Elongation of the proximal descending thoracic aorta and associated hemodynamics increase the risk of acute type B aortic dissection.

BACKGROUND: Acute type B aortic dissection (ATBAD) is a life-threatening aortic disease. However, little information is available on predicting and understanding of ATBAD. OBJECTIVE: The study sought to explore the underlying mechanism of ATBAD by analyzing the morphological and hemodynamic characteristics related to aortic length. METHODS: The length and tortuosity of the segment and the whole aorta in the ATBAD group (n= 163) and control group (n= 120) were measured. A fixed anatomic landmark from the distal of left subclavian artery (LSA) to the superior border of sixth thoracic vertebra was proposed as the proximal descending thoracic aorta (PDTA), and the dimensionless parameter, length ratio, was introduced to eliminate the individual differences. The significant morphological parameters were filtrated and the associations between parameters were investigated using statistical approaches. Furthermore, how aortic morphology influenced ATBAD was explored based on idealized aortic models and hemodynamic-related metrics. RESULTS: The PDTA length was significantly increased in the ATBAD group compared with the control group and had a strong positive correlation with the whole aortic length (r= 0.89). The length ratio (LR2) and tortuosity (T2) of PDTA in the ATBAD group were significantly increased (0.15 ± 0.02 vs 0.12 ± 0.02 and 1.73 ± 0.48 vs 1.50 ± 0.36; P< 0.001), and LR2 was positive correlation with T2 (r= 0.73). In receiver-operating curve analysis, the area under the curve was 0.835 for LR2 and 0.641 for T2. Low and oscillatory shear (LOS) was positive correlation with LR2, and the elevated LOS occurred in the distal of LSA. CONCLUSION: Elongation of PDTA is associated with ATBAD, and the length ratio is a novel predictor. Elongated PDTA induced more aggressive hemodynamic forces, and high LOS regions may correspond to the entry tear location. The synergy of the morphological variation and aggressive hemodynamics creates contributory conditions for ATBAD.

Hemodynamics of different configurations of the left subclavian artery parallel stent graft for thoracic endovascular aortic repair.

BACKGROUND AND OBJECTIVE: Parallel (chimney and periscope) graft technique is an effective approach for left subclavian artery (LSA) reconstruction in patients treated by thoracic endovascular aortic repair (TEVAR) for the inadequate landing zone. However, certain stent graft (SG) configurations may promote thrombosis and reduce distal blood flow, increasing risks of cerebral infarction and reintervention. METHODS: In this paper, we first attempt to systematically evaluate the hemodynamic performances of different parallel graft techniques as potential determinants of complication risks. Based on the patient-specific 3D aortic geometry undergoing parallel graft technique, fifteen models in total for five kinds of LSA branched SG configurations (Forward, Backward, Extended, Elliptical and Periscopic) were designed virtually, and the hemodynamic discrepancies between them were analyzed by computational fluid dynamics. RESULTS: Results show that flow rate of patients undergoing periscope technique reduces by half compared with chimney technique, suggesting that periscope SG may cause more serious flow obstruction to LSA, leading to stroke. For chimney stent structure, the extension length 0has little influence on energy loss and other parameters. Conversely, hemodynamic differences between the retrograde curvature and the antegrade curvature are significant (time average WSS: 47.07%), so the retrograde curvature might prompt SG displacement. Furthermore, the flatter chimney SG induces more aggressive hemodynamic forces, among which the difference of the maximum WSS between the flatter SG and nearly round SG reaches 65.56%, leading to the greater risk of vascular wall damage. CONCLUSIONS: Results obtained might provide suggestions for physicians to formulate appropriate parallel graft technique schemes in TEVAR.

Enlarged Lumen Volume of Proximal Aortic Segment and Acute Type B Aortic Dissection: A Computer Fluid Dynamics Study of Ideal Aortic Models.

BACKGROUND: Recent study has revealed that enlarged diameters of the ascending aorta and proximal aortic arch enhance the probability of ATBAD. However, little is understood about the relation to ATBAD. OBJECTIVE: This study explored the differences in proximal aortic segment (PAS) morphology in patients with acute type B aortic dissection (ATBAD), and performed hemodynamic simulations to provide proof of principle. MATERIALS AND METHODS: The morphological characteristics of PAS in the ATBAD group (n = 163) and corresponding segment in the control group (n = 120) were retrospectively measured. The morphological parameters were analyzed using comprehensive statistical approaches. Ridge regression analysis was also performed to determine the association between independent variable and dependent variable. P < 0.01 was considered significant. Idealized aortic models were established based on variables of statistical significance, and hemodynamic simulations were performed to evaluate blood flow changes caused by morphology. RESULTS: Diameters at landmarks of PAS were significantly larger in the ATBAD group. The lumen volume (VPAS) of PAS in the ATBAD group was significantly enlarged than that of the control group (124,659.07 ± 34,089.27 mm3 vs 89,796.65 ± 30,334.40 mm3; P < 0.001). Furthermore, the VPAS was positively correlated to diameters. As the VPAS increased, the fluid kinetic energy in PAS enhanced linearly, and time-averaged wall shear stress and oscillatory shear index at the distal area of the left subclavian artery increased significantly. CONCLUSION: In the ATBAD group, the enlarged VPAS and increased diameters of PAS are positively correlated. Meanwhile, the enlarged VPAS leads to more aggressive hemodynamic parameters at the distal area of the left subclavian artery, which may create a contributory condition for ATBAD.

Quantitative analysis of renal blood flow during thoracic endovascular aortic repair in type B aortic dissection using syngo iFlow.

BACKGROUND: Currently, the thoracic endovascular aortic repair is the recommended clinical treatment for type B aortic dissections. Unfortunately, malperfusion or ischemia of the kidneys is a major complication of type B aortic dissections. Despite this, few studies have focused on the effects of thoracic endovascular aortic repair on blood flow in renal arteries and parenchyma. This current investigation used novel real-time imaging software to quantitatively analyze the hemodynamic changes in renal artery blood flow and perfusion before and after stent graft placement. METHODS: A total of 51 patients with type B aortic dissection undergoing thoracic endovascular aortic repair between April 2017 and September 2019 were retrospectively recruited. The pre-and post-procedural digital subtraction angiography images were converted into color-coded maps using syngo iFlow for quantitative comparison. Time-intensity curves and related parameters, including the average peak ratio (avg.Pr), average delayed time to peak (avg.dTTP), and average area under the curve ratio (avg.AUCr) of the renal arteries and renal cortex were obtained and analyzed. Wilcoxon signed-rank test was used to compare iFlow parameters before and after endovascular repair. Spearman correlation analyses were performed to study iFlow parameters and renal function parameters and the estimated glomerular filtration rate (eGFR) and blood urea nitrogen (BUN). RESULTS: A total of 102 images including 51 pre-operative and 51 post-operative image datasets were successfully post-processed. Following endovascular repair, syngo iFlow showed a significant 33.0% increase in avg.Pr (P<0.001) and a significant 35.1% increase in avg.AUCr (P<0.001) in the renal artery. Additionally, there was a significant 12.2% decrease in the avg.dTTP (P=0.001), a significant 24.5% increase in avg.Pr (P=0.004), and a significant 38.3% increase in avg.AUCr (P=0.009) in the renal cortex. Spearman correlation analysis showed that after endovascular repair there was a significant correlation between the avg.Pr of the renal artery and eGFR (r=0.30; P=0.0349), the avg.Pr of the renal cortex and eGFR (r=0.30; P=0.0300), and the avg. AUCr of the renal cortex and BUN (r=0.31; P=0.0289). CONCLUSIONS: syngo iFlow provided a novel quantitative method for evaluating renal hemodynamic changes in patients with type B aortic dissection undergoing endovascular treatment. Time-intensity curve parameters may facilitate the intraprocedural evaluation of renal blood flow and perfusion to complement the color-coded map.

Numerical investigations of temperature and hemodynamics in carotid arteries with and without atherosclerotic plaque during open surgery.

Intraoperative monitoring is essential for providing safe and effective care during open surgery. In this paper, numerical simulation is performed to track the flow and heat transfer of carotid arteries with and without atherosclerotic plaque in a real physiological system during surgery, in which the heat transport is first considered to couple to the blood flow due to the temperature dependence of the blood viscosity. The impacts of the operating room temperature and hematocrit (H) on the viscosity, velocity, temperature, wall shear stress (WSS), pressure drop and oscillation are investigated. The results demonstrate that the presence of plaque in the carotid artery induces a greater blood flow velocity, pressure drop, WSS, and oscillation, as well as a smaller viscosity and temperature variations. A decreasing ambient temperature leads to a decrease in the temperature and an increase in the low-WSS area, which implies a greater risk of hypothermia and atherosclerosis. As H increases, the high-WSS areas substantially expand; when H varies from 65% to 80%, WSSave increases by 70.02% and 68.57% for the arteries with and without plaque, respectively, which indicates a higher risk of vascular injury. The results obtained can serve as a guideline regarding the selection of an operating room temperature for carotid disease patients with distinct hematocrits.

Numerical simulation of magnetic nano drug targeting to atherosclerosis: Effect of plaque morphology (stenosis degree and shoulder length).

BACKGROUND AND OBJECTIVE: Nanoparticle-mediated targeted drug delivery is a promising option for treatment of atherosclerosis. However, the drug targeting may be affected by multiple factors. Considerable attentions have been focused on the influences of external factors, e.g., magnetic field, drug-loaded particle, but internal factors, e.g., plaque morphology (stenosis degree and shoulder length), have not received any attention yet. Therefore, we investigate the impact of plaque morphology on magnetic nanoparticles targeting under the action of an external field. METHOD: Numerical simulation, based on Eulerian-Lagrangian coupled Fluid-Solid Interaction, is performed in ANSYS Workbench platform. Blood flow is solved by Navier-Stokes equation, particles are tracked by discrete phase model, and the incorporated effect is obtained by two-way method. Plaques with varying stenosis degrees and shoulder lengths are acquired by manually modifying the geometry of patient-specific. The quantified variables include targeted delivery efficiency (deposition+adhesive strength) of particles and plaque injury characterized by temporal-spatial averaged shear stress (TAWSS¯) during the process of drug transport, in which the critical deposition velocity is determined by plaques and particles, the DEFINE_DPM_BC and User Defined Memory are employed to evaluate whether the particles are deposited, and to store the total number and the adhesive strength of particles deposited on the plaque. RESULTS: Results signify that, with an increment of plaque stenosis degree, the deposition of particle and the adhesive strength between particle and plaque decrease, while the TAWSS¯ increases. Furthermore, for the same stenosis degree, with the increase of plaque shoulder length, the deposition and the adhesive strength of particle increase, and the TAWSS¯ decreases. CONCLUSIONS: Results demonstrates that the plaque with smaller stenosis degree or longer shoulder length may achieve a better treatment effect in view of the higher targeted delivery efficiency of particles and the lighter shear damage to plaque itself during the process of drug transport.

Analysis of the formation mechanism and occurrence possibility of Post-Stenotic Dilatation of the aorta by CFD approach.

BACKGROUND AND OBJECTIVE: Post-Stenotic Dilatation (PSD), the common complication of coarctation of the aorta (COA), is a progressive disease involving aortic aneurysm and even rupture. However, there has been no definitive method that could investigate the mechanism of PSD formation, progression and rupture. The purpose of the present work is to analyze the mechanism behind PSD formation and to further assess the risk of COA patients with different coarctation degrees deteriorating into PSD. METHOD: Three-dimensional non-Newtonian (Carreau-Yasuda) hemodynamic simulations are performed throughout the cardiac cycle, and a novel parameter (λci¯ intensity) is proposed to evaluate the intensity of vortices within the aorta. The PSD geometry is reconstructed from Computed Tomography scans. To analyze the formation mechanism and occurrence possibility of PSD, the computer technology is utilized to restore the expansive and/or narrow regions to obtain its previous state (COA) and control group (Normal), and to modify the minimum diameter to obtain the aortas with different coarctation degrees. The clinical cases of pre- and post-operation are further introduced to verify the analysis. RESULTS: Compared with the Normal, the vortical structures with higher swirling strength are generated and accumulated at the downstream of the coarctation segment after COA occurrence, and partially disappear in the wake of PSD formation. The sequence of λci¯ intensity is COA > PSD > Normal and pre-operation > post-operation. With increasing the degree of coarctation, the λci¯ intensity is higher and the jet-flow becomes more drastic. CONCLUSIONS: The formation of PSD is caused by the vortical structures with higher swirling strength accumulating at the downstream of the coarctation segment. An increase in coarctation degree elevates the risk of PSD occurrence and even aneurysmal dilatation.

A novel investigation of a micropolar fluid characterized by nonlinear constitutive diffusion model in boundary layer flow and heat transfer.

The rheological and heat-conduction constitutive models of micropolar fluids (MFs), which are important non-Newtonian fluids, have been, until now, characterized by simple linear expressions, and as a consequence, the non-Newtonian performance of such fluids could not be effectively captured. Here, we establish the novel nonlinear constitutive models of a micropolar fluid and apply them to boundary layer flow and heat transfer problems. The nonlinear power law function of angular velocity is represented in the new models by employing generalized "n-diffusion theory," which has successfully described the characteristics of non-Newtonian fluids, such as shear-thinning and shear-thickening fluids. These novel models may offer a new approach to the theoretical understanding of shear-thinning behavior and anomalous heat transfer caused by the collective micro-rotation effects in a MF with shear flow according to recent experiments. The nonlinear similarity equations with a power law form are derived and the approximate analytical solutions are obtained by the homotopy analysis method, which is in good agreement with the numerical solutions. The results indicate that non-Newtonian behaviors involving a MF depend substantially on the power exponent n and the modified material parameter [Formula: see text] introduced by us. Furthermore, the relations of the engineering interest parameters, including local boundary layer thickness, local skin friction, and Nusselt number are found to be fitted by a quadratic polynomial to n with high precision, which enables the extraction of the rapid predictions from a complex nonlinear boundary-layer transport system.

Fractal aggregation kinetics contributions to thermal conductivity of nano-suspensions in unsteady thermal convection.

Nano-suspensions (NS) exhibit unusual thermophysical behaviors once interparticle aggregations and the shear flows are imposed, which occur ubiquitously in applications but remain poorly understood, because existing theories have not paid these attentions but focused mainly on stationary NS. Here we report the critical role of time-dependent fractal aggregation in the unsteady thermal convection of NS systematically. Interestingly, a time ratio λ = tp/tm (tp is the aggregate characteristic time, tm the mean convection time) is introduced to characterize the slow and fast aggregations, which affect distinctly the thermal convection process over time. The increase of fractal dimension reduces both momentum and thermal boundary layers, meanwhile extends the time duration for the full development of thermal convection. We find a nonlinear growth relation of the momentum layer, but a linear one of the thermal layer, with the increase of primary volume fraction of nanoparticles for different fractal dimensions. We present two global fractal scaling formulas to describe these two distinct relations properly, respectively. Our theories and methods in this study provide new evidence for understanding shear-flow and anomalous heat transfer of NS associated non-equilibrium aggregation processes by fractal laws, moreover, applications in modern micro-flow technology in nanodevices.

A Novel Equivalent Agglomeration Model for Heat Conduction Enhancement in Nanofluids.

We propose a multilevel equivalent agglomeration (MEA) model in which all particles in an irregular cluster are treated as a new particle with equivalent volume, the liquid molecules wrapping the cluster and in the gaps are considered to assemble on the surface of new particle as mixing nanolayer (MNL), the thermal conductivity in MNL is assumed to satisfy exponential distribution. Theoretical predictions for thermal conductivity enhancement are highly in agreement with the classical experimental data. Also, we first try to employ TEM information quantitatively to offer probable reference agglomeration ratio (not necessary a very precise value) to just test rational estimations range by present model. The comparison results indicate the satisfactory priori agglomeration ratio estimations range from renovated model.

The exterior unsteady viscous flow and heat transfer due to a porous expanding or contracting cylinder.

Since the vessels in the biological tissues are characterized by low seepage Reynolds numbers and contracting or expanding walls, more attention is paid on the viscous flow outside the porous pipe with small expansion or contraction. This paper presents a numerical solution of the flow and heat transfer outside an expanding or contracting porous cylinder. The coupled nonlinear similarity equations are solved by Bvp4c, which is a collocation method with MATLAB. The effects of the different physical parameters, namely the permeability Reynolds number,the expansion ratio and the Prandtl number, on the velocity and temperature distribution are obtained and the results are shown graphically.