A new imaging technique to study 3-D plaque and shear stress distribution in human coronary artery bifurcations in vivo

Patient-specific computational modeling of upper extremity arteriovenous fistula creation: its feasibility to support clinical decision-making

Introduction

Inadequate flow enhancement on the one hand, and excessive flow enhancement on the other hand, remain frequent complications of arteriovenous fistula (AVF) creation, and hamper hemodialysis therapy in patients with end-stage renal disease. In an effort to reduce these, a patient-specific computational model, capable of predicting postoperative flow, has been developed. The purpose of this study was to determine the accuracy of the patient-specific model and to investigate its feasibility to support decision-making in AVF surgery.

Methods

Patient-specific pulse wave propagation models were created for 25 patients awaiting AVF creation. Model input parameters were obtained from clinical measurements and literature. For every patient, a radiocephalic AVF, a brachiocephalic AVF, and a brachiobasilic AVF configuration were simulated and analyzed for their postoperative flow. The most distal configuration with a predicted flow between 400 and 1500 ml/min was considered the preferred location for AVF surgery. The suggestion of the model was compared to the choice of an experienced vascular surgeon. Furthermore, predicted flows were compared to measured postoperative flows.

Results

Taken into account the confidence interval (25^th and 75^th percentile interval), overlap between predicted and measured postoperative flows was observed in 70% of the patients. Differentiation between upper and lower arm configuration was similar in 76% of the patients, whereas discrimination between two upper arm AVF configurations was more difficult. In 3 patients the surgeon created an upper arm AVF, while model based predictions allowed for lower arm AVF creation, thereby preserving proximal vessels. In one patient early thrombosis in a radiocephalic AVF was observed which might have been indicated by the low predicted postoperative flow.

Conclusions

Postoperative flow can be predicted relatively accurately for multiple AVF configurations by using computational modeling. This model may therefore be considered a valuable additional tool in the preoperative work-up of patients awaiting AVF creation.

Impact of plaques in the left coronary artery on wall shear stress and pressure gradient in coronary side branches

In this study, we investigate plaques located at the left coronary bifurcation. We focus on the effect that the resulting changes in wall shear stress (WSS) and wall pressure stress gradient (WPSG) have on atherosclerotic progress in coronary artery disease. Coronary plaques were simulated and placed at the left main stem and the left anterior descending to produce >50% narrowing of the coronary lumen. Computational fluid dynamics analysis was carried out, simulating realistic physiological conditions that show the in vivo cardiac haemodynamic. WSS and WPSG in the left coronary artery were calculated and compared in the left coronary models, with and without the presence of plaques during cardiac cycles. Our results showed that WSS decreased while WPSG was increased in coronary side branches due to the presence of plaques. There is a direct correlation between coronary plaques and subsequent WSS and WPSG variations based on the bifurcation plaques simulated in the realistic coronary models.

Computational fluid dynamics analysis of the effect of plaques in the left coronary artery

This study was to investigate the hemodynamic effect of simulated plaques in left coronary artery models, which were generated from a sample patient's data. Plaques were simulated and placed at the left main stem and the left anterior descending (LAD) to produce at least 60% coronary stenosis. Computational fluid dynamics analysis was performed to simulate realistic physiological conditions that reflect the in vivo cardiac hemodynamics, and comparison of wall shear stress (WSS) between Newtonian and non-Newtonian fluid models was performed. The pressure gradient (PSG) and flow velocities in the left coronary artery were measured and compared in the left coronary models with and without presence of plaques during cardiac cycle. Our results showed that the highest PSG was observed in stenotic regions caused by the plaques. Low flow velocity areas were found at postplaque locations in the left circumflex, LAD, and bifurcation. WSS at the stenotic locations was similar between the non-Newtonian and Newtonian models although some more details were observed with non-Newtonian model. There is a direct correlation between coronary plaques and subsequent hemodynamic changes, based on the simulation of plaques in the realistic coronary models.

Image-based modeling of hemodynamics in coronary artery aneurysms caused by Kawasaki disease

Kawasaki Disease (KD) is the leading cause of acquired pediatric heart disease. A subset of KD patients develops aneurysms in the coronary arteries, leading to increased risk of thrombosis and myocardial infarction. Currently, there are limited clinical data to guide the management of these patients, and the hemodynamic effects of these aneurysms are unknown. We applied patient-specific modeling to systematically quantify hemodynamics and wall shear stress in coronary arteries with aneurysms caused by KD. We modeled the hemodynamics in the aneurysms using anatomic data obtained by multi-detector computed tomography (CT) in a 10-year-old male subject who suffered KD at age 3 years. The altered hemodynamics were compared to that of a reconstructed normal coronary anatomy using our subject as the model. Computer simulations using a robust finite element framework were used to quantify time-varying shear stresses and particle trajectories in the coronary arteries. We accounted for the cardiac contractility and the microcirculation using physiologic downstream boundary conditions. The presence of aneurysms in the proximal coronary artery leads to flow recirculation, reduced wall shear stress within the aneurysm, and high wall shear stress gradients at the neck of the aneurysm. The wall shear stress in the KD subject (2.95-3.81 dynes/sq cm) was an order of magnitude lower than the normal control model (17.10-27.15 dynes/sq cm). Particle residence times were significantly higher, taking 5 cardiac cycles to fully clear from the aneurysmal regions in the KD subject compared to only 1.3 cardiac cycles from the corresponding regions of the normal model. In this novel quantitative study of hemodynamics in coronary aneurysms caused by KD, we documented markedly abnormal flow patterns that are associated with increased risk of thrombosis. This methodology has the potential to provide further insights into the effects of aneurysms in KD and to help risk stratify patients for appropriate medical and surgical interventions.

Clinical Study Protocol for the ARCH Project Computational Modeling for Improvement of Outcome after Vascular Access Creation

Despite clinical guidelines and the possibility of diagnostic vascular imaging, creation and maintenance of a vascular access (VA) remains problematic: avoiding short- and long-term VA dysfunction is challenging. Although prognostic factors for VA dysfunction have been identified in previous studies, their potential interplay at a systemic level is disregarded. Consideration of multiple prognostic patient specific factors and their complex interaction using dedicated computational modeling tools might improve outcome after VA creation by enabling a better selection of VA configuration. These computational modeling tools are developed and validated in the ARCH project: a joint initiative of four medical centers and three industrial partners (FP7-ICT-224390). This paper reports the rationale behind computational modeling and presents the clinical study protocol designed for calibrating and validating these modeling tools. The clinical study is based on the pre-operative collection of structural and functional data at a vascular level, as well as a VA functional evaluation during the follow-up period. The strategy adopted to perform the study and for data collection is also described here.

Non-dimensional modeling in flow simulation studies of coronary arteries including side-branches: a novel diagnostic tool in coronary artery disease

AIMS

Blood flow, vascular shape and size and local remodeling of the vascular wall are linked through wall shear stress (WSS) signaling. Inter-individual comparison of shape and WSS is hampered by large differences in size of flow and shape. We performed non-dimensional modeling to discriminate different types of coronary artery remodeling based on WSS patterns and vessel morphology.

METHODS AND RESULTS

Blood flow was simulated in three-dimensional reconstructed right coronary artery trees from seven controls, five patients with coronary artery disease (CAD) and five patients with aneurysmatic CAD (AnCAD) classified by expert visual diagnosis. A discriminant model using low WSS area, a remodeling index, and cross-correlation of WSS in main trunks and complete trees (K) as non-dimensional parameters classified CAD and AnCAD correctly and identified three patients with high risk profile and functional disease in controls. The new model was compared with discriminant analysis of identical cases simulated without side-branches. The inclusion of K (information from side-branches) and replacement of the mean diameter by a non-dimensional remodeling index improved the model. We found significant (p<0.005) gender differences in the remodeling index.

CONCLUSION

The combination of non-dimensional modeling and WSS profiling should be further investigated as a novel diagnostic tool in CAD beyond local stenosis.

Flow simulation studies in coronary arteries--impact of side-branches

AIMS

Wall shear stress (WSS) may induce local remodeling of the vascular wall and the WSS pattern in turn depends on vascular geometry. We aimed to elucidate the impact of side-branches on local WSS.

METHODS AND RESULTS

Steady numerical flow simulation studies were performed in three-dimensional reconstructed right coronary artery (RCA) trees. RCA from seven controls, five patients with coronary artery disease (CAD) and five patients with aneurysmatic CAD (AnCAD) classified by expert visual diagnosis were studied. Then three transient flow simulations were performed with cases representative for each group in order to evaluate the impact of pulsatile flow simulation. As vascular size and flow rates vary considerably between patients, non-dimensional approaches were applied for group comparison. A point-to-point comparison of the WSS in the same tree with and without side-branches revealed local differences in WSS of up to 12.0 Pa. This was caused by a reduction of volume flow of up to 78.7% in the trunk. Differences are not only limited to bifurcation sites but also affect local narrowings and strongly curved segments. The point-to-point comparison of steady and transient simulations found an average increase of WSS of below 7% in transient simulations. No significant differences were found between histograms of pulsatile and steady simulations, showing a high cross-correlation of >0.97.

CONCLUSION

Side-branches must not be neglected in numerical flow simulation (steady and transient) studies. Steady simulations are valid for an assessment of time-averaged WSS distributions.

Open Problems in Computational Vascular Biomechanics: Hemodynamics and Arterial Wall Mechanics

The vasculature consists of a complex network of vessels ranging from large arteries to arterioles, capillaries, venules, and veins. This network is vital for the supply of oxygen and nutrients to tissues and the removal of carbon dioxide and waste products from tissues. Because of its primary role as a pressure-driven chemomechanical transport system, it should not be surprising that mechanics plays a vital role in the development and maintenance of the normal vasculature as well as in the progression and treatment of vascular disease. This review highlights some past successes of vascular biomechanics, but emphasizes the need for research that synthesizes complementary advances in molecular biology, biomechanics, medical imaging, computational methods, and computing power for purposes of increasing our understanding of vascular physiology and pathophysiology as well as improving the design of medical devices and clinical interventions, including surgical procedures. That is, computational mechanics has great promise to contribute to the continued improvement of vascular health.

Patient-specific modeling of cardiovascular mechanics

Advances in numerical methods and three-dimensional imaging techniques have enabled the quantification of cardiovascular mechanics in subject-specific anatomic and physiologic models. Patient-specific models are being used to guide cell culture and animal experiments and test hypotheses related to the role of biomechanical factors in vascular diseases. Furthermore, biomechanical models based on noninvasive medical imaging could provide invaluable data on the in vivo service environment where cardiovascular devices are employed and on the effect of the devices on physiologic function. Finally, patient-specific modeling has enabled an entirely new application of cardiovascular mechanics, namely predicting outcomes of alternate therapeutic interventions for individual patients. We review methods to create anatomic and physiologic models, obtain properties, assign boundary conditions, and solve the equations governing blood flow and vessel wall dynamics. Applications of patient-specific models of cardiovascular mechanics are presented, followed by a discussion of the challenges and opportunities that lie ahead.