Treatment planning for a TCPC test case: a numerical investigation under rigid and moving wall assumptions

Engineering Perspective on Cardiovascular Simulations of Fontan Hemodynamics: Where Do We Stand with a Look Towards Clinical Application

BACKGROUND

Cardiovascular simulations for patients with single ventricles undergoing the Fontan procedure can assess patient-specific hemodynamics, explore surgical advances, and develop personalized strategies for surgery and patient care. These simulations have not yet been broadly accepted as a routine clinical tool owing to a number of limitations. Numerous approaches have been explored to seek innovative solutions for improving methodologies and eliminating these limitations.

PURPOSE

This article first reviews the current state of cardiovascular simulations of Fontan hemodynamics. Then, it will discuss the technical progress of Fontan simulations with the emphasis of its clinical impact, noting that substantial improvements have been made in the considerations of patient-specific anatomy, flow, and blood rheology. The article concludes with insights into potential future directions involving clinical validation, uncertainty quantification, and computational efficiency. The advancements in these aspects could promote the clinical usage of Fontan simulations, facilitating its integration into routine clinical practice.

A Tribute to Ajit Yoganathan's Cardiovascular Fluid Mechanics Lab: A Survey of Its Contributions to Our Understanding of the Physiology and Management of Single-Ventricle Patients

INTRODUCTION

Among patients with congenital heart disease, those born with only a single working ventricle represent a particularly complex sub-population, typically requiring multiple surgeries and suffering from high levels of mortality and morbidity. Their cardiac care is complex and has evolved considerably since surgical palliation was first introduced more than 50 years ago. Improvements in treatment have been driven both by growing clinical experience and by knowledge gained through experimental and computational studies of blood flow in these patients. The Cardiovascular Fluid Mechanics Lab at the Georgia Institute of Technology, founded 30 years ago by Dr. Ajit Yoganathan, has pioneered work in this field.

METHODS

In this review, key contributions of Dr. Yoganathan's Cardiovascular Fluid Dynamics Lab are surveyed, including experimental flow loop studies as well as computational fluid dynamics analyses that address many of the critical challenges that cardiologists and surgeons face in treating these patients, including how to reconstruct cardiovascular anatomy to minimize power loss, balance blood flow distribution at key bifurcation points, and avoid other unfavorable hemodynamic conditions.

CONCLUSIONS

Among many contributions in this field, work from the Cardiovascular Fluid Mechanics Lab has led to novel medical devices and patient-specific computational modeling workflows and software tools. These key contributions from this group have enhanced our understanding of the physiology and management of single-ventricle patients.

Fluid-Structure Interaction Simulation of an Intra-Atrial Fontan Connection

Simple Summary

A fluid-structure interaction (FSI) simulation of an intra-atrial Fontan connection was performed. Power loss and pressure drop results fluctuated less during the FSI simulation than during the simulation run with rigid walls, but there were no observable differences in time-averaged pressure drop, connection power loss or hepatic flow distribution. These results suggested that employing a rigid wall is a reasonable assumption when evaluating time-averaged hemodynamic quantities of the Fontan connection under resting breath-held flow conditions.

Abstract

Total cavopulmonary connection (TCPC) hemodynamics has been hypothesized to be associated with long-term complications in single ventricle heart defect patients. Rigid wall assumption has been commonly used when evaluating TCPC hemodynamics using computational fluid dynamics (CFD) simulation. Previous study has evaluated impact of wall compliance on extra-cardiac TCPC hemodynamics using fluid-structure interaction (FSI) simulation. However, the impact of ignoring wall compliance on the presumably more compliant intra-atrial TCPC hemodynamics is not fully understood. To narrow this knowledge gap, this study aims to investigate impact of wall compliance on an intra-atrial TCPC hemodynamics. A patient-specific model of an intra-atrial TCPC is simulated with an FSI model. Patient-specific 3D TCPC anatomies were reconstructed from transverse cardiovascular magnetic resonance images. Patient-specific vessel flow rate from phase-contrast magnetic resonance imaging (MRI) at the Fontan pathway and the superior vena cava under resting condition were prescribed at the inlets. From the FSI simulation, the degree of wall deformation was compared with in vivo wall deformation from phase-contrast MRI data as validation of the FSI model. Then, TCPC flow structure, power loss and hepatic flow distribution (HFD) were compared between rigid wall and FSI simulation. There were differences in instantaneous pressure drop, power loss and HFD between rigid wall and FSI simulations, but no difference in the time-averaged quantities. The findings of this study support the use of a rigid wall assumption on evaluation of time-averaged intra-atrial TCPC hemodynamic metric under resting breath-held condition.

Comparison of Fontan Surgical Options for Patients with Apicocaval Juxtaposition

Apicocaval juxtaposition (ACJ) is a rare form of viscerocardiac malpositions in association with single-ventricle congenital heart defects. The Fontan surgery is the common palliation, and possible surgical options include ipsilateral, contralateral, and intra-atrial conduits. Concerns include lower hemodynamic performances or risks of conduit compression by the cardiac mass. This study investigates the hemodynamics and clinical outcomes of ACJ patients and potential surgical improvements. Ten consecutive ACJ patients were included, along with a reference cohort of ten non-ACJ patients. Magnetic resonance images were acquired at 6 ± 0.6 year follow-up for anatomical analysis and hemodynamic assessments using computational fluid dynamics. Metrics of interest are deformation index (DI), indexed power loss (iPL), and hepatic flow distribution (HFDoff). A "virtual" surgery was performed to explore potential hemodynamic improvements using a straightened conduit. DI for ACJ patients fell within the DI range of non-ACJ patients. Contralateral conduits had insignificantly higher iPL (0.070 [0.032,0.137]) than ipsilateral conduits (0.041 [0.013,0.095]) and non-ACJ conduits (0.034 [0.011,0.061]). HFDoff was similar for the ipsilateral (21 [12,35]), contralateral (26 [7,41]), and non-ACJ Fontan conduits (17 [0,48]). Virtual surgery demonstrated that a straightened conduit reduced HFDoff and iPL for the contralateral and ipsilateral conduits, potentially leading to improved clinical outcomes. In this limited sample, the hemodynamic performance of ACJ patients was not significantly different from their non-ACJ counterparts. The use of a straightened conduit option could potentially improve patient outcomes. Additionally, the fear of significant compression of conduits for ACJ patients was unsupported.

Hemodynamic Effects of Additional Pulmonary Blood Flow on Glenn and Fontan Circulation

PURPOSE

Additional pulmonary blood flow (APBF) can provide better pulsating blood flow and systemic arterial oxygen saturation, while low blood pulsation and low oxygen saturation are defects of the Fontan and Glenn procedure. Studying the hemodynamic effect of APBF is beneficial for clinical decisions. This study aimed to explore the effect on particle washout, as well as the differences among the sensitivities of both different hemodynamic parameters and different procedures to APBF.

METHODS

The patient-specific clinical datasets of a patient who underwent bilateral bidirectional Glenn (BBDG) with APBF were enrolled in this study, and using these datasets, Glenn- and Fontan-type artery models were reconstructed. A series of parameters, including the total caval flow pulsatility index (TCPI), indexed energy loss (iPL), wall shear stress (WSS), systemic arterial oxygen saturation (Sat_art), particle washout time (WOT), pressure in the right superior vena cava (P_RSVC), pulmonary flow distribution (PFD) and hepatic flow distribution (HFD), were computed from computational fluid dynamic (CFD) simulation to evaluate the hemodynamic effect of APBF.

RESULTS

The result showed that APBF led to better iPL and Sat_art but worse P_RSVC and heart load accompanied by a great impact on HFD, making hepatic flow easier to perfuse the side without MPA and APBF. The increase in the APBF rate also effectively results in larger flow pulsation, region velocity, and wall shear stress and lower WOT, and this effect may be more effective for patients with persistent left superior vena cava (PLSVC). However, APBF might have little effect on PFD. Furthermore, APBF might affect WOT, iPL and HFD more significantly than P_RSVC and has a greater improvement effect in patients with poorer iPL and WOT.

CONCLUSIONS

Moderate APBF is not only a measure to promote pulmonary artery growth and systemic arterial oxygen saturation but also an effective method against endothelial dysfunction and thrombosis. However, moderate APBF is patient-specific and should be determined based on hemodynamic preference that leads to desired patient outcomes, and care should be taken to prevent P_RSVC and heart load from being too high as well as an imbalance in HFD.

Physiological Fontan Procedure

Objective: The conventional Fontan circulation deviates the superior vena cava (SVC = 1/3 of the systemic venous return) toward the right lung (3/5 of total lung volume) and the inferior vena cava (IVC = 2/3 of the systemic venous return) toward the left lung (2/5 of total lung volume). A "physiological" Fontan deviating the SVC toward the left lung and the IVC toward the right lung was compared with the conventional setting by computational fluid dynamics, studying whether this setting achieves a more favorable hemodynamics than the conventional Fontan circulation. Materials and Methods: An in-silico 3D parametric model of the Fontan procedure was developed using idealized vascular geometries with invariant sizes of SVC, IVC, right pulmonary artery (RPA), and left pulmonary artery (LPA), steady inflow velocities at IVC and SVC, and constant equal outflow pressures at RPA and LPA. These parameters were set to perform finite-volume incompressible steady flow simulations, assuming a single-phase, Newtonian, isothermal, laminar blood flow. Numerically converged finite-volume mass and momentum flow balances determined the inlet pressures and the outflow rates. Numerical closed-path integration of energy fluxes across domain boundaries determined the flow energy loss rate through the Fontan circulation. The comparison evaluated: (1) mean IVC pressure; (2) energy loss rate; (3) kinetic energy maximum value throughout the domain volume. Results: The comparison of the physiological vs. conventional Fontan provided these results: (1) mean IVC pressure 13.9 vs. 14.1 mmHg (= 0.2 mmHg reduction); (2) energy loss rate 5.55 vs. 6.61 mW (= 16% reduction); (3) maximum kinetic energy 283 vs. 396 J/m3 (= 29% reduction). Conclusions: A more physiological flow distribution is accompanied by a reduction of mean IVC pressure and by substantial reductions of energy loss rate and of peak kinetic energy. The potential clinical impact of these hemodynamic changes in reducing the incidence and severity of the adverse long-term effects of the Fontan circulation, in particular liver failure and protein-losing enteropathy, still remains to be assessed and will be the subject of future work.

The effect of respiration-driven flow waveforms on hemodynamic metrics used in Fontan surgical planning

OBJECTIVE

Poor total cavopulmonary connection (TCPC) hemodynamics have been hypothesized to be associated with long-term complications in Fontan patients. Image-based Fontan surgical planning has shown great potential as a clinical tool because it can pre-operatively evaluate patient-specific hemodynamics. Current surgical planning paradigms commonly utilize cardiac-gated phase contrast magnetic resonance (MR) imaging to acquire vessel flows. These acquisitions are often taken under breath-held (BH) conditions and ignore the effect of respiration on blood flow waveforms. This study investigates the effect of respiration-driven flow waveforms on patient-specific hemodynamics using real-time MR acquisitions.

METHODS

Patient-specific TCPCs were reconstructed from cardiovascular MR images. Real-time phase contrast MR images were acquired under both free-breathing (FB) and breath-held conditions for 9 patients. Numerical simulations were employed to assess flow structures and hemodynamics used in Fontan surgical planning including hepatic flow distribution (HFD) and indexed power loss (iPL), which were then compared between FB and BH conditions.

RESULTS

Differences in TCPC flow structures between FB and BH conditions were observed throughout the respiratory cycle. However, the average differences (BH - FB values for each patient, which are then averaged) in iPL and HFD between these conditions were 0.002 ± 0.011 (p = 0.40) and 1 ± 3% (p = 0.28), respectively, indicating no significant difference in clinically important hemodynamic metrics.

CONCLUSIONS

Respiration affects blood flow waveforms and flow structures, but might not significantly influence the values of iPL or HFD. Therefore, breath-held MR acquisition can be adequate for Fontan surgical planning when focusing on iPL and HFD.

Numerical Investigation of the Effect of Additional Pulmonary Blood Flow on Patient-Specific Bilateral Bidirectional Glenn Hemodynamics

The effect of additional pulmonary blood flow (APBF) on the hemodynamics of bilateral bidirectional Glenn (BBDG) connection was marginally discussed in previous studies. This study assessed this effect using patient-specific numerical simulation. A 15-year-old female patient who underwent BBDG was enrolled in this study. Patient-specific anatomy, flow waveforms, and pressure tracings were obtained using computed tomography, Doppler ultrasound technology, and catheterization, respectively. Computational fluid dynamic simulations were performed to assess flow field and derived hemodynamic metrics of the BBDG connection with various APBF. APBF showed noticeable effects on the hemodynamics of the BBDG connection. It suppressed flow mixing in the connection, which resulted in a more antegrade flow structure. Also, as the APBF rate increases, both power loss and reflux in superior venae cavae (SVCs) monotonically increases while the flow ratio of the right to the left pulmonary artery (RPA/LPA) monotonically decreases. However, a non-monotonic relationship was observed between the APBF rate and indexed power loss. A high APBF rate may result in a good flow ratio of RPA/LPA but with the side effect of bad power loss and remarkable reflux in SVCs, and vice versa. A moderate APBF rate could be favourable because it leads to an optimal indexed power loss and achieves the acceptable flow ratio of RPA/LPA without causing severe power loss and reflux in SVCs. These findings suggest that patient-specific numerical simulation should be used to assist clinicians in determining an appropriate APBF rate based on desired outcomes on a patient-specific basis.

Using a Novel In Vitro Fontan Model and Condition-Specific Real-Time MRI Data to Examine Hemodynamic Effects of Respiration and Exercise

Several studies exist modeling the Fontan connection to understand its hemodynamic ties to patient outcomes (Chopski in: Experimental and Computational Assessment of Mechanical Circulatory Assistance of a Patient-Specific Fontan Vessel Configuration. Dissertation, 2013; Khiabani et al. in J Biomech 45:2376-2381, 2012; Taylor and Figueroa in Annu Rev Biomed 11:109-134, 2009; Vukicevic et al. in ASAIO J 59:253-260, 2013). The most patient-accurate of these studies include flexible, patient-specific total cavopulmonary connections. This study improves Fontan hemodynamic modeling by validating Fontan model flexibility against a patient-specific bulk compliance value, and employing real-time phase contrast magnetic resonance flow data. The improved model was employed to acquire velocity field information under breath-held, free-breathing, and exercise conditions to investigate the effect of these conditions on clinically important Fontan hemodynamic metrics including power loss and viscous dissipation rate. The velocity data, obtained by stereoscopic particle image velocimetry, was visualized for qualitative three-dimensional flow field comparisons between the conditions. Key hemodynamic metrics were calculated from the velocity data and used to quantitatively compare the flow conditions. The data shows a multi-factorial and extremely patient-specific nature to Fontan hemodynamics.

Simulation of aortopulmonary collateral flow in Fontan patients for use in prediction of interventional outcomes

PURPOSE

Patients with complex congenital heart disease may need to be converted to a Fontan circulation with systemic venous return surgically connected to the pulmonary circulation. These patients frequently form aortopulmonary collaterals (APC), that is arterial inflows to the pulmonary artery vascular tree. The aim of this study was to develop a method to calculate the effect of APC on the pulmonary flow distribution based on magnetic resonance imaging (MRI) measurements and computational fluid dynamics simulations in order to enable prediction of interventional outcomes in Fontan patients.

METHODS

Patient-specific models of 11 patients were constructed in a 3D-design software based on MRI segmentations. APC flow was quantified as the difference between pulmonary venous flow and pulmonary artery flow, measured by MRI. A method was developed to include the modulating effect of the APC flow by calculating the patient-specific relative pulmonary vascular resistance. Simulations, including interventions with a Y-graft replacement and a stent dilatation, were validated against MRI results.

RESULTS

The bias between simulated and MRI-measured fraction of blood to the left lung was 2·9 ± 5·3%. Including the effects of the APC flow in the simulation (n = 6) reduced simulation error from 9·8 ± 7·0% to 5·2 ± 6·3%. Preliminary findings in two patients show that the effect of surgical and catheter interventions could be predicted using the demonstrated methods.

CONCLUSIONS

The work demonstrates a novel method to include APC flow in predictive simulations of Fontan hemodynamics. APC flow was found to have a significant contribution to the pulmonary flow distribution in Fontan patients.

Role of Computational Modelling in Planning and Executing Interventional Procedures for Congenital Heart Disease

Increasingly, computational modelling and numerical simulations are used to help plan complex surgical and interventional cardiovascular procedures in children and young adults with congenital heart disease. From its origins more than 30 years ago, surgical planning with analysis of flow hemodynamics and energy loss/efficiency has helped design and implement many modifications to existing techniques. On the basis of patient-specific medical imaging, surgical planning allows accurate model production that can then be manipulated in a virtual surgical environment, with the proposed solutions finally tested with advanced computational fluid dynamics to evaluate the results. Applications include a broad range of congenital heart disease, including patients with single-ventricle anatomy undergoing staged palliation, those with arch obstruction, with double outlet right ventricle, or with tetralogy of Fallot. In the present work, we focus on clinical applications of this exciting field. We describe the framework for these techniques, including brief descriptions of the engineering principles applied and the interaction between "benchtop" data with medical decision-making. We highlight some early insights learned from pioneers over the past few decades, including refinements in Fontan baffle geometries and configurations. Finally, we offer a glimpse into exciting advances that are presently being explored, including use of modelling for transcatheter interventions. In this era of personalized medicine, computational modelling and surgical planning allows patient-specific tailoring of interventions to optimize clinical outcomes.

A Multiscale Model of Cardiovascular System Including an Immersed Whole Heart in the Cases of Normal and Ventricular Septal Defect (VSD)

A mathematical and computational model combining the heart and circulatory system has been developed to understand the hemodynamics of circulation under normal conditions and ventricular septal defect (VSD). The immersed boundary method has been introduced to describe the interaction between the moving two-dimensional heart and intracardiac blood flow. The whole-heart model is governed by the Navier-Stokes system; this system is combined with a multi-compartment model of circulation using pressure-flow relations and the linearity of the discretized Navier-Stokes system. We investigate the velocity field, flowmeters, and pressure-volume loop in both normal and VSD cases. Simulation results show qualitatively good agreements with others found in the literature. This model, combining the heart and circulation, is useful for understanding the complex, hemodynamic mechanisms involved in normal circulation and cardiac diseases.

Patient-Specific Surgical Planning, Where Do We Stand? The Example of the Fontan Procedure

The Fontan surgery for single ventricle heart defects is a typical example of a clinical intervention in which patient-specific computational modeling can improve patient outcome: with the functional heterogeneity of the presenting patients, which precludes generic solutions, and the clear influence of the surgically-created Fontan connection on hemodynamics, it is acknowledged that individualized computational optimization of the post-operative hemodynamics can be of clinical value. A large body of literature has thus emerged seeking to provide clinically relevant answers and innovative solutions, with an increasing emphasis on patient-specific approaches. In this review we discuss the benefits and challenges of patient-specific simulations for the Fontan surgery, reviewing state of the art solutions and avenues for future development. We first discuss the clinical impact of patient-specific simulations, notably how they have contributed to our understanding of the link between Fontan hemodynamics and patient outcome. This is followed by a survey of methodologies for capturing patient-specific hemodynamics, with an emphasis on the challenges of defining patient-specific boundary conditions and their extension for prediction of post-operative outcome. We conclude with insights into potential future directions, noting that one of the most pressing issues might be the validation of the predictive capabilities of the developed framework.

Patient-specific assessment of cardiovascular function by combination of clinical data and computational model with applications to patients undergoing Fontan operation

The assessment of cardiovascular function is becoming increasingly important for the care of patients with single-ventricle defects. However, most measurement methods available in the clinical setting cannot provide a separate measure of cardiac function and loading conditions. In the present study, a numerical method has been proposed to compensate for the limitations of clinical measurements. The main idea was to estimate the parameters of a cardiovascular model by fitting model simulations to patient-specific clinical data via parameter optimization. Several strategies have been taken to establish a well-posed parameter optimization problem, including clinical data-matched model development, parameter selection based on an extensive sensitivity analysis, and proper choice of parameter optimization algorithm. The numerical experiments confirmed the ability of the proposed parameter optimization method to uniquely determine the model parameters given an arbitrary set of clinical data. The method was further tested in four patients undergoing the Fontan operation. Obtained results revealed a prevalence of ventricular abnormalities in the patient cohort and at the same time demonstrated the presence of marked inter-patient differences and preoperative to postoperative changes in cardiovascular function. Because the method allows a quick assessment and makes use of clinical data available in clinical practice, its clinical application is promising.

Perspective on CFD studies of coronary artery disease lesions and hemodynamics: a review

Coronary artery disease (CAD) is the most common cardiovascular disease. Early diagnosis of CAD's physiological significance is of utmost importance for guiding individualized risk-tailored treatment strategies. In this paper, we first review the state-of-the-art clinical diagnostic indices to quantify the severity of CAD and the associated invasive and noninvasive imaging technologies in order to quantify the anatomical parameters of diameter stenosis, area stenosis, and hemodynamic indices of coronary flow reserve and fractional flow reserve. With the development of computational technologies and CFD methods, tremendous progress has been made in applying image-based CFD simulation techniques to elucidate the effects of hemodynamics in vascular pathophysiology toward the initialization and progression of CAD. So then, we review the advancements of CFD technologies in patient-specific modeling, involving the development of geometry reconstruction, boundary conditions, and fluid-structure interaction. Next, we review the applications of CFD to stenotic sites, in order to compute their hemodynamic parameters and study the relationship between the hemodynamic conditions and the clinical indices, to thereby assess the amount of viable myocardium and candidacy for percutaneous coronary intervention. Finally, we review the strengths and limitations of current researches of applying CFD to CAD studies.

Magnetic resonance imaging-guided surgical design: can we optimise the Fontan operation?

The Fontan procedure, although an imperfect solution for children born with a single functional ventricle, is the only reconstruction at present short of transplantation. The haemodynamics associated with the total cavopulmonary connection, the modern approach to Fontan, are severely altered from the normal biventricular circulation and may contribute to the long-term complications that are frequently noted. Through recent technological advances, spear-headed by advances in medical imaging, it is now possible to virtually model these surgical procedures and evaluate the patient-specific haemodynamics as part of the pre-operative planning process. This is a novel paradigm with the potential to revolutionise the approach to Fontan surgery, help to optimise the haemodynamic results, and improve patient outcomes. This review provides a brief overview of these methods, presents preliminary results of their clinical usage, and offers insights into its potential future directions.

Validation of an open source framework for the simulation of blood flow in rigid and deformable vessels

We discuss in this paper the validation of an open source framework for the solution of problems arising in hemodynamics. The proposed framework is assessed through experimental data for fluid flow in an idealized medical device with rigid boundaries and a numerical benchmark for flow in compliant vessels. The core of the framework is an open source parallel finite element library that features several algorithms to solve both fluid and fluid-structure interaction problems. The numerical results for the flow in the idealized medical device (consisting of a conical convergent, a narrow throat, and a sudden expansion) are in good quantitative agreement with the measured axial components of the velocity and pressures for three different flow rates corresponding to laminar, transitional, and turbulent regimes. We emphasize the crucial role played by the accuracy in performing numerical integration, mesh, and time step to match the measurements. The numerical fluid-structure interaction benchmark deals with the propagation of a pressure wave in a fluid-filled elastic tube. The computed pressure wave speed and frequency of oscillations, and the axial velocity of the fluid on the tube axis are close to the values predicted by the analytical solution associated with the benchmark. A detailed account of the methods used for both benchmarks is provided.