Fluid-structure interaction simulation of visceral perfusion and impact of different cannulation methods on aortic dissection

Impact of Residual Intimal Flap Displacement Post-TEVAR on TBAD Haemodynamics in Compliant, Patient-specific CFD Simulations Informed by MRI

We propose a novel formulation of a moving boundary method to account for the motion of the intimal flap (IF) in a TBAD post-thoracic endovascular aortic repair using patient-specific compliant computational fluid dynamics simulations. The simulations were informed by non-invasive 4D flow MRI sequences. Predicted flow waveforms, aortic wall, and IF displacements were validated against in vivo 4D flow MRI and cine-MRI data. The patient-specific simulation showed that at peak systole, the dynamic interplay between high IF displacement and high transmural pressures promoted true lumen compression and false lumen expansion, whilst luminal patterns were reversed at the deceleration phase. High vorticity and swirling flow patterns were observed throughout the cardiac cycle at the primary entry tear, the descending aorta and proximal to the visceral aortic branches, correlating with high relative residence time, which could indicate an increased localised risk of aortic growth proximal to the IF. A rigid IF simulation revealed significant discrepancies in haemodynamic metrics, highlighting the potential mispredictions when using a rigid wall assumption to assess disease progression. Simulations assuming a more compliant IF highlighted potential increased risks of visceral branches malperfusion and localised aortic wall degeneration. The study underscores the necessity of patient-specific compliant IF simulations for accurate TBAD haemodynamic assessments. These insights can improve disease understanding and inform future treatment strategies.

Mechanisms of aortic dissection: From pathological changes to experimental and in silico models

Aortic dissection continues to be responsible for significant morbidity and mortality, although recent advances in medical data assimilation and in experimental and in silico models have improved our understanding of the initiation and progression of the accumulation of blood within the aortic wall. Hence, there remains a pressing necessity for innovative and enhanced models to more accurately characterize the associated pathological changes. Early on, experimental models were employed to uncover mechanisms in aortic dissection, such as hemodynamic changes and alterations in wall microstructure, and to assess the efficacy of medical implants. While experimental models were once the only option available, more recently they are also being used to validate in silico models. Based on an improved understanding of the deteriorated microstructure of the aortic wall, numerous multiscale material models have been proposed in recent decades to study the state of stress in dissected aortas, including the changes associated with damage and failure. Furthermore, when integrated with accessible patient-derived medical data, in silico models prove to be an invaluable tool for identifying correlations between hemodynamics, wall stresses, or thrombus formation in the deteriorated aortic wall. They are also advantageous for model-guided design of medical implants with the aim of evaluating the deployment and migration of implants in patients. Nonetheless, the utility of in silico models depends largely on patient-derived medical data, such as chosen boundary conditions or tissue properties. In this review article, our objective is to provide a thorough summary of medical data elucidating the pathological alterations associated with this disease. Concurrently, we aim to assess experimental models, as well as multiscale material and patient data-informed in silico models, that investigate various aspects of aortic dissection. In conclusion, we present a discourse on future perspectives, encompassing aspects of disease modeling, numerical challenges, and clinical applications, with a particular focus on aortic dissection. The aspiration is to inspire future studies, deepen our comprehension of the disease, and ultimately shape clinical care and treatment decisions.

Impact of direct mesenteric perfusion on malperfusion in acute type A aortic dissection repair

OBJECTIVES

Mesenteric malperfusion in acute aortic dissection remains a life-threatening complication with no standardized treatment strategy. This study aimed to describe and evaluate the outcomes of our integrated approach combining exploratory laparotomy, immediate mesenteric reperfusion, and central aortic repair.

METHODS

We retrospectively reviewed patients with acute aortic dissection with a preoperative diagnosis of mesenteric malperfusion who were treated between August 2011 and November 2022. Our surgical approach was to establish cardiopulmonary bypass, followed by exploratory laparotomy with mesenteric artery flow assessment using Doppler ultrasound and direct perfusion if needed, central aortic repair, and subsequent mesenteric artery reconstruction. The primary end-point was the 30-day operative mortality.

RESULTS

Among 217 patients with acute aortic dissection, 12 (5.5%) had mesenteric malperfusion on preoperative computed tomography. Ten patients underwent exploratory laparotomy, where Doppler ultrasonography revealed reduced mesenteric blood flow in five patients (2.3% of the total 217 patients). These patients underwent direct perfusion of the mesenteric artery via a side branch of the cardiopulmonary bypass circuit. Doppler ultrasound confirmed the restoration of mesenteric blood flow in all perfused patients. No bowel resections were required. The operative mortality in patients with mesenteric malperfusion was 20%. The causes of death were stroke (n = 1) and acute myocardial infarction (n = 1).

CONCLUSIONS

Our integrated surgical strategy combining central aortic repair with concurrent exploratory laparotomy and immediate mesenteric perfusion demonstrated technical feasibility in managing mesenteric malperfusion during aortic repair. Further prospective studies with larger cohorts are warranted to validate these findings.

Neo-Hookean modeling of nonlinear coupled behavior in circular plates supported by micro-pillars

In the contemporary era, the enhancement of wearable capacitive sensors is achieved through the utilization of polymeric micropillars as filler materials between electrode plates. To gain a deeper understanding of the dynamic response of the system, nonlinear coupled governing equations of a circular microplate motion resting on an array of polymeric micropillars have been derived. These equations are used to model the system's behavior. In addition, the squeezing motion of the micro-pillars is characterized using the incompressible Neo-Hookean model. Both static and dynamic responses, including transient and steady-state solutions, are investigated in detail by discretizing over spatial coordinates using a weak formulation approach. A frequency response analysis is conducted using a continuation-based method. This entails expanding the steady-state solution using a Fourier transform and employing the energy balance principle. The unknown coefficients of the expansion series are calculated using a gradient descent-based learning approach that is physically motivated. Furthermore, a dynamic step size strategy for frequency increments is employed to effectively follow the solution path. This strategy is implemented via the ARC length method. In this study, we examine the impact of varying PDMS (polydimethylsiloxane) hydrogel mechanical and geometrical configurations. It can be reasonably concluded that the mechanical properties of the pillars and the geometrical configuration of the circular plate and micropillars have a significant impact on the maximum tolerable pressure, fast transient response, and frequency response analysis.

Investigation of Type A Aortic Dissection Using Computational Modelling

Aortic dissection is a catastrophic failure of the endothelial wall that could lead to malperfusion or rupture. Computational modelling tools may help detect arterial damage. Technological advancements have led to more sophisticated forms of modelling being made available to low-grade computers. These devices can create 3D models with clinical data, where the clinical blood pressure waveforms' model can be used to form boundary conditions for assessing hemodynamic parameters, modelling blood flow propagation along the aorta to predict the development of cardiovascular disease. This study presents patient-specific data for a rare case of severe Type A aortic dissection. CT scan images were taken nine months apart, consisting of the artery both before and after dissection. The results for the pre-dissection CT showed that the pressure waveform at the ascending aorta was higher, and the systolic pressure was lagging at the descending aorta. For the post-dissection analysis, we observed the same outcome; however, the amplitude for the waveform (systolic pressure) at the ascending aorta increased in the false lumen by 25% compared to the true lumen by 3%. Also, the waveform peak (systolic) was leading by 0.01 s. The hemodynamic parameter of wall shear stress (WSS) predicted the aneurysm's existence at the ascending aorta, as well as potential aortic dissection. The high WSS contours were located at the tear location at the peak blood flow of 0.14 s, which shows the potential of this tool for earlier diagnosis of aortic dissection.

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.

Is axillary artery cannulation necessary in type II hybrid aortic arch repair for acute type A aortic dissection?

INTRODUCTION

Axillary artery cannulation (AAC) has been widely employed in total arch replacement surgeries using the frozen elephant trunk (FET) technique for acute type A aortic dissection (ATAAD), showing better clinical results than femoral artery cannulation (FAC). Nevertheless, in type II hybrid arch repair (HAR), FAC is crucial for lower body perfusion. Hence, it is unclear whether AAC remains necessary or if AAC represents a more advantageous method for initiating cardiopulmonary bypass.

METHODS

We conducted a study involving patients diagnosed with ATAAD who underwent type II HAR from August 2021 to December 2022. Demographic baseline and intraoperative data were collected, and the postoperative outcomes of patients receiving FAC only were compared with those receiving AAC.

RESULTS

There were no significant differences in baseline demographics between patients who underwent FAC alone (n = 46) and those who underwent AAC (n = 39). Patients who underwent AAC showed a lower incidence of transient neurological dysfunction (TND) post-surgery compared to those who underwent FAC (12.8% vs 32.6%, p = .032). There were no significant differences between the groups in terms of postoperative mortality within 30 days, permanent neurological dysfunction (PND), length of stay in the intensive care unit (ICU) and postoperative ward, duration of mechanical ventilation, and other complications.

CONCLUSIONS

Axillary artery cannulation may decrease the incidence of postoperative transient neurological dysfunction (TND) in type II HAR for ATAAD. Nonetheless, studies with larger sample sizes are necessary.

Cannulation Strategies in Type A Aortic Dissection: Overlooked Details and Novel Approaches

Aortic dissection type A is a life-threatening condition that frequently necessitates surgical intervention. This review focuses on central aortic cannulation, arch branch vessel (ABV) cannulation, and proximal arch cannulation as key techniques during aortic surgery. It discusses innovative solutions for addressing these challenges. The review synthesizes findings from recent studies and emphasizes the significance of meticulous planning and execution of cannulation in aortic dissection repair. This review aims to contribute to the advancement of surgical practices and the enhancement of patient outcomes in the management of type A aortic dissection (AAD) by addressing these frequently overlooked details.

Assessing the impact of turbulent kinetic energy boundary conditions on turbulent flow simulations using computational fluid dynamics

Computational fluid dynamics has been widely used to study hemodynamics, but accurately determining boundary conditions for turbulent blood flow remains challenging. This study aims to investigate the effect of patient-specific turbulence boundary conditions on the accuracy of turbulent flow simulation. Using a stenosis model with 50% severity in diameter, the post-stenosis turbulence flow region was simulated with different planes to obtain inlet boundary conditions and simulate downstream flows. The errors of simulated flow fields obtained with turbulence kinetic energy (TKE) boundary data and arbitrary turbulence intensity were compared. Additionally, the study tested various TKE data resolutions and noise levels to simulate experimental environments. The mean absolute error of velocity and TKE was investigated with various turbulence intensities and TKE mapping. While voxel size and signal-to-noise ratio of the TKE data affected the results, simulation with SNR > 5 and voxel size < 10% resulted in better accuracy than simulations with turbulence intensities. The simulation with appropriate TKE boundary data resulted in a more accurate velocity and turbulence field than those with arbitrary turbulence intensity boundary conditions. The study demonstrated the potential improvement of turbulent blood flow simulation with patient-specific turbulence boundary conditions, which can be obtained from recent measurement techniques.