Coronary artery bypass grafting hemodynamics and anastomosis design: a biomedical engineering review

Hemodynamics in arterial bypass graft anastomoses with varying cuff sizes and proximal flow paths: a fluid-structure interaction study

This article investigates the effect of the cuff size of arterial bypass grafts and the flow conditions on the hemodynamics in the anastomosis (connection) to the artery, using numerical simulations. We consider a fluid-structure interaction problem which is solved based on a partitioned scheme. Additionally, we employ computational fluid dynamics to investigate the effect of a rigid wall assumption. The work focuses on clinically relevant hemodynamic quantities associated with the development of intimal hyperplasia. We also include a model for the prediction of hemolysis into the simulation. The results show that even minor changes of the cuff size can result into significant differences in the corresponding quantities of interest. The importance of the inflow path is shown to be lower than that of the cuff size. The usually employed rigid wall assumption is found to be adequate to address wall shear stress oscillations but falls short on predicting maximum and minimum wall shear stress-related quantities of interest.

Hemodynamics of vascular shunts: trends, challenges, and prospects

Vascular bypass surgery takes a significant place in the treatment of vascular disease. According to various assessments, this type of surgery is associated with almost 20 % of all vascular surgery episodes (up to 23 % according to the Federal Neurosurgical Center of Novosibirsk). Even though the problem of using of vascular grafts is obvious and natural, many problems associated with them are not still elucidated. From the mechanics' point of view, a vascular bypass is a converging or diverging tee, and the functioning of such structures still does not have strict mathematical formulations and proofs in the general case, which forces many researchers to solve specific engineering problems associated with shunting. Mathematical modeling, which is the gold standard for virtual simulations of industrial and medical problems, faces great difficulties and limitations in solving problems for vascular bypasses. Complications in the treatment of the vascular disease may follow the difficulties in mathematical modeling, and the price can be a cardiac arrest or a stroke. This work is devoted to the main aspects of the medical application of vascular bypasses and their functioning as a mechanical system, as well the mathematical aspects of their possible setup.

A review on the treatment of intimal hyperplasia with perivascular medical devices: role of mechanical factors and drug release kinetics

INTRODUCTION

Intimal hyperplasia (IH) is a significant factor limiting the success of revascularization surgery for blood flow restoration. IH results from a foreign body response and mechanical disparity that involves complex biochemical reactions resulting in graft failure. The available treatment option utilizes either different pharmacological interventions or mechanical support to the vascular grafts with limited success.

AREAS COVERED

This review explains the pathophysiology of IH, responsible mechanical and biological factors, and treatment options, emphasizing perivascular devices. They are designed to provide mechanical support and pharmacology actions. The perivascular drug delivery concept has successfully demonstrated efficacy in various animal studies. Accurate projections of drug release mechanisms using mathematical modeling could be used to formulate prolonged drug elution devices. Numerical modeling aspects for the prediction of design outcomes have been given due importance that fulfills the unmet clinical need for better patient care.

EXPERT OPINION

IH could be effectively prevented by simultaneous mechanical scaffolding and sustained local drug delivery. Future perivascular medical devices could be designed to integrate these essential features. Numerical modeling for device performance prediction should be utilized in the development of next-generation perivascular devices.

Acute Mechanical Consequences of Vessel-Specific Coronary Bypass Combinations

PURPOSE

Premature coronary artery bypass graft (CABG) failure has been linked to geometric, mechanical, and compositional discrepancies between host and graft tissues. Acute hemodynamic disturbances and the introduction of wall stress gradients trigger a myriad of mechanobiological processes at the anastomosis that can be associated with restenosis and graft failure. Although the origins of coronary artery disease dictate the anastomotic target, an opportunity exists for graft-vessel optimization through rationale graft selection.

METHODS

Here we explored the four distinct regions of the left (L) and right (R) ITA (1 = proximal, 2 = submuscular, 3 = middle, 4 = distal), and four common target vessels in the coronary circulation including the proximal and distal left anterior descending (PLAD & DLAD), right coronary (RCA), and left circumflex (LCX) arteries. Benchtop biaxial mechanical data was used to acquire constitutive model parameters of these tissues and enable vessel-specific computational models to elucidate the mechanical consequences of 32 unique graft-target combinations.

RESULTS

Simulations revealed the maximum principal wall stresses for the PLAD, RCA, and LCX occurred when anastomosed with LITA1, and the maximum flow-induced shear stress occurred with LITA4. The DLAD, on the other hand, reached stress maximums when anastomosed to LITA4. Using a normalized objective function of simulation output variables, we found LITA2 to be the best graft choice for both LADs, RITA3 for the RCA, and LITA3 for the LCX.

CONCLUSION

Although mechanical compatibility is just one of many factors determining bypass graft outcomes, our data suggests improvements can be made to the grafting process through vessel-specific regional optimization.

Multi-Objective Optimisation of a Novel Bypass Graft with a Spiral Ridge

The low long-term patency of bypass grafts is a major concern for cardiovascular treatments. Unfavourable haemodynamic conditions in the proximity of distal anastomosis are closely related to thrombus creation and lumen lesions. Modern graft designs address this unfavourable haemodynamic environment with the introduction of a helical component in the flow field, either by means of out-of-plane helicity graft geometry or a spiral ridge. While the latter has been found to lack in performance when compared to the out-of-plane helicity designs, recent findings support the idea that the existing spiral ridge grafts can be further improved in performance through optimising relevant design parameters. In the current study, robust multi-objective optimisation techniques are implemented, covering a wide range of possible designs coupled with proven and well validated computational fluid dynamics (CFD) algorithms. It is shown that the final set of suggested design parameters could significantly improve haemodynamic performance and therefore could be used to enhance the design of spiral ridge bypass grafts.

Intra-atrial coronary artery reconstruction during surgery for cardiac angiosarcoma

A cardiac angiosarcoma is usually located in the right atrium and tends to involve the right coronary artery. Our goal was to report a novel reconstruction technique after en bloc resection of a cardiac angiosarcoma with right coronary artery invasion. This technique includes orthotopic reconstruction of the invaded artery and atrial patch suturing onto the epicardium lateral to the reconstructed right coronary artery. Intra-atrial reconstruction with an end-to-end anastomosis can enhance the graft patency compared to a distal side-to-end anastomosis and can reduce the risk of anastomotic stenosis. Moreover, suturing the graft patch to the epicardium did not increase the risk of bleeding because the pressure in the right atrium was low.

Patient-specific computational simulation of coronary artery bypass grafting

INTRODUCTION

Coronary artery bypass graft surgery (CABG) is an intervention in patients with extensive obstructive coronary artery disease diagnosed with invasive coronary angiography. Here we present and test a novel application of non-invasive computational assessment of coronary hemodynamics before and after bypass grafting.

METHODS AND RESULTS

We tested the computational CABG platform in n = 2 post-CABG patients. The computationally calculated fractional flow reserve showed high agreement with the angiography-based fractional flow reserve. Furthermore, we performed multiscale computational fluid dynamics simulations of pre- and post-CABG under simulated resting and hyperemic conditions in n = 2 patient-specific anatomies 3D reconstructed from coronary computed tomography angiography. We computationally created different degrees of stenosis in the left anterior descending artery, and we showed that increasing severity of native artery stenosis resulted in augmented flow through the graft and improvement of resting and hyperemic flow in the distal part of the grafted native artery.

CONCLUSIONS

We presented a comprehensive patient-specific computational platform that can simulate the hemodynamic conditions before and after CABG and faithfully reproduce the hemodynamic effects of bypass grafting on the native coronary artery flow. Further clinical studies are warranted to validate this preliminary data.

A bioactive compliant vascular graft modulates macrophage polarization and maintains patency with robust vascular remodeling

Conventional synthetic vascular grafts are associated with significant failure rates due to their mismatched mechanical properties with the native vessel and poor regenerative potential. Though different tissue engineering approaches have been used to improve the biocompatibility of synthetic vascular grafts, it is still crucial to develop a new generation of synthetic grafts that can match the dynamics of native vessel and direct the host response to achieve robust vascular regeneration. The size of pores within implanted biomaterials has shown significant effects on macrophage polarization, which has been further confirmed as necessary for efficient vascular formation and remodeling. Here, we developed biodegradable, autoclavable synthetic vascular grafts from a new polyurethane elastomer and tailored the grafts' interconnected pore sizes to promote macrophage populations with a pro-regenerative phenotype and improve vascular regeneration and patency rate. The synthetic vascular grafts showed similar mechanical properties to native blood vessels, encouraged macrophage populations with varying M2 to M1 phenotypic expression, and maintained patency and vascular regeneration in a one-month rat carotid interposition model and in a four-month rat aortic interposition model. This innovative bioactive synthetic vascular graft holds promise to treat clinical vascular diseases.

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Small diameter vascular grafts were fabricated from a new elastomeric polyurethane designed for vascular tissue engineering.

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The grafts combined excellent elasticity, strength, porosity, hemocompatibility, degradability, and biocompatibility.

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In vivo, grafts maintained patency for four months and supported tissue regeneration resembling the native arterial wall.

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Pore size was found to influence graft characteristics and efficacy.

1-Year Patency of Biorestorative Polymeric Coronary Artery Bypass Grafts in an Ovine Model

Many attempts have been made to inhibit or counteract saphenous vein graft (SVG) failure modes; however, only external support for SVGs has gained momentum in clinical utility. This study revealed the feasibility of implantation, and showed good patency out to 12 months of the novel biorestorative graft, in a challenging ovine coronary artery bypass graft model. This finding could trigger the first-in-man trial of using the novel material instead of SVG. We believe that, eventually, this novel biorestorative bypass graft can be one of the options for coronary artery bypass graft patients who have difficulty harvesting SVG.

A predictive patient-specific computational model of coronary artery bypass grafts for potential use by cardiac surgeons to guide selection of graft configurations

Cardiac surgeons face a significant degree of uncertainty when deciding upon coronary artery bypass graft configurations for patients with coronary artery disease. This leads to significant variation in preferred configuration between different surgeons for a particular patient. Additionally, for the majority of cases, there is no consensus regarding the optimal grafting strategy. This situation results in the tendency for individual surgeons to opt for a “one size fits all” approach and use the same grafting configuration for the majority of their patients neglecting the patient-specific nature of the diseased coronary circulation. Quantitative metrics to assess the adequacy of coronary bypass graft flows have recently been advocated for routine intraoperative use by cardiac surgeons. In this work, a novel patient-specific 1D-0D computational model called “COMCAB” is developed to provide the predictive haemodynamic parameters of functional graft performance that can aid surgeons to avoid configurations with grafts that have poor flow and thus poor patency. This model has significant potential for future expanded applications.

Hemocompatibility of micropatterned biomaterial surfaces is dependent on topographical feature size

Small-diameter synthetic vascular grafts that have improved hemocompatibility and patency remain an unmet clinical need due to thrombosis. A surface modification that has potential to attenuate these failure mechanisms while promoting an endothelial layer is the micropatterning of luminal surfaces. Anisotropic features have been shown to downregulate smooth muscle cell proliferation, direct endothelial migration, and attenuate platelet adhesion and activation. However, the effect of micropatterning feature size and orientation relative to whole blood flow has yet to be investigated within a systematic study. In this work, hemocompatibility of micropattern grating sizes of 2, 5, and 10 µm were investigated. The thrombogenicity of the micropattern surface modifications were characterized by quantifying FXIIa activity, fibrin formation, and static platelet adhesion in vitro. Additionally, dynamic platelet attachment and end-point fibrin formation were quantified using an established, flowing whole blood ex vivo non-human primate shunt model without antiplatelet or anticoagulant therapies. We observed a higher trend in platelet attachment and significantly increased fibrin formation for larger features. We then investigated the orientation of 2 µm gratings relative to whole blood flow and found no significant differences between the various orientations for platelet attachment, rate of linear platelet attachment, or end-point fibrin formation. MicroCT analysis of micropatterned grafts was utilized to quantify luminal patency. This work is a significant step in the development of novel synthetic biomaterials with improved understanding of hemocompatibility for use in cardiovascular applications.

Numerical simulation in the abdominal aorta and the visceral arteries with or without stenosis based on 2D PCMRI

It is important to obtain accurate boundary conditions (BCs) in hemodynamic simulations. This article aimed to improve the accuracy of BCs in computational fluid dynamics (CFD) simulation and analyze the differences in hemodynamics between healthy volunteers and patients with visceral arterial stenosis (VAS). The geometric models of seven cases were reconstructed using the magnetic resonance angiogram (MRA) or computed tomography angiogram (CTA) imaging data. The physiological flow waveforms obtained from 2D Phase Contrast Magnetic Resonance Imaging (PCMRI) were imposed on the aortic inlet and the visceral arteries' outlets. The individualized RCR values of the three-element Windkessel model were imposed on the aortic outlet. CFD simulations were run in the open-source software: svSolver. Two specific time points were selected to compare the hemodynamics of healthy volunteers and patients with VAS. The results suggested that blood in the stenotic visceral arteries flowed at high speed throughout the cardiac cycle. The low pressure is distributed at stenotic lesions. The wall shear stress (WSS) reached 4 Pa near stenotic locations. The low time average wall shear stress (TAWSS), high oscillatory shear index (OSI), and high relative residence time (RRT) concentrated in the abdominal aorta. Besides, the ratios of the areas with low TAWSS, high OSI, and high RRT to the computational domain were higher in patients with VAS than which in the healthy volunteers. The individualized BCs were used for hemodynamic simulations and results suggest that patients with stenosis have a higher risk of blood retention and atherosclerosis formation in the abdominal aorta.

Case report: asymptomatic pseudoaneurysm of the native coronary soon after the graft anastomosis treated with off-pump repair

Background

Pseudoaneurysms (PSAs) of native coronary arteries are rare but potentially lethal complications occurring after coronary artery graft anastomosis mainly secondary to median sternotomy.

Case summary

A 61-year-old man underwent coronary artery bypass grafting because of stable angina. After the surgery, the patient was asymptomatic. A routine pre-discharge transthoracic echocardiogram was performed showing a haematoma of the apex partially involving the right ventricle with systolic colour Doppler flow going from the left ventricle to the pericardium. A coronary computed tomography scan was ordered and it revealed the presence of a PSA of the left anterior descending (LAD) artery distal to the graft anastomosis with the left internal mammary artery. An off-pump direct suture of the LAD injury through a redo sternotomy was successfully performed.

Discussion

The development of a PSA of a native coronary artery after bypass grafting is a very rare and potentially fatal condition. A correct and prompt diagnosis is crucial to avoid lethal complication.

Sutureless versus Hand-Sewn Coronary Anastomoses: A Systematic Review and Meta-Analysis

Background: Sutureless coronary anastomotic devices are intended to facilitate minimally invasive coronary artery bypass grafting (MICS-CABG) by easing and eventually standardizing the anastomotic technique. Within this systematic review and meta-analysis, we aim to determine patency and to evaluate safety outcomes for the sutureless anastomoses. Methods: CENTRAL, MEDLINE, and EMBASE were searched from database start till August 2021 in a predefined search strategy combining the key concepts: ‘coronary artery bypass grafting’, ‘sutureless coronary anastomoses’, and ‘hand-sewn coronary anastomoses’ by the Boolean operation ‘AND’. Study characteristics, patient demographics, interventional details, and all available outcome data were extracted. A meta-analysis was performed on patency at longest follow-up. Safety outcomes were presented. Results: A total of eleven trials towards six sutureless anastomotic devices were included, comprising 3724 patients (490 sutureless and 3234 hand-sewn). There was no significant difference in patency at a mean follow-up duration of 546.3 (range 1.5–2691) days, with a risk ratio of 0.77 (95% CI 0.55–1.06). MACE was reported in 4.5% sutureless and 3.9% hand-sewn patients, including all-cause mortality (resp. 1.3 vs. 1.9%), myocardial infarction (resp. 1.6 vs. 1.7%), and coronary revascularization (resp. 1.8 vs. 0.5%). Incomplete hemostasis occurred in 24.8% of the sutureless anastomoses. Intra-operative device failure forced conversion to hand-sewn or redo-anastomosis in 5.8% of the sutureless cases. Conclusion: Based on the systematic review and meta-analysis including six devices, we conclude that sutureless coronary anastomotic devices appear safe and effective when used by well-trained and dedicated surgical teams.

Reduced order methods for parametric optimal flow control in coronary bypass grafts, toward patient-specific data assimilation

Coronary artery bypass grafts (CABG) surgery is an invasive procedure performed to circumvent partial or complete blood flow blockage in coronary artery disease. In this work, we apply a numerical optimal flow control model to patient-specific geometries of CABG, reconstructed from clinical images of real-life surgical cases, in parameterized settings. The aim of these applications is to match known physiological data with numerical hemodynamics corresponding to different scenarios, arisen by tuning some parameters. Such applications are an initial step toward matching patient-specific physiological data in patient-specific vascular geometries as best as possible. Two critical challenges that reportedly arise in such problems are: (a) lack of robust quantification of meaningful boundary conditions required to match known data as best as possible and (b) high computational cost. In this work, we utilize unknown control variables in the optimal flow control problems to take care of the first challenge. Moreover, to address the second challenge, we propose a time-efficient and reliable computational environment for such parameterized problems by projecting them onto a low-dimensional solution manifold through proper orthogonal decomposition-Galerkin.

Long-Term Sustained Drug Delivery via 3D Printed Masks for the Development of a Heparin-Loaded Interlayer in Vascular Tissue Engineering Applications

Current approaches in small-diameter vascular grafts for coronary artery bypass surgeries fail to address physiological variations along the graft that contribute to thrombus formation and ultimately graft failure. We present an innovative interlayer drug delivery system that can be utilized for the sustained delivery of heparin through a graft with a high degree of temporal and spatial control. A heparin-loaded gelatin methacrylate (gelMA) interlayer sits within a biohybrid composed of decellularized bovine pericardium (dECM) and poly(propylene fumarate) (PPF), and its UV crosslinking is controlled via three-dimensional (3D) printed shadow masks. The masks can be readily designed to modulate the incident light intensity on the graft, enabling us to control the resultant gelMA crosslinking and properties. A high heparin loading efficiency was obtained in gelMA and was independent of crosslinking. We achieved sustained heparin release over the course of 2 weeks within the biohybrid material using the 3D printed mask patterns. High doses of heparin were observed to have detrimental effects on endothelial cell function. However, when exposed to heparin in a slower, more sustained manner consistent with the masks, endothelial cells behave similarly to untreated cells. Further, slower release profiles cause significantly more release of tissue factor pathway inhibitor, an anticoagulant, than a faster release profile. The heparin-loaded gelMA interlayer we have developed is a useful tool for the temporal and spatial control of heparin release that supports endothelial function and promotes an antithrombotic environment.

Impact of coronary artery bypass grafting (CABG) on coronary collaterals in patients with a chronic total occlusion (CTO)

Chronic total occlusions (CTO) are found commonly in patients with prior coronary artery bypass grafting (CABG). We sought to determine the effect of CABG on collateral robustness in patients with a CTO. Patients with a CTO diagnosed on coronary angiography between July 2010 and December 2019 were included in this study. Patients were classified as either CTO supplied by a functional graft, CTO supplied by collaterals from a non-grafted donor vessel (non-grafted) or a CTO supplied by collaterals from a grafted donor vessel (grafted). The degree of collateral robustness was determined by the Rentrop classification and collateral connection (CC) grade. Demographic, angiographic and clinical outcomes were recorded. A total of 2088 CTO lesions were identified, of which 878 (42.0%) were supplied by a functional graft, 994 (47.6%) CTOs were supplied by a non-grafted donor vessel and 216 (10.3%) CTOs were supplied by a grafted donor vessel. CTOs supplied by a grafted donor vessel had lower rates of robust collaterals (37.0% vs 83.0%, p < 0.0001) with less mature collaterals as determined by the Rentrop grade (p < 0.0001) and CC grade (p < 0.0001) as compared to CTOs supplied by a non-grafted donor vessel. In patients with a previous CABG, a grafted donor vessel results in less robust coronary collaterals with lower Rentrop and CC grade compared to an ungrafted donor vessel. This may be attributable to changes in coronary blood flow and shear stress, and may be a factor in the lower procedural success rates for CTO intervention in patients with prior CABG.

Dilation-Responsive Microshape Programing Prevents Vascular Graft Stenosis

Shape memory materials have been successfully applied to minimally invasive implantation of medical devices. However, organ-movement-specific shape programing at a microscale level has never been demonstrated despite significant unmet needs. As vein-to-artery grafting induces vein dilation and stenosis, a polymeric self-enclosable external support (SES) is designed to wrap the vascular out-wall. Its micropores are programmed to increase sizes and interconnections upon dilation. Vessel dilation promotes venous maturation, but overdilation induces stenosis by disturbed blood flow. Therefore, the unique elastic shape-fixity of SES provides a foundation to enable a stable microscale shape transition by maintaining the vein dilation. The shape transition of micropore architecture upon dilation induces beneficial inflammation, thereby regenerating vasa vasorum and directing smooth muscle cell migration toward adventitia with the consequent muscle reinforcement of veins. This game-changer approach prevents the stenosis of vein-to-artery grafting by rescuing ischemic disorders and promoting arterial properties of veins.

On the relevance of boundary conditions and viscosity models in blood flow simulations in patient-specific aorto-coronary bypass models

Physiologically realistic results are the aim of every blood flow simulation. This is not different in aorto-coronary bypasses where the properties of the coronary circulation may significantly affect the relevance of the performed simulations. By considering three patient-specific bypass geometries, the present article focuses on two aspects of the coronary blood flow - its phasic flow pattern and its behaviour affected by blood rheology. For the phasic flow property, a multiscale modelling approach is chosen as a means to assess the ability of five different types of coronary boundary conditions (mean arterial pressure, Windkessel model and three lumped parameter models) to attain realistic coronary haemodynamics. From the analysed variants of boundary conditions, the best option in terms of physiological characteristics and its potential for use in patient-based simulations, is utilised to account for the effect of shear-dependent viscosity on the resulting haemodynamics and wall shear stress stimulation. Aside from the Newtonian model, the blood rheology is approximated by two non-Newtonian models in order to determine whether the choice of a viscosity model is important in simulations involving coronary circulation. A comprehensive analysis of obtained results demonstrated notable superiority of all lumped parameter models, especially in comparison to the constant outlet pressure, which regardless of bypass type gave overestimated and physiologically misleading results. In terms of rheology, it was noted that blood in undamaged coronary arteries behaves as a Newtonian fluid, whereas in vessels with atypical lumen geometry, such as that of anastomosis or stenosis, its shear-thinning behaviour should not be ignored.

Small diameter polycaprolactone vascular grafts are patent in sheep carotid bypass but require antithrombotic therapy

Background: Polycaprolactone (PCL) scaffolds exhibit high biocompatibility and are attractive as vascular conduits. Materials & methods: PCL tubes were cultivated in bioreactor with human adipose regenerative cells to assess ex vivo cytocompatibility, whereas in vivo PCL tube patency was evaluated in sheep carotid bypass with and without antithrombotic treatment. Results: Ex vivo results revealed increasing adipose regenerative cells on PCL using dynamic bioreactor culturing. In vivo data showed that 67% (2/3) of grafts in the antithrombotic group were patent at day 28, while 100% (3/3) of control grafts were occluded already during the first week due to thrombosis. Histology showed that patent PCL grafts were recellularized by host cells. Conclusion: PCL tubes may work as small diameter vascular scaffolds under antithrombotic treatment.

Longitudinal histomechanical heterogeneity of the internal thoracic artery

The internal thoracic artery (ITA) is the principal choice for coronary artery bypass grafting (CABG) due to its mechanical compatibility, histological composition, anti-thrombogenic lumen, and single anastomotic junction. Originating at the subclavian artery, traversing the thoracic cavity, and terminating at the superior epigastric and musculophrenic bifurcation, bilateral ITAs follow a protracted circuitous pathway. The physiological hemodynamics, anatomical configuration, and perivascular changes that occur throughout this length influence the tissue's microstructure and gross mechanical properties. Since histomechanics play a major role in premature graft failure we used inflation-extension testing to quantify the regional material and biaxial mechanical properties at four distinct locations along the left (L) and right (R) ITA and fit the results to a structurally-motivated constitutive model. Our comparative analysis of 44 vessel segments revealed a significant increase in the amount of collagen but not smooth muscle and a significant decrease in elastin and elastic lamellae present with distance from the heart. A subsequent decrease in the total deformation energy and isotropic contribution to the strain energy was present in the LITA but not RITA. Circumferential stress and compliance generally decreased along the length of the LITA while axial stress increased in the RITA. When comparing RITAs to LITAs, some morphological and histological differences were found in proximal sections while distal sections revealed differences predominantly in compliance and axial stress. Overall, this information can be used to better guide graft selection, graft preparation, and xenograft-based tissue-engineering strategies for CABG.

A Thesis Proposal Development Course for Engineering Graduate Students

Helping engineering graduate students to write their thesis can be a difficult and time-consuming undertaking for a thesis advisor. Efficiency can be gained by having an experienced graduate student thesis advisor help multiple students at the same time. This article describes the philosophy, methods, and course design details used to develop and conduct a graduate level course on "thesis proposal development" for engineering students. The course provides structure to encourage students to engage in research and write their thesis proposal. The thesis proposal contains the student's detailed research plans and serves as the foundation for the student's final thesis. Each element of the course is described in detail with enough information that readers can implement the course at their own institution using this article as a guide. It includes detailed descriptions of individual assignments, reasons for including the assignment in the course, and Supplemental Material on the ASME Digital Collection which is downloadable from the journal. Since implementing this at our university, we have observed improvements in graduate student research projects, better written theses, and earlier thesis defense dates. The changes were implemented without altering the number of credit hours needed to graduate and we believe that the change has been beneficial.

Computational fluid dynamics of internal mammary artery-left anterior descending artery anastomoses

OBJECTIVES

The aim of this study was to elucidate the remodelling of the internal mammary artery (IMA)-left anterior descending artery anastomosis and compare 2 different anastomosis techniques (end-to-side versus side-to-side) using computational fluid dynamics.

METHODS

This study included 9 patients. Computed tomography (CT) angiography was performed immediately after coronary artery bypass grafting (CABG) and at 3-6 months later. The computational fluid dynamics models were made using the CT data. The pulsatile 3-dimensional blood flow was achieved with the finite volume method to evaluate the postoperative morphological and haemodynamic changes at the anastomosis in each patient. Flow velocity distribution, wall shear stress (WSS) and its fluctuation oscillatory shear index were measured.

RESULTS

No early or mid-term graft occlusion was observed in the study series. In the side-to-side anastomosis, pouch formation at the distal end of IMA caused a vortex flow with low WSS immediately after CABG. However, at 3-6 months after surgery, this pouch disappeared. As a result, the laminar straight flow with uniform WSS distribution was achieved inside the anastomosis. In the end-to-side anastomosis, the anastomosis shape was remodelled, resulting in a laminar flow pattern with uniform WSS distribution. A patchy high oscillatory shear index was detected at the IMA wall on the top of anastomosis in either anastomosis techniques immediately after the surgery, but it disappeared at 3-6 months after surgery.

CONCLUSIONS

Regardless of the anastomosis technique used, a successful remodelling of the IMA-left anterior descending artery anastomosis shape was achieved a few months after surgery, resulting in a straightforward flow streamline, with uniform WSS distribution and minimal oscillatory shear index.

Application of tissue-engineered interventions for coronary artery bypass grafts

Coronary artery bypass graft is one of the extensively conducted procedures to release occlusion in the coronary vessel. Various biological grafts are used for this purpose, superiorly, saphenous vein graft, if unavailable, other vessels in the body, with likewise characteristics are exploited for the purpose. The choice of graft is yet under discovery that could impeccably meet all the requirements. Variation in perioperative and postoperative results have given uneven clinical inferences of these conduits. Alternatively, tissue-engineering is also being applied in this area for clinical improvements. This review underlines some of the commonly used grafts for coronary artery bypass graft and advancements in tissue engineering for this purpose.

Saphenous vein graft disease, pathophysiology, prevention, and treatment. A review of the literature

BACKGROUND

The saphenous vein remains the most frequently used conduit for coronary artery bypass grafting, despite reported unsatisfactory long-term patency rates. Understanding the pathophysiology of vein graft failure and attempting to improve its longevity has been a significant area of research for more than three decades. This article aims to review the current understanding of the pathophysiology and potential new intervention strategies.

METHODS

A search of three databases: MEDLINE, Web of Science, and Cochrane Library, was undertaken for the terms "pathophysiology," "prevention," and "treatment" plus the term "vein graft failure."

RESULTS

Saphenous graft failure is commonly the consequence of four different pathophysiological mechanisms, early acute thrombosis, vascular inflammation, intimal hyperplasia, and late accelerated atherosclerosis. Different methods have been proposed to inhibit or attenuate these pathological processes including modified surgical technique, topical pretreatment, external graft support, and postoperative pharmacological interventions. Once graft failure occurs, the available treatments are either surgical reintervention, angioplasty, or conservative medical management reserved for patients not eligible for either procedure.

CONCLUSION

Despite the extensive amount of research performed, the pathophysiology of saphenous vein graft is still not completely understood. Surgical and pharmacological interventions have improved early patency and different strategies for prevention seem to offer some hope in improving long-term patency.

Computational fluid dynamic study of multiple sequential coronary artery bypass anastomoses in a native coronary stenosis model

BACKGROUND

The objective of this study was to evaluate the hemodynamic characteristics of multiple sequential coronary artery bypass grafting using a computational fluid dynamics study.

METHODS

First anastomosis was configured into parallel and diamond anastomoses, and the second anastomosis was set as end-side anastomosis. The anastomosis incision lengths were fixed at 2 mm. Various combinations of the degree of first and second stenoses were studied. The diameter of both the native and graft vessels was set at 2 mm. The inlet boundary condition was set by a sample of the transient time flow measurement, which was measured intraoperatively.

RESULTS

Both swirl and stagnation were observed at the outlets of the stenosis and the anastomosis sites. When the severity of the second stenosis was larger than that of the first, the flow at the outlet of the second stenosis was more unstable. Higher wall shear stress and larger oscillatory shear index regions were observed when the severe stenosis was bypassed by the first anastomosis, especially with diamond anastomoses. Less energy loss and higher energy efficiency were present when the vessel with more severe stenosis was bypassed as the second anastomosis. Energy loss was lower and energy efficiency was higher with parallel anastomosis than diamond anastomosis when the severity of the two stenoses was the same.

CONCLUSIONS

It is ideal to bypass the less severe stenosis vessel first with a parallel anastomosis method when employing multiple sequential bypass grafting. This improves hemodynamic stability and energy efficiency, according to a computational fluid dynamics model.

Role of Vessel Microstructure in the Longevity of End-to-Side Grafts

Compliance mismatch between the graft and the host artery of an end-to-side (ETS) arterial bypass graft anastomosis increases the intramural stress in the ETS graft-artery junction, and thus may compromise its long-term patency. The present study takes into account the effects of collagen fibers to demonstrate how their orientations alter the stresses. The stresses in an ETS bypass graft anastomosis, as a man-made bifurcation, are compared to those of its natural counterpart with different fiber orientations. Both of the ETS bypass graft anastomosis and its natural counterpart have identical geometric and material models and only their collagen fiber orientations are different. The results indicate that the fiber orientation mismatch between the graft and the host artery may increase the stresses at both the heel and toe regions of the ETS anastomosis (the maximum principal stress at the heel and toe regions increased by 72% and 12%, respectively). Our observations, thus, propose that the mismatch between the collagen fiber orientations of the graft and the host artery, independent of the effect of the suture line, may induce aberrant stresses to the anastomosis of the bypass graft.

The effect of anastomotic angle and diameter ratio on flow field in the distal end-to-side anastomosis

Flow fields in the distal end-to-side anastomosis of coronary artery bypass graft are associated with intimal hyperplasia and bypass failure. This work aims to demonstrate the effect of anastomotic angle and diameter ratio on flow field of coronary artery bypass graft. The flow fields inside polydimethylsiloxane models of coronary artery bypass graft with various anastomotic angles (α = 30°, 45°, 60° and 75°) and different diameter ratios (Φ = 0.78 and 1.11) are investigated using particle image velocimetry and computational fluid dynamics method under pulsatile flow condition. The results show that the anastomotic angle is positively correlated with the number and area of the recirculation zone, and the flow field disturbance at the anastomosis will develop in the same direction. Compared with that of Φ = 0.78, when Φ = 1.11, the flow fields at the anastomosis are relatively smoother with less turbulence.

Functional remodeling of an electrospun polydimethylsiloxane-based polyether urethane external vein graft support device in an ovine model

Saphenous vein graft (SVG) failure rates are unacceptably high, and external mechanical support may improve patency. We studied the histologic remodeling of a conformal, electrospun, polydimethylsiloxane-based polyether urethane external support device for SVGs and evaluated graft structural evolution in adult sheep to 2 years. All sheep (N = 19) survived to their intended timepoints, and angiography showed device-treated SVG geometric stability over time (30, 90, 180, 365, or 730 days), with an aggregated graft patency rate of 92%. There was minimal inflammation associated with the device material at all timepoints. By 180 days, treated SVG remodeling was characterized by minimal/nonprogressive intimal hyperplasia; polymer fragmentation and integration; as well as the development of a neointima, and a confluent endothelium. By 1-year, the graft developed a media-like layer by remodeling the neointima, and elastic fibers formed well-defined structures that subtended the neo-medial layer of the remodeled SVG. Immunohistochemistry showed that this neo-media was populated with smooth muscle cells, and the intima was lined with endothelial cells. These data suggest that treated SVGs were structurally remodeled by 180 days, and developed arterial-like features by 1 year, which continued to mature to 2 years. Device-treated SVGs remodeled into arterial-like conduits with stable long-term performance as arterial grafts in adult sheep.

A mathematical model to evaluate hemodynamic effects of the graft anastomosis in coronary surgery

The durability and patency of a coronary graft can be negatively affected by technical factors that induce thrombosis of the graft and poor prognosis of patients undergoing coronary artery bypass grafting. Technical factors include the inclination angle of the coronary anastomosis and the alignment between the main vessel and the inserted vessel as graft. We have studied a mathematical model aimed to assess the best angulation of the anastomosis and the influence of a correct alignment in order to prevent the risk of early graft occlusion. From data obtained from the mathematical model, in our opinion an inclination of the anastomotic angle of at least 30° seems to be a right choice when performing a coronary artery bypass graft. In addition, the incision of the coronary vessel should be done perfectly on the same axis as that performed on the graft, since even a deviation of the axis of the anastomosis of only 10° can create turbulence of the flow in the anastomosis site, which is accentuated when the deviation reaches 20°.

Role of smooth muscle cells in coronary artery bypass grafting failure

Atherosclerosis is the underlying pathology of many cardiovascular diseases. The formation and rupture of atherosclerotic plaques in the coronary arteries results in angina and myocardial infarction. Venous coronary artery bypass grafts are designed to reduce the consequences of atherosclerosis in the coronary arteries by diverting blood flow around the atherosclerotic plaques. However, vein grafts suffer a high failure rate due to intimal thickening that occurs as a result of vascular cell injury and activation and can act as 'a soil' for subsequent atherosclerotic plaque formation. A clinically-proven method for the reduction of vein graft intimal thickening and subsequent major adverse clinical events is currently not available. Consequently, a greater understanding of the underlying mechanisms of intimal thickening may be beneficial for the design of future therapies for vein graft failure. Vein grafting induces inflammation and endothelial cell damage and dysfunction, that promotes vascular smooth muscle cell (VSMC) migration, and proliferation. Injury to the wall of the vein as a result of grafting leads to the production of chemoattractants, remodelling of the extracellular matrix and cell-cell contacts; which all contribute to the induction of VSMC migration and proliferation. This review focuses on the role of altered behaviour of VSMCs in the vein graft and some of the factors which critically lead to intimal thickening that pre-disposes the vein graft to further atherosclerosis and re-occurrence of symptoms in the patient.

Biomechanics and Mechanobiology of Saphenous Vein Grafts

Within several weeks of use as coronary artery bypass grafts (CABG), saphenous veins (SV) exhibit significant intimal hyperplasia (IH). IH predisposes vessels to thrombosis and atherosclerosis, the two major modes of vein graft failure. The fact that SV do not develop significant IH in their native venous environment coupled with the rapidity with which they develop IH following grafting into the arterial circulation suggests that factors associated with the isolation and preparation of SV and/or differences between the venous and arterial environments contribute to disease progression. There is strong evidence suggesting that mechanical trauma associated with traditional techniques of SV preparation can significantly damage the vessel and might potentially reduce graft patency though modern surgical techniques reduces these injuries. In contrast, it seems possible that modern surgical technique, specifically endoscopic vein harvest, might introduce other mechanical trauma that could subtly injure the vein and perhaps contribute to the reduced patency observed in veins harvested using endoscopic techniques. Aspects of the arterial mechanical environment influence remodeling of SV grafted into the arterial circulation. Increased pressure likely leads to thickening of the medial wall but its role in IH is less clear. Changes in fluid flow, including increased average wall shear stress, may reduce IH while disturbed flow likely increase IH. Nonmechanical stimuli, such as exposure to arterial levels of oxygen, may also have a significant but not widely recognized role in IH. Several potentially promising approaches to alter the mechanical environment to improve graft patency are including extravascular supports or altered graft geometries are covered.

NUMERICAL INVESTIGATION OF THE EFFECTS OF BED SHAPE ON THE END-TO-SIDE CABG HEMODYNAMICS

Disrupted flow initiates and aggravates intimal thickening in the end-to-side (ETS) coronary artery bypass grafting (CABG), which may lead to failure. To enhance the post-intervention hemodynamics, the geometry is either optimized or totally reconfigured. Majority of configurations proposed by researchers have not suited CABG surgery, for they entailed rigorous manipulation on conventional grafts in situ, which was neither swift nor straightforward. The aim of the present study is, thus, to introduce a slight, yet effective, modification to a conventional ETS CABG configuration, and numerically investigate its effects on updated hemodynamic and structural environment, anticipating the longevity of proposed configuration and CABG success. This fairly simple modification may easily be made positioning a pre-designed anastomotic device between the bed of host artery in the conventional ETS CABG and its surrounding tissues. Conducting comprehensive numerical simulations, performance of the proposed configuration was assessed using idealized and patient-specific geometries of the conventional ETS CABG. Blood flow was simulated in a conventional and an updated CABG configuration considering 2-way fluid–structure interaction. Results revealed that, although the proposed configuration may induce higher structural stresses in vessels walls, it may improve important hemodynamic metrics such as wall shear stress gradient, oscillatory shear index, and relative residence time on host artery bed reducing disruption of flow. This study may also set the stage for design engineers and regulatory officials to evolve ETS CABG toward more hemodynamics-friendly approaches. Further in vitro, preclinical, and clinical experiments are, yet, entailed to accomplish ideal designs of procedural guidelines/grafts.

Long-term graft patency after coronary artery bypass grafting: Effects of morphological and pathophysiological factors

OBJECTIVE

The aim of the present study was to identify morphological and pathophysiological factors associated with long-term patency of grafts used in coronary artery bypass grafting (CABG).

METHODS

A total of 127 patients who underwent CABG between 2000 and 2006 and presented for computed tomography evaluation of graft patency at 139.78±36.64 months post-CABG were analyzed. Patients received 340 grafts (2.68 grafts/patient), 399 distal anastomoses (3.14 anastomoses/ patient), 220 (55.14%) performed using arterial grafts, and 179 (44.86%) using saphenous vein grafts (SVGs).

RESULTS

Graft patency varied according to vessel type and coronary territory. Overall graft patency was 90.16% for the left internal thoracic artery (LITA), 75.55% for the right internal thoracic artery (RITA), 79.25% for the radial artery (RA), and 74.3% for the SVG. The maximum patency rate was obtained with the RA (80.65%) for the right coronary territory, RITA (92.86%) for the anterolateral territory, and SVG (82.54%) for the circumflex territory. The LITA-left anterior descending artery graft occluded in 13 (7.93%) cases, 7 due to competitive flow. The influence of graft length on patency rates after indexing to height was not significant. The target vessel degree of stenosis influenced arterial graft patency rates with an occlusion odds ratio (OR) of 3.02 when anastomosed to target vessels with <90% stenosis. Target vessel caliber also influenced patency rates with occlusion ORs of 2.63 for SVGs and 2.31 for arterial grafts when anastomosed to ≤1.5 mm target vessels.

CONCLUSION

Morphological parameters, such as graft type, target territory, target vessel caliber, and degree of stenosis, are important factors conditioning long-term graft patency.

Computational fluid dynamic study of different incision length of coronary artery bypass grafting in a native coronary stenosis model

Background

The objective of this study was to evaluate hemodynamic patterns in end-side coronary artery bypass grafting with different anastomosis length by computational fluid dynamic study in the native coronary stenosis model.

Methods

The fluid dynamic computations were carried out using ANSYS CFX. Incision length was set to be 2, 4, 6, 8, 10 mm. The angle between the two blood vessels corresponded to the length of the incision. Native vessels were set to be 90% stenosis. The radius of both native and graft vessels was set to be 2 mm. The inlet boundary condition was set by the sample of the transient time flow which was measured intraoperatively.

Results

The energy efficiency was higher and energy loss was lower when the anastomosis length was longer until 8 mm. However, energy efficiency was lowest and energy loss was highest in the 10-mm model. In the 10-mm incision model, the streamline showed the scanty bypass flow in the bottom. Vortex showed that only 10-mm model showed the vortex just distal to the stenosis in the native inlet, and more vortex in native outlet than other length models. The oscillatory shear index (OSI) was higher in the outlet top in all models. And only 10-mm model showed high oscillatory index just distal to the stenosis.

Conclusions

In the end-side anastomosis, an anastomosis length of 8 mm was the ideal length with less flow complexity, low OSI, and less energy loss and high energy efficiency in the native 90% stenosis model.

Long-term graft patency after coronary artery bypass grafting: Effects of surgical technique

The aim of the current study was to identify surgical factors associated with long-term patency of grafts used in coronary artery bypass grafting (CABG). The present study analyzed data from 127 patients who underwent CABG at our institute between 2000 and 2006 and presented for ambulatory examination and coronary computed tomography angiography evaluation of graft patency in 2016 (139.78±36.64 months post-CABG). The 127 patients received 340 grafts (2.68 grafts/patient) and 399 distal anastomoses (3.14 anastomoses/patient), 220 (55.14%) with arterial grafts and 179 (44.86%) with saphenous vein grafts. Graft patency varied according to coronary territory, proximal anastomosis type (in situ graft, composite graft, graft anastomosed to the ascending aorta), Y anastomosis angle (47.21° for patent arterial grafts vs. 56° for occluded), and distal anastomosis angle (in sequential anastomoses irrespective to graft type, 48.60° for patent side-to-side anastomosis vs. 53.97° for occluded, 65.12° for patent end-to-side anastomosis vs. 90.80° for occluded; in single end-to-side anastomosis of arterial grafts, 39.46° for patent and 44.94° for occluded). A single end-to-side anastomosis angle 60° or greater was associated with a 5.149 occlusion odds ratio (OR) (P<0.001) for arterial grafts. Venous grafts were not sensitive to single end-to-side anastomosis angle. In conclusion, a small anastomosis angle for proximal Y and distal anastomoses is associated with a higher long-term patency of the free graft. Radial artery grafts registered higher patency rates when anastomosed to the ascending aorta compared with composite grafting with the left internal thoracic artery, whereas in situ right internal thoracic artery (RITA) anastomosed to the right coronary territory is associated with a lower patency rate compared with free RITA used to revascularise the anterolateral or circumflex territory in composite grafting.

Patient-Specific Multi-Scale Model Analysis of Hemodynamics Following the Hybrid Norwood Procedure for Hypoplastic Left Heart Syndrome: Effects of Reverse Blalock-Taussig Shunt Diameter

INTRODUCTION

The hybrid Norwood (HN) is a relatively new first stage palliative procedure for neonates with hypoplastic left heart syndrome, in which a sustainable uni-ventricular circulation is established in a less invasive manner than with the standard Norwood procedure. A computational multiscale model of the circulation following the HN procedure was used to obtain detailed hemodynamics. Implementation of a reverse-BT shunt (RBTS), a synthetic bypass from the main pulmonary to the innominate artery placed to counteract aortic arch stenosis, and its effects on local and global hemodynamics were studied.

METHODS

A post-op patient-derived anatomy of the HN procedure was utilized with varying degrees of distal arch obstruction, or stenosis, (nominal and 90% lumenal area reduction) and varying RBTS diameters (3.0, 3.5, 4.0 mm). A closed lumped parameter model (LPM) for the proximal and peripheral circulations was coupled to a 3D computational fluid dynamics (CFD) model in order to obtain converged flow fields for analysis.

RESULTS

CFD analyses of patient-derived anatomic configurations demonstrated consistent trends of vascular bed perfusion, vorticity, oscillatory shear index and wall shear stress levels. In the models with severe stenosis, implementation of the RBTS resulted in a restoration of arterial perfusion to near-nominal levels regardless of the shunt diameter. Shunt flow velocity, vorticity, and overall wall shear stress levels decreased with increasing shunt diameter, while shunt flow and systemic oxygen delivery increased with increased shunt diameter. In the absence of distal arch stenosis, large (4.0 mm) grafts may risk thrombosis due to low velocities and flow patterns.

CONCLUSION

Among the three graft sizes, the best option seems to be the 3.5 mm RBTS which provides a more organized flow similar to that of the 3.0 mm configuration with lower levels of wall shear stress. As such, in the setting of this study and for comparable HN physiologies our results suggest that: (1) the 4.0 mm shunt is a generous shunt diameter choice that may be problematic particularly when implemented prophylactically in the absence of stenosis, and (2) the 3.5 mm shunt may be a more suitable alternative since it exhibits more favorable hemodynamics at lower levels of wall shear stress.

Image-Based Computational Fluid Dynamic Analysis for Surgical Planning of Sequential Grafts in Coronary Artery Bypass Grafting

Coronary bypass grafting (CABG) is a surgical procedure for anastomosing small grafts to the coronary vessels. The bypass graft bridges the occluded or diseased coronary artery, allowing sufficient blood flow to deliver oxygen and nutrients to the heart muscles. Patient-specific (PS) anatomy obtained from coronary computed tomography angiography (CCTA) was used to generate a 3D aorto-coronary model (pre-surgery). Additionally, three more models with idealized grafts (individual and sequential grafts), were created using Boolean operations to represent post-surgery configuration. Fractional flow reserve (FFR) and wall shear stress (WSS) were estimated from the computational fluid dynamics (CFD). The pre-surgical FFR values for all the three left coronary arteries were significant (FFR<.80). The flow was restored (FFR>0.80) distal to stenosis in all the three post- surgical idealized graft models. Peak WSS values of 468, 336 and 295 dynes/cm² were observed at the toe of the individual end-to-side anastomosis for the three graft models. More importantly, low WSS (< 100 dynes/cm²) prevails at the heel and the walls opposite to the anastomosis in the sequential graft models. The prevailing low WSS at the heel and the wall bed opposite to anastomosis, in a sequential graft model, reduces restenosis rates and promotes a uniform hemodynamic environment for a better long-term patency of the graft. PS- CFD simulations based on CCTA can be helpful in assessing the hemodynamic parameters of graft models for optimal surgical planning.

Studies on the effects of microencapsulated human mesenchymal stem cells in RGD-modified alginate on cardiomyocytes under oxidative stress conditions using in vitro biomimetic co-culture system

Stem cell therapy has been recognized as a promising approach for myocardium regeneration post myocardial infarction (MI); however, it unfortunately often remains a challenge because of poor survival of transplanted cells and a lack of clear understanding of their interactions with host cells. High oxidative stress at heart tissues post MI is considered one of the important factors damaging transplanted cells and native cells/tissues. Here, we employed an in vitro co-culture system, capable of mimicking cases of stem cell transplantation into the myocardium presenting high oxidative stress, using human mesenchymal stem cells (hMSCs) encapsulated in alginate or cell interactive Arg-Gly-Asp (RGD) peptide-modified alginate micro-hydrogels. Under H₂O₂-induced oxidative stress conditions, viabilities of hMSCs and CMs were significantly higher in their co-culture than in their individual monolayer cultures. Expression of cardiac muscle markers remained high even with H₂O₂ treatment when cardiomyocytes (CMs) were co-cultured with hMSCs in RGD-alginate. Higher levels of various growth factors (associated with angiogenesis, cardiac regeneration, and contractility) were found in co-culture (noticeably with RGD-alginate) compared to monolayer cultures of CMs or hMSCs. These results can benefit the study of in vivo MI progression with transplanted stem cells and the development of effective stem cell-based therapeutic strategies for various oxidative stress-related diseases.

Lund exhaust on hemodynamic parameters and inflammatory mediators in patients undergoing cardiac valve replacement under cardiopulmonary bypass

The effect of Lund exhaust technique on hemodynamics and inflammatory mediators in patients undergoing cardiac valve replacement under cardiopulmonary bypass was evaluated. A total of 60 patients with heart disease undergoing elective heart valve replacement under elective cardiopulmonary bypass were randomly divided into Lund exhaust group (group A) and control group (group B), with 30 patients in each group. Group A underwent Lund exhaust during cardiopulmonary bypass, while group B was identical to group A except for not using the Lund exhaust technique during cardiopulmonary bypass. The hemodynamic parameters at different time-points showed that the indexes of MAP, PASP, CO, CI, PCWP, CVP and SVR in T₁, T₂, T₃ and T₄ moments between group A and group B were statistically significant (p<0.05). There was no statistical significance in IL-6, IL-8, IL-10, TNF-α and TIMP-1 between group A and group B patients at the T₀ moment (p>0.05). The levels of IL-6, IL-8, IL-10, TNF-α and TIMP-1 in group B patients at T₁, T₂, T₃ and T₄ moments were statistically significant compared with those in group A (p<0.05). The IL-6, IL-8, TNF-α indexes of group B patients were statistically significant at the T₅ moment compared with those in group A (p<0.05). The IL-10 and TIMP-1 of two groups were not statistically significant at the T₅ moment. The operating time, CPB time, aortic clamp time, intraoperative blood loss, postoperative tube time, ICU stay time, hospital stay time and pulmonary infection of patients in group A were significantly less than those in group B. In conclusion, Lund exhaust technology can significantly reduce the fluctuation of hemodynamics, decrease the expression of inflammatory factors, improve lung function, and is conducive to the rehabilitation of patients.

Tunable Elastomers with an Antithrombotic Component for Cardiovascular Applications

This study reports the development of a novel family of biodegradable polyurethanes for use as tissue engineered cardiovascular scaffolds or blood-contacting medical devices. Covalent incorporation of the antiplatelet agent dipyridamole into biodegradable polycaprolactone-based polyurethanes yields biocompatible materials with improved thromboresistance and tunable mechanical strength and elasticity. Altering the ratio of the dipyridamole to the diisocyanate linking unit and the polycaprolactone macromer enables control over both the drug content and the polymer cross-link density. Covalent cross-linking in the materials achieves significant elasticity and a tunable range of elastic moduli similar to that of native cardiovascular tissues. Interestingly, the cross-link density of the polyurethanes is inversely related to the elastic modulus, an effect attributed to decreasing crystallinity in the more cross-linked polymers. In vitro characterization shows that the antiplatelet agent is homogeneously distributed in the materials and is released slowly throughout the polymer degradation process. The drug-containing polyurethanes support endothelial cell and vascular smooth muscle cell proliferation, while demonstrating reduced levels of platelet adhesion and activation, supporting their candidacy as promising substrates for cardiovascular tissue engineering.

A comparison of postoperative morphometric and hemodynamic changes between saphenous vein and left internal mammary artery grafts

There is higher long‐term failure of the saphenous vein graft (SVG) compared with the left internal mammary artery (LIMA) graft, which is affected by the hemodynamic environment. A comprehensive analysis of postoperative structure‐function changes is important to study the atherogenesis in the SVG. A comparison of morphometric and hemodynamic parameters was carried out between LIMA grafts and SVGs and between different time points postoperatively. Various parameters were obtained from the image reconstruction and flow simulation in patients, who underwent CT exams for ~1 year, 5 and 10 years after revascularization. Morphometric data showed a decrease in lumen size in the entire SVG and anastomosis of different patients in a sequence of ~1 year, 5 and 10 years postoperatively despite negligible changes of LIMA size. Computational results indicated the fourfold increased surface area ratio (SAR) of low time‐averaged wall shear stress (TAWSS) in the SVG and anastomosis at postoperative 10 years than that at postoperative ~1 year. The SAR of high TAWSS gradient (TAWSSG) at the distal anastomosis between SVG and coronary arteries was significantly higher (14 ± 9% vs. 6 ± 8%) than that in the LIMA group at postoperative ~1 year. There were strong correlations between morphometric and hemodynamic parameters in the SVG and distal anastomosis at various time points postoperatively, which showed deterioration relevant to persistent diffuse diseases at postoperative ~10 years.

Hyaluronan enhancement of expanded polytetrafluoroethylene cardiovascular grafts

Heart disease continues to be the leading cause of death in the United States. The demand for cardiovascular bypass procedures increases annually. Expanded polytetrafluoroethylene is a popular material for replacement implants, but it does have drawbacks such as high thrombogenicity and low patency, particularly in small diameter grafts. Hyaluronan, a naturally occurring polysaccharide in the human body, is known for its wound healing and anticoagulant properties. In this work, we demonstrate that treating the luminal surface of expanded polytetrafluoroethylene grafts with hyaluronan improves hemocompatibility without notably changing its mechanical properties and without significant cytotoxic effects. Surface characterization such as ATR-FTIR and contact angle goniometry demonstrates that hyaluronan treatment successfully changes the surface chemistry and increases hydrophilicity. Tensile properties such as elastic modulus, tensile strength, yield stress and ultimate strain are unchanged by hyaluronan enhancement. Durability data from flow loop studies demonstrate that hyaluronan is durable on the expanded polytetrafluoroethylene inner lumen. Hemocompatibility tests reveal that hyaluronan-treated expanded polytetrafluoroethylene reduces blood clotting and platelet activation. Together our results indicate that hyaluronan-enhanced expanded polytetrafluoroethylene is a promising candidate material for cardiovascular grafts.

Development of an electrospun biomimetic polyurea scaffold suitable for vascular grafting

The optimization of biomechanical and biochemical properties of a vascular graft to render properties relevant to physiological environments is a major challenge today. These critical properties of a vascular graft not only regulate its stability and integrity, but also control invasion of cells for scaffold remodeling permitting its integration with native tissue. In this work, we have synthesized a biomimetic scaffold by electrospinning a blend of a polyurea, poly(serinol hexamethylene urea) (PSHU), and, a polyester, poly-ε-caprolactone (PCL). Mechanical properties of the scaffold were varied by varying polymer blending ratio and electrospinning flow rate. Mechanical characterization revealed that scaffolds with lower PSHU content relative to PCL content resulted in elasticity close to native mammalian arteries. We also found that increasing electrospinning flow rates also increased the elasticity of the matrix. Optimization of elasticity generated scaffolds that enabled vascular smooth muscle cells (SMCs) to adhere, grow and maintain a SMC phenotype. The 30/70 scaffold also underwent slower degradation than scaffolds with higher PSHU content, thereby, providing the best option for in vivo remodeling. Further, Gly-Arg-Gly-Asp-Ser (RGD) covalently conjugated to the polyurea backbone in 30/70 scaffold resulted in significantly increased clotting times. Reducing surface thrombogenicity by the conjugation of RGD is critical to avoiding intimal hyperplasia. Hence, biomechanical and biochemical properties of a vascular graft can be balanced by optimizing synthesis parameters and constituent components. For these reasons, the optimized RGD-conjugated 30/70 scaffold electrospun at 2.5 or 5 mL/h has great potential as a suitable material for vascular grafting applications. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 278-290, 2018.

Vascularisation for cardiac tissue engineering: the extracellular matrix

Cardiovascular diseases present a major socio-economic burden. One major problem underlying most cardiovascular and congenital heart diseases is the irreversible loss of contractile heart muscle cells, the cardiomyocytes. To reverse damage incurred by myocardial infarction or by surgical correction of cardiac malformations, the loss of cardiac tissue with a thickness of a few millimetres needs to be compensated. A promising approach to this issue is cardiac tissue engineering. In this review we focus on the problem of in vitro vascularisation as implantation of cardiac patches consisting of more than three layers of cardiomyocytes (> 100 μm thick) already results in necrosis. We explain the need for vascularisation and elaborate on the importance to include non-myocytes in order to generate functional vascularised cardiac tissue. We discuss the potential of extracellular matrix molecules in promoting vascularisation and introduce nephronectin as an example of a new promising candidate. Finally, we discuss current biomaterial- based approaches including micropatterning, electrospinning, 3D micro-manufacturing technology and porogens. Collectively, the current literature supports the notion that cardiac tissue engineering is a realistic option for future treatment of paediatric and adult patients with cardiac disease.