Pseudo-organ boundary conditions applied to a computational fluid dynamics model of the human aorta

Comparative Analysis of Patient-Specific Aortic Dissections through Computational Fluid Dynamics Suggests Increased Likelihood of Degeneration in Partially Thrombosed False Lumen

Aortic dissection is a life-threatening vascular disease associated with high rates of morbidity and mortality, especially in medically underserved communities. Understanding patients’ blood flow patterns is pivotal for informing evidence-based treatment as they greatly influence the disease outcome. The present study investigates the flow patterns in the false lumen of three aorta dissections (fully perfused, partially thrombosed, and fully thrombosed) in the chronic phase, and compares them to a healthy aorta. Three-dimensional geometries of aortic true and false lumens (TLs and FLs) are reconstructed through an ad hoc developed and minimally supervised image analysis procedure. Computational fluid dynamics (CFD) is performed through a finite volume unsteady Reynolds-averaged Navier–Stokes approach assuming rigid wall aortas, Newtonian and homogeneous fluid, and incompressible flow. In addition to flow kinematics, we focus on time-averaged wall shear stress and oscillatory shear index that are recognized risk factors for aneurysmal degeneration. Our analysis shows that partially thrombosed dissection is the most prone to false lumen degeneration. In all dissections, the arteries connected to the false lumen are generally poorly perfused. Further, both true and false lumens present higher turbulence levels than the healthy aorta, and critical stagnation points. Mesh sensitivity and a thorough comparison against literature data together support the reliability of the CFD methodology. Image-based CFD simulations are efficient tools to assess the possibility of aortic dissection to lead to aneurysmal degeneration, and provide new knowledge on the hemodynamic characteristics of dissected versus healthy aortas. Similar analyses should be routinely included in patient-specific hemodynamics investigations, to plan and design tailored therapeutic strategies, and to timely assess their effectiveness.

A detailed analysis in thoracic aorta by means of the entropy generation rate: Prediction of the atherosclerotic lesion

A detailed numerical analysis is carried out in a real human thoracic aorta by means of the Computational Fluid Dynamics (CFD) for the prediction of the atherosclerosis lesion. Common hemodynamics parameters, such as, the oscillatory shear index (OSI) and the time average wall shear stress (TAWSS) are used for the prediction of the atherosclerosis lesion. Furthermore, the entropy generation rate is considered to obtain the main irreversibilities that occurs inside the thoracic aorta for the prediction of the atherosclerosis lesion. The model considers the blood flow inside the thoracic aorta in an unsteady state. The results show contours of velocity, streams lines, velocity profiles and the comparison of the hemodynamics parameters OSI versus TAWSS. Moreover, contours of the entropy generation rate are showed inside the aorta. The time averaged entropy generation rate (TAEGR) is obtained as a result of the entropy generation analysis. Finally, TAEGR index is compared and discussed with the common hemodynamics parameters, OSI and TAWSS. The accuracy to detect prone locations to atherosclerotic development in the real aorta using the TAEGR in comparison to the OSI and the TAWSS is in good agreement.

Non-invasive Prediction of Peak Systolic Pressure Drop across Coarctation of Aorta using Computational Fluid Dynamics

This paper proposes a novel method to noninvasively measure the peak systolic pressure difference (PSPD) across coarctation of the aorta for diagnosing the severity of coarctation. Traditional non-invasive estimates of pressure drop from the ultrasound can underestimate the severity and invasive measurements by cardiac catheterization can carry risks for patients. To address the issues, we employ computational fluid dynamics (CFD) computation to accurately predict the PSPD across a coarctation based on cardiac magnetic resonance (CMR) imaging data and cuff pressure measurements from one arm. The boundary conditions of a patient-specific aorta model are specified at the inlet of the ascending aorta by using the time-dependent blood velocity, and the outlets of descending aorta and supra aortic branches by using a 3-element Windkessel model. To estimate the parameters of the Windkessel model, steady flow simulations were performed using the time-averaged flow rates in the ascending aorta, descending aorta, and two of the three supra aortic branches. The mean cuff pressure from one arm was specified at the outlet of one of the supra aortic branches. The CFD predicted PSPDs of 5 patients (n=5) were compared with the invasively measured pressure drops obtained by catheterization. The PSPDs were accurately predicted (mean µ=0.3mmHg, standard deviation σ =4.3mmHg) in coarctation of the aorta using completely non-invasive flow and cuff pressure data. The results of our study indicate that the proposed method could potentially replace invasive measurements for estimating the severity of coarctations.Clinical relevance-Peak systolic pressure drop is an indicator of the severity of coarctation of the aorta. It can be predicted without any additional risks to patients using non-invasive cuff pressure and flow data from CMR.

Numerical simulation of blood flow in femoral perfusion: comparison between side-armed femoral artery perfusion and direct femoral artery perfusion

Computational numerical analysis was performed to elucidate the flow dynamics of femoral artery perfusion. Numerical simulation of blood flow was performed from the right femoral artery in an aortic model. An incompressible Navier-Stokes equation and continuity equation were solved using computed flow dynamics software. Three different perfusion models were analyzed: a 4.0-mm cannula (outer diameter 15 French size), a 5.2-mm cannula (18 French size) and an 8-mm prosthetic graft. The cannula was inserted parallel to the femoral artery, while the graft was anastomosed perpendicular to the femoral artery. Shear stress was highest with the 4-mm cannula (172 Pa) followed by the graft (127 Pa) and the 5.2-mm cannula (99 Pa). The cannula exit velocity was high, even when the 5.2-mm cannula was used. Although side-armed perfusion with an 8-mm graft generated a high shear stress area near the point of anastomosis, flow velocity at the external iliac artery was decreased. The jet speed decreased due to the Coanda effect caused by the recirculation behind sudden expansion of diameter, and the flow velocity maintains a constant speed after the reattachment length of the flow. This study showed that iliac artery shear stress was lower with the 5.2-mm cannula than with the 4-mm cannula when used for femoral perfusion. Side-armed graft perfusion generates a high shear stress area around the anastomotic site, but flow velocity in the iliac artery is slower in the graft model than in the 5.2-mm cannula model.

Finite element modelling of pulsatile blood flow in idealized model of human aortic arch: study of hypotension and hypertension

A three-dimensional computer model of human aortic arch with three branches is reproduced to study the pulsatile blood flow with Finite Element Method. In specific, the focus is on variation of wall shear stress, which plays an important role in the localization and development of atherosclerotic plaques. Pulsatile pressure pulse is used as boundary condition to avoid flow entry development, and the aorta walls are considered rigid. The aorta model along with boundary conditions is altered to study the effect of hypotension and hypertension. The results illustrated low and fluctuating shear stress at outer and inner wall of aortic arch, proximal wall of branches, and entry region. Despite the simplification of aorta model, rigid walls and other assumptions results displayed that hypertension causes lowered local wall shear stresses. It is the sign of an increased risk of atherosclerosis. The assessment of hemodynamics shows that under the flow regimes of hypotension and hypertension, the risk of atherosclerosis localization in human aorta may increase.

Hemodynamic Modeling of Surgically Repaired Coarctation of the Aorta

PURPOSE: Late morbidity of surgically repaired coarctation of the aorta includes early cardiovascular and cerebrovascular disease, shortened life expectancy, abnormal vasomodulator response, hypertension and exercise-induced hypertension in the absence of recurrent coarctation. Observational studies have linked patterns of arch remodeling (Gothic, Crenel, and Romanesque) to late morbidity, with Gothic arches having the highest incidence. We evaluated flow in native and surgically repaired aortic arches to correlate respective hemodynamic indices with incidence of late morbidity. METHODS: Three dimensional reconstructions of each remodeled arch were created from an anatomic stack of magnetic resonance (MR) images. A structured mesh core with a boundary layer was generated. Computational fluid dynamic (CFD) analysis was performed assuming peak flow conditions with a uniform velocity profile and unsteady turbulent flow. Wall shear stress (WSS), pressure and velocity data were extracted. RESULTS: The region of maximum WSS was located in the mid-transverse arch for the Crenel, Romanesque and Native arches. Peak WSS was located in the isthmus of the Gothic model. Variations in descending aorta flow patterns were also observed among the models. CONCLUSION: The location of peak WSS is a primary difference among the models tested, and may have clinical relevance. Specifically, the Gothic arch had a unique location of peak WSS with flow disorganization in the descending aorta. Our results suggest that varied patterns and locations of WSS resulting from abnormal arch remodeling may exhibit a primary effect on clinical vascular dysfunction.

Evaluation of local flow conditions in jailed side branch lesions using computational fluid dynamics

Background and Objectives

Lesions of vascular bifurcation and their treatment outcomes have been evaluated by anatomical and physiological methods, such as intravascular ultrasound and fractional flow reserve (FFR). However, local changes in flow dynamics in lesions of bifurcation have not been well evaluated. This study aimed at evaluating changes in the local flow patterns of bifurcation lesions.

Materials and Methods

Eight (n=8) representative simulation-models were constructed: 1 normal bifurcation, 5 main-branch (MB) stenting models with various side-branch (SB) stenoses (ostial or non-ostial 75% diameter stenosis with 1- or 2-cm lesion lengths, ostial 75% diameter stenosis caused by carina shift), and 2 post-kissing models (no or 50% SB residual stenosis). Pressure, velocity, and wall shear stress (WSS) profiles around the bifurcation sites were investigated using computational fluid dynamics.

Results

Post-stenting models revealed significant pressure drop in the SB (FFR<0.75), excluding the carina shift model (FFR=0.89). In the post-kissing models, there was no significant pressure drop. All post-stenting models revealed eccentric low velocity flow patterns and areas of low WSS, primarily in the lateral wall on distal MB. Post-kissing angioplasty improved pressure drop in the SB but resulted in alteration of flow distribution in the MB. In the carina shift model, kissing ballooning resulted in deteriorated local flow conditions due to increased area of low velocity and WSS.

Conclusion

This study suggests that the most commonly used bifurcation intervention strategy may cause local flow disturbances, which may partially explain high restenosis and event rates in patients with bifurcation lesions.