Sam Chitsaz

Comparison of Mechanical Properties of Human Ascending Aorta and Aortic Sinuses

BACKGROUND Computational finite element models of the aortic root have previously used material properties of the ascending aorta to describe both aortic sinuses and ascending aorta. We have previously demonstrated significant material property differences between ascending aorta and sinuses in pigs. However, it is unknown whether these regional material property differences exist in humans. The main objective of this study was to investigate biomechanics of fresh human ascending aorta and aortic sinuses and compare nonlinear material properties of these regions. METHODS Fresh human aortic root specimens obtained from the California Transplant Donor Network (Oakland, CA) were subjected to displacement-controlled equibiaxial stretch testing within 24 hours of harvest. Stress-strain data recorded were used to derive strain energy functions for each region. Tissue behavior was quantified by tissue stiffness and a direct comparison was made between different regions of aortic root at physiologic stress levels. RESULTS All regions demonstrated a nonlinear response to strain during stretch testing in both circumferential and longitudinal directions. No significant difference in tissue stiffness was found between anterior and posterior regions of the ascending aorta or among the three sinuses in both directions. However, our results demonstrated that human ascending aorta is significantly more compliant than aortic sinuses in both circumferential and longitudinal directions within the physiologic stress range. CONCLUSIONS Significant material and structural differences were observed between human ascending aorta and aortic sinuses. Regionally specific material properties should be employed in computational models used to assess treatments of structural aortic root disease.

Biomechanical Properties of Human Ascending Thoracic Aortic Aneurysms

BACKGROUND Surgical management of ascending thoracic aortic aneurysms (aTAAs) relies on maximum diameter, growth rate, and presence of connective tissue disorders. However, dissection and rupture do occur in patients who do not meet criteria for surgical repair. This study investigated the mechanical properties of aTAAs compared with normal human ascending aortas for eventual development of biomechanical aTAA risk models. METHODS aTAA specimens (n = 18) were obtained from patients undergoing surgical aneurysm repair, and fresh, healthy ascending aortas (n = 19) as controls were obtained from the transplant donor network. Biaxial stretch testing was performed to obtain tissue mechanical properties. Patient-specific aTAA physiologic stress was calculated based on preoperative computed tomography diameter. aTAA and ascending aorta tissue stiffness at respective physiologic stress were determined. RESULTS Physiologic stress of aTAA was significantly greater (241.6 ± 59.4 kPa) than the 74 kPa for normal controls. Tissue stiffness of aTAAs was significantly greater than that of the ascending aortas at their respective physiologic stresses in the circumferential (3041.4 ± 1673.7 vs 905.1 ± 358.9 kPa, respectively; p < 0.001) and longitudinal (3498.2 ± 2456.8 vs 915.3 ± 368.9 kPa, respectively; p < 0.001) directions. Tissue stiffness of aTAAs positively correlated with aTAA diameter but did not correlate with patient age. No correlation was found between aTAA physiologic stress level and maximum aTAA diameter. CONCLUSIONS aTAAs are much stiffer than normal ascending aortas at their respective physiologic stress, which was also significantly greater in ATAAs than ascending aortas. Patient-specific physiologic stress did not correlate with maximum aTAA diameter, and patient-specific aTAA wall stress may be a useful variable to predict adverse aTAA events.

Valve-in-Valve Hemodynamics of 20-mm Transcatheter Aortic Valves in Small Bioprostheses

BACKGROUND Transcatheter aortic valve (TAV) implantation is a treatment for selected patients with failing bioprostheses. We previously showed that currently available SAPIEN (Edwards Lifesciences, Irvine, CA) TAV sizes did not yield acceptable valve-in-valve (VIV) hemodynamics in small degenerated bioprostheses because optimal TAV function requires full stent expansion to its nominal size. The study objective was to determine (1) if 20-mm TAVs provide acceptable hemodynamics in small degenerated bioprostheses and (2) the effect of TAV spatial orientation on valvular hemodynamics and coronary flows. METHODS Twelve 20-mm TAVs were created for implantation within 19-mm and 21-mm degenerated Carpentier-Edwards Perimount (Edwards Lifesciences) and porcine bioprostheses. Degenerated valves were sutured into human homograft roots and mounted in a pulse duplicator. TAVs were implanted within bioprostheses as VIV in standard orientation, in which TAV and bioprosthetic commissures were aligned, and later with 60-degree rotation. RESULTS The 20-mm TAVs migrated retrograde into the left ventricle after VIV in the 21-mm Perimount bioprostheses. However, 20-mm TAVs in 19-mm Perimount (54.9±5.4 to 23.5±3.9 mm Hg, p=0.006) and 21-mm porcine bioprostheses (35.2±8.9 to 16.8±4.1 mm Hg, p=0.03) significantly reduced mean gradients. No significant reduction in pressure gradient occurred after VIV in 19-mm degenerated porcine bioprostheses. Mild regurgitation was observed after VIV. VIV with standard and 60-degree TAV orientation did not significantly alter hemodynamics or coronary flows. CONCLUSIONS Valve-in-valve hemodynamics with 20-mm TAV improved for 19-mm Perimount and 21-mm porcine but not 19-mm porcine bioprostheses. No significant differences in hemodynamics were noted by orientation with TAV and bioprosthesis commissural alignment or 60-degree rotation.

Operative Risks and Survival in Veterans With Severe Aortic Stenosis: Surgery Versus Medical Therapy

BACKGROUND Transcatheter aortic valves were developed as an alternative to surgery for the one third to two thirds of patients with severe aortic stenosis who do not undergo aortic valve replacement. In this study, we examined reasons for medical management of aortic stenosis in relation to operative risks and outcomes for veterans with and without valve replacement. METHODS The echocardiography database was screened from 2000 to 2007 for severe aortic stenosis. The Society of Thoracic Surgeons risk scores and survival were determined for patients with and without aortic valve replacement. RESULTS Of 132 severe aortic stenosis patients included, 42% were medically managed. Predicted operative mortality risk was lower for surgical patients than for medical patients (4.5%±4.2% versus 6.8%±5.1%, p=0.002). Overall, the most common reason for medical management of aortic stenosis was assumption that the patient was high risk for surgery (30.4%). The surgery group had significantly higher median survival (92.2 versus 32.4 months) and 5-year survival (71% versus 37%, p<0.001) than the medical group. Cardiac surgery was not consulted in 61% of medically managed patients, of whom only 18% had Society of Thoracic Surgeons risk score of 10 or greater. Aortic valve replacement was an independent predictor of lower mortality (hazard ratio 0.43, p=0.008). CONCLUSIONS Although operative risk was higher among patients who did not undergo surgery, most were not the 10% or greater required for transcatheter valves. Given the significantly lower survival with medical therapy, aortic valve replacement should be carefully considered for most severe aortic stenosis patients whereas transcatheter aortic valves should be reserved for patients with high operative risks.

Late MitraClip Embolization: A New Cause of ST-Segment–Elevation Myocardial Infarction

Percutaneous edge-to-edge mitral valve repair using the MitraClip device (Abbott Vascular) is currently indicated for high-risk patients with symptomatic severe mitral valve regurgitation. The percutaneous approach, although inferior to surgical mitral valve repair, has an excellent safety profile. We report the first case of ST-segment–elevation myocardial infarction caused by late embolization of MitraClip to the right coronary ostium. An 80-year-old gentleman with severe symptomatic mitral regurgitation and multiple cardiac risk factors including coronary artery disease, moderate aortic stenosis, atrial fibrillation, hypertension, and moderately reduced left ventricular systolic function underwent percutaneous mitral valve repair with 3 MitraClips at another medical center. The first clip was deployed in A2-P2 position with a 12 to 6 o’clock orientation, despite having difficulties grasping the posterior leaflet because of significant degree of flail (Movie I in the Data Supplement). Given a moderate residual regurgitation (Movie II in the Data Supplement), a second clip was placed just lateral to the first one. After full release of the second clip, there appeared to be a single leaflet attachment of both clips with one free clip attached to the anterior leaflet and one free clip attached to the posterior leaflet (Movie III in the Data Supplement …

Comprehensive evaluation of culture-negative endocarditis with use of cardiac and 4-dimensional-flow magnetic resonance imaging.

A 42-year-old, otherwise healthy man with exposure to domesticated buffalo presented with a 3-day history of intermittent headaches and word-finding difficulties. Magnetic resonance images (MRI) revealed an ischemic infarct in the left temporal lobe. A high-pitched, intense, pandiastolic murmur was detected at the right upper sternal border. A transthoracic echocardiogram showed moderate-to-severe aortic insufficiency. A transesophageal echocardiogram (TEE) showed a bicuspid aortic valve with possible leaflet perforation. Cardiac magnetic resonance (CMR) images confirmed regurgitation through a perforated bicuspid leaflet, excluded an abscess, and revealed a dilated ascending thoracic aorta (Fig. 1). The eccentric regurgitant jet and asymmetric aortic dilation were best seen by means of time-resolved, 3-dimensional, phase-contrast MRI (4-dimensional [4D]-flow MRI), with use of instantaneous streamline and dynamic pathline views of blood flow (Fig. 2). The patients blood cultures remained negative; however, serologic studies subsequently were positive for Coxiella burnetii, suggesting a chronic, active infection. The patient was treated with doxycycline and hydroxychloroquine and underwent successful surgical aortic root and valve replacement (Fig. 3). Fig. 1. Cardiac magnetic resonance image shows the dilated ascending aorta, and significant aortic regurgitation (arrow) through the perforated bicuspid aortic valve leaflet. Fig. 2. Four-dimensional-flow magnetic resonance imaging of the thoracic aorta. Streamlines are color-coded for velocity and reveal instantaneous flow across the aortic valve (arrow) and in the aorta during A) systole and B) diastole. Fig. 3. Photograph shows surgical specimen of the infected, bicuspid aortic valve with leaflet perforation (arrow).

Material properties of CorCap passive cardiac support device.

BACKGROUND Myocardial function deteriorates during ventricular remodeling in patients with congestive heart failure (HF). Ventricular restraint therapy using a cardiac support device (CSD) is designed to reduce the amount of stress inside the dilated ventricles, which in turn halts remodeling. However, as an open mesh surrounding the heart, it is unknown what the mechanical properties of the CSD are in different fiber orientations. METHODS Composite specimens of CorCap (Acorn Cardiovascular, Inc, St. Paul, MN) CSD fabric and silicone were constructed in different fiber orientations and tested on a custom-built biaxial stretcher. Silicone controls were made and stretched to detect the parameters of the matrix. CSD coefficients were calculated using the composite and silicone matrix stress-strain data. Stiffness in different fiber orientations was determined. RESULTS Silicone specimens exerted a linear behavior, with stiffness of 2.57 MPa. For the composites with 1 fiber set aligned with respect to the stretch axes, stiffness in the direction of the aligned fiber set was higher than that in the cross-fiber direction (14.39 MPa versus 5.66 MPa), indicating greater compliance in the cross-fiber direction. When the orientation of the fiber sets in the composite were matched to the expected clinical orientation of the implanted CorCap, the stiffness in the circumferential axis (with respect to the heart) was greater than in the longitudinal axis (10.55 MPa versus 9.70 MPa). CONCLUSIONS The mechanical properties of the CorCap demonstrate directionality with greater stiffness circumferentially than longitudinally. Implantation of the CorCap clinically should take into account the directionality of the biomechanics to optimize ventricular restraint.

PATIENT-PROSTHESIS MISMATCH IN THE ERA OF TRANSCATHETER AORTIC VALVE REPLACEMENT: IS THIS A REAL PROBLEM?

Body surface area-indexed effective orifice area (EOAi) measured by transthoracic echocardiography has been recommended for assessment of patient-prosthesis mismatch (PPM) post aortic valve replacement (AVR). Recent studies have shown a high prevalence of PPM in both post surgical and transcatheter