Objectives

The authors sought to measure the turbulent kinetic energy (TKE) in the ascending aorta of patients with aortic stenosis and to assess its relationship to irreversible pressure loss.

Background

Irreversible pressure loss caused by energy dissipation in post-stenotic flow is an important determinant of the hemodynamic significance of aortic stenosis. The simplified Bernoulli equation used to estimate pressure gradients often misclassifies the ventricular overload caused by aortic stenosis. The current gold standard for estimation of irreversible pressure loss is catheterization, but this method is rarely used due to its invasiveness. Post-stenotic pressure loss is largely caused by dissipation of turbulent kinetic energy into heat. Recent developments in magnetic resonance flow imaging permit noninvasive estimation of TKE.

Methods

The study was approved by the local ethics review board and all subjects gave written informed consent. Three-dimensional cine magnetic resonance flow imaging was used to measure TKE in 18 subjects (4 normal volunteers, 14 patients with aortic stenosis with and without dilation). For each subject, the peak total TKE in the ascending aorta was compared with a pressure loss index. The pressure loss index was based on a previously validated theory relating pressure loss to measures obtainable by echocardiography.

Results

The total TKE did not appear to be related to global flow patterns visualized based on magnetic resonance-measured velocity fields. The TKE was significantly higher in patients with aortic stenosis than in normal volunteers (p < 0.001). The peak total TKE in the ascending aorta was strongly correlated to index pressure loss (R(2) = 0.91).

Conclusions

Peak total TKE in the ascending aorta correlated strongly with irreversible pressure loss estimated by a well-established method. Direct measurement of TKE by magnetic resonance flow imaging may, with further validation, be used to estimate irreversible pressure loss in aortic stenosis.

Background

Equipoise of transcatheter aortic valve replacement with surgical aortic valve replacement in intermediate-risk patients has been demonstrated. As transcatheter aortic valve replacement usage expands, questions regarding long-term durability become paramount. Valve design impacts durability with regions of increased leaflet stress being vulnerable to early failure. However, transcatheter aortic valve (TAV) leaflet stresses are unknown. The objective of this study was to determine stent and leaflet stresses of second-generation balloon-expandable TAV.

Methods

Commercial 29-mm Edwards Sapien XT (Edwards Lifesciences, Irvine, CA) valves underwent high-resolution microcomputed tomography scanning to develop precise three-dimensional geometric mesh. Compressed and uncompressed TAVs were modeled under systemic pressure using finite element software. Material properties of stent were based on cobalt-chromium, whereas those for leaflets were obtained from surgical bioprostheses.

Results

Maximum and minimum principal stresses on uncompressed Sapien XT TAV were 1.63 MPa and -0.36 MPa on leaflets and 93.3 MPa and -105.6 MPa on stent at diastolic pressure. Peak leaflet stress was observed at commissural tips where leaflets connected to the stent. For compressed TAV to 26 mm, maximum and minimum principal stresses were 1.55 MPa and -0.63 MPa on leaflets and 526.1 MPa and -902.2 MPa on stent at diastolic pressure. Peak leaflet stress was located at similar position and also along the suture line with the Dacron (C. R. Bard, Haverhill, PA).

Conclusions

Stress analysis of two extreme deployed geometries of 29-mm Edwards Sapien XT using exact geometry from high-resolution scans demonstrated that peak stresses for TAV leaflets were present at commissural tips where leaflets were attached. These regions would be mostly likely to initiate degeneration.

Background

Flow displacement quantifies eccentric flow, a potential risk factor for aneurysms in the ascending aorta, but only at a single anatomic location. The aim of this study is to extend flow displacement analysis to 3D in patients with aortic and aortic valve pathologies.

Methods

43 individuals were studied with 4DFlow MRI in 6 groups: healthy, tricuspid aortic valve (TAV) with aortic stenosis (AS) but no dilatation, TAV with dilatation but no AS, and TAV with both AS and dilatation, BAV without AS or dilatation, BAV without AS but with dilation. The protocol was approved by our institutional review board, and informed consent was obtained. Flow displacement was calculated for multiple planes along the ascending aorta, and 2D and 3D analyses were compared.

Results

Good correlation was found between 2D flow displacement and both maximum and average 3D values (r>0.8). Healthy controls had significantly lower flow displacement values with all approaches (p<0.05). The highest flow displacement was seen with stenotic TAV and aortic dilation (0.24±0.02 with maximum flow displacement). The 2D approach underestimated the maximum flow displacement by more than 20% in 13 out of 36 patients (36%).

Conclusions

The extended 3D flow displacement analysis offers a more comprehensive quantitative evaluation of abnormal systolic flow in the ascending aorta than 2D analysis. Differences between patient subgroups are better demonstrated, and maximum flow displacement is more reliably assessed.

Objective

Transcatheter aortic valve replacement is established therapy for high-risk and inoperable patients with severe aortic stenosis, but questions remain regarding long-term durability. Valve design influences durability. Increased leaflet stresses in surgical bioprostheses have been correlated with degeneration; however, transcatheter valve leaflet stresses are unknown. From 2007 to 2014, a majority of US patients received first-generation balloon-expandable transcatheter valves. Our goal was to determine stent and leaflet stresses in this valve design using finite element analyses.

Methods

A 26-mm Sapien Transcatheter Heart Valve (Edwards Lifesciences, Inc, Irvine, Calif) underwent high-resolution microcomputed tomography scanning to develop precise 3-dimensional geometry of the leaflets, the stent, and the polyethylene terephthalate elements. The stent was modeled using 3-dimensional elements and the leaflets were modeled using shell elements. Stent material properties were based on stainless steel, whereas those for leaflets were obtained from surgical bioprostheses. Noncylindrical Sapien valve geometry was also simulated. Pressure loading to 80 mm Hg and 120 mm Hg was performed using ABAQUS finite element software (Dassault Systèmes, Waltham, Mass).

Results

At 80 mm Hg, maximum principal stresses on Sapien leaflets were 1.31 megaspascals (MPa). Peak leaflet stress was observed at commissural tips where leaflets connected to the stent. Maximum principal stresses for the stent were 188.91 MPa and located at stent tips where leaflet commissures were attached. Noncylindrical geometry increased peak principal leaflet stresses by 16%.

Conclusions

Using exact geometry from high-resolution scans, the 26-mm Sapien Transcatheter Heart Valve showed that peak stresses for both stent and leaflets were present at commissural tips where leaflets were attached. These regions would be prone to leaflet degeneration. Understanding stresses in first-generation transcatheter valves allows comparison to future designs for relative durability.

Objectives

Transcatheter aortic valve replacement (TAVR) is established therapy for severe aortic stenosis patients with intermediate-, high- and prohibitive-risk for surgery. A significant challenge when expanding TAVR to low-risk and younger patients is the unknown long-term durability. High leaflet stresses have been associated with surgical bioprosthetic valve degeneration. In this study, we examined the impact of changes in valve design across 3 generations of same-sized TAVR devices on stent and leaflet stresses.

Methods

The 26-mm Edwards SAPIEN, 23, 26 and 29 mm SAPIEN XT (XT) and 26 mm SAPIEN 3 (S3) (n = 1 each) underwent micro-computed tomography (micro-CT) scanning. Dynamic finite element computational simulations of 23-26 mm SAPIEN, 23-29 mm XT and 23-29 mm S3 were performed with physiological loading and micro-CT or scaled geometries.

Results

Peak stresses were concentrated in the commissure area and along the bottom of the suture, representing areas most likely to develop structural valve degeneration across TAVR generations. Latest-generation S3 showed greatest 99th percentile principal stress on commissural leaflets for 26 and 29 mm, and increased stresses over XT for 23 mm. Percentage of higher stress areas within the leaflets steadily increased across generations, 3.8%, 3.9% and 5.7%, respectively, for 26 mm SAPIEN, XT and S3 with similar trend for 29-mm valves.

Conclusions

Using computational simulations based on high-fidelity modelling of balloon-expandable TAVRs, our study demonstrated that maximum stress areas existed in similar leaflet locations across SAPIEN generations, while the latest model S3 had the highest magnitude for both 26- and 29-mm valves. S3 also had the largest area of higher stresses than other generations, which would be prone to degeneration. Our study coupled with future long-term clinical outcomes >10 years will provide insight on biomechanics of TAVR degeneration.

Background

Finite element analysis (FEA) can be used to determine ascending thoracic aortic aneurysm (aTAA) wall stress as a potential biomechanical predictor of dissection. FEA is dependent upon zero-pressure three-dimensional geometry, patient-specific material properties, wall thickness, and hemodynamic loading conditions. Unfortunately, determining material properties on unoperated patients using non-invasive means is challenging; and we have previously demonstrated significant material property differences among aTAA patients. Our study objective was to determine the impact of patient-specific material properties on aTAA wall stress. Using FEA, we investigated if patient-specific wall stress could be reasonably predicted using population-averaged material properties, which would greatly simplify dissection prediction.

Methods

ATAA patients (n=15) with both computed tomography (CT) imaging and surgical aTAA specimens were recruited. Patient-specific aTAA CT geometries were meshed and pre-stress geometries determined as previously described. Patient-specific material properties were derived from biaxial stretch testing of aTAA tissue and incorporated into a fiber-enforced hyper-elastic model, while group-averaged material properties were estimated using mean values of each parameter. Population-averaged material properties were also calculated from literature and studied. Wall stress distribution and its magnitude were determined using LS-DYNA FEA software. Peak and averaged stresses and stress distributions were compared between patient-specific and both group- and population-averaged material property models.

Results

Patient-specific material properties had minimal influence on either peak or averaged wall stress compared to use of group- or population-averaged material properties. Stress distribution was also nearly superimposed among models with patient-specific vs. group- or population-averaged material properties and provided similar prediction of sites most prone to rupture.

Conclusions

FEA using population-averaged material properties likely provides reliable stress prediction to indicate sites most prone to rupture. Population-averaged material properties may be reliably used in computational models to assess wall stress and significantly simplify risk prediction of aTAA dissection.

Background

Pulmonary autograft dilatation after Ross operation often necessitates reoperation. To understand autograft remodeling, a biomechanical understanding of human autografts after exposure to systemic pressure is required. We previously developed an ex vivo human pulmonary autograft finite element (FE) model to predict wall stress after exposure to systemic pressure. However, autograft material properties vary significantly among individuals. Our study aim was to quantify range of wall stress changes in a human autograft after Ross operation prior to remodeling based upon normal variation in human autograft mechanical properties.

Methods

A normal human autograft FE model was loaded to pulmonary and systemic arterial pressures. Stress-strain data of normal human autografts (n=24) were incorporated into an Ogden hyper-elastic model to describe autograft mechanical behavior. Autograft wall stresses at pulmonary vs. systemic pressures were examined. Autograft volume-based stress analysis was performed, based on percentage of autograft element volume exceeding 1 standard deviation (SD) above group mean stress at systemic systole.

Results

Mean first principal wall stresses (FPS) at systole of systemic versus pulmonary pressures were 129.29±17.47kPa versus 24.42±3.85kPa (p<0.001) at the annulus, 187.53±20.06kPa versus 35.98±2.15kPa at sinuses (p<0.001), and 268.68±23.40kPa versus 50.15±5.90kPa (p<0.001) at sinotubuluar junction (STJ). The percentage of autograft element volume that exceeded one SD above the group mean was 14.3±5.6% for FPS and 12.6±10.1% for second principal stresses.

Conclusion

We quantified normal human autograft biomechanical responses to systemic pressure based on patient-specific material properties. Regions of peak stresses were observed in autograft sinuses and STJ regions, which corresponded clinically to locations of autograft dilation. Our results provide valuable information on predicting variations in patient-specific ex vivo FE models when population-based material properties are used in settings where patient-specific properties are unknown.

Objective

The majority of minimally invasive surgical aortic valve replacements (MIAVRs) are performed at high-volume cardiac surgery centers. However, outcomes at lower volume federal facilities are not yet reported in the literature and not captured in the national Society of Thoracic Surgeons (STS) database. Our study objective was to describe the evolution of MIAVR at a Veterans Affairs Medical Center (VAMC).

Methods

A single-center retrospective cohort study was performed of 114 patients who underwent MIAVR for isolated aortic valvular disease between January 2011 and August 2018. Preoperative STS risk factors were determined and perioperative outcomes were analyzed.

Results

By 2016, 100% of isolated surgical aortic valve replacements were performed as MIAVRs at our VAMC. Introduction of automatic knot-fastening devices, single-shot del Nido cardioplegia, and rapid deployment valves decreased aortic cross-clamp (AXC) times from a median of 96 (interquartile range [IQR]: 84 to 103) to 53 minutes (38 to 61, P < 0.001, Kruskal-Wallis). Thirty-day mortality was 0.9%. Median length of hospital stay was 9 days (7 to 13). Postoperative atrial fibrillation occurred in 54% of patients, stroke occurred in 1.8% of patients, and 7.1% of patients required permanent pacemakers. Transition to rapid deployment valves decreased postoperative mean pressure gradient from median 14 mmHg (10 to 17) to 7 mmHg (4.7 to 10, P < 0.001, Mann-Whitney). At median 1.5-year follow-up echocardiogram, mean gradient was 10.8 mmHg with mild paravalvular leak rate of 1.8%.

Conclusions

Facilitating technologies decreased operative times during MIAVR adoption at our VAMC. For patients with isolated aortic valve pathology, MIAVR can be performed with low morbidity and mortality at lower volume federal institutions, with outcomes comparable to those reported from higher volume centers.

Background

Elective repair of bicuspid aortic valve (BAV)-associated ascending thoracic aortic aneurysm (aTAA) is recommended at lower size limits than tricuspid aortic valve (TAV)-associated aTAA. Rupture/dissection can occur when wall stress exceeds wall strength. Previously, a validated computational method was developed for determining aTAA wall stress, but to date this method has not applied to a patient-specific BAV aTAA. The study aim was to develop a patient-specific BAV aTAA computational model to determine regional wall stress, using the required zero-pressure geometry, wall thickness, material properties, and residual stress.

Methods

A BAV aTAA specimen was excised intact during elective repair, and zero-pressure geometry generated using micro-computed tomography. Residual stress was determined from the aTAA opening angle. aTAA material properties determined using biaxial stretch testing were incorporated into an Ogden hyperelastic model. Finite element analyses (FEAs) were performed in LS-DYNA to determine wall stress distribution and magnitudes at systemic pressure.

Results

The left aTAA region had the highest stiffness, followed by the right, and then anterior/posterior walls, suggesting regional variability in mechanical properties. During systole, the mean principal wall stresses were 108.8 kPa (circumferential) and 59.9 kPa (longitudinal), while peak wall stresses were 789.4 kPa (circumferential) and 618.8 kPa (longitudinal). Elevated wall stress pockets were seen in anatomic left aTAA regions.

Conclusions

To the present authors' knowledge, this was the first patient-specific BAV aTAA model based on surgical specimens to be developed. Surgical specimens serve as the 'gold standard' for determining wall stress to validate models based on in-vivo imaging data alone. Regions of maximal wall stress may indicate sites most prone to rupture, and are crucial for evaluating rupture risk based on the wall stress/strength relationship.