A model for blood flow through a stenotic tube

This article deals with the introduction of the modified Casson's fluid model as the true representation for the blood for the steady laminar flow through a small diameter artery with axi-symmetric identical double stenoses in series. The governing equations are solved by using the finite element method. The results for the velocity profiles, the pressure and the wall shear stress distributions in addition to the location and length of the flow reversal zones have been brought out and discussed in reference to the severity of the disease. It has been observed that the non-Newtonian nature of the blood helps in reducing the magnitude of the peak wall shear stress at the throat and the length of the reversed flow regions in the post stenotic dilatation.     

A mathematical model for blood flow through an arterial bifurcation

By introducing the finite element technique, a study of blood flow through an arterial bifurcation is presented in this paper. The blood is represented by a modified model of thixotropic power-law fluids, for which the parametric values for blood, both in normal and pathological states, have already been established. The results for the velocity profiles, pressure and wall shear stress distributions are elucidated and discussed for normal old and diseased states. The separation and reattachment points are also located for different values of the Reynolds number and the flow behaviour index (n) of the model representing the blood. The analysis identifies low shear stress zones behind the stenosis along the outer wall and high shear stresses downstream of the apex. The increasing percentage of the stenosis and the increasing values of the Reynolds number facilitate the high shear stress zones, whereas the thixotropy of the blood depicts an inbuilt mechanism of reducing high shear stresses as well as flow reversal regions.     

Computational analysis of flow in a curved tube model of the coronary arteries: effects of time-varying curvature

The flow through a curved tube whose radius of curvature varies with time was studied in order to better understand flow patterns in coronary arteries. A computational flow model was constructed using commercially available software. The artery model featured a uniform circular cross section, and the curvature was assumed to be constant along the tube, and in one plane. The computational model was verified with the use of a dynamically similar in vitro apparatus. A steady uniform velocity was prescribed at the entrance at a Reynolds number of 300. Two sets of results were obtained: one in which the curvature was held constant at the mean, maximum and minimum radii of curvature (quasistatic), and another in which the curvature was varied sinusoidally in time at a frequency of I Hz (dynamic). The results of the dynamic analysis showed that the wall shear rates varied as much as 52% of the static mean wall shear rate within a region of 10 tube diameters from the inlet. The results of the dynamic analysis were within 6% of the quasistatic predictions. Realistic modeling of the deforming geometry is important in determining which locations in the coronary arteries are subjected to low and oscillating wall shear stresses, flow patterns that have been associated with atherogenesis.     

The papaverine test for blood flow potential of ileo-femoral arteries

Cells of Saccharomyces carlsbergensis contain proteins assembling to crystals and bundles of tubules (tb) in the cyto- and karyoplasm by osmotic shock with hypertonic solutions. The cellular concentration of these proteins is regulated by oxygen pressure during growth. In cells grown at optimal aeration the protein level is low and crystals and tb cannot be induced. After a short period of O2-limitation or anerobic growth conditions the protein concentration increases and induction of crystals and tb is possible.     

Doppler flow velocity waveforms of the umbilical arteries correlate with intravillous blood volume

OBJECTIVE: My purpose was to measure the volume of the fetoplacental vessel tree and to relate findings to Doppler flow patterns of the umbilical arteries. STUDY DESIGN: One hundred sixty placentas were examined by means of standardized random block placental histomorphometry after delivery and the results were compared with antenatal Doppler findings. RESULTS: There was a high correlation (r = -0.703) between the intravillous blood volume obtained from measurements of intermediate and terminal villi and the Doppler flow velocity waveforms detected within the last week before delivery. Moreover, the reduced size of a vessel tree less than 85 mL is highly predictive of perinatal complications, such as fetal growth restriction, low umbilical artery pH values after birth, reduced Apgar scores, and cesarean section for fetal distress. CONCLUSION: These data suggest that reduced end-diastolic flow velocities in the umbilical arteries are associated with elevated fetoplacental impedance owing to reduced vascularization of intermediate and terminal villi.     

A computational model for the prediction of steel hardenability

A computational model is presented in this article for the prediction of microstructural development during heat treating of steels and resultant room-temperature hardness. This model was applied in this study to predict the hardness distribution in end-quench bars (Jominy hardness) of heat treatable steels. It consists of a thermodynamics model for the computation of equilibria in multicomponent Fe-C-M systems, a finite element model to simulate the heat transfer induced by end quenching of Jominy bars, and a reaction kinetics model for austenite decomposition. The overall methodology used in this study was similar to the one in the original work of Kirkaldy. Significant efforts were made to reconstitute the reaction kinetics model for austenite decomposition in order to better correlate the phase transformation theory with empiricism and to allow correct phase transformation predictions under continuous cooling conditions. The present model also expanded the applicable chemical composition range. The predictions given by the present model were found to be in good agreement with experimental measurements and showed considerable improvement over the original model developed by Kirkaldy et al.     

Differential response of arteries and vein grafts to blood flow reduction

BACKGROUND: Because the relative efficacy of antiarrhythmic agents on halothane-epinephrine arrhythmias has not been well characterized, this study was undertaken to comparatively evaluate the antiarrhythmic action of Na(+)-, K(+)- and Ca(2+)-channel blockers on epinephrine-induced ventricular arrhythmias during halothane anesthesia in rats. METHODS: Rats were anesthetized at random with either halothane (1.5%), isoflurane (2.0%), or pentobarbital (50 mg/kg intraperitoneally), and the lungs were mechanically ventilated with oxygen. The rats were studied in three consecutive protocols. Protocol I determined the arrhythmogenic thresholds of epinephrine during the three types of anesthesia in 33 rats. Protocol II determined the arrhythmogenic thresholds of epinephrine during halothane anesthesia in 64 rats receiving saline (control) or one of five antiarrhythmic agents. Protocol III measured the duration of epinephrine-induced arrhythmias during halothane anesthesia in 42 rats receiving saline (control) or one of five antiarrhythmic agents. RESULTS: In protocol I, the arrhythmogenic doses of epinephrine during halothane, isoflurane, or pentobarbital anesthesia were 1.7 +/- 3.2, 11.1 +/- 0.6, and 39.0 +/- 3.9 micrograms/kg, respectively, and the corresponding plasma concentrations were 4.3 +/- 0.8, 103.7 +/- 9.2, and 246.7 +/- 28.9 ng/ml, respectively. In protocol II, the arrhythmogenic doses were similar in rats receiving saline and in those receiving lidocaine. The arrhythmogenic doses in rats receiving verapamil, flecainide (Na(+)- and K(+)-channel blocker), E-4031 (K(+)-channel blocker), or amiodarone(K(+)-channel blocker with Na(+)-, Ca(2+)-, and beta-blocking activity) increased significantly, i.e., 4.2, 4.2, 5.5, and 31.7 times control (P < 0.01). In protocol III, lidocaine had no effect on the duration of arrhythmias. Flecainide, E-4031, and verapamil markedly reduced the duration of arrhythmias induced by epinephrine, 8 micrograms/kg intravenously (P < 0.01), whereas only amiodarone markedly reduced the duration of arrhythmias induced by epinephrine, 16 micrograms/kg intravenously (P < 0.01). CONCLUSIONS: It was concluded that agents with K(+)-channel blocking properties were the most effective in preventing halothane-epinephrine arrhythmias in rats.     

Validation of a new transit time ultrasound flowmeter for measuring blood flow in colonic mesenteric arteries

OBJECTIVE: To validate a transit time ultrasound flowmeter (CardioMed CM 4000) for measuring blood flow in isolated colonic mesenteric arteries. DESIGN: Experimental and clinical study. SETTING: Teaching hospital, Denmark. ANIMALS AND SUBJECTS: One female pig, and 6 patients being operated on for carcinoma of the sigmoid colon and rectum. INTERVENTIONS: Volume blood flow measured by Cardiomed CM 4000 and pump withdrawal flow recordings. MAIN OUTCOME MEASURES: Correlation between the two methods. RESULTS: There was good agreement between transit time flow recordings and pump withdrawal flow recordings (correlation coefficient of 1.0). Of the differences between the two methods, 95% were between -0.16 ml min(-1) and 1.29 ml min(-1), mean 0.57 ml min(-1), or (in percentages) 105, 95% lying between 97-115. There was also good reproducibility in transit time flow recordings, the mean difference between repeated measurements being 0.06 ml min(-1), 95% lying between -0.66 ml min(-1) and 0.78 ml min(-1). CONCLUSION: Ultrasound transit time flow recordings gave precise measurements of blood flow in isolated colonic mesenteric arteries.     

A computational model for tracking subsurface tissue deformation during stereotactic neurosurgery

Recent advances in the field of stereotactic neurosurgery have made it possible to coregister preoperative computed tomography (CT) and magnetic resonance (MR) images with instrument locations in the operating field. However, accounting for intraoperative movement of brain tissue remains a challenging problem. While intraoperative CT and MR scanners record concurrent tissue motion, there is motivation to develop methodologies which would be significantly lower in cost and more widely available. The approach we present is a computational model of brain tissue deformation that could be used in conjunction with a limited amount of concurrently obtained operative data to estimate subsurface tissue motion. Specifically, we report on the initial development of a finite element model of brain tissue adapted from consolidation theory. Validations of the computational mathematics in two and three dimensions are shown with errors of 1%-2% for the discretizations used. Experience with the computational strategy for estimating surgically induced brain tissue motion in vivo is also presented. While the predicted tissue displacements differ from measured values by about 15%, they suggest that exploiting a physics-based computational framework for updating preoperative imaging databases during the course of surgery has considerable merit. However, additional model and computational developments are needed before this approach can become a clinical reality.     

Myocardial blood flow and coronary flow reserve late after anatomical correction of transposition of the great arteries

OBJECTIVES: Myocardial blood flow (MBF) in children late after arterial switch operation (ASO) was investigated quantitatively by positron emission tomography (PET). BACKGROUND: In children with transposition of the great arteries (TGA), ASO is widely accepted as the management of choice. The long-term patency of coronary arteries after surgical transfer to the neo-aorta, however, remains a concern. METHODS: Twenty-two normally developed, symptom-free children were investigated by PET with nitrogen-13 ammonia at rest and during adenosine vasodilation 10+/-1 years after ASO. A subgroup of 15 children (9+/-1 years; group A) had simple TGA and underwent ASO within 20 days after birth while 7 (13+/-3 years; group B) had complex TGA and underwent ASO and correction of associated anomalies later after birth. Ten young, healthy adults (26+/-6 years) served as the control group. RESULTS: Resting MBF was not different between groups. After correction for the rate-pressure product as an index of cardiac work, younger children of group A had significantly higher MBF at rest compared to healthy adults (102+/-29 vs. 77+/-6 ml/100 g/min; p = 0.012) while flow in group B was not different from the other groups (85+/-22 ml/100 g/min; p = NS). Hyperemic blood flows were significantly lower in both groups after ASO compared to normals (290+/-42 ml/100 g/min for group A, 240+/-28 for group B, 340+/-57 for normals; p < 0.01); thus, coronary flow reserve was significantly lower in both groups after ASO compared to healthy adults (3.0+/-0.6 for group A, 2.9+/-0.6 for group B, 4.6+/-0.9 for normals; p < 0.01). CONCLUSIONS: Blood flow measurements suggest decreased coronary reserve in the absence of ischemic symptoms in children late after arterial switch repair of TGA. The global impairment of stress flow dynamics may indicate altered vasoreactivity; however, the prognostic significance of these findings needs to be determined.     

Assessment of blood flow distal to coronary artery stenoses. Correlations between myocardial positron emission tomography and poststenotic intracoronary Doppler flow reserve

BACKGROUND: Previous studies have correlated quantitative coronary angiographic stenosis severity with positron emission tomography (PET) myocardial perfusion and proximal measurements of intracoronary flow velocities in normal and diseased coronary arteries. The aim of this study was to correlate regional myocardial blood flow (RMBF) derived from [15O]H2O PET with directly measured poststenotic intracoronary Doppler flow velocity data acquired under basal conditions and dipyridamole-induced hyperemia. METHODS AND RESULTS: Eleven consecutive patients 53 +/- 13 years old with ischemic chest pain and isolated proximal left coronary artery stenoses (left anterior descending, 9; left circumflex, 2; mean, 59 +/- 23% diameter stenosis) underwent [15O]H2O myocardial PET and intracoronary Doppler flow velocity studies within 1 week. PET RMBF (mL.g-1.min-1) and myocardial perfusion reserve (MPR) were calculated in poststenotic and normal reference vascular beds. Poststenotic Doppler average peak flow velocities (APV; cm/s) and coronary flow velocity reserve (CFR) were compared with corresponding PET data and quantitative angiographic lesional parameters. PET RMBF and Doppler APV were linearly correlated (r = .60; P < .001), as were poststenotic PET MPR and Doppler CFR (r = .76; P < .0002). Relative coronary flow velocity and MPR ratios between poststenotic and angiographically normal vascular beds were comparably reduced (0.83 +/- 0.25 versus 0.86 +/- 0.21, respectively; P = NS). CONCLUSIONS: Intracoronary Doppler flow velocities acquired distal to isolated left coronary artery stenoses correlated with [15O]H2O PET regional myocardial perfusion and are useful for assessment of the physiological significance of coronary stenoses in humans.     

A computational model of hippocampal place fields.

Notes that there are neurons in the hippocampus that become active only when an animal is near a particular location in a specific environment. The activity of some of these units is governed by the configuration of a small set of discrete landmarks. To respond in this fashion, these neurons must, in effect, be able to recognize particular locations. A model of this recognition process is described that is able to make quantitative predictions about how the response of these place-field units varies as properties of the environmental landmarks are manipulated. Computer simulations of the model show that it is consistent with the available quantitative data. These simulations also predict large, characteristic changes in place-field location and size with manipulations of the environmental landmarks. Experiments to test the validity of the model are described. (6 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A model of blood interaction with optical-fluid guide for laser angioplasty

A mathematical model is developed to describe the flow and mixing of blood and optical fluid used in liquid-guided light for laser angioplasty. The model is based on a two-fluid formulation in which separate transport equations are solved for the blood and the optical fluid. Empirical relations, established in prior work, are used to represent interfluid transport of momentum. Both steady and phasic inflow conditions are considered. Parametric calculations are performed showing effect of relative flow rates of blood and optical fluid, on the mixing phenomena. The relative velocity considered (based on average blood velocity) ranged from 0.08 to 0.28 m/sec. No allowance has been made for ablation of the plaque. The predicted results include spatial distribution of the velocity field and the existence probabilities (volume fractions) that provide a measure of the extent of mixing between the fluids. It is found that the degree of mixing is adversely affected by the relative inflow velocity between blood and optical fluid and the pulsatility of blood inflow. Deep penetration of the optical fluid is predicted at high relative velocity and at the end of diastolic and early systolic stages of the cardiac cycle.     

A unified computational model for surface texturing process of strip casting rollers

A new unified computational method is proposed for modeling the relationship between the parameters of the high-speed particle-impact texturing process and the final surface morphology of the casting roller. The whole surface-texturing process is divided into three parts. The first part is the acceleration process of particles inside and outside the nozzle,which can be analyzed using the computational fluid dynamics method to obtain the particle impact velocities. The second part is a simulation of the bombardment process of particles onto the roller surface using the LS-DYNA software as the analysis tool and the results obtained in the first part as input parameters. The last part is the continuously random impact process of particles on the casting roller surface to form a functional surface,which is characterized by a simplified roughness model. Finally,the above three parts are combined to establish a unified computational model,the performance of which is successfully verified in a series of experiments.     

非对称控流结构中间包流场一致性的物理模拟

国内某钢厂使用的两流板坯连铸中间包因受固定位置排渣口的限制,包内控流装置采用左右不对称布置。生产实践发现,排渣口侧的水口对应铸坯大型夹杂物含量高、热轧卷探伤合格率低,疑与中间包流场的一致性有关。为此,采用1∶3.5的水模型对中间包流场进行了模拟研究,并基于流体动力学原理对其控流效果进行了优化。结果表明,原型中间包两个水口的滞止时间差高达36 s,钢液在排渣口侧的1号水口形成短路流,因而导致两流铸坯洁净度的差异。经水模优化后,方案F1下两流平均停留时间标准差和滞止时间标准差分别可降到0.12和0.35 s,明显改善了中间包内两流浇铸流动特性的一致性,且死区比例较原型降低8.87%、平均停留时间延长了30 s。     

Arterial blood pressure and blood flow velocity in major cerebral and visceral arteries. I. Interindividual differences

The growth hormone-releasing hormone receptor (GHRHR) is a member of the family of G protein-coupled receptors that is expressed on pituitary somatotrope cells and mediates the actions of GHRH in stimulating growth hormone (GH) synthesis and secretion. We report that the Ghrhr gene is located in the middle of mouse chromosome 6 in the same region as the little mutation. Mice homozygous for this mutation have reduced GH secretion and a dwarf phenotype. A missense mutation was identified in the extracellular domain of the little GHRHR that disrupts receptor function, suggesting that the growth deficit in these mice results from a defect in the GHRHR. Similar alterations in GHRHR might explain some isolated GH deficiencies in humans.     

A simple technique for measuring relative renal blood flow

To determine whether externally monitored early renal uptake of 131I-hippurate is proportional to renal blood flow, the renal uptake of 131I-hippurate at 1--2 min after injection was compared with the renal accumulation of radioactive carbonized microspheres in dogs. A renal artery catheter equipped with a balloon was used to decrease renal blood flow unilaterally. One minute after the intravenous injection of 100 muCi of 131I-hippurate, about 1 muCi of either 85Sr- or 98Nb-labeled carbon microspheres was injected into the left ventricle. Radioactivity was measured over both kidneys. The total radioactivity within each kidney region of interest was corrected for background and integrated over the 1--2 min after injection. Thirteen measurements of relative renal blood flow were made for seven dogs. The dogs were then killed and both kidneys were excised and counted for the radioactivity of the microspheres. The 1--2-min relative renal uptake of 131I-hippurate correlated well with relative microsphere uptake, suggesting that relative renal blood flow can be simply determined from the external measurement of renal uptake of 131I-hippurate.     

A nonlinear model for myogenic regulation of blood flow to bone: equilibrium states and stability characteristics

A simple compartmental model for myogenic regulation of interstitial pressure in bone is developed, and the interaction between changes in interstitial pressure and changes in arterial and venous resistance is studied. The arterial resistance is modeled by a myogenic model that depends on transmural pressure, and the venous resistance is modeled by using a vascular waterfall. Two series capacitances model blood storage in the vascular system and interstitial fluid storage in the extravascular space. The static results mimic the observed effect that vasodilators work less well in bone than do vasoconstrictors. The static results also show that the model gives constant flow rates over a limited range of arterial pressure. The dynamic model shows unstable behavior at small values of bony capacitance and at high enough myogenic gain. At low myogenic gain, only a single equilibrium state is present, but a high enough myogenic gain, two new equilibrium states appear. At additional increases in gain, one of the two new states merges with and then separates from the original state, and the original state becomes a saddle point. The appearance of the new states and the transition of the original state to a saddle point do not depend on the bony capacitance, and these results are relevant to general fluid compartments. Numerical integration of the rate equations confirms the stability calculations and shows limit cycling behavior in several situations. The relevance of this model to circulation in bone and to other compartments is discussed.