High Wall Shear Stress can Predict Wall Degradation in Ascending Aortic Aneurysms: An Integrated Biomechanics Study

Computational fluid dynamics in cardiac surgery and perfusion: A review

Cardiovascular diseases persist as a leading cause of mortality and morbidity, despite significant advances in diagnostic and surgical approaches. Computational Fluid Dynamics (CFD) represents a branch of fluid mechanics widely used in industrial engineering but is increasingly applied to the cardiovascular system. This review delves into the transformative potential for simulating cardiac surgery procedures and perfusion systems, providing an in-depth examination of the state-of-the-art in cardiovascular CFD modeling. The study first describes the rationale for CFD modeling and later focuses on the latest advances in heart valve surgery, transcatheter heart valve replacement, aortic aneurysms, and extracorporeal membrane oxygenation. The review underscores the role of CFD in better understanding physiopathology and its clinical relevance, as well as the profound impact of hemodynamic stimuli on patient outcomes. By integrating computational methods with advanced imaging techniques, CFD establishes a quantitative framework for understanding the intricacies of the cardiac field, providing valuable insights into disease progression and treatment strategies. As technology advances, the evolving synergy between computational simulations and clinical interventions is poised to revolutionize cardiovascular care. This collaboration sets the stage for more personalized and effective therapeutic strategies. With its potential to enhance our understanding of cardiac pathologies, CFD stands as a promising tool for improving patient outcomes in the dynamic landscape of cardiovascular medicine.

Blood flow assessment technology in aortic surgery: a narrative review

Background and Objective

Blood flow assessment is an emerging technique that allows for assessment of hemodynamics in the heart and blood vessels. Recent advances in cardiovascular imaging technologies have made it possible for this technique to be more accessible to clinicians and researchers. Blood flow assessment typically refers to two techniques: measurement-based flow visualization using echocardiography or four-dimensional flow magnetic resonance imaging (4D flow MRI), and computer-based flow simulation based on computational fluid dynamics modeling. Using these methods, blood flow patterns can be visualized and quantitative measurements of mechanical stress on the walls of the ventricles and blood vessels, most notably the aorta, can be made. Thus, blood flow assessment has been enhancing the understanding of cardiac and aortic diseases; however, its introduction to clinical practice has been negligible yet. In this article, we aim to discuss the clinical applications and future directions of blood flow assessment in aortic surgery. We then provide our unique perspective on the technique's translational impact on the surgical management of aortic disease.

Methods

Articles from the PubMed database and Google Scholar regarding blood flow assessment in aortic surgery were reviewed. For the initial search, articles published between 2013 and 2023 were prioritized, including original articles, clinical trials, case reports, and reviews. Following the initial search, additional articles were considered based on manual searches of the references from the retrieved literature.

Key Content and Findings

In aortic root pathology and ascending aortic aneurysms, blood flow assessment can elucidate postoperative hemodynamic changes after surgical reconfiguration of the aortic valve complex or ascending aorta. In cases of aortic dissection, analysis of blood flow can predict future aortic dilatation. For complicated congenital aortic anomalies, surgeons may use preoperative imaging to perform "virtual surgery", in which blood flow assessment can predict postoperative hemodynamics for different surgical reconstructions and assist in procedural planning even before entering the operating room.

Conclusions

Blood flow assessment and computational modeling can evaluate hemodynamics and flow patterns by visualizing blood flow and calculating biomechanical forces in patients with aortic disease. We anticipate that blood flow assessment will become an essential tool in the treatment planning and understanding of the progression of aortic disease.

Patient-Specific Haemodynamic Analysis of Virtual Grafting Strategies in Type-B Aortic Dissection: Impact of Compliance Mismatch

INTRODUCTION

Compliance mismatch between the aortic wall and Dacron Grafts is a clinical problem concerning aortic haemodynamics and morphological degeneration. The aortic stiffness introduced by grafts can lead to an increased left ventricular (LV) afterload. This study quantifies the impact of compliance mismatch by virtually testing different Type-B aortic dissection (TBAD) surgical grafting strategies in patient-specific, compliant computational fluid dynamics (CFD) simulations.

MATERIALS AND METHODS

A post-operative case of TBAD was segmented from computed tomography angiography data. Three virtual surgeries were generated using different grafts; two additional cases with compliant grafts were assessed. Compliant CFD simulations were performed using a patient-specific inlet flow rate and three-element Windkessel outlet boundary conditions informed by 2D-Flow MRI data. The wall compliance was calibrated using Cine-MRI images. Pressure, wall shear stress (WSS) indices and energy loss (EL) were computed.

RESULTS

Increased aortic stiffness and longer grafts increased aortic pressure and EL. Implementing a compliant graft matching the aortic compliance of the patient reduced the pulse pressure by 11% and EL by 4%. The endothelial cell activation potential (ECAP) differed the most within the aneurysm, where the maximum percentage difference between the reference case and the mid (MDA) and complete (CDA) descending aorta replacements increased by 16% and 20%, respectively.

CONCLUSION

This study suggests that by minimising graft length and matching its compliance to the native aorta whilst aligning with surgical requirements, the risk of LV hypertrophy may be reduced. This provides evidence that compliance-matching grafts may enhance patient outcomes.

Aortic valve neocuspidization and bioprosthetic valves: Evaluating turbulence haemodynamics

Aortic valve disease is often treated with bioprosthetic valves. An alternative treatment is aortic valve neocuspidization which is a relatively new reparative procedure whereby the three aortic cusps are replaced with patient pericardium or bovine tissues. Recent research indicates that aortic blood flow is disturbed, and turbulence effects have yet to be evaluated in either bioprosthetic or aortic valve neocuspidization valve types in patient-specific settings. The aim of this study is to better understand turbulence production in the aorta and evaluate its effects on laminar and turbulent wall shear stress. Four patients with aortic valve disease were treated with either bioprosthetic valves (n=2) or aortic valve neocuspidization valvular repair (n=2). Aortic geometries were segmented from magnetic resonance images (MRI), and 4D flow MRI was used to derive physiological inlet and outlet boundary conditions. Pulsatile large-eddy simulations were performed to capture the full range of laminar, transitional and turbulence characteristics in the aorta. Turbulence was produced in all aortas with highest levels occurring during systolic deceleration. In the ascending aorta, turbulence production is attributed to a combination of valvular skew, valvular eccentricity, and ascending aortic dilation. In the proximal descending thoracic aorta, turbulence production is dependent on the type of arch-descending aorta connection (e.g., a narrowing or sharp bend) which induces flow separation. Laminar and turbulent wall shear stresses are of similar magnitude throughout late systolic deceleration and diastole, although turbulent wall shear stress magnitudes exceed laminar wall shear stresses between 27.3% and 61.1% of the cardiac cycle. This emphasises the significance of including turbulent wall shear stress to improve our comprehension of progressive arterial wall diseases. The findings of this study recommend that aortic valve treatments should prioritise minimising valvular eccentricity and skew in order to mitigate turbulence generation.

The study of DIEA three dimensional reconstruction technique based on CTA in preoperative perforator selection

Preoperative selection of perforator is one of the key steps for successful surgery. The purpose of this study is to simulate the selection process of the perforator of the flap using the 3D models of the deep inferior epigastric artery (DIEA). A retrospective study was performed of women who underwent deep inferior epigastric perforator flap breast reconstruction from January 2011 to July 2021. Construct 3D models of the DIEA using computerized tomography angiography images, and then computational fluid dynamics simulations were performed. Correlation and regression analyses were used to analyze the geometric and hemodynamic parameters. Statistical analysis suggested that the outlet flow was positively correlated with the inlet area (r = 0.338, p = 0.000), outlet area (r = 0.840, p = 0.000), the average radius of the perforator (r = 0.592, p = 0.000), and negatively correlated with the length of perforator(r = -0.210, p = 0.024). The results of linear regression analysis showed that the outlet area (p = 0.000), the average radius (p = 0.000), and the length (p = 0.044) of the perforator were the influencing factors on outlet flow. In multiple perforators analysis, there was a significant difference in the total outlet flow among single perforator, double perforators, and triple perforators (p = 0.002). The successful implementation of this experiment provides a new approach for the selection of dominant perforators in the future.

Stress Load and Ascending Aortic Aneurysms: An Observational, Longitudinal, Single-Center Study Using Computational Fluid Dynamics

Ascending aortic aneurysm (AAoA) is a silent disease with high mortality; however, the factors associated with a worse prognosis are not completely understood. The objective of this observational, longitudinal, single-center study was to identify the hemodynamic patterns and their influence on AAoA growth using computational fluid dynamics (CFD), focusing on the effects of geometrical variations on aortic hemodynamics. Personalized anatomic models were obtained from angiotomography scans of 30 patients in two different years (with intervals of one to three years between them), of which 16 (53%) showed aneurysm growth (defined as an increase in the ascending aorta volume by 5% or more). Numerically determined velocity and pressure fields were compared with the outcome of aneurysm growth. Through a statistical analysis, hemodynamic characteristics were found to be associated with aneurysm growth: average and maximum high pressure (superior to 100 Pa); average and maximum high wall shear stress (superior to 7 Pa) combined with high pressure (>100 Pa); and stress load over time (maximum pressure multiplied by the time interval between the exams). This study provides insights into a worse prognosis of this serious disease and may collaborate for the expansion of knowledge about mechanobiology in the progression of AAoA.

Integrative analysis reveals chemokines CCL2 and CXCL5 mediated shear stress-induced aortic dissection formation

Background

Aortic dissection (AD) is a critical emergency in cardiovascular disease. AD occurs only in specific sites of the aorta, and the variation of shear stress in different aortic segments is a possible cause not reported. This study investigated the key molecules involved in shear stress-induced AD through quantitative bioinformatic analysis of a public RNA sequencing database and clinical tissue sample validation.

Methods

Gene expression data from the GSE153434, GSE147026, and GSE52093 datasets were downloaded from the Gene Expression Omnibus. Next, differently expressed genes (DEGs) in each dataset were identified and integrated to identify common AD DEGs. STRING, Cytoscape, and MCODE were used to identify hub genes and crucial clustering modules, and Connectivity Map (CMap) was used to identify positive and negative agents. The same procedure was performed for the GSE160611 dataset to obtain shear stress-induced human aortic endothelial cell (HAEC) DEGs. After the integration of these two DEGs sets to identify shear stress-associated hub DEGs in AD, Gene Ontology Enrichment Analysis was performed. The common chemokine receptors and ligands in AD were identified by analyzing AD's three RNA sequencing datasets. Their origin was verified by analyzing AD single-cell sequencing data and validated by immunoblotting and immunofluorescence.

Results

We identified 100 down-regulated and 50 up-regulated AD common DEGs. Enrichment results showed that common DEGs were closely related to blood vessel morphogenesis, muscle structure development, muscle tissue development, and chemotaxis. Among those DEGs, MYC, CCL2, and SPP1 are the three molecules with the highest degree. A crucial cluster of 15 genes was identified using MCODE, which contained inflammation-related genes with elevated expression and muscle cell-related genes with decreased expression, and CCL2 is central to immune-related genes. CMap confirmed MEK inhibitors and ALK inhibitors as possible therapeutic agents for AD. Moreover, 366 shear stress-associated DEGs in HAEC were identified in the GSE160611 dataset. After taking the intersection, we identified five shear stress-associated hub DEGs in AD (ANGPTL4, SNAI2, CCL2, GADD45B, and PROM1), and the enrichment analysis indicated they were related to the endothelial cell apoptotic process. Chemokine CCL2 was the molecule with a high degree in both DEG sets. Besides CCL2, CXCL5 was the only chemokine ligand differentially expressed in the three datasets. Additionally, immunoblotting confirmed the increased expression of CCL2 and CXCL5 in clinical tissue samples. Further research at the single-cell level revealed that CCL2 has multiple origins, and CXCL5 is macrophage-derived.

Conclusion

Through integrative analysis, we identified core common AD DEGs and possible therapeutic agents based on these DEGs. We elucidated that the chemokine CCL2 and CXCL5-mediated "Endothelial-Monocyte-Neutrophil" axis may contribute to the development of shear stress-induced AD. These findings provide possible therapeutic targets for the prevention and treatment of AD.

Does the AVNeo valve reduce wall stress on the aortic wall? A cardiac magnetic resonance analysis with 4D-flow for the evaluation of aortic valve replacement with the Ozaki technique

OBJECTIVES

Aortic valve neocuspidalization aims to replace the 3 aortic cusps with autologous pericardium pre-treated with glutaraldehyde, and it is a surgical alternative to the classical aortic valve replacement (AVR). Image-based patient-specific computational fluid dynamics allows the derivation of shear stress on the aortic wall [wall shear stress (WSS)]. Previous studies support a potential link between increased WSS and histological alterations of the aortic wall. The aim of this study is to compare the WSS of the ascending aorta in patients undergoing aortic valve neocuspidalization versus AVR with biological prostheses.

METHODS

This is a prospective nonrandomized clinical trial. Each patient underwent a 4D-flow cardiac magnetic resonance scan after surgery, which informed patient-specific computational fluid dynamics models to evaluate WSS at the ascending aortic wall. The adjusted variables were calculated by summing the residuals obtained from a multivariate linear model (with ejection fraction and left ventricle outflow tract-aorta angle as covariates) to the mean of the variables.

RESULTS

Ten patients treated with aortic valve neocuspidalization were enrolled and compared with 10 AVR patients. The aortic valve neocuspidalization group showed a significantly lower WSS in the outer curvature segments of the proximal and distal ascending aorta as compared to AVR patients (P = 0.0179 and 0.0412, respectively). WSS levels remained significantly lower along the outer curvature of the proximal aorta in the aortic valve neocuspidalization population, even after adjusting the WSS for the ejection fraction and the left ventricle outflow tract-aorta angle [2.44 Pa (2.17-3.01) vs 1.94 Pa (1.72-2.01), P = 0.02].

CONCLUSIONS

Aortic valve neocuspidalization hemodynamical features are potentially associated with a lower WSS in the ascending aorta as compared to commercially available bioprosthetic valves.

Stiffness matters: Improved failure risk assessment of ascending thoracic aortic aneurysms

Objectives

Rupture and dissection are feared complications of ascending thoracic aortic aneurysms caused by mechanical failure of the wall. The current method of using the aortic diameter to predict the risk of wall failure and to determine the need for surgical resection lacks accuracy. Therefore, this study aims to identify reliable and clinically measurable predictors for aneurysm rupture or dissection by performing a personalized failure risk analysis, including clinical, geometrical, histologic, and mechanical data.

Methods

The study cohort consisted of 33 patients diagnosed with ascending aortic aneurysms without genetic syndromes. Uniaxial tensile tests until failure were performed to determine the wall strength. Material parameters were fitted against ex vivo planar biaxial data and in vivo pressure-diameter relationships at diastole and systole, which were derived from multiphasic computed tomography (CT) scans. Using the resulting material properties and in vivo data, the maximal in vivo stress at systole was calculated, assuming a thin-walled axisymmetric geometry. The retrospective failure risk was calculated by comparing the peak wall stress at suprasystolic pressure with the wall strength.

Results

The distensibility coefficient, reflecting aortic compliance and derived from blood pressure measurements and multiphasic CT scans, outperformed predictors solely based on geometrical features in assessing the risk of aneurysm failure.

Conclusions

In a clinical setting, multiphasic CT scans followed by the calculation of the distensibility coefficient are of added benefit in patient-specific, clinical decision-making. The distensibility derived from the aneurysm volume change has the best predictive power, as it also takes the axial stretch into account.

Biomechanical properties of the aortic root are distinct from those of the ascending aorta in both normal and aneurysmal states

Background

Although aneurysms of the ascending aorta and the aortic root are treated similarly in clinical guidelines, how biomechanical properties differ between these 2 segments of aorta is poorly defined.

Methods

Biomechanical testing was performed on tissue collected from the aortic root (normal = 11, aneurysm = 51) and the ascending aorta (normal = 21, aneurysm = 76). Energy loss, tangent modulus of elasticity, and delamination strength were evaluated. These biomechanical properties were then compared between (1) normal ascending and normal root tissue, (2) normal and aneurysmal root tissue, (3) normal and aneurysmal ascending tissue, and (4) aneurysmal root and aneurysmal ascending tissue. Propensity score matching was performed to further compare aneurysmal root and aneurysmal ascending aortic tissue. Clinical and biomechanical variables associated with decreased delamination strength in the aortic root were evaluated.

Results

The normal aortic root demonstrated greater viscoelastic behavior (energy loss 0.08 [0.06, 0.10] vs 0.05 [0.04, 0.06], P = .008), and greater resistance against delamination (93 [58, 126] mN/mm vs 54 [40, 63] mN/mm, P = .05) compared with the ascending aorta. Delamination strength was significantly reduced in aneurysms in both the root and the ascending aorta compared with their normal states. Aneurysms of the aortic root matched to the ascending aortic aneurysms in terms of baseline characteristics including size, were characterized by a larger decrease in delamination strength from baseline (Δ -59 mN/mm vs Δ -24 mN/mm). Aging (P = .003) and the presence of hypertension (P = .02) were associated with weakening of the aortic root, while diameter did not have this association (P = .29).

Conclusions

The normal aortic root was found to have distinct biomechanical properties compared with the ascending aorta. When aneurysms form in the aortic root, there is less strength against delamination, without other biomechanical changes such as increased energy loss observed in aneurysmal ascending aortas. Age and hypertension were associated decreased aortic wall strength in the aortic root, whereas diameter had no such association.

Significant unfavorable geometrical changes in ascending aorta despite stable diameter at follow-up

BACKGROUND

The clinical importance of following-up the ascending aortic diameter lies on the fundamental presumption that wall pathology eventually manifests as change in shape. However, the diameter describes the vessel locally, and the 55mm criterion fails to prevent most dissections. We hypothesized that geometric changes across the ascending aorta are not necessarily imprinted on its diameter; i.e. the maximum diameter correlates weakly and insignificantly with elongation, surface stretching, engorgement, and tortuosity.

METHODS

Two databases were interrogated for patients who had undergone at least 2 ECG-gated CT scans. The absence of motion artifacts permitted the generation of exact copies of the ascending aorta which then underwent three-dimensional analysis producing objective and accurate measurements of the centreline length, surface, volume, and tortuosity. The correlations of these global variables with the diameter were explored.

RESULTS

Twenty-two patients, 13 male and 9 females, were included. The mean age at the first and last scan was 63.7 and 67.1y, respectively. The mean diameter increase was approximately 1mm/y. There were no dissections, while 7 patients underwent preemptive surgery. The yearly change rate of the global variables, normalized to height if applicable, showed statistically insignificant, weak or negligible correlation with diameter increments at follow-up. Most characteristically, a patient's aorta maintained its diameter, while undergoing 1mm/y elongation, 151mm2/(y∙m) stretching, 2366mm3/(y∙m) engorgement, and 0.02/y tortuosity.

CONCLUSIONS

Maximum diameter provides a local description for the ascending aorta and cannot fully portray the pathological process across this vessel. Following-up the diameter is not suggestive of length, surface, volume and tortuosity changes.

The Role of ADAMTS Proteoglycanases in Thoracic Aortic Disease

Thoracic aortic aneurysm and dissection (TAAD) are complex disease states with high morbidity and mortality that pose significant challenges to early diagnosis. Patients with an aneurysm are asymptomatic and typically present to the emergency department only after the development of a dissection. The extracellular matrix (ECM) plays a crucial role in regulating the aortic structure and function. The histopathologic hallmark termed medial degeneration is characterised by smooth muscle cell (SMC) loss, the degradation of elastic and collagen fibres and proteoglycan (PG) accumulation. Covalently attached to the protein core of PGs are a number of glycosaminoglycan chains, negatively charged molecules that provide flexibility, compressibility, and viscoelasticity to the aorta. PG pooling in the media can produce discontinuities in the aortic wall leading to increased local stress. The accumulation of PGs is likely due to an imbalance between their synthesis by SMCs and decreased proteolysis by A Disintegrin-like and Metalloproteinase with Thrombospondin motifs (ADAMTS) proteoglycanases in the ECM. Mouse models of TAAD indicated that these proteases exert a crucial, albeit complex and not fully elucidated, role in this disease. This has led to a mounting interest in utilising ADAMTS proteoglycanases as biomarkers of TAAD. In this review, we discuss the role of ADAMTSs in thoracic aortic disease and their potential use in facilitating the clinical diagnosis of TAAD and disease progression.

The Development of Totally Percutaneous Aortic Arch Repair With Inner-Branch Endografts: Experience From 2 Centers

OBJECTIVE

The main objective of this study is to present the experience of 2 centers undertaking total percutaneous aortic arch-branched graft endovascular repair using combination of femoral and axillary routes. The report summarizes the procedural steps, outcomes achieved, and the benefits of this approach, which eliminates the need for direct open surgical exposure of the carotid, subclavian, or axillary arteries, thereby reducing the unnecessary associated surgical risks.

METHODS

Retrospectively collected data of 18 consecutive patients (15M:3F) undergoing aortic arch endovascular repair using a branched device between February 2021 and June 2022 at 2 aortic units. Six patients were treated for a residual aortic arch aneurysm following previous type A dissection with size range of (58-67 mm in diameter), 10 were treated for saccular or fusiform degenerative atheromatous aneurysm with size range of (51.5-80 mm in diameter), and 2 were treated for penetrating aortic ulcer (PAU) with size range of (50-55 mm). Technical success was defined as completion of the procedure and satisfactory placement of the bridging stent grafts (BSGs) in the supra-aortic vessels percutaneously including the brachiocephalic trunk (BCT), left common carotid artery (LCCA), and left subclavian artery (LSA) without the need for carotid, subclavian, or axillary cut down. The primary technical success was examined as primary outcome well as any other related complications and reinterventions as secondary outcomes.

RESULTS

The primary technical success with our alternative approach was achieved in all 18 cases. There was one access site complication (groin haematoma), which was managed conservatively. There was no incidence of death, stroke, or cases of paraplegia. No other immediate complications were noted. Postoperative imaging confirmed supra-aortic branch patency, with satisfactory position of the BSGs and immediate aneurysm exclusion except in 4 patients who had type 1C endoleak (Innominate: 2, LSA 2) detected on the first postoperative scan. Three of them were treated with relining/extension, and 1 spontaneously resolved after 6 weeks.

CONCLUSIONS

Total percutaneous aortic arch repair with antegrade and retrograde inner-branch endografts can be performed with promising early results. Dedicated steerable sheaths and appropriate BSG would optimize the percutaneous approach for aortic arch endovascular repairs.

CLINICAL IMPACT

This article provides an alternative and innovative approach to improve the minimally invasive techniques in the endovascular treatment of the aortic arch conditions.

A clinician's guide to understanding aortic 4D flow MRI

Four-dimensional flow magnetic resonance imaging is an emerging technique which may play a role in diagnosis and risk-stratification of aortic disease. Some knowledge of flow dynamics and related parameters is necessary to understand and apply this technique in clinical workflows. The purpose of the current review is to provide a guide for clinicians to the basics of flow imaging, frequently used flow-related parameters, and their relevance in the context of aortic disease.Clinical relevance statement Understanding normal and abnormal aortic flow could improve clinical care in patients with aortic disease.

A computational study of the effects of size, location, and number of tears on haemodynamics in surgically repaired type A aortic dissection

Objective

This study aimed to comprehensively examine the roles of size, location, and number of tears in the progression of surgically repaired type A aortic dissection (TAAD) by assessing haemodynamic changes through patient-specific computational fluid dynamic (CFD) simulations.

Methods

Two patient-specific TAAD geometries with replaced ascending aorta were reconstructed based upon computed 15 tomography (CT) scans, after which 10 hypothetical models (5 per patient) with different tear configurations were artificially created. CFD simulations were performed on all the models under physiologically realistic boundary conditions.

Results

Our simulation results showed that increasing either the size or number of the re-entry tears reduced the luminal pressure difference (LPD) and maximum time-averaged wall shear stress (TAWSS), as well as areas exposed to abnormally high or low TAWSS values. Models with a large re-entry tear outperformed the others by reducing the maximum LPD by 1.88 mmHg and 7.39 mmHg, for patients 1 and 2, respectively. Moreover, proximally located re-entry tears in the descending aorta were more effective at reducing LPD than distal re-entry tears.

Discussion

These computational results indicate that the presence of a relatively large re-entry tear in the proximal descending aorta might help stabilize post-surgery aortic growth. This finding has important implications for the management and risk stratification of surgically repaired TAAD patients. Nevertheless, further validation in a large patient cohort is needed.

Influence of aortic aneurysm on the local distribution of NO and O2 using image-based computational fluid dynamics

There is a pressing need to establish novel biomarkers to predict the progression of thoracic aortic aneurysm (TAA) dilatation. Aside from hemodynamics, the roles of oxygen (O₂) and nitric oxide (NO) in TAA pathogenesis are potentially significant. As such, it is imperative to comprehend the relationship between aneurysm presence and species distribution in both the lumen and aortic wall. Given the limitations of existing imaging methods, we propose the use of patient-specific computational fluid dynamics (CFD) to explore this relationship. We have performed CFD simulations of O₂ and NO mass transfer in the lumen and aortic wall for two cases: a healthy control (HC) and a patient with TAA, both acquired using 4D-flow magnetic resonance imaging (MRI). The mass transfer of O₂ was based on active transport by hemoglobin, while the local variations of the wall shear stress (WSS) drove NO production. Comparing hemodynamic properties, the time-averaged WSS was considerably lower for TAA, while the oscillatory shear index and endothelial cell activation potential were notably elevated. O₂ and NO showed a non-uniform distribution within the lumen and an inverse correlation between the two species. We identified several locations of hypoxic regions for both cases due to lumen-side mass transfer limitations. In the wall, NO varied spatially, with a clear distinction between TAA and HC. In conclusion, the hemodynamics and mass transfer of NO in the aorta exhibit the potential to serve as a diagnostic biomarker for TAA. Furthermore, hypoxia may provide additional insights into the onset of other aortic pathologies.

Magnetic resonance imaging-based hemodynamic wall shear stress alters aortic wall tissue biomechanics in bicuspid aortic valve patients

OBJECTIVE

In this study we aimed to conclusively determine whether altered aortic biomechanics are associated with wall shear stress (WSS) independent of region of tissue collection. Elevated WSS in the ascending aorta of patients with bicuspid aortic valve has been shown to contribute to local maladaptive aortic remodeling and might alter biomechanics.

METHODS

Preoperative 4-dimensional flow magnetic resonance imaging was performed on 22 patients who underwent prophylactic aortic root and/or ascending aorta replacement. Localized elevated WSS was identified in patients using age-matched healthy atlases (n = 60 controls). Tissue samples (n = 78) were collected and categorized according to WSS (elevated vs normal) and region. Samples were subjected to planar biaxial testing. To fully quantify the nonlinear biomechanical response, the tangential modulus (local stiffness) at a low-stretch (LTM) and high-stretch (HTM) linear region and the onset (TZo) and end stress of the nonlinear transition zone were measured. A linear mixed effect models was implemented to determine statistical relationships.

RESULTS

A higher LTM in the circumferential and axial direction was associated with elevated WSS (P = .007 and P = .018 respectively) independent of collection region. Circumferential TZo and HTM were higher with elevated WSS (P = .024 and P = .003); whereas the collection region was associated with variations in axial TZo (P = .013), circumferential HTM (P = .015), and axial HTM (P = .001).

CONCLUSIONS

This study shows strong evidence that biomechanical changes in the aorta are strongly associated with hemodynamics, and not region of tissue collection for bicuspid valve aortopathy patients. Elevated WSS is associated with tissue behavior at low stretch ranges (ie, LTM and TZo).

Different hemodynamic factors cause the occurrence of superior mesenteric atherosclerotic stenosis and superior mesenteric artery dissection

Objective

To compare the hemodynamic factors involved in the occurrence of superior mesenteric atherosclerotic stenosis (SMAS) and superior mesenteric artery (SMA) dissection (SMAD).

Methods

Hospital records were searched to identify consecutive patients who were diagnosed with SMAS or SMAD between January 2015 and December 2021. A computational fluid dynamics (CFD) simulation method was used to assess the hemodynamic factors of the SMA in these patients. Histologic analysis was also performed on SMA specimens obtained from 10 cadavers, and scanning electron microscopy was used to evaluate collagen microstructure.

Results

A total of 124 patients with SMAS and 61 patients with SMAD were included. Most SMASs were circumferentially distributed at the SMA root, whereas the origin of most SMADs was located on the anterior wall of the curved segment of the SMA. Vortex, higher turbulent kinetic energy (TKE), and lower wall shear stress (WSS) were observed near plaques; higher TKE and WSS were seen near dissection origins. The intima in the SMA root (388.5 ± 202.3 µm) was thicker than in the curved (243.8 ± 100.5 µm; p = .007) and distal (183.7 ± 88.0 µm; p < .001) segments. The media in the anterior wall (353.1 ± 37.6 µm) was thinner than that in the posterior wall (473.7 ± 142.8 µm; p = .02) in the curved segment of the SMA. The gaps in the lamellar structure in the SMA root were larger than in the curved and distal segments. The collagen microstructure was more substantially disturbed in the anterior wall than in the posterior wall in the curved segment of the SMA.

Conclusion

Different hemodynamic factors in different portions of the SMA are related to local pathological changes in the SMA wall and may lead to the occurrence of SMAS or SMAD.

Modelling blood flow in patients with heart valve disease using deep learning: A computationally efficient method to expand diagnostic capabilities in clinical routine

Introduction

The computational modelling of blood flow is known to provide vital hemodynamic parameters for diagnosis and treatment-support for patients with valvular heart disease. However, most diagnosis/treatment-support solutions based on flow modelling proposed utilize time- and resource-intensive computational fluid dynamics (CFD) and are therefore difficult to implement into clinical practice. In contrast, deep learning (DL) algorithms provide results quickly with little need for computational power. Thus, modelling blood flow with DL instead of CFD may substantially enhances the usability of flow modelling-based diagnosis/treatment support in clinical routine. In this study, we propose a DL-based approach to compute pressure and wall-shear-stress (WSS) in the aorta and aortic valve of patients with aortic stenosis (AS).

Methods

A total of 103 individual surface models of the aorta and aortic valve were constructed from computed tomography data of AS patients. Based on these surface models, a total of 267 patient-specific, steady-state CFD simulations of aortic flow under various flow rates were performed. Using this simulation data, an artificial neural network (ANN) was trained to compute spatially resolved pressure and WSS using a centerline-based representation. An unseen test subset of 23 cases was used to compare both methods.

Results

ANN and CFD-based computations agreed well with a median relative difference between both methods of 6.0% for pressure and 4.9% for wall-shear-stress. Demonstrating the ability of DL to compute clinically relevant hemodynamic parameters for AS patients, this work presents a possible solution to facilitate the introduction of modelling-based treatment support into clinical practice.

Assessment of Pseudocoarctation of the Aorta with Saccular Aneurysms by Four-Dimensional Flow Magnetic Resonance Imaging and Histological Analysis

In this study, we present the case of a 21-year-old woman with pseudocoarctation of the aorta with saccular aneurysms that were evaluated by four-dimensional flow magnetic resonance imaging and histological analysis. We observed complete occupation of the aneurysm sacs by vortex flow and high peak wall shear stress in the proximal region of the kinked aorta. The aortic replacement was performed for the thoracic aortic aneurysms and the clinical course was uneventful. The aneurysms were histopathologically diagnosed as pseudoaneurysms based on the disappearance of all three layers and their replacement with collagen-rich connective tissues. These findings indicate that abnormal flow dynamics and the resulting abnormal shear stress in the aorta may play central roles in the formation and development of a saccular aneurysm.

The pathogenesis of superior mesenteric artery dissection: An in-depth study based on fluid-structure interaction and histology analysis

OBJECTIVE

To explore the role of hemodynamic factors in the occurrence of superior mesenteric artery (SMA) dissection (SMAD) using a fluid-structure interaction (FSI) simulation method, and to identify histopathologic changes occurring in the wall of the SMA.

METHODS

A total of 122 consecutive patients diagnosed with SMAD and 122 controls were included in this study. Hemodynamic factors were calculated using a FSI simulation method. Additionally, SMA specimens obtained from 12 cadavers were stained for histological quantitative analysis.

RESULTS

The mean aortomesenteric angle (59.7° ± 21.4° vs 48.2° ± 16.8°; p < .001) and SMA maximum curvature (0.084 ± 0.078 mm^-1 vs 0.032 ± 0.023 mm^-1; p < .001) were higher in SMAD patients than the controls. Larger aortomesenteric angles and SMA curvatures were associated with higher and more concentrated wall shear stress at anterior wall of the SMA curve segment, co-located with the dissection origins. The mean thickness of media (325.18 ± 44.87 µm vs 556.92 ± 138.32 µm; p = .003) was thinner in the anterior wall of the SMA curve than in the posterior wall. The area fractions of elastin (17.96% ± 3.36% vs 27.06% ± 4.18%; p = .002) and collagen (45.43% ± 6.89% vs 55.57% ± 7.57%; p = .036) were lower in anterior wall of the SMA curve than in posterior wall.

CONCLUSION

Increased aortomesenteric angle and SMA curvature are risk factors for SMAD. Both of these factors can cause local hemodynamic abnormalities, which can lead to histopathologic changes in anterior wall of SMA.

Clinical implications of the biomechanics of bicuspid aortic valve and bicuspid aortopathy

Bicuspid aortic valve (BAV), which affects up to 2% of the general population, results from the abnormal fusion of the cusps of the aortic valve. Patients with BAV are at a higher risk for developing aortic dilatation, a condition known as bicuspid aortopathy, which is associated with potentially life-threatening sequelae such as aortic dissection and aortic rupture. Although BAV biomechanics have been shown to contribute to aortopathy, their precise impact is yet to be delineated. Herein, we present the latest literature related to BAV biomechanics. We present the most recent definitions and classifications for BAV. We also summarize the current evidence pertaining to the mechanisms that drive bicuspid aortopathy. We highlight how aberrant flow patterns can contribute to the development of aortic dilatation. Finally, we discuss the role cardiac magnetic resonance imaging can have in assessing and managing patient with BAV and bicuspid aortopathy.

Wall Shear Stress Predicts Media Degeneration and Biomechanical Changes in Thoracic Aorta

Objectives: In thoracic aortic aneurysm (TAA) of the ascending aorta (AA), AA is progressively dilating due to the weakening of the aortic wall. Predicting and preventing aortic dissections and ruptures in TAA continues to be challenging, and more accurate assessment of the AA dilatation, identification of high-risk patients, and timing of repair surgery are required. We investigated whether wall shear stress (WSS) predicts pathological and biomechanical changes in the aortic wall in TAA.

Methods: The study included 12 patients with bicuspid (BAV) and 20 patients with the tricuspid aortic valve (TAV). 4D flow magnetic resonance imaging (MRI) was performed a day before aortic replacement surgery. Biomechanical and histological parameters, including assessing of wall strength, media degeneration, elastin, and cell content were analyzed from the resected AA samples.

Results: WSSs were greater in the outer curves of the AA compared to the inner curves in all TAA patients. WSSs correlated with media degeneration of the aortic wall (ρ = -0.48, p < 0.01), elastin content (ρ = 0.47, p < 0.01), and aortic wall strength (ρ = -0.49, p = 0.029). Subsequently, the media of the outer curves was thinner, more rigid, and tolerated lower failure strains. Failure values were shown to correlate with smooth muscle cell (SMC) density (ρ = -0.45, p < 0.02), and indicated the more MYH10⁺ SMCs the lower the strength of the aortic wall structure. More macrophages were detected in patients with severe media degeneration and the areas with lower WSSs.

Conclusion: The findings indicate that MRI-derived WSS predicts pathological and biomechanical changes in the aortic wall in patients with TAA and could be used for identification of high-risk patients.

Large Vessel Cell Heterogeneity and Plasticity: Focus in Aortic Aneurysms

Smooth muscle cells and endothelial cells have a remarkable level of plasticity in vascular pathologies. In thoracic and abdominal aortic aneurysms, smooth muscle cells have been suggested to undergo phenotypic switching and to contribute to degradation of the aortic wall structure in response to, for example, inflammatory mediators, dysregulation of growth factor signaling or oxidative stress. Recently, endothelial-to-mesenchymal transition, and a clonal expansion of degradative smooth muscle cells and immune cells, as well as mesenchymal stem-like cells have been suggested to contribute to the progression of aortic aneurysms. What are the factors driving the aortic cell phenotype changes and how vascular flow, known to affect aortic wall structure and to be altered in aortic aneurysms, could affect aortic cell remodeling? In this review, we summarize the current literature on aortic cell heterogeneity and phenotypic switching in relation to changes in vascular flow and aortic wall structure in aortic aneurysms in clinical samples with special focus on smooth muscle and endothelial cells. The differences between thoracic and abdominal aortic aneurysms are discussed.

Evaluation of Computational Methodologies for Accurate Prediction of Wall Shear Stress and Turbulence Parameters in a Patient-Specific Aorta

Background: Recent studies suggest that blood flow in main arteries is intrinsically disturbed, even under healthy conditions. Despite this, many computational fluid dynamics (CFD) analyses of aortic haemodynamics make the assumption of laminar flow, and best practices surrounding appropriate modelling choices are lacking. This study aims to address this gap by evaluating different modelling and post-processing approaches in simulations of a patient-specific aorta. Methods: Magnetic resonance imaging (MRI) and 4D flow MRI from a patient with aortic valve stenosis were used to reconstruct the aortic geometry and derive patient-specific inlet and outlet boundary conditions. Three different computational approaches were considered based on assumed laminar or assumed disturbed flow states including low-resolution laminar (LR-Laminar), high-resolution laminar (HR-Laminar) and large-eddy simulation (LES). Each simulation was ran for 30 cardiac cycles and post-processing was conducted on either the final cardiac cycle, or using a phase-averaged approach which utilised all 30 simulated cycles. Model capabilities were evaluated in terms of mean and turbulence-based parameters. Results: All simulation types, regardless of post-processing approach could correctly predict velocity values and flow patterns throughout the aorta. Lower resolution simulations could not accurately predict gradient-derived parameters including wall shear stress and viscous energy loss (largest differences up to 44.6% and 130.3%, respectively), although phase-averaging these parameters improved predictions. The HR-Laminar simulation produced more comparable results to LES with largest differences in wall shear stress and viscous energy loss parameters up to 5.1% and 11.6%, respectively. Laminar-based parameters were better estimated than turbulence-based parameters. Conclusion: Our findings suggest that well-resolved laminar simulations can accurately predict many laminar-based parameters in disturbed flows, but there is no clear benefit to running a HR-Laminar simulation over an LES simulation based on their comparable computational cost. Additionally, post-processing "typical" laminar simulation results with a phase-averaged approach is a simple and cost-effective way to improve accuracy of lower-resolution simulation results.