A modeling framework for computational simulations of thoracic endovascular aortic repair.

Thoracic endovascular aortic repair (TEVAR) is a minimally invasive treatment for thoracic aortic conditions including aneurysms and is associated with a number of postoperative stent graft related complications. Computational simulations of TEVAR have the potential to predict surgical outcomes and complications preoperatively. When using simulations for stent graft design and prediction of complications in a population, it is difficult to generalize patient-specific TEVAR computational models due to patient variability. This study proposes a novel modeling framework for creating realistic population-based computational models of TEVAR focused on aneurysms that allow for developing various clinically relevant geometric configurations and scenarios that are not easily attainable with limited patient data. The framework includes a methodology for developing population-based thoracic aortic geometries and defining age-dependent aortic tissue material models, as well as detailed steps and boundary conditions for finite element modeling of stent graft deployment during TEVAR. The simulation framework is illustrated for predicting the formation of a bird-beak configuration, a wedge-shaped gap at the proximal end of the deployed stent graft in TEVAR that leads to incomplete seal. A baseline TEVAR simulation model was developed along with three simulations in which the value of aortic curvature, aortic arch angle, or aortic tissue properties varied from the baseline model. Analyzing the length and angle of the bird-beak configuration in each case shows that the bird-beak size is sensitive to different values of the aortic geometry highlighting the importance of using realistic parameter values.

Lumped parameter models for two-ventricle and healthy and failing extracardiac Fontan circulations.

Fontan circulations are surgical strategies to treat infants born with single ventricle physiology. Clinical and mathematical definitions of Fontan failure are lacking, and understanding is needed of parameters indicative of declining physiologies. Our objective is to develop lumped parameter models of two-ventricle and single-ventricle circulations. These models, their mathematical formulations and a proof of existence of periodic solutions are presented. Sensitivity analyses are performed to identify key parameters. Systemic venous and systolic left ventricular compliances and systemic capillary and pulmonary venous resistances are identified as key parameters. Our models serve as a framework to study the differences between two-ventricle and single-ventricle physiologies and healthy and failing Fontan circulations.

The focal mechanical properties of normal and diseased porcine aortic valve tissue measured by a novel microindentation device.

Cells sense and respond to the heterogeneous mechanical properties of their tissue microenvironment, with implications for the development of many diseases, including cancer, fibrosis, and aortic valve disease. Characterization of tissue mechanical heterogeneity on cellular length scales of tens of micrometers is thus important for understanding disease mechanobiology. In this study, we developed a low-cost bench-top microindentation system to readily map focal microscale soft tissue mechanical properties. The device was validated by comparison with atomic force microscopy nanoindentation of polyacrylamide gels. To demonstrate its utility, the device was used to measure the focal microscale elastic moduli of normal and diseased porcine aortic valve leaflet tissue. Consistent with previous studies, the fibrosa layer of intact leaflets was found to be 1.91-fold stiffer than the ventricularis layer, with both layers exhibiting significant heterogeneity in focal elastic moduli. For the first time, the microscale compressive moduli of focal proteoglycan-rich lesions in the fibrosa of early diseased porcine aortic valve leaflets were measured and found to be 2.44-fold softer than those of normal tissue. These data provide new insights into the tissue micromechanical environment in valvular disease and demonstrate the utility of the microindentation device for facile measurement of the focal mechanical properties of soft tissues.

Calibration of an Electrical Analog Model of Liver Hemodynamics in Fontan Patients.

Fontan associated liver disease is a common complication in patients with Fontan circulation, who were born with a single functioning heart ventricle. The hepatic venous pressure gradient (HVPG) is used to assess liver health and is a surrogate measure of the pressure gradient across the entire liver (portal pressure gradient (PPG)). However, it is thought to be inaccurate in Fontan patients. The main objectives of this study were (1) to apply an existing detailed lumped parameter model (LPM) of the liver to Fontan patients using patient-specific clinical data and (2) to determine whether HVPG is a suitable measurement of PPGs in these patients. An existing LPM of the liver blood circulation was applied and tuned to simulate patient-specific liver hemodynamics. Geometries were collected from seven adult Fontan patients and used to evaluate model parameters. The model was solved and tuned using waveform measurements of flows, inlet and outlet pressures. The predicted ratio of portal to hepatic venous pressures is comparable to in vivo measurements. The results confirmed that HVPG is not suitable for Fontan patients, as it would underestimate the portal pressures gradient by a factor of 3 to 4. Our patient-specific liver model provides an estimate of the pressure drop across the liver, which differs from the clinically used metric HVPG. This work represents a first step toward models suitable to assess liver health in Fontan patients and improve its long-term management.

Cardiovascular and abdominal flow alterations in adults with morphologic evidence of liver disease post Fontan palliation.

BACKGROUND: Although morphologic abnormalities in the liver are commonly encountered post Fontan palliation, the relationships between hepatic morphology, vascular flows, and clinical status remain incompletely understood. We therefore aimed to explore flow characteristics in hepatic and intestinal vessels and to examine cardiovascular associations with liver disease. METHODS: This was a retrospective study of adults post Fontan palliation undergoing clinically indicated cardiovascular magnetic resonance imaging (MRI). Patients were included if MRI flow quantification was available for cardiac, hepatic and intestinal vessels; patients were excluded if phase-contrast flow imaging was insufficient for analysis. RESULTS: Thirty patients were studied (median age at MRI 28.5 years [range 19-47]). Eighteen subjects (60%) were classified as having morphologic liver disease according to validated criteria based on available MRI imaging. Abdominal and cardiovascular flows were quantified. Patients with morphologic liver disease had a 41% reduction in superior mesenteric artery (211 ± 124 versus 358 ± 181 mL/min/m2, p = .004), a 36% reduction in hepatic vein (496 ± 247 versus 778 ± 220 mL/min/m2, p = .01), a 31% reduction in portal vein (399 ± 133 versus 580 ± 159 mL/min/m2, p = .004), and an 18% reduction in Fontan pathway flows (1358 ± 429 versus 1651 ± 270 mL/min/m2, p = .04) compared with the remaining population. Adverse cardiovascular events were not associated with morphologic liver disease. CONCLUSION: Morphologic liver disease appears to be associated with flow alterations within the heart, liver and intestine post Fontan palliation. These novel observations suggest that a potential relationship exists between morphologic disease and vascular flows thereby providing further insights into the pathophysiology of liver disease in this high-risk population.

Impact of Insertion Technique and Iliac Artery Anatomy on Fenestrated Endovascular Aneurysm Repair.

Purpose: To develop a mechanically realistic aortoiliac model to evaluate anatomic variables associated with stent-graft rotation and to assess common deployment techniques that may contribute to rotation. Materials and Methods: Idealized aortoiliac geometries were constructed either through direct 3-dimensional (3D) printing (rigid) or through casting with polyvinyl alcohol using 3D-printed molds (flexible). Flexible model bending rigidity was controlled by altering wall thickness. Three flexible patient-specific models were also created based on the preoperative computed tomography angiograms. Zenith infrarenal and fenestrated devices were used in this study. The models were pressurized to 100 mm Hg with normal saline. Deployments were performed under fluoroscopy at 37°C. Rotation was calculated by tracking the change in position of gold markers affixed to the devices. Results: In the rigid idealized models, stent-graft rotation increased with increasing torsion; torsion levels of 1.6, 2.6, and 3.6 mm-1 had mean rotations of 5.2°±0.03°, 11.2°±4.8°, and 27.6°±13.0°, respectively (p<0.001). In the flexible models, the highest rotation (58°±3.0°) was observed in models with high torsion and high rigidity (7.5 mm-1 net torsion and 254 N·m2 flexural rigidity). No rotation was observed in the absence of torsion. Applying torque to the device during insertion significantly increased stent-graft rotation by an average of 28° across all levels of torsion (p<0.01). Multiple device insertions prior to deployment did not change the observed device rotation. The patient-specific models accurately predicted the degree of rotation seen intraoperatively to within 5°. Conclusion: Insertion technique plays an important role in the degree of stent-graft rotation during deployment. Our model suggests that in vivo correction of device orientation can increase the observed rotation and supports the concept of fully removing the device, adjusting the orientation, and subsequently reinserting. Additionally, increasing iliac artery torsion in the presence of increased vessel rigidity results in stent-graft rotation.

Comparison of Qualitative and Quantitative Assessments of Iliac Artery Tortuosity and Calcification.

INTRODUCTION: During endovascular aneurysm repair, the iliac artery typically serves as a conduit for device delivery. The degree of tortuosity and calcification in the iliac artery ultimately determines whether the device can successfully traverse the vessel. These 2 parameters can be assessed using qualitative approaches or calculated using quantitative methods based on the Society for Vascular Surgery (SVS) reporting standards. The objective of this study was to determine whether qualitative methods are sufficient to accurately assess iliac artery tortuosity and calcification by calculating interobserver variability and comparing them to the SVS Reporting Standards. METHODS: Three vascular surgeons reviewed preoperative computed tomography scans for 50 patients who underwent fenestrated endovascular aneurysm repair and qualitatively assessed left and right iliac artery tortuosity and calcification. Iliac artery geometries were segmented from these image sets. Tortuosity index and calcification length ratio were calculated and categorized based on the SVS Reporting Standards. RESULTS: Interobserver variability was calculated for the qualitative assessments using interclass correlation coefficients. For tortuosity index, among the 3 observers, good agreement was found for the left iliac artery and fair agreement was found for the right. For calcification length ratio, excellent agreement was found for both iliac arteries. When compared to the quantitative assessment, the qualitative assessments underpredicted tortuosity in 2.3% of cases, matched the quantitative values in 16.7% of cases, and overpredicted tortuosity in 81.0% of cases. The qualitative assessments underpredicted calcification in 46.3% of cases, matched the quantitative values in 49.3% of cases, and overpredicted calcification in 4.3% of cases. CONCLUSION: Qualitative assessment of iliac artery tortuosity showed fair-to-good interobserver agreement and poor agreement to SVS Reporting Standards. Qualitative assessment of iliac artery calcification showed excellent interobserver agreement and fair agreement to SVS Reporting Standards. These trends should be considered when qualitative reporting methodologies are used.

Impact of fenestrated stent graft misalignment on patient outcomes.

OBJECTIVE: During fenestrated endovascular aneurysm repair (FEVAR), accurate alignment of the fenestration and the target artery is necessary to prevent complications. This study's objective is to determine the incidence of clinical outcomes following fenestration misalignment during FEVAR. METHODS: A single-center, retrospective chart review was performed for all elective FEVARs between January 2008 and April 2015. Data were gathered from patient records and intraoperative imaging. Native vessel angles were calculated using the vessel centerlines. Intraoperative stent graft orientation was determined by changing the angle of the image intensifier as the fenestration was profiled for cannulation. Vertical fenestration misalignment was defined as 4 mm or greater and is the distance from the center of the fenestration markers to the center of the target vessel ostium at the time of cannulation. Horizontal stent graft misalignment was defined as a difference between the native vessel angle and the intraoperative fenestration angle of 15° or more. Early and late clinical outcomes were analyzed with respect to the presence of either vertical or horizontal misalignment of the renal artery fenestrations. RESULTS: The study cohort includes 65 patients who underwent FEVAR during this study period. A horizontal misalignment of 15° or more occurred in 40% of patients (n = 26) and 30° or more in 9.2% of patients (n = 6). A vertical misalignment of 4 mm or greater occurred in 32.3% of patients (n = 21). The incidence of severe postoperative complications, defined as any in-hospital end-organ ischemia and/or death, was significantly greater for patients with stent graft misalignment-31% (n = 11) vs 3% (n = 1) in the aligned group. There was a trend toward higher rates of target vessel cannulation failure in patients with stent graft misalignment 3% (n = 99 fenestrations) vs 0% (n = 76 fenestrations). The combined incidence of any intraoperative target vessel complication was significantly higher in patients with misalignment, 8.1% (4, 15) vs 1.3% (0, 8). Long-term survival was significantly lower in patients with stent graft misalignment, which was primarily driven by high intraoperative and in-hospital mortality rates (P < .05). CONCLUSIONS: Intraoperative stent graft misalignment is associated with higher rates of procedural target vessel complications and severe postoperative adverse events. Patients with stent graft misalignment should be considered at high risk for early postoperative complications. These results highlight an important need for improved FEVAR planning.

Computational fluid dynamic simulations of a cavopulmonary assist device for failing Fontan circulation.

OBJECTIVES: Adult patients who have undergone the Fontan procedure are highly vulnerable to gradual, progressive circulatory failure, and options to reverse this situation are few. A cavopulmonary assist device could decongest the venous and lymphatic systems, overcome elevated pulmonary vascular resistance, increase cardiac output, and support some of these patients to heart transplant. This study characterizes the performance and challenges of a novel multilumen cannula coupled to an external blood pump proposed as a potential Fontan cavopulmonary assist strategy. METHODS: Computational fluid dynamic simulations were conducted for 3 extracardiac Fontan geometries consisting of 1 idealized model and 2 patient-specific models. A range of physiologic flow rates and pump assist levels were simulated to calculate the pressure gain provided by the multilumen cannula. Hemolysis index was estimated for the idealized model with Lagrangian particle tracking and 2 variations of the power-law. Wall shear stresses were also examined. RESULTS: Pressure gains up to 4 and 9 mm Hg were achieved for the idealized and patient-specific models, respectively. Pressure gains increased with both higher cardiac output and larger pump intake through the external pump. Flow-weighted hemolysis show hemoglobin damage levels to be several times lower than the 2% threshold at the highest pump intake flow cases. Wall shear stress predictions depict elevated areas in the pulmonary vessels and regions of the cannula device. CONCLUSIONS: The cannula tested in this study shows promise as a percutaneous option to bridge support in some patients with a failing extracardiac Fontan. Limitations identified will be addressed in future design iterations and in ongoing experimental tests.

Microdevice arrays with strain sensors for 3D mechanical stimulation and monitoring of engineered tissues.

Native and engineered tissue development are regulated by the integrative effects of multiple microenvironmental stimuli. Microfabricated bioreactor array platforms can efficiently dissect cue-response networks, and have recently integrated critical 2D and 3D mechanical stimulation for greater physiological relevance. However, a limitation of these approaches is that assessment of tissue functional properties is typically limited to end-point analyses. Here we report a new deformable membrane platform with integrated strain sensors that enables mechanical stretching or compression of 3D cell-hydrogel arrays and simultaneous measurement of hydrogel construct stiffness in situ. We tested the ability of the integrated strain sensors to measure the evolution of the stiffness of cell-hydrogel constructs for two cases. First, we demonstrated in situ stiffness monitoring of degradable poly (ethylene glycol)-norbornene (PEG-NB) hydrogels embedded with mesenchymal stromal cells (MSCs) and cultured with or without cyclic tensile stimulation for up to 15 days. Whereas statically-cultured hydrogels degraded and softened throughout the culture period, mechanically-stimulated gels initially softened and then recovered their stiffness corresponding to extensive cell network and collagen production. Second, we demonstrated in situ measurement of compressive stiffening of MSC-seeded PEG-NB gels cultured statically under osteogenic conditions, corresponding to increased mineralization and cellularization. This measurement technique can be generalized to other relevant bioreactor and organ-on-a-chip platforms to facilitate online, non-invasive, and high-throughput functional analysis, and to provide insights into the dynamics of engineered tissue development that are otherwise not available.

Predicting Rotation in Fenestrated Endovascular Aneurysm Repair Using Finite Element Analysis.

Fenestrated endovascular aneurysm repair (FEVAR) is a minimally invasive method of abdominal aortic aneurysm (AAA) repair utilized in patients with complex vessel anatomies. Stent grafts (SG) used in this process contain fenestrations within the device that need to be aligned with the visceral arteries upon successful SG deployment. Proper alignment is crucial to maintain blood flow to these arteries and avoid surgical complications. During fenestrated SG deployment, rotation of the SG can occur during the unsheathing process. This leads to misalignment of the vessels, and the fenestrations and is associated with poor clinical outcomes. The aim of this study was to develop a computational model of the FEVAR process to predict SG rotation. Six patient-specific cases are presented and compared with surgical case data. Realistic material properties, frictional effects, deployment methods, and boundary conditions are included in the model. A mean simulation error of 2 deg (range 1-4 deg) was observed. This model was then used to conduct a parameter study of frictional properties to see if rotation could be minimized. This study showed that increasing or decreasing the coefficients of friction (COF) between the sheath and the vessel walls would decrease the amount of rotation observed. Our model accurately predicts the amount of SG rotation observed during FEVAR and can be used as a preoperative planning tool within the surgical workflow.

Analysis of Iliac Artery Geometric Properties in Fenestrated Aortic Stent Graft Rotation.

INTRODUCTION: A complication of fenestrated endovascular aneurysm repair is the potential for stent graft rotation during deployment causing fenestration misalignment and branch artery occlusion. The objective of this study is to demonstrate that this rotation is caused by a buildup of rotational energy as the device is delivered through the iliac arteries and to quantify iliac artery geometric properties associated with device rotation. METHODS: A retrospective clinical study was undertaken in which iliac artery geometric properties were assessed from preoperative imaging for 42 cases divided into 2 groups: 27 in the nonrotation group and 15 in the rotation group. Preoperative computed tomography scans were segmented, and the iliac artery centerlines were determined. Iliac artery tortuosity, curvature, torsion, and diameter were calculated from the centerline and the segmented vessel geometry. RESULTS: The total iliac artery net torsion was found to be higher in the rotation group compared to the nonrotation group (23.5 ± 14.7 vs 14.6 ± 12.8 mm-1; P = .05). No statistically significant differences were found for the mean values of tortuosity, curvature, torsion, or diameter between the 2 groups. CONCLUSION: Stent graft rotation occurred in 36% of the cases considered in this study. Cases with high iliac artery total net torsion were found to be more likely to have stent graft rotation upon deployment. This retrospective study provides a framework for prospectively studying the influence of iliac artery geometric properties on fenestrated stent graft rotation.

Prediction of advanced endovascular stent graft rotation and its associated morbidity and mortality.

OBJECTIVE: Advanced endovascular aneurysm repair (EVAR) with fenestrated and branched stent grafts is increasingly being used to repair complex aortic aneurysms; however, these devices can rotate unpredictably during deployment, leading to device misalignment. The objectives of this study were to quantify the short-term clinical outcomes in patients with intraoperative stent graft rotation and to identify quantitative anatomic markers of the arterial geometry that can predict stent graft rotation preoperatively. METHODS: A prospective study evaluating all patients undergoing advanced EVAR was conducted at two university-affiliated hospitals between November 2015 and December 2016. Stent graft rotation (defined as ≥10 degrees) was measured on intraoperative fluoroscopic video of the deployment sequence. Standard preoperative computed tomography angiography imaging was used to calculate the geometric properties of the arterial anatomy. Any in-hospital and 30-day complications were prospectively documented, and a composite outcome of any end-organ ischemia or death was used as the primary end point. RESULTS: Thirty-nine patients undergoing advanced EVAR were enrolled in the study with a mean age of 75 years (interquartile range [IQR], 71-80 years) and a mean aneurysm diameter of 64 mm (IQR, 59-65 mm). The incidence of stent graft rotation was 37% (n = 14), with a mean rotation of 25 degrees (IQR, 21-28 degrees). A nominal logistic regression model identified iliac artery torsion, volume of iliac artery calcification, and stent graft length as the primary predictive factors. The total net torsion and the total volume of calcific plaque were higher in patients with stent graft rotation, 8.9 ± 0.8 mm-1 vs 4.1 ± 0.5 mm-1 (P < .0001) and 1054 ± 144 mm3 vs 525 ± 83 mm3 (P < .01), respectively. The length of the implanted stent grafts was also higher in patients with intraoperative rotation, 172 ± 9 mm vs 156 ± 8 mm (P < .01). The composite outcome of any end-organ ischemia or death was also substantially higher in patients with stent graft rotation (36% vs 0%; P = .004). In addition, patients with stent graft rotation had significantly higher combined rates of type Ib and type III endoleaks (43% vs 8%; P = .03). CONCLUSIONS: Patients with intraoperative stent graft rotation have a significantly higher rate of severe postoperative complications, and this is strongly associated with higher levels of iliac artery torsion, calcification, and stent graft length. These findings suggest that preoperative quantitative analysis of iliac artery torsion and calcification may improve risk stratification of patients before advanced EVAR.

'In-stock' fenestrated stent graft for the urgent repair of an abdominal aortic aneurysm.

Endovascular aneurysm repair (EVAR) is a minimally invasive method for the treatment of abdominal aortic aneurysms; however, the implementation of this technique is often limited by the aortic pathology, especially in the urgent or emergent setting. An 82-year-old male with a 7.3 cm symptomatic juxtarenal aneurysm presented at our centre for assessment. He was diagnosed as a high-risk candidate for open repair and therefore, not suitable for a conventional EVAR. Fortunately, a custom two-vessel fenestrated stent graft, which was originally constructed for another patient, was available. This device was implanted with no complications and all branches remain unobstructed; clear of aneurysms at 1 year. We present the use of 'in-stock' fenestrated grafts as a potential option to be considered in the urgent or emergent repair of abdominal aortic aneurysms.

Clinical outcomes and material properties of in situ fenestration of endovascular stent grafts.

OBJECTIVE: In situ fenestration of endovascular stent grafts has been used as a method for branch vessel revascularization in urgent and emergent settings. The objective of this manuscript was to review the clinical and experimental evidence related to this technique. METHODS: PubMed, MEDLINE, and Embase databases were searched for papers published until December 2015 describing in situ fenestration of aortic stent grafts. Benchtop, animal, and human studies were included. RESULTS: The literature review identified 118 articles, of which 28 studies were selected for inclusion. These included 16 clinical papers (2 case series and 14 case reports) reporting in situ fenestration of 46 aortic branch vessels in 44 patients. There were 42 retrograde and 4 antegrade instances of in situ fenestration. The most frequent target vessel for in situ fenestration was the left subclavian artery (72%), and the most frequent indication for stent graft implantation was a degenerative aortic aneurysm (43%). Technical success was reported in 44 of 46 attempted fenestrations (96%). The combined rate of perioperative mortality, stroke, and paralysis was 7%. In situ fenestration was predominantly performed with the Talent (Medtronic, Santa Rosa, Calif) stent graft (54%), followed by the Zenith (Cook Medical, Bloomington, Ind) stent graft (37%) and the TAG (W. L. Gore & Associates, Newark, Del) stent graft (9%). In vitro benchtop evaluations of in situ fenestration showed minimal change in fenestration size after 1 year of pulsatile fatigue testing. The use of energy-based fenestration techniques (radiofrequency or laser) has been associated with less fabric fraying than in needle-based techniques. The larger caliber initial fenestration created by these devices also avoids the need for cutting balloons, which have also been linked with increased fabric tears and fraying of the fibers surrounding the fenestration. In addition, the Zenith stent graft was shown in benchtop testing to be the strongest in postfenestration mechanical testing, but it was also the most resistant to balloon dilation. CONCLUSIONS: In the short to moderate term, in situ fenestration appears to be a reasonable and effective method to extend the proximal landing zone for revascularization of the left subclavian artery. However, longer follow-up is needed to fully assess the long-term durability of this procedure. Based on studies of material properties, an energy-based fenestration technique (radiofrequency or laser) is recommended, along with the avoidance of cutting balloons for dilation of the fenestration.

Increasing angulation decreases measured aortic stent graft pullout forces.

OBJECTIVE: Experimentally measured pullout forces for stent grafts (SGs) are used in clinical discussions and as reference values in bench studies and computer simulations. Previous values of these forces are available from studies in which the SG was pulled out in the straight caudal direction. However, clinical and numerical studies have suggested that displacement forces acting on SGs are directed more anteriorly. The objective of this study was to measure pullout forces as a function of angulation and to test the hypothesis that pullout forces decrease with increasing angulation. METHODS: Six different SGs (Bolton Treovance, Cook Zenith Flex, Cook Zenith LP, Medtronic Endurant, Medtronic Talent, and Vascutek Anaconda) were deployed in fresh bovine aortas, then pulled out by an electronic motor at 1 mm/s, while tension force was measured continuously with a digital load cell. The SG off-axis angulation was changed from 0 to 90 degrees in increments of 10 degrees. The test system was submerged in a custom-built temperature-controlled saline bath at 37°C. At least three tests were performed for each device at each angle (with the exception of the Cook Zenith Flex, which experienced plastic deformation of its barbs after a single test per device). Each aortic specimen was used only once and then discarded. Hand-sutured graft anastomoses were also tested at 0 degrees to provide a reference value. RESULTS: A total of 374 pullout tests were performed for the SGs and anastomoses. Sixty-four tests were excluded because of failure of the aorta or apparatus before device pullout. The remaining 310 tests showed pullout forces that demonstrated a decrease in the average pullout force for all six devices from 0 to 90 degrees (Bolton Treovance from 39.3 N to 23.9 N; Cook Zenith Flex from 59.8 N to 48.9 N; Cook Zenith LP from 50.3 N to 41.8 N; Medtronic Endurant from 29.9 N to 25.8 N; Medtronic Talent from 6.0 N to 5.5 N; and Vascutek Anaconda from 37.0 N to 30.3 N). For reference, the mean pullout force for the hand-sutured anastomoses was 63 N. CONCLUSIONS: This study reports for the first time the change in pullout force with angulation, showing a general pullout force decrease with increasing angle. With a larger number of samples than in previous studies, our results provide updated benchmark data that can be used for clinical discussions, computational and experimental studies, and future device design.

Adaptation of a rabbit myocardium material model for use in a canine left ventricle simulation study.

The myocardium of the left ventricle (LV) of the heart comprises layers of muscle fibers whose orientation varies through the heart wall. Because of these fibers, accurate modeling of the myocardium stress-strain behavior requires models that are nonlinear, anisotropic, and time-varying. This article describes the development and testing of a material model of the canine LV myocardium, which will be used in ongoing simulations of the mechanics of the LV with fluid-structure interaction. The model assumes that myocardium deformation has two extreme states: one during which the muscle fibers are fully relaxed, and another during which the muscle fibers are fully contracted. During the second state, the "total" stresses are assumed to be the sum of "passive" stresses, which represent the fully relaxed muscle fibers, and "active" stresses, which are additional stresses due to the contraction of the muscle fibers. The canine LV myocardium is modeled as a transversely isotropic material for which material properties vary in the fiber and cross-fiber directions. The material behavior is considered to be hyperelastic and is modeled by a strain-energy density function in a manner that is an adaptation of an approach based on measurements of the stress-strain behavior of rabbit LV myocardia. A numerical method has been developed to calculate suitable parameter values for the passive material model using previous passive canine LV myocardium stress measurements and taking into account existing physical and numerical constraints. In the absence of published measurements of total canine LV myocardium stresses, a method has been developed to estimate these stresses from available passive and total rabbit LV myocardium stresses and then to calculate active material parameter values. Material parameter values were calculated for passive and active canine LV myocardium. Passive stresses calculated using the model compare well to previous stress measurements while active stresses calculated using the model compare well with those approximated from rabbit measurements. The adapted material model of the canine LV myocardium is deemed to be suitable for use in simulations of the operation of both idealized and realistic canine hearts. The estimated model parameter values can be easily revised to more appropriate ones if measurements of active canine LV myocardium stresses become available. The extension of this material model to a fully orthotropic one is also possible but determination of its parameters would require stress-stretch measurements in the fiber and both cross-fiber directions.

Numerical simulations of blood flow in artificial and natural hearts with fluid-structure interaction.

This article describes two ongoing numerical studies of fluid-structure interaction in the cardiovascular system: an idealized pulsatile ventricular assist device (VAD), consisting of two fluid chambers separated by a flexible diaphragm; and blood flow and heart wall motion during passive filling of a canine heart. Simulations have been performed for the VAD and compared with the results of a previous study and to our own preliminary experimental results. Detailed measurements of the flow field in the VAD model and additional simulations are in progress. Preliminary simulations using both an idealized model of the natural heart as well as a realistic model have identified the limitations of the current numerical methods in dealing with large three-dimensional deformations. Ongoing research aims at extending the range of simulations to include large deformations and to incorporate an anisotropic material model for the heart wall to account for the muscle fibers.