Quantitative comparison of CFD predicted and MRI measured velocity fields in a carotid bifurcation phantom

Steady flow of a blood mimicking fluid in a physiologically realistic model of the human carotid bifurcation was studied using both magnetic resonance imaging (MRI) and computational fluid dynamics (CFD) modelling techniques. Quantitative comparisons of the 3D velocity field in the bifurcation phantom were made between phase contrast MRI measurements and CFD predictions. The geometry for the CFD model was reconstructed from T(1) weighted MR imaging of the test phantom. It was found that the predicted velocity fields were in fair agreement with MR measured velocities. In both the internal and external carotid arteries, the agreement between CFD predictions and MRI measurements was better along the inner-outer wall axis with a correlation factor C>0.897 (average 0.939) where the velocity profiles were skewed, than along the anterior-posterior axis (average correlation factor 0.876) where the velocity profiles were in M-shape.     

In vivo two-photon excited fluorescence microscopy reveals cardiac- and respiration-dependent pulsatile blood flow in cortical blood vessels in mice

Subtle alterations in cerebral blood flow can impact the health and function of brain cells and are linked to cognitive decline and dementia. To understand hemodynamics in the three-dimensional vascular network of the cerebral cortex, we applied two-photon excited fluorescence microscopy to measure the motion of red blood cells (RBCs) in individual microvessels throughout the vascular hierarchy in anesthetized mice. To resolve heartbeat- and respiration-dependent flow dynamics, we simultaneously recorded the electrocardiogram and respiratory waveform. We found that centerline RBC speed decreased with decreasing vessel diameter in arterioles, slowed further through the capillary bed, and then increased with increasing vessel diameter in venules. RBC flow was pulsatile in nearly all cortical vessels, including capillaries and venules. Heartbeat-induced speed modulation decreased through the vascular network, while the delay between heartbeat and the time of maximum speed increased. Capillary tube hematocrit was 0.21 and did not vary with centerline RBC speed or topological position. Spatial RBC flow profiles in surface vessels were blunted compared with a parabola and could be measured at vascular junctions. Finally, we observed a transient decrease in RBC speed in surface vessels before inspiration. In conclusion, we developed an approach to study detailed characteristics of RBC flow in the three-dimensional cortical vasculature, including quantification of fluctuations in centerline RBC speed due to cardiac and respiratory rhythms and flow profile measurements. These methods and the quantitative data on basal cerebral hemodynamics open the door to studies of the normal and diseased-state cerebral microcirculation.     

Early effects of tobacco smoke exposure on vascular dynamics in the microcirculation.

The purpose of these studies is to examine the early effects of chronic tobacco smoke exposure on vascular dynamics in the mesenteric microcirculation. Female rats were exposed daily to tobacco smoke from five reference cigarettes for a period of 2 mo. At the end of this period the smoke-treated rats had gained 12 g less than sham-treated controls, and arterial blood pressure in the smoke-treated animals was slightly less than pressure in the sham-treated animals. These are characteristic effects of tobacco smoke exposure on rats. Following the treatment period, red blood cell (RBC) velocity in single mesenteric capillaries and microvascular pressures in arterioles and venules were measured in accordance to established methods. There was no significant difference in pressure distribution on the arterial side of the mesenteric vascular network, but pressure in the venules of the smoke-treated animals was significantly higher than that of the sham-treated group. In association with the higher venular pressure in the smoke-treated animals, capillary RBC velocity (an index of capillary flow) was significantly lower. The reduction in velocity was in proportion to the decrease in pressure drop (arteriole-venule) across the capillary network.     

Propagation properties of vasomotion at terminal arterioles and precapillaries in the rabbit mesentery

A vasomotion activity in the mesentery of anesthetized rabbits were studied by simultaneous measurements of inside diameters at multiple sites in arterioles, precapillaries and their bifurcations. A frame-by-frame diameter determination technique was used with a microcomputer-assisted laser video disk recorder and video-image analysis system. Simultaneous intensity profiles across microvessels were continuously obtained. Applying an automatic wall surface tracer and a graphic editor to construct temporal sequences of intensity profiles, we obtained digitized data of inside diameters of microvessels, and implemented cross-correlation analysis between data sets to calculate phase differences of vasomotion at separated sites. The present analysis of the propagation of vasomotion showed that the vasomotion originated from the orifice of precapillaries at the bifurcations, spreading downstream in the precapillary. The vasomotion wave spreads both upstream and downstream along single arterioles from various origins of the vasomotion activity. The propagation velocity of vasomotion was 0.17 +/- 0.03 mm/sec (n = 19), and it became significantly slower through the branching points than along the arterioles. It is suggested that the vasomotion in terminal arterioles and precapillaries may spread through some mechanical transmission factors.     

Radial displacement of red blood cells during hemodilution and the effect on arteriolar oxygen profile

In this study, we assessed the magnitude of the erratic deviations in the radial position of red blood cells (RBCs) in the laminar flow regime of arterioles in a hamster window preparation and the intraluminal Po(2) profile to determine whether this variability affects the intraluminal distribution of oxygen in conditions of normal hematocrit and hemodilution. A gated image intensifier was used to visualize fluorescently labeled RBCs in tracer quantities and obtain multiple measurements of RBC radial and longitudinal positions at time intervals on the order of 5 ms within single arterioles (diameter range 40-95 microm). RBCs in the velocity range of 0.3-14 mm/s exhibit a mean coefficient of variation of velocity of 16.9 +/- 10.5% and a SD of the radial position of 1.98 +/- 0.98 microm. Both quantities were inversely related to hematocrit, and the former was significantly lowered by hemodilution. Our experimental results presented very similar values and shape compared with the intraluminal oxygen profile derived theoretically for normal hematocrit, suggesting that shear-augmented diffusion due to the measured radial displacement of RBCs did not significantly affect oxygen diffusion from blood into the arteriolar vessel wall. Po(2) profiles in the arterioles assumed an increasingly parabolic configuration with increasing levels of hemodilution.     

Numerical investigation of the non-Newtonian blood flow in a bifurcation model with a non-planar branch

The non-Newtonian fluid flow in a bifurcation model with a non-planar daughter branch is investigated by using finite element method to solve the three-dimensional Navier–Stokes equations coupled with a non-Newtonian constitutive model, in which the shear thinning behavior of the blood fluid is incorporated by the Carreau–Yasuda model. The objective of this study is to investigate the influence of the non-Newtonian property of fluid as well as of curvature and out-of-plane geometry in the non-planar daughter vessel on wall shear stress (WSS) and flow phenomena. In the non-planar daughter vessel, the flows are typified by the skewing of the velocity profile towards the outer wall, creating a relatively low WSS at the inner wall. In the downstream of the bifurcation, the velocity profiles are shifted towards the flow divider. The low WSS is found at the inner walls of the curvature and the lateral walls of the bifurcation. Secondary flow patterns that swirl fluid from the inner wall of curvature to the outer wall in the middle of the vessel are also well documented for the curved and bifurcating vessels. The numerical results for the non-Newtonian fluid and the Newtonian fluid with original Reynolds number and the corresponding rescaled Reynolds number are presented. Significant difference between the non-Newtonian flow and the Newtonian flow is revealed; however, reasonable agreement between the non-Newtonian flow and the rescaled Newtonian flow is found. Results of this study support the view that the non-planarity of blood vessels and the non-Newtonian properties of blood are an important factor in hemodynamics and may play a significant role in vascular biology and pathophysiology.     

The distribution of blood flow velocity in the terminal bed of the rat mesentery

Blood flow velocities in microvessels of the rat intestinal mesentery were determined by means of prism-grating method. Mean velocity values in arterioles were 1.9 +/- 0.1, in venules 1.2 +/- 0.2, in capillaries 0.82 +/- 0.06 and in arteriole-venule anastomoses 1.7 +/- 0.2 mm/s. These values do not vary significantly in arterioles with internal diameter from 23.2 to 6.9 mm and in venules from 7.2 to 28.2 mm. The most significant velocity changes appear in the passage of arterioles into capillaries (50%) and between capillaries and venules (40%).     

Oxygen diffusion through the venule walls in the rat cerebral cortex during breathing with pure oxygen

Using oxygen microelectrodes, distribution of oxygen tension (pO2) has been studied in venules of the rat brain cortex at normobaric hyperoxia (spontaneous breathing with pure oxygen). It has been shown that inhalation of oxygen results in sharp increase of pO2 in majority of the venules under study. The pO2 distribution along the length of venous microvessels of 7-280 microns in diameter is best approximated by equation: pO2 = 76.44 e-0.0008D, n = 407. The pO2 distribution was characterised by extremely high pO2 values (180-240 mm Hg) in some minute venules. Heterogeneity of pO2 distribution in venous microvessels at hyperoxia was shown to be significantly increased. Profiles of pO2 between neighbouring arterioles and venules were for the first time measured. The data clearly evidenced that O2 diffusional shunting took place between cortical arterioles and venules, provided they were distanced from each other for not over 80-100 microns. Distribution of pO2 in venules has been shown to be dependent on the blood flow in the brain cortical microvessels.     

Experimental determination of velocity profiles and wall shear rate along the rabbit aortoiliac bifurcation: relationship to vessel wall low-density lipoprotein (LDL) metabolism.

We have determined the velocity profiles and wall shear rates along the New Zealand White (NZW) rabbit aortoiliac bifurcation. A pulsatile perfusion apparatus was used to impose physiologic pressure and flow waveforms on nine freshly excised NZW bifurcation segments. Pulsed Doppler velocimetry (PDV) was utilized to construct velocity profiles at five measurement sites: within the infrarenal aorta; immediately distal to the apex of the bifurcation; and, more distally along the iliac arteries. Wall shear rate was derived from a numerical differentiation of the experimental velocity profiles. The results of this study indicate that the average shear rate was lower along the lateral (approximately 40 s-1) vs medial (approximately 240 s-1) wall of the proximal iliac branch. The degree of flow reversal along the proximal lateral walls (20 +/- 2%) exceeded that along the proximal flow divider wall (1 +/- 1%). Flow at the distal iliac measurement sites and within the infrarenal aorta was approximately symmetric. These findings complement our companion in vivo study [Berceli et al., Arteriosclerosis 10, 688-694 (1990)] wherein we determined the rates of low-density lipoprotein (LDL) incorporation and catabolism along this symmetrically bifurcating conduit. Taken together, these studies provide original information regarding the effects of hemodynamics on one presumed atherogenic risk factor, namely, LDL metabolism.     

Serotonin metabolism in thrombocytes and microvascular hemostasis in spontaneous arterial hypertension

Serotonin content and accumulation in platelets and its release from them, as well as changes in thrombus formation in mesenteric arterioles and venules of the small intestine have been investigated in control rats and rats with spontaneous hypertension (SHR). Serotonin accumulation in platelets was determined upon its incubation with platelets. Disodium ADP salt was used as an inductor of release. Laser-induced thrombosis was caused by microvessels exposure to impulse laser irradiation. The control animals revealed a significant difference between the initial serotonin platelet level and serotonin level upon incubation and release; in values, the values of basic thrombus-forming parameters were higher than in arterioles. In SHR there is a decrease in biogenic amine content in platelets, a depression in its accumulation and release, an increase in the time of thrombus growth, its size up to the separation of the first embolus and its length along the vascular wall. It is concluded that spontaneous hypertension is characterized by decreased functional activity of platelets and depressed resistance of arterioles and venules to thrombus formation.     

Velocity pulse advances pressure pulse by close to 45 degrees in the rat pial arterioles

In order to clarify the phase relationship between velocity pulse and pressure pulse propagating along microvessels, the red cell velocity and intravascular pressure were simultaneously measured in the rat pial arterioles of 41-53 microm in diameter with a high temporal resolution by a laser-Doppler anemometer and a servo-null micropressure system. It was found that the velocity pulse preceded the pressure pulse in all the measured arterioles by 18.7-35.6 ms. The corresponding phase difference was 43.6+/-6.9 degrees (mean +/- SD), which is not statistically different from 45 degrees. The value is consistent with the phase difference predicted for the blood flow in microvessels with a small reflection coefficient at frequencies as low as the heart rate of the rats. The present results suggest that the upstream changes in blood flow are transmitted by the velocity pulse faster than by the pressure pulse in the microvasculature.     

Effect of ocular shape and vascular geometry on retinal hemodynamics: a computational model

A computational model for retinal hemodynamics accounting for ocular curvature is presented. The model combines (i) a hierarchical Darcy model for the flow through small arterioles, capillaries and small venules in the retinal tissue, where blood vessels of different size are comprised in different hierarchical levels of a porous medium; and (ii) a one-dimensional network model for the blood flow through retinal arterioles and venules of larger size. The non-planar ocular shape is included by (i) defining the hierarchical Darcy flow model on a two-dimensional curved surface embedded in the three-dimensional space; and (ii) mapping the simplified one-dimensional network model onto the curved surface. The model is solved numerically using a finite element method in which spatial domain and hierarchical levels are discretized separately. For the finite element method, we use an exterior calculus-based implementation which permits an easier treatment of non-planar domains. Numerical solutions are verified against suitably constructed analytical solutions. Numerical experiments are performed to investigate how retinal hemodynamics is influenced by the ocular shape (sphere, oblate spheroid, prolate spheroid and barrel are compared) and vascular architecture (four vascular arcs and a branching vascular tree are compared). The model predictions show that changes in ocular shape induce non-uniform alterations of blood pressure and velocity in the retina. In particular, we found that (i) the temporal region is affected the least by changes in ocular shape, and (ii) the barrel shape departs the most from the hemispherical reference geometry in terms of associated pressure and velocity distributions in the retinal microvasculature. These results support the clinical hypothesis that alterations in ocular shape, such as those occurring in myopic eyes, might be associated with pathological alterations in retinal hemodynamics.     

Dynamic curvature strongly affects wall shear rates in a coronary artery bifurcation model

This study was motivated by the need for a better understanding of coronary artery blood flow patterns and their possible role in atherosclerosis formation. Of particular interest in this study was the effects of the dynamic deformation due to myocardial contraction on wall shear rate patterns in the coronary arteries. A better understanding of these effects on wall shear rate in a bifurcation geometry and an evaluation of the importance of these effects was desired. A three-dimensional computer model of a bifurcation lying on the surface of a sphere with time-varying radius of curvature was employed to simulate the motion and deformation of the arteries. The results indicated low mean shear rates along the myocardial wall and very high shear rate variations (over 100% of the static mean shear rate) along the outer wall. The results obtained using a quasi-static analysis were found to underestimate the dynamic wall shear rate variation along the myocardial and outer walls. It was concluded that dynamic geometry effects are important in determining sites of low mean and oscillating wall shear that have been associated with atherogenesis in curved, bifurcating arteries.     

Intravital microscopy of the murine pulmonary microcirculation.

Intravital microscopy (IVM) is considered as the gold standard for in vivo investigations of dynamic microvascular regulation. The availability of transgenic and knockout animals has propelled the development of murine IVM models for various organs, but technical approaches to the pulmonary microcirculation are still scarce. In anesthetized and ventilated BALB/c mice, we established a microscopic access to the surface of the right upper lung lobe by surgical excision of a window of 7- to 10-mm diameter from the right thoracic wall. The window was covered by a transparent polyvinylidene membrane and sealed with alpha-cyanoacrylate. Removal of intrathoracic air via a trans-diaphragmal intrapleural catheter coupled the lung surface to the window membrane. IVM preparations were hemodynamically stable for at least 120 min, with mean arterial blood pressure above 70 mmHg, and mean arterial Po(2) and arterial Pco(2) in the range of 90-100 Torr and 30-40 Torr, respectively. Imaged lungs did not show any signs of acute lung injury or edema. Following infusion of FITC dextran, subpleural pulmonary arterioles and venules of up to 50-microm diameter and alveolar capillary networks could be visualized during successive expiratory plateau phases over a period of at least 2 h. Vasoconstrictive responses to hypoxia (11% O(2)) or infusion of the thromboxane analog U-46619 were prominent in medium-sized arterioles (30- to 50-microm diameter), minor in small arterioles <30 microm, and absent in venules. The presented IVM model may constitute a powerful new tool for investigations of pulmonary microvascular responses in mice.     

TNF-alpha activation of arterioles and venules alters distribution and levels of ICAM-1 and affects leukocyte-endothelial cell interactions

The observation that leukocyte-endothelial cell (EC) interactions are localized to specific regions on the microvessel wall suggests that adhesion molecule distribution is not uniform. We investigated ICAM-1 distribution and leukocyte-EC interactions in blood-perfused microvessels (<80 mum) in cremaster muscle of anesthetized mice, using intravital confocal microscopy and immunofluorescent labeling. Variability of ICAM-1 expression directly determines leukocyte adhesion distribution within the venular microcirculation and contributes to leukocyte rolling in arterioles during inflammation. The number of rolling interactions increased with ICAM-1 intensity (r(2) = 0.69, P < 0.05), and rolling velocity was lower in regions of higher ICAM-1 intensity. In controls, venular ICAM-1 expression was approximately twofold higher than in arterioles. After TNF-alpha treatment, ICAM-1 expression was significantly increased, 2.8 +/- 0.2-fold in arterioles and 1.7 +/- 0.2-fold in venules (P < 0.05). ICAM-1 expression on activated arteriolar ECs only reached the level of control venular ICAM-1. Arteriolar but not venular ECs underwent redistribution of ICAM-1 among cells; some cells increased and some decreased ICAM-1 expression, magnifying the variability of ICAM-1. TNF-alpha treatment increased the length of bright fluorescent regions per unit vessel length (42%, control; 70%, TNF-alpha) along the arteriolar wall, whereas no significant change was observed in venules (60%, control; 63%, TNF-alpha). The spatial distribution and expression levels of adhesion molecules in the microcirculation determine the timing and placement of leukocyte interactions and hence significantly impact the inflammatory response. That arteriolar ECs respond to TNF-alpha by upregulation of ICAM-1, although in a different way compared with venules, suggests an explicit role for arterioles in inflammatory responses.     

Fringe mode transmittance laser Doppler microscope anemometer: its adaptation for measurement in the microcirculation

Blood flow analysis in the microcirculation requires accurate measurement of velocity, volume flow and shear-rate versus shear-stress relationships. The resolution of most anemometers is too limited to obtain useful measurements, especially near the blood vessel wall and at branches and bifurcations. To make such measurements possible with a noninvasive, high resolution, accurate technique, we have developed a fringe mode, transmittance laser Doppler microscope anemometer (LDMA). This system has an intrinsically high spatial resolution (10 × 12 μm), and does not require a high concentration (10⁶/cm³) of scatterers or red blood cells (RBC) as in our application. Preliminary measurements of water flow in a rectangular channel were conducted to ascertain the reliability and accuracy of velocity measurements using the LDMA. Velocity profiles were then measured by the LDMA system in arterioles 38–135 μm in diameter, in the transparent, everted cheek pouch of the anaesthelized hamster. The extremely high resolution of the optical system, and the ultra-fine traversing mechanism of the microscope slage, made velocity readings larger than 0.02 mm/s with accuracy and reproducibility better than 1%, possible near the wall to within 7–10 μm.     

Immunohistochemical characteristics of suburothelial microvasculature in the mouse bladder

The morphological characteristics of smooth muscle cells (SMCs) and their innervation of the suburothelial microvasculature of the mouse bladder were investigated by immunohistochemistry. Whole mount bladder mucosal preparations were immune-stained for α-smooth muscle actin (α-SMA) and/or neuronal markers and examined using confocal laser scanning microscopy. Suburothelial arterioles consisted of α-SMA-immunopositive circular smooth muscle cells, while the venular wall composed of α-SMA-positive SMCs that displayed several processes which extended from their cell bodies to form an extensive meshwork. In larger venules, a complex meshwork of stellate-shaped SMCs were observed. NG2 chondroitin sulphate proteoglycan-immunoreactive cell bodies of capillary pericytes were not immunoreactive for α-SMA. In the rat bladder suburothelial venules, circular SMCs were the dominant cell type expressing α-SMA-immunoreactivity. Since α-SMA-positive SMCs in suburothelial arterioles and venules in the mouse bladder had quite distinct morphologies, the innervation of both vessels could be examined by double labelling for α-SMA and various neuronal markers. Varicose nerve bundles immunoreactive for tyrosine hydroxylase (sympathetic nerves), choline acetyltransferase (cholinergic nerves) or substance P (primary afferent nerves) were all detected along side suburothelial arterioles. Single varicose nerve fibres positive for these three neuronal markers were also detected around the venules. Thus, whole mount preparations are useful when examining the morphology of α-SMA-positive SMCs of the microvasculature in the suburothelium of mouse bladder as well as their relationship with their innervations. In conclusion, arterioles and venules of the bladder suburothelium are the target of sympathetic, cholinergic and primary afferent nerve fibres.     

The features of thrombus in a microvessel injury model and the antithrombotic efficacy of heparin,urokinase, and prostaglandin E1

In failed flap transfers and in burn injuries, superoxides and thrombi generated in the microcirculation are considered responsible for tissue injury. A dynamic and morphologic analysis of thrombus formation was conducted in a model of microvessel injury, and an analysis was made of the different antithrombotic effects of heparin, urokinase, and prostaglandin E(1). The dye-light method was used (i.e., injury of the endothelium by reactive oxygen species) to induce thrombus formation in both the arterioles and venules of the rabbit ear chamber under an intravital microscope-television system. The dynamic course of thrombus formation was observed, and the period from irradiation to complete obstruction of blood flow (i.e., time to stasis) was measured and compared in relation to various treatment conditions. Arteriolar thrombi were formed by platelet aggregation. Venular thrombi were composed of platelets and erythrocytes that gathered and adhered around leukocytes stuck to the vessel wall. Heparin treatment prolonged the time to stasis in both the arterioles and the venules. Urokinase extended the time to stasis in the venules but not in the arterioles. Prostaglandin E(1)-treatment significantly prolonged the time to stasis in the arterioles, but only high-dose prostaglandin E(1) prolonged the time to stasis in the venules. The results of this study show that endothelial damage caused by superoxides promotes the formation of thrombi that differ in composition between the arteriole and the venule and that the effectiveness of each drug varies accordingly. The authors believe that these agents can be used with increased efficacy if the two types of thrombi and the specific antithrombotic effects of each agent are considered.     

Effects of wall shear stress and its gradient on tumor cell adhesion in curved microvessels

Tumor cell adhesion to vessel walls in the microcirculation is one critical step in cancer metastasis. In this paper, the hypothesis that tumor cells prefer to adhere at the microvessels with localized shear stresses and their gradients, such as in the curved microvessels, was examined both experimentally and computationally. Our in vivo experiments were performed on the microvessels (post-capillary venules, 30–50 μm diameter) of rat mesentery. A straight or curved microvessel was cannulated and perfused with tumor cells by a glass micropipette at a velocity of ~1mm/s. At less than 10 min after perfusion, there was a significant difference in cell adhesion to the straight and curved vessel walls. In 60 min, the averaged adhesion rate in the curved vessels (n = 14) was ~1.5-fold of that in the straight vessels (n = 19). In 51 curved segments, 45% of cell adhesion was initiated at the inner side, 25% at outer side, and 30% at both sides of the curved vessels. To investigate the mechanical mechanism by which tumor cells prefer adhering at curved sites, we performed a computational study, in which the fluid dynamics was carried out by the lattice Boltzmann method , and the tumor cell dynamics was governed by the Newton’s law of translation and rotation. A modified adhesive dynamics model that included the influence of wall shear stress/gradient on the association/dissociation rates of tumor cell adhesion was proposed, in which the positive wall shear stress/gradient jump would enhance tumor cell adhesion while the negative wall shear stress/gradient jump would weaken tumor cell adhesion. It was found that the wall shear stress/gradient, over a threshold, had significant contribution to tumor cell adhesion by activating or inactivating cell adhesion molecules. Our results elucidated why the tumor cell adhesion prefers to occur at the positive curvature of curved microvessels with very low Reynolds number (in the order of 10⁻²) laminar flow.