Autograft remodeling after the Ross procedure by cardiovascular magnetic resonance imaging: Aortic stenosis versus insufficiency.

BACKGROUND: Studies suggest that patients undergoing the Ross procedure for aortic insufficiency are at greater risk of autograft dilatation than those with aortic stenosis. By using a tailored Ross technique to mitigate autograft dilatation in patients with aortic insufficiency, we aimed to compare the biomechanical and morphologic remodeling of the autograft at 1 year between patients with aortic insufficiency and patients with aortic stenosis. METHODS: A total of 210 patients underwent a Ross procedure (2011-2016). Of those, 86 patients (mean age 43 ± 13 years; 32% were female) completed preoperative and postoperative cardiovascular magnetic resonance imaging. A total of 71 studies were suitable for analysis: 41 patients with aortic stenosis and 30 patients with aortic insufficiency. Nine healthy adults were used as controls. Autograft root dimensions, individual sinus volumes, and distensibility were measured using cardiovascular magnetic resonance. RESULTS: At 1 year, there was no difference in autograft root dimensions between patients with aortic stenosis (mean annulus 25.1 ± 3.1 mm and sinus diameters 35 ± 4.1 mm) and patients with aortic insufficiency (26.6 ± 3 mm and 37.1 ± 3.5 mm; P = .12 and .06, respectively). Relative sinus of Valsalva volumes were symmetrical in the aortic stenosis (right 34.8% ± 4%, left 33.7% ± 3.5%, noncoronary 31.4% ± 3.2%) and aortic insufficiency groups (34.8% ± 3.9%, 33.8% ± 2.8%, 31.3% ± 3.7%, P = .85, .92, and .82), and similar to those of healthy adults. Aortic root distensibility was reduced in both groups compared with healthy adults (P = .003), but was similar between aortic stenosis (3.12 ± 1.58 × 10-3 mm Hg-1) and aortic insufficiency (3.04 ± 1.15 × 10-3 mm Hg-1; P = .9). CONCLUSIONS: Using a tailored technique, there were no differences in the morphologic or biomechanical remodeling of the autograft root 1 year after the Ross procedure between patients with aortic stenosis and patients with aortic insufficiency. However, autograft roots are stiffer than native aortic roots.

Wall stresses of early remodeled pulmonary autografts.

OBJECTIVE: The Ross procedure is an excellent option for children or young adults who need aortic valve replacement because it can restore survival to that of the normal aged-matched population. However, autograft remodeling can lead to aneurysmal formation and reoperation, and the biomechanics of this process is unknown. This study investigated postoperative autograft remodeling after the Ross procedure by examining patient-specific autograft wall stresses. METHODS: Patients who have undergone the Ross procedure who had intraoperative pulmonary root and aortic specimens collected were recruited. Patient-specific models (n = 16) were developed using patient-specific material property and their corresponding geometry from cine magnetic resonance imaging at 1-year follow-up. Autograft ± Dacron for aneurysm repair and ascending aortic geometries were reconstructed to develop patient-specific finite element models, which incorporated material properties and wall thickness experimentally measured from biaxial stretching. A multiplicative approach was used to account for prestress geometry from in vivo magnetic resonance imaging. Pressure loading to systemic pressure (120/80) was performed using LS-DYNA software (LSTC Inc, Livermore, Calif). RESULTS: At systole, first principal stresses were 809 kPa (25%-75% interquartile range, 691-1219 kPa), 567 kPa (485-675 kPa), 637 kPa (555-755 kPa), and 382 kPa (334-413 kPa) at the autograft sinotubular junction, sinuses, annulus, and ascending aorta, respectively. Second principal stresses were 360 kPa (310-426 kPa), 355 kPa (320-394 kPa), 272 kPa (252-319 kPa), and 184 kPa (147-222 kPa) at the autograft sinotubular junction, sinuses, annulus, and ascending aorta, respectively. Mean autograft diameters were 29.9 ± 2.7 mm, 38.3 ± 5.3 mm, and 26.6 ± 4.0 mm at the sinotubular junction, sinuses, and annulus, respectively. CONCLUSIONS: Peak first principal stresses were mainly located at the sinotubular junction, particularly when Dacron reinforcement was used. Patient-specific simulations lay the foundation for predicting autograft dilatation in the future after understanding biomechanical behavior during long-term follow-up.

3D printed ascending aortic simulators with physiological fidelity for surgical simulation.

Introduction: Three-dimensional (3D) printed multimaterial ascending aortic simulators were created to evaluate the ability of polyjet technology to replicate the distensibility of human aortic tissue when perfused at physiological pressures. Methods: Simulators were developed by computer-aided design and 3D printed with a Connex3 Objet500 printer. Two geometries were compared (straight tube and idealised aortic aneurysm) with two different material variants (TangoPlus pure elastic and TangoPlus with VeroWhite embedded fibres). Under physiological pressure, ß Stiffness Index was calculated comparing stiffness between our simulators and human ascending aortas. The simulators' material properties were verified by tensile testing to measure the stiffness and energy loss of the printed geometries and composition. Results: The simulators' geometry had no effect on measured ß Stiffness Index (p>0.05); however, ß Stiffness Index increased significantly in both geometries with the addition of embedded fibres (p<0.001). The simulators with rigid embedded fibres were significantly stiffer than average patient values (41.8±17.0, p<0.001); however, exhibited values that overlapped with the top quartile range of human tissue data suggesting embedding fibres can help replicate pathological human aortic tissue. Biaxial tensile testing showed that fiber-embedded models had significantly higher stiffness and energy loss as compared with models with only elastic material for both tubular and aneurysmal geometries (stiffness: p<0.001; energy loss: p<0.001). The geometry of the aortic simulator did not statistically affect the tensile tested stiffness or energy loss (stiffness: p=0.221; energy loss: p=0.713). Conclusion: We developed dynamic ultrasound-compatible aortic simulators capable of reproducing distensibility of real aortas under physiological pressures. Using 3D printed composites, we are able to tune the stiffness of our simulators which allows us to better represent the stiffness variation seen in human tissue. These models are a step towards achieving better simulator fidelity and have the potential to be effective tools for surgical training.

Distinct Expression of Nonmuscle Myosin IIB in Pulmonary Arteries of Patients With Aortic Stenosis vs Insufficiency Undergoing a Ross Procedure.

BACKGROUND: Clinical studies have revealed a greater risk of pulmonary autograft dilation after the Ross procedure in patients with preoperative aortic insufficiency (AI). The present study examined whether the morphologic, biomechanical, and cellular properties of the pulmonary artery (PA) from patients with AI were phenotypically different compared with patients diagnosed with aortic stenosis (AS). METHODS: PA segments were harvested from patients undergoing the Ross procedure for AS (n = 16) and AI (n = 6). Preoperative aortic annulus was significantly larger (P < 0.05) in patients with AI (28.5 ± 1.8 mm) vs AS (22.8 ± 1.2 mm). Morphologic, biomechanical, and cellular phenotypes of the PA were analyzed. RESULTS: Collagen and elastin content in the media of the PA wall were similar in patients with AS and AI. Elastic modulus and energy loss of the PA were not significantly different between the groups. In the media of the PA, expression of a panel of vascular smooth muscle cell-specific proteins were similar in patients with AS and AI. In contrast, nonmuscle myosin IIB protein levels in the PA of AS patients were significantly higher compared with AI patients, and immunofluorescence identified staining in α-smooth muscle actin-positive vascular smooth muscle cells. CONCLUSIONS: Despite similar morphological and biomechanical properties, the disparate expression of nonmuscle myosin IIB protein distinguishes the PA of patients with AI from patients with AS. The biological role in vascular smooth muscle cells and the potential contribution of nonmuscle myosin IIB to pulmonary autograft dilation in a subset of AI patients after the Ross procedure remain to be determined.

Increased MMP activity in curved geometries disrupts the endothelial cell glycocalyx creating a proinflammatory environment.

Wall shear stress gradients (WSSGs) induce an inflammatory phenotype in endothelial cells (ECs) which is hypothesized to be mediated by mechanotransduction through the EC glycocalyx (GCX). We used a three-dimensional in vitro cell culture model with a 180o curved geometry to investigate if WSSGs created by curvature can cause EC inflammation and disruption of the GCX. The hydrodynamics of the model elicited a morphological response in ECs as well as a pattern of leukocyte adhesion towards the inner wall of curvature that was attenuated with enzymatic removal of GCX components. GCX degradation was also observed in regions of curvature which corresponded to increased activity of MMPs. Together, these results support the hypothesis that the EC GCX is involved in mechanotransduction of WSSGs and that components of the GCX are regulated by MMP activity in regions of curvature.

Transesophageal echocardiographic strain imaging predicts aortic biomechanics: Beyond diameter.

BACKGROUND: Clinical guidelines recommend resection of ascending aortic aneurysms at diameters 5.5 cm or greater to prevent rupture or dissection. However, approximately 40% of all ascending aortic dissections occur below this threshold. We propose new transesophageal echocardiography strain-imaging moduli coupled with blood pressure measurements to predict aortic dysfunction below the surgical threshold. METHODS: A total of 21 patients undergoing aortic resection were recruited to participate in this study. Transesophageal echocardiography imaging of the aortic short-axis and invasive radial blood pressure traces were taken for 3 cardiac cycles. By using EchoPAC (GE Healthcare, Madison, Wis) and postprocessing in MATLAB (MathWorks, Natick, Mass), circumferential stretch profiles were generated and combined with the blood pressure traces. From these data, 2 in vivo stiffness moduli were calculated: the Cardiac Cycle Pressure Modulus and Cardiac Cycle Stress Modulus. From the resected aortic ring, testing squares were isolated for ex vivo mechanical analysis and histopathology. Each square underwent equibiaxial tensile testing to generate stress-stretch profiles for each patient. Two ex vivo indices were calculated from these profiles (energy loss and incremental stiffness) for comparison with the Cardiac Cycle Pressure Modulus and Cardiac Cycle Stress Modulus. RESULTS: The echo-derived stiffness moduli demonstrate positive significant covariance with ex vivo tensile biomechanical indices: energy loss (vs Cardiac Cycle Pressure Modulus: R2 = 0.5873, P < .0001; vs Cardiac Cycle Stress Modulus: R2 = 0.6401, P < .0001) and apparent stiffness (vs Cardiac Cycle Pressure Modulus: R2 = 0.2079, P = .0378; vs Cardiac Cycle Stress Modulus: R2 = 0.3575, P = .0042). Likewise, these transesophageal echocardiography-derived moduli are highly predictive of the histopathologic composition of collagen and elastin (collagen/elastin ratio vs Cardiac Cycle Pressure Modulus: R2 = 0.6165, P < .0001; vs Cardiac Cycle Stress Modulus: R2 = 0.6037, P < .0001). CONCLUSIONS: Transesophageal echocardiography-derived stiffness moduli correlate strongly with aortic wall biomechanics and histopathology, which demonstrates the added benefit of using simple echocardiography-derived biomechanics to stratify patient populations.

Histopathological and biomechanical properties of the aortic wall in 2 patients with chronic type A aortic dissection.

Type A aortic dissection is an acute condition that requires urgent surgical intervention. However, in a subset of patients, aortic dissections go undiagnosed and become chronic, thereby allowing the dissected wall to undergo a distinct remodeling process from that of the surrounding intact wall. Here, we observe the biomechanical and histological changes in the aortic wall of two patients with chronic Type A aortic dissection. Partial or complete disruption of the elastic structure of the medial layer was observed in the dissected wall of both patients; however, aortic stiffness in the region of dissection covaried with a change in collagen content. A ~50% increase in viscous energy loss was observed in the region of dissection of both patients which suggests an impaired elastic recoil and Windkessel function of the proximal aorta. MMP expression (2 and 9) differed between the dissected and intact wall and was distinct between the two patients. Our observations suggest that an active remodeling process occurs in the dissected aortic wall resulting in a vastly different biomechanical behavior.

Obtaining the biomechanical behavior of ascending aortic aneurysm via the use of novel speckle tracking echocardiography.

INTRODUCTION: Ex vivo measurement of ascending aortic biomechanical properties may help understand the risk for rupture or dissection of dilated ascending aortas. A validated in vivo method that can predict aortic biomechanics does not exist. Speckle tracking transesophageal echocardiography (TEE) has been used to measure ventricular stiffness; we sought to determine whether speckle TEE could be adapted to estimate aortic stiffness in vivo and compare these findings with those obtained by ex vivo tissue measurements. METHODS: A total of 17 patients undergoing ascending aortic resection were recruited to with a mean aortic diameter was 56.16 ± 15 mm. Intraoperative speckle TEE tracking analysis was used to calculate aortic stiffness index using the following equation: ß2=ln(SBP/DBP)/AoS, where ß2 is the stiffness index; SBP is systolic blood pressure; DBP is diastolic blood pressure; and AoS is the circumferential strain. Ex vivo stiffness was obtained by mechanical tissue testing according to previously described methods. The aortic ring at the pulmonary trunk was divided into 4 equal quadrants. RESULTS: The in vivo stiffness index for the inner curvature, anterior wall, outer curvature, and posterior wall were 0.0544 ± 0.0490, 0.0295 ± 0.0199, 0.0411 ± 0.0328, and 0.0502 ± 0.0320, respectively. The mean ex vivo 25% apparent stiffness for inner curvature, anterior wall, outer curvature, and posterior wall were 0.0616 ± 0.0758 MPa, 0.0352 ± 0.00992 MPa, 0.0405 ± 0.0199 MPa, and 0.0327 ± 0.0106 MPa, respectively. The patient-matched ex vivo 25% apparent stiffness and in vivo stiffness index were not significantly different (P = .8617, 2-way analysis of variance with repeated measures). CONCLUSIONS: The use of speckle TEE appears to be a promising technique to estimate ex vivo mechanical properties of the ascending aortic tissue.

Evaluating ascending aortic aneurysm tissue toughness: Dependence on collagen and elastin contents.

Ascending thoracic aortic aneurysms (ATAAs) can lead to a dissection or rupture of the aorta, causing death or disability of the patients. Surgical interventions used to treat this disease are associated with risks of mortality and morbidity. Several studies have investigated the rupture mechanisms of ATAAs; however, underlying reasons behind aortic rupture (failure) have not been fully elucidated and further investigations are necessary. The rupture of pathological aortic tissue is a local phenomenon resulting from defects or tears in the vessel wall. In this work, the toughness-based rupture properties of ATAAs have been examined. The toughness, biaxial tensile properties, and histological properties of aneurysmal and control human ascending thoracic aortas (ATAs) were characterized from four quadrants of surgically excised aortic rings. The aneurysmal tissue population included aortas from patients with bicuspid aortic valves (BAV) and tricuspid aortic valves (TAV). The toughness, incremental modulus, and thickness properties of the aortas were determined and compared regionally. Additionally, to further explore the rupture propensity of ATAAs, the inter-correlation of the toughness properties with histological characteristics have been explored. We found no correlation between toughness and incremental modulus. However, toughness decreased significantly with the amount of collagen. In the outer curvature, there was an increase in incremental modulus with collagen+elastin content, but a decrease in toughness. These results suggest tissue remodeling could affect toughness and stiffness differently in ascending aortic aneurysms.

The Ross procedure: biomechanical properties of the pulmonary artery according to aortic valve phenotype.

OBJECTIVES: The aim of this study is to determine whether patients undergoing the Ross procedure with bicuspid aortic valves have pulmonary artery biomechanical properties different from those with tricuspid valves. METHODS: Thirty-two pulmonary arteries and 20 aortas were obtained from patients undergoing the Ross procedure at the time of surgery, from a cohort of 32 patients. The aortic valve was tricuspid in 5 patients (16%), bicuspid in 18 patients (56%) and unicuspid in 9 patients (28%). Histological analysis and ex vivo equi-biaxial tensile testing completed within 8 hours of surgery were used to evaluate differences in patient groups and between the pulmonary artery and the ascending aorta. RESULTS: There was no difference in thickness among pulmonary arteries when compared according to aortic valve phenotype (P = 0.94). There was no difference in the tensile tissue properties among aortas and pulmonary arteries when compared according to aortic valve phenotype, in either the circumferential or longitudinal axis. When compared according to the main surgical indication, pulmonary artery walls from patients with pure aortic regurgitation were less stiff than their counterparts (aortic regurgitation: 0.055 ± 0.037 MPa, aortic stenosis: 0.103 ± 0.051 MPa, mixed disease: 0.110 ± 0.044 MPa and aortic valve endocarditis: 0.216 ± 0.033 MPa, P = 0.002). There was no difference in the number of elastic lamellae in pulmonary artery specimens from the three different aortic valve phenotypes, as well as in the aortic specimens. CONCLUSIONS: No significant differences were observed in the biomechanical properties of pulmonary arteries when compared according to aortic valve phenotype.

Biomechanics of the Ascending Thoracic Aorta: A Clinical Perspective on Engineering Data.

Aneurysms of the ascending thoracic aorta often require prophylactic surgical intervention to resect and replace the aortic wall with a synthetic graft to avoid the risk of dissection or rupture. The main criterion for surgical intervention is the size of the aneurysm, with elective surgery recommended with a maximal aortic diameter of 4.2-5.5 cm depending on valve type and other patient risk factors. Although the risk of dissection and rupture increases with the size of aneurysm, different pathologies, including aortic valve phenotype and connective tissue disorders uniquely influence the mechanical dysfunction of the aortic wall. Dissection and rupture are mechanical modes of failure caused by an inability of the tissue to withstand local tissue stresses. Tensile testing of aortic tissues, therefore, has been used to reveal the mechanical parameters of diseased and healthy tissues to better characterize the mechanical function of aortic tissues in different patient groups. In this review, we highlight the principles and methods of ex vivo tensile analysis as well as the composition and structural properties that contribute to the mechanical behaviour of the ascending aorta. We also present a clinically oriented description of mechanical testing along with insight into the characterization of aneurysm. Finally, we highlight recent advances in echocardiography, computer tomographic angiography, and magnetic resonance angiography that have the potential to measure biomechanical properties noninvasively and therefore help select aortas at risk.

Investigation on the Regional Loss Factor and Its Anisotropy for Aortic Aneurysms.

An aortic aneurysm is a lethal arterial disease that mainly occurs in the thoracic and abdominal regions of the aorta. Thoracic aortic aneurysms are prevalent in the root/ascending parts of the aorta and can lead to aortic rupture resulting in the sudden death of patients. Understanding the biomechanical and histopathological changes associated with ascending thoracic aortic aneurysms (ATAAs), this study investigates the mechanical properties of the aorta during strip-biaxial tensile cycles. The loss factor-defined as the ratio of dissipated energy to the energy absorbed during a tensile cycle-the incremental modulus, and their anisotropy indexes were compared with the media fiber compositions for aneurysmal (n = 26) and control (n = 4) human ascending aortas. The aneurysmal aortas were categorized into the aortas with bicuspid aortic valves (BAV) and tricuspid aortic valves (TAV). The strip-biaxial loss factor correlates well with the diameter of the aortas with BAV and TAV (for the axial direction, respectively, R² = 0.71, p = 0.0022 and R² = 0.54, p = 0.0096). The loss factor increases significantly with patients' age in the BAV group (for the axial direction: R² = 0.45, p = 0.0164). The loss factor is isotropic for all TAV quadrants, whereas it is on average only isotropic in the anterior and outer curvature regions of the BAV group. The results suggest that loss factor may be a useful surrogate measure to describe the histopathology of aneurysmal tissue and to demonstrate the differences between ATAAs with the BAV and TAV.

Efficient processing of procathepsin K to the mature form.

The proteolysis of collagen fibrils by cathepsin K is a hallmark of bone catabolism and tissue degeneration. The production of active recombinant cathepsin K is central for our ability to study the mechanisms by which these processes occur. Here we report an efficient processing method for the preparation of recombinant cathepsin K expressed in Pichia pastoris. Methanol precipitation of crude media and autoactivation in the absence of a reducing agent allows for the reversible inhibition of the enzyme prior to subsequent purification steps. The resultant purified enzyme is both resistant to autolysis and effective at cleaving collagen.

Laminar shear stress prevents simvastatin-induced adhesion molecule expression in cytokine activated endothelial cells.

In addition to lowering cholesterol, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, or statins, have been shown to modulate gene expression in endothelial cells. The effect of statins on cell adhesion molecule expression is unclear and largely unexplored in endothelial cells exposed to shear stress, an important regulator of endothelial cell function. In this study, the effect of simvastatin on vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) expression was evaluated in human abdominal aortic endothelial cells (HAAEC) conditioned with various levels of laminar wall shear stress with or without tumor necrosis factor alpha (TNFα). As expected, TNFα alone greatly enhanced both VCAM-1 and ICAM-1 mRNA and protein. In static culture, simvastatin potentiated the TNFα-induced increase in VCAM-1 and ICAM-1 mRNA but not total protein at 24 h. Mevalonate, a precursor to cholesterol biosynthesis, eliminated the effect of simvastatin. Exposure of endothelial cells to elevated levels of laminar shear stress during simvastatin treatment prevented the potentiating effect of simvastatin on cell adhesion molecule mRNA. A shear stress of 12.5 dyn/cm² eliminated the increase in VCAM-1 by simvastatin, while 25 dyn/cm² was needed for ICAM-1. We conclude that simvastatin enhances VCAM-1 and ICAM-1 gene expression in TNFα-activated endothelial cells through inhibition of HMG-CoA reductase. High levels of laminar shear stress prevented the upregulation of VCAM-1 and ICAM-1 by simvastatin suggesting that an induction of cell adhesion molecules by statins may not occur in endothelial cells exposed to shear stress from blood flow.