused lungs from rats exposed to room air (normoxic) or 85% O₂ for 7 days (hyperoxic). The moments (mean transit time and variance) of the measured FITC-dex and CoQ₁H₂ outflow curves were determined for each lung, and were then used in a mathematical model [Audi et al. J. Appl. Physiol. 77: 332-351, 1994] to estimate t(c) and the relative dispersion (RD(c)) of h (c)(t). Data analysis reveals that exposure to hyperoxia decreases lung t(c) by 42% and increases RD(c), a measure h(c)(t) heterogeneity, by 40%.

Microvascular Rheology and Hemodynamics

The goal of elucidating the biophysical and physiological basis of pressure-flow relations in the microcirculation has been a recurring theme since the first observations of capillary blood flow in living tissues. At the birth of the Microcirculatory Society, seminal observations on the heterogeneous distribution of blood cells in the microvasculature and the rheological properties of blood in small bore tubes raised many questions on the viscous properties of blood flow in the microcirculation that captured the attention of the Society's membership. It is now recognized that blood viscosity in small bore tubes may fall dramatically as shear rates are increased, and increase (dramatically with elevations in hematocrit. These relationships are strongly affected by blood cell deformability and concentration, red cell aggregation, and white cell interactions with the red cells anti endothelium. Increasing strength of red cell aggregation may result in sequestration of clumps of red cells with either reductions or increases in microvascular hematocrit dependent upon network topography. During red cell aggregation, resistance to flow may thus decrease with hematocrit reduction or increase due to redistribution of red cells. Blood cell adhesion to the microvessel wall may initiate flow reductions, as, for example, in the case of red cell adhesion to the endothelium in sickle cell disease, or leukocyte adhesion in inflammation. The endothelial glycocalyx has been shown to result from a balance of the biosynthesis of new glycans, and the enzymatic or shear-dependent alterations in its composition. Flow-dependent reductions in the endothelial surface layer may thus affect the resistance to flow and/or the adhesion of red cells and/or leukocytes to the endothelium. Thus, future studies aimed at the molecular rheology of the endothelial surface layer may provide new insights into determinants of the resistance to flow.

Normal endothelium

In recent decades, it has become evident that the endothelium is by no means a passive inner lining of blood vessels. This 'organ' with a large surface (approximately 350 m2) and a comparatively small total mass (approximately 110 g) is actively involved in vital functions of the cardiovascular system, including regulation of perfusion, fluid and solute exchange, haemostasis and coagulation, inflammatory responses, vasculogenesis and angiogenesis. The present chapter focusses on two central aspects of endothelial structure and function: (1) the heterogeneity in endothelial properties between species, organs, vessel classes and even within individual vessels and (2) the composition and role of the molecular layer on the luminal surface of endothelial cells. The endothelial lining of blood vessels in different organs differs with respect to morphology and permeability and is classified as 'continuous', 'fenestrated' or 'discontinuous'. Furthermore, the mediator release, antigen presentation or stress responses of endothelial cells vary between species, different organs and vessel classes. Finally there are relevant differences even between adjacent endothelial cells, with some cells exhibiting specific functional properties, e.g. as pacemaker cells for intercellular calcium signals. Organ-specific structural and functional properties of the endothelium are marked in the vascular beds of the lung and the brain. Pulmonary endothelium exhibits a high constitutive expression of adhesion molecules which may contribute to the margination of the large intravascular pool of leucocytes in the lung. Furthermore, the pulmonary microcirculation is less permeable to protein and water flux as compared to large pulmonary vessels. Endothelial cells of the blood-brain barrier exhibit a specialised phenotype with no fenestrations, extensive tight junctions and sparse pinocytotic vesicular transport. This barrier allows a strict control of exchange of solutes and circulating cells between the plasma and the interstitial space. It was observed that average haematocrit levels in muscle capillaries are much lower as compared to systemic haematocrit, and that flow resistance of microvascular beds is higher than expected from in vitro studies of blood rheology. This evidence stimulated the concept of a substantial layer on the luminal endothelial surface (endothelial surface layer, ESL) with a thickness in the range of 0.5-1 microm. In comparison, the typical thickness of the glycocalyx directly anchored in the endothelial plasma membrane, as seen in electron micrographs, amounts to only about 50-100 microm. Therefore it is assumed that additional components, e.g. adsorbed plasma proteins or hyaluronan, are essential in constituting the ESL. Functional consequences of the ESL presence are not yet sufficiently understood and acknowledged. However, it is evident that the thick endothelial surface layer significantly impacts haemodynamic conditions, mechanical stresses acting on red cells in microvessels, oxygen transport, vascular control, coagulation, inflammation and atherosclerosis.

The haemodynamic response to submaximal exercise during isovolaemic haemodialysis

INTRODUCTION

Exercise during haemodialysis has potential benefits but may compromise cardiovascular stability. We studied its acute effects on relative blood volume (RBV) and other haemodynamic parameters.

METHODS

Two groups of 10 patients were exercised submaximally using a stationary cycle during isovolaemic dialysis whilst RBV was monitored continuously. In study 1, patients exercised for two 10 min periods separated by 10 min rest. Cardiac output (CO), peripheral vascular resistance (PVR), central blood volume (CBV) and stroke volume were measured using ultrasound dilution immediately before and after each exercise session. In study 2, haemoglobin, serum total protein and albumin levels were measured before and immediately after the exercise session and at the nadir of the RBV trace.

RESULTS

RBV fell immediately on exercise initiation, the maximum reduction being 2.0+/-1.1% (after 5.9+/-1.4 min of exercise 1: P<0.001) and 2.0+/-1.2% (after 4.7+/-2.3 min of exercise 2: P<0.001). CO increased significantly after both periods of exercise (4.5+/-0.96 and 5.1+/-1.1 to 7.2+/-2.1 and 7.9+/-2.4 l/min, P<0.001 in both). Stroke volume increased significantly and PVR fell significantly during exercise. CBV increased in absolute terms but fell as a proportion of CO. Mean haemoglobin level at the RBV nadir was significantly higher than baseline (12.3+/-1.8 vs 11.8+/-1.7 g/dl: P<0.05: mean change 4.4+/-2.3%), as was mean total protein concentration (66.0+/-6.9 vs 62.0+/-8.1 g/l: P = 0.001: mean change 6.8+/-5.9%) and mean serum albumin concentration (36.0+/-3.9 vs 34.1+/-3.9 g/l: P<0.001: mean change 5.8+/-3.5%).

CONCLUSION

The haemodynamic response to exercise during haemodialysis is comparable with that in normal individuals. The rapid reduction in RBV on exercise occurs in spite of a significant increase in CO, mainly as a consequence of fluid shifts from the microvasculature to the interstitium.

Pancreatic capillary blood flow in an improved model of necrotizing pancreatitis in the rat

INTRODUCTION

The development of acute pancreatitis is characterized by profound changes in pancreatic microcirculation. Using in vivo microscopy with fluorescent-labeled erythrocytes as tracers we studied changes in pancreatic microcirculation in an improved rat model of necrotizing pancreatitis (NP) in comparison to edematous pancreatitis (EP) and healthy controls.

METHODS

Twenty-one male Wistar rats had their pancreatae exteriorized in a temperature-controlled immersion chamber followed by intravenous administration of fluorescent-labeled autologous erythrocytes. EP was induced by intraductal saline and intravenous caerulein (5 microg/kg/h) for 6 h (n = 7) and NP by controlled intraductal infusion of glycodeoxycholic acid (10 mmol/L) followed by intravenous caerulein (n = 7). Control animals received intraductal and intravenous saline (n = 7). The determination of pancreatic microcirculation was performed before as well as 1, 3, and 6 h after intraductal infusion by correlating the number of passing labeled erythrocytes/capillary/min with their concentration per microliter of arterial blood.

RESULTS

Pancreatic capillary flow in control animals remained constant over the 6-h observation period. Pancreatic capillary flow in the EP group rapidly increased to 188% of baseline after 3 h and remained significantly elevated throughout the experiments (P = 0.0001). In contrast, pancreatic capillary flow decreased significantly in the group suffering NP with values 46.7% of baseline after 6 h (P = 0.0001). Complete capillary stasis developed in 38% of investigated capillaries in the NP group compared to 0-1% in both other groups (P = 0.0001).

CONCLUSION

Pancreatic microcirculation in mild edematous pancreatitis is significantly increased while the evolution of necrotizing pancreatitis in the model studied herein is characterized by a dramatic reduction in pancreatic capillary flow in conjunction with areas of capillary stasis. These results underline the pathophysiologic relevance of the model and of therapeutic measures aimed at an improvement of pancreatic microcirculation in clinical necrotizing pancreatitis.

The relative safety of MRI contrast agent in acute necrotizing pancreatitis

OBJECTIVE

To validate the safety of gadolinium-diethylenetriamine pentaacetic acid (GD-DTPA) by measuring its effect on pancreatic capillary perfusion and acinar injury in acute pancreatitis.

BACKGROUND

Contrast-enhanced computed tomography (CECT) is proposed as a gold standard for early evaluation of acute necrotizing pancreatitis. However, iodinated contrast media used for CECT have been shown in these circumstances to reduce pancreatic capillary flow and increase necrosis and mortality. Recent reports suggest that post-GD MRI provides images comparable to CECT in the assessment of severe acute pancreatitis.

METHODS

Necrotizing pancreatitis was induced in 14 Wistar rats by intraductal glycodeoxycholic acid (10 mM/L) and intravenous caerulein (5 microg/kg/h) over 6 hours. Intravital microscopic quantitation of pancreatic capillary blood flow was performed using fluorescein isothiocyanate-labeled erythrocytes after induction of pancreatitis and 30 and 60 minutes after an intravenous bolus of either Ringer's solution or GD-DTPA (0.2 mL/kg).

RESULTS

The two study groups were comparable with regard to mean arterial pressure, heart rate, arterial blood gases, hematocrit, amylase, lipase, and trypsinogen activation peptide production throughout the experiment. GD-DTPA did not reduce capillary flow (1.93 +/- 0.05 nL/capillary/min) compared to animals infused with Ringer's solution (1.90 +/- 0.06 nL/capillary/min).

CONCLUSIONS

Intravenous injection of GD-DTPA does not further impair pancreatic microcirculation or increase acinar injury in acute necrotizing pancreatitis. Because of this advantage over CT contrast medium, further development of MRI as a staging tool in acute pancreatitis seems desirable.

Dobutamine maintains intestinal villus blood flow during normotensive endotoxemia: an intravital microscopic study in the rat

PURPOSE

The gut plays a pivotal role in sepsis. Intestinal hypoperfusion with subsequent ischemia leads to translocation of endotoxin. Dobutamine has been demonstrated to increase mesenteric blood flow during endotoxic shock; however, its effects on mucosal blood flow especially in intestinal villi is not known. Therefore, we investigated its influence on the blood flow and the arteriolar diameters in intestinal villi in a model of normotensive endotoxemia.

MATERIALS AND METHODS

Twenty-one male Wistar rats were divided into three groups: (1) control, saline; (2) endotoxin, endotoxin 1.5 mg/kg during 60 minutes; and (3) dobutamine, endotoxin 1.5 mg/kg (60 minutes) and dobutamine 2.5 micrograms/kg/min during 120 minutes. Villus blood flow and arteriolar diameters were determined at 0 minutes, 60 minutes, and 120 minutes in each group using intravital microscopy.

RESULTS

Villus blood flow was constant in the control group, significantly reduced at 120 minutes in the endotoxin group (120 minutes, 55.1 +/- 7.4%), and remained at baseline values in the dobutamine group. The arteriolar diameters remained constant in the control and the dobutamine groups, but they were significantly reduced in the endotoxin group at 120 minutes (7.8 +/- 0.2 to 6.5 +/- 0.7 micron).

CONCLUSION

Our results indicate that in rats with normotensive endotoxemia, arteriolar diameters and blood flow in intestinal villi were reduced. Dobutamine prevented arteriolar constriction and maintained villus blood flow at preendotoxemic values.

Capillary recruitment and transit time in the rat lung

Increasing pulmonary blood flow and the associated rise in capillary perfusion pressure cause capillary recruitment. The resulting increase in capillary volume limits the decrease in capillary transit time. We hypothesize that small species with relatively high resting metabolic rates are more likely to utilize a larger fraction of gas-exchange reserve at rest. Without reserve, we anticipate that capillary transit time will decrease rapidly as pulmonary blood flow rises. To test this hypothesis, we measured capillary recruitment and transit time in isolated rat lungs. As flow increased, transit time decreased, and capillaries were recruited. The decrease in transit time was limited by an increase in the homogeneity of the transit time distribution and an increased capillary volume due, in part, to recruitment. The recruitable capillaries, however, were nearly completely perfused at flow rates and pressures that were less than basal for the intact animal. This suggests that a limited reserve of recruitable capillaries in the lungs of species with high resting metabolic rates may contribute to their inability to raise O2 consumption manyfold above basal values.

Dopexamine maintains intestinal villus blood flow during endotoxemia in rats

OBJECTIVE

To determine the influence of dopexamine, a synthetic catecholamine ligand for dopaminergic and beta 2-adrenergic receptors, on alterations of the intestinal villus microcirculation in a model of normotensive endotoxemia.

DESIGN

Randomized, controlled trial.

SETTING

Experimental laboratory.

SUBJECTS

Twenty-one male Wistar rats.

INTERVENTIONS

Rats were treated with a continuous infusion of dopexamine (2.5 micrograms/kg/min; N = 7; group A) or 0.9% saline (n = 7; group B) during a study period of 120 mins. Both groups were given endotoxin (Escherichia coli lipopolysaccharide; 1.5 mg/kg Iv) over 60 mins. Animals in the control group (n = 7; group C) received a volume-equivalent infusion of 0.9% saline. Total volume substitution in all groups was 15 mL/kg/hr.

MEASUREMENTS AND MAIN RESULTS

Blood flow in the intestinal villi of the distal ileum was determined using in vivo videomicroscopy at baseline, and 60 and 120 mins after the endotoxin challenge. These blood flow determinations were done by an observer who was unaware of the previous treatment of the animals. In addition, mean arterial pressure was monitored at baseline, and 15, 30, 45, 60, 75, 90, 105, and 120 mins later. The administration of 1.5 mg/kg endotoxin alone (group B) resulted in a reduction of the intestinal villus blood flow to 74.8 +/- 9.5% of baseline after 60 mins, and to 61.1 +/- 8.5% of baseline after 120 mins (baseline: 7.4 +/- 0.6 nL/min; 60 mins: 5.3 +/- 0.8 nL/min; 120 mins: 4.4 +/- 0.5 nL/min; p < .05). This reduction of blood flow was associated with a decrease in the arteriolar diameters by 13.8 +/- 2.5% after 60 mins, and by 17.1 +/- 4.3% after 120 mins (p < .05 vs. baseline). In contrast, villus blood flow in the dopexamine group (group A) did not show statistically significant changes during the entire study period, despite the administration of endotoxin (baseline: 8.2 +/- 0.6 nL/min; 60 mins: 7.3 +/- 0.8 nL/min; 120 mins: 7.8 +/- 0.5 nL/min). No vasoconstriction of the villus arterioles was noted in this group. In control animals (group C), the blood flow (baseline: 8.1 +/- 1.6 nL/min; 60 mins: 7.6 +/- 1.4 nL/min: 120 mins: 7.8 +/- 1.4 nL/min) and the arteriolar diameters remained unchanged throughout the observation period. Mean arterial pressure did not differ between groups: it remained unaltered in all groups during the entire study period.

CONCLUSIONS

Dopexamine maintains intestinal villus arterial perfusion and prevents the vasoconstriction in villus arterioles during early normotensive endotoxemia. Therefore, further studies in critically ill patients will have to determine whether the early prophylactic use of dopexamine can limit ischemia and prevent the development of multiple organ failure.

Intravenous contrast medium aggravates the impairment of pancreatic microcirculation in necrotizing pancreatitis in the rat

BACKGROUND

Previous reports demonstrated that radiographic contrast medium, as used in contrast-enhanced computed tomography, increases acinar necrosis and mortality in experimental pancreatitis. The authors studied the possibility that these changes may be related to an additional impairment of pancreatic microcirculation.

METHODS

Fifty Wistar rats had acute pancreatitis induced by intraductal glycodeoxycholic acid (10 mmol/L for 10 min) and intravenous cerulein (5 micrograms/kg/hr for 6 hrs). After rehydration (16 mL/kg), pancreatic capillary perfusion was quantified by means of intravital microscopy at baseline before intravenous infusion of contrast medium (n = 25) or saline (n = 25), and 30 and 60 minutes thereafter. In addition to total capillary flow, capillaries were categorized as high- or low-flow (> or < 1.6 nL/min).

RESULTS

Pancreatic capillary flow did not change in either high- or low-flow capillaries after saline infusion. However, contrast medium infusion induced a significant decrease of total capillary flow (p < 0.001). Analysis according to the relative flow rate revealed that this was primarily because of a significant additional reduction of perfusion in low-flow capillaries (p < 0.0001). Furthermore, complete capillary stasis was observed in 15.9 +/- 3.4% after contrast medium as compared with 3.2 +/- 1.2% after saline infusion (p < 0.006).

CONCLUSION

Radiographic contrast medium aggravates the impairment of pancreatic microcirculation in experimental necrotizing pancreatitis.

Isovolemic hemodilution with dextran prevents contrast medium induced impairment of pancreatic microcirculation in necrotizing pancreatitis of the rat

BACKGROUND

Previous studies demonstrated that intravenous contrast medium (CM), as used in contrast enhanced computed tomography, aggravates the impairment of pancreatic microcirculation (PM) characteristic of severe pancreatitis and increases necrosis and mortality in necrotizing pancreatitis (NP) in rats. This study evaluates the use of isovolemic hemodilution, which can enhance the microcirculation in severe pancreatitis, for preventing CM-induced injury.

METHODS

NP was induced in 30 dextran-tolerant Wistar rats by intraductal glycodeoxycholic acid and intravenous cerulein for 6 hours. PM was quantified by intravital microscopy using fluorescein isothiocyanate labeled erythrocytes. Based on previous results, areas with low blood flow (< 1.6 nL/min/cap) were identified and baseline recordings of capillary blood flow taken. A reduction of hematocrit to 75% of baseline was achieved by replacement of 5 mL/kg of blood with 25 mL/kg Ringer's lactate (RL) or by exchange of 8 mL/kg of blood for the same amount of dextran 70.6%. Thereafter, the nonionic CM iopamidol (Solutrast, Byk Gulden, Konstanz, Germany) was injected during 1 minute and PM measurements repeated after 30 and 60 minutes.

RESULTS

Despite hemodilution with RL, pancreatic capillary perfusion was significantly decreased to 87% of baseline (0.83 +/- 0.04 mL/min/cap; n = 216) 60 minutes after CM infusion (P < 0.05). In contrast, capillary blood flow was significantly increased to 161% (1.56 +/- 0.05 nL/min/cap; n = 278) in the group treated with dextran. Moreover, the percentage of capillaries developing complete stasis was significantly lower in the dextran group (2.3 +/- 1.2%) compared to animals diluted with RL (22.3 +/- 4.8%) (P < 0.002).

CONCLUSION

Isovolemic hemodilution with dextran prevents the additional impairment of pancreatic microcirculation induced by CM in NP.

Redistribution of red blood cell flow in microcirculatory networks by hemodilution

The effect of isovolemic hemodilution on red blood cell flow distribution was studied in complete self-contained microvessel networks of the rat mesentery. Hematocrit, diameter, and length of all vessel segments as well as the topological structure were determined in control networks (systemic hematocrit, 0.54) and after hemodilution (systemic hematocrit, 0.30). Hemodilution was performed by exchanging blood with hydroxyethyl starch (MW 450,000; 6%) or homologous plasma. With hemodilution, the decrease of microvessel hematocrit exceeded that of systemic hematocrit. The average discharge hematocrit in capillaries was 79% of systemic hematocrit in the control group and 73% with hemodilution (p less than 0.001). The heterogeneity of capillary hematocrit within the network, expressed by the coefficient of variation, increased from 0.4 to 0.7. By using the morphological and topological data of four networks, the distribution of hematocrits was also calculated using a hydrodynamic flow model. The modeling results were found to be in close agreement with the experimental data. This indicates that the observed changes can be deduced from established rheological phenomena, most of all phase separation at arteriolar bifurcations. The changes in hematocrit distribution after hemodilution are accompanied by a redistribution of red blood cell flow within the network: relative to total red blood cell flow, red blood cell flow in the distal capillaries of the network increases by about 40% at the expense of the proximal capillaries that are close to the feeding arteriole and that exhibit the highest red blood cell flow under control conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

[99mTc]diphosphonate uptake and hemodynamics in arthritis of the immature dog knee

The relationship between [99mTc]diphosphonate uptake and bone hemodynamics was studied in canine carrageenan-induced juvenile chronic arthritis. Blood flow was determined with microspheres, plasma and red cell volumes were measured by labeled fibrinogen and red cells, and the microvascular volume and mean transit time of blood were calculated. Normal femoral epiphyses had lower central and higher subchondral blood flow and diphosphonate uptake values. Epiphyseal vascular volume was uniform, resulting in a greater transit time of blood centrally. In arthritis, blood flow and diphosphonate uptake were increased subchondrally and unaffected centrally, while epiphyseal vascular volume was increased throughout, leading to prolonged transit time centrally. The normal metaphyses had low blood flow and diphosphonate uptake values in cancellous bone and very high values in growth plates, but a large vascular volume throughout. The mean transit time therefore was low in growth plates and high in adjacent cancellous bone. Arthritis caused decreased blood flow and diphosphonate uptake in growth plates but increased vascular volume and transit time of blood. Diphosphonate uptake correlated positively with blood flow and plasma volume and negatively with red cell volume in a nonlinear fashion. Thus, changes in diphosphonate uptake and microvascular hemodynamics occur in both epiphyseal and metaphyseal bone in chronic synovitis of the immature knee. The [99mTc]diphosphonate bone scan seems to reflect blood flow, plasma volume, and red cell volume of bone.

Microvascular hemodynamics in experimental arthritis: disparity between the distribution of microspheres and plasma flow in bone

The microcirculation in normal and arthritic juxtaarticular bone was studied in 16 young dogs with carragheenan-induced arthritis of one knee. The regional blood flow was determined by the tissue uptake of intracardially injected 15-microns 141Ce-labeled microspheres, and the microvascular plasma volume was determined by the distribution space of circulating 125I-fibrinogen. Disparities between the distribution of plasma flow and microspheres, introduced by plasma skimming or nonentrapment of spheres in the intraosseous circulation, were estimated by 59Fe-transferrin, a third intravascular tracer, injected as a bolus intracardially and trapped peripherally after 15 sec by prompt circulatory arrest. The tissue uptake of the plasma flow tracer was compared to that of microspheres by the ratio between observed and expected activity of 59Fe-transferrin, the expected activity being calculated from the microsphere distribution. The transferrin and microsphere uptake agreed well in patella, marginal epiphyseal bone, and cortical bone, whereas observed activity of transferrin was twice the expected in central epiphyseal bone, three times higher in marrow, and up to eightfold higher in metaphyses adjacent to growth plates. This discrepancy was significantly greater in arthritic bone when the metaphyses were examined in toto. The microsphere method thus appears to underestimate blood flow to cancellous bone and marrow due to uneven distribution of plasma and formed elements from profound plasma skimming and perhaps also by AV shunting.

Visualization of renal autoregulation in the split hydronephrotic kidney of rats

The autoregulatory ability of the renal vascular system in the split hydronephrotic kidney was quantitated with intravital microscopy. The luminal diameters of the arcuate and interlobular arteries as well as the afferent and efferent arterioles were measured. Glomerular blood flow was determined by the dual slit technique. In the first series of experiments, the renal perfusion pressure was reduced by graded clamping of the abdominal aorta. Pressure reduction from 118 mm Hg to 95 mm Hg induced dilation of all preglomerular vessels except for the distal afferent arteriole; there was no change in the efferent arteriole and the blood flow was maintained. Further pressure reductions to 71 and 43 mm Hg caused additional dilations of the preglomerular vessels, a marginal reduction in diameter of proximal efferent arterioles and flow reductions by 15% and 41%, respectively. In the second series, systemic blood pressure was increased by continuous i.v. infusions of norepinephrine (NE). NE constricted pre- and postglomerular vessels except for the distal afferent arteriole; glomerular flow was decreased. Reduction of renal perfusion pressure during NE infusion to the preinfusion value did not diminish glomerular blood flow, but reduced the constrictor response to NE in preglomerular vessels. In a third series of experiments we examined the effect of atrial natriuretic factor (ANF) on renal autoregulation. Addition of ANF (10(-9) to 10(-7) M) to the renal bath induced a dose-dependent dilation of all preglomerular vessels and a constriction of the efferent arteriole. Pressure reduction from 120 to 95 mm Hg resulted in a further preglomerular vasodilation. These experiments demonstrate that autoregulation is mediated primarily by diameter changes in all preglomerular vessels excluding the distal segment of the afferent arteriole. Further, these data suggest that ANF induced dilation of preglomerular vessels does not impair the myogenic response of these vessels.