Biomechanics of Transcatheter Aortic Valve Implant

Abstract

Transcatheter aortic valve implantation (TAVI) has grown exponentially within the cardiology and cardiac surgical spheres. It has now become a routine approach for treating aortic stenosis. Several concerns have been raised about TAVI in comparison to conventional surgical aortic valve replacement (SAVR). The primary concerns regard the longevity of the valves. Several factors have been identified which may predict poor outcomes following TAVI. To this end, the lesser-used finite element analysis (FEA) was used to quantify the properties of calcifications which affect TAVI valves. This method can also be used in conjunction with other integrated software to ascertain the functionality of these valves. Other imaging modalities such as multi-detector row computed tomography (MDCT) are now widely available, which can accurately size aortic valve annuli. This may help reduce the incidence of paravalvular leaks and regurgitation which may necessitate further intervention. Structural valve degeneration (SVD) remains a key factor, with varying results from current studies. The true incidence of SVD in TAVI compared to SAVR remains unclear due to the lack of long-term data. It is now widely accepted that both are part of the armamentarium and are not mutually exclusive. Decision making in terms of appropriate interventions should be undertaken via shared decision making involving heart teams.

Keywords: structural valve degeneration; surgical aortic valve replacement; transcatheter aortic valve implantation; transcatheter heart valves.

Figure 1

Decision Tree for treatment of severe AVS based on international guidelines and the VARC-2 consensus document. Recommendations from the 2020 international guidelines (ACC/AHA/ESC) for the treatment of patients with valvular heart disease. Clinical factors and imaging findings are shown in green and yellow boxes as well as AVR recommendations according to Class (Strength) of Recommendation and Level (Quality) of Evidence. Treatment recommendations are shown in red boxes. 1A, 1B-NR, 2aB-NR, 2a B-R, and 2b NR are the CORs which indicate the strength of recommendation, including the estimated magnitude and assurance of advantage in relation to risk. The LOE rates the quality of scientific evidence supporting the intervention based on the type, quantity, and consistency of data from clinical trials and other sources. Computational biomodelling is a suitable method for a predictive evaluation of TAVI performance. Abbreviations: AVS, aortic valve stenosis; COR, class of recommendation; LOE, level of evidence; SAVR, standard aortic valve replacement; HF, heart failure; LVF, left ventricular function; TAVI, transcatheter aortic valve implant, VARC, Valve Academic Research Consortium.

Figure 2

The computational framework aimed to simulate the implantation of TAVI. Four sections from Steps 1 to 4 of the worked-out modeling strategy are identified. The extrapolated images (ECHO and CT) allow biomodelling on which to perform the simulations to be established. The data obtained from the simulations are compared to the data that emerged in the follow-up. Abbreviations; CT; computed tomography.

Figure 3

The 3D CT scan protocol for patients receiving TAVI. It is designed to ensure all steps of the transcatheter procedure. The figure reports an example in which the exam was conducted in 6.8 s with only 640 mGy.cm. Precise planning to support the intervention was conferred. Abbreviations; DLP, Dose Length Product; mGy, microgray.

Figure 4

DICOM from a 3D CT scan serves to extract the RAW data that allow definition of functional aortic valve assessment (A), morphological aortic valve features (B), and anatomical aortic valve characteristics (C); abbreviations in other figures.

Figure 5

Up: With the use of ITK-Snapv 2.4 the data extracted from the CT images, (A,C) are processed to highlight the images of the aortic lumen (B, red) and calcium deposits (D, yellow/orange). Down: A first generation of balloon-expandable TAVI Sapien (E; Edwards Lifesciences, Irvine, CA, USA) is used to treat severe AVS (F). G: Aortic lumen morphology as well as the calcium conglomerates are extracted with the use of STL files. Left: The lumen of the aortic root (red) and the calcifications (yellow) are superimposed to the aortic wall model (gray). Center: The enclosed native leaflets (blue mesh) correspond perfectly to the real leaflets with calcifications obtained by processing CT images (A,C). Right: The top view is shown. Abbreviations: AVS, aortic valve stenosis; CT, computed tomography; STL, stereolithographic; TAVI, transcatheter aortic valve implantation.

Figure 6

Balloon-expandable THV. (A–C) The SAPIEN 3 balloon-expandable device is constituted by a cobalt–chromium alloy frame valve with bovine pericardium leaflets. The device is available on the market in the following sizes: 20 mm, 23 mm, 26 mm, and 29 mm (A). The Commander Delivery System is 14 F expandable introducer sheath compatible with 20–26 mm valves and 16 F expandable introducer sheath compatible with 29 mm valves (B,C). (D–G) Self-expandable THV. The bioprosthesis is manufactured by suturing 3 valve leaflets and a skirt, made from a single layer of the porcine pericardium, onto a self-expanding, multi-level, radiopaque frame made of Nitinol (D–F). CoreValve (D), Evolut R (E), and Evolut PRO (F) in the following sizes: 20 mm, 23 mm, 26 mm, and 29 mm. ©. The loading system. The outer diameter of the catheter is 15 Fr (AccuTrak™ stability layer) and 12 Fr, and the outer diameter of the valve capsule is 18 Fr. The catheter can be used for femoral, subclavian/axillary, or ascending aortic (direct aortic) access sites. (H) The Portico re-sheathable transcatheter aortic valve system (Abbott Structural Heart, St Paul, MN, USA). (I) The ACURATE neo (Boston Scientific, Marlborough, MA, USA) self-expanding THV. (L) Lotus mechanically expanded valve (Lotus Valve System (MEV; Boston Scientific Corp., Natick, MA, USA) (H): Portico valve is designed with large, open cells and intra-annular leaflet placement to preserve flow and access to the coronary arteries after deployment The Portico valve was delivered by a flexible, first-generation Portico Delivery system, which had an 18 F outer diameter for the small valves (23 and 25 mm) and a 19 F outer diameter for the larger valves (27 and 29 mm). (I): the ACURATE neo bioprosthesis consists of a self-expanding nitinol frame with three porcine pericardial leaflets and a stent body with an outer and inner pericardial skirt. (L): The MEV is constituted by 3 bovine pericardial tissue valve leaflets and a braided nitinol frame with a polycarbonate-based urethane adaptive seal. λ From Willson AB et al., transcatheter aortic valve replacement with the St. Jude Medical Portico valve: first-inhuman experience. J. Am. Coll. Cardiol. 2012; 60: 581–86; † From Mollmann H, EuroIntervention 2013; 9 (suppl): S107–10. from Meredith IT et al. Boston Scientific Lotus valve. EuroIntervention. 2012; 8 (suppl Q): Q70–Q74. Abbreviation. MEV = mechanically expanded valve. THV = transcatheter heart valve.

Figure 6

Balloon-expandable THV. (A–C) The SAPIEN 3 balloon-expandable device is constituted by a cobalt–chromium alloy frame valve with bovine pericardium leaflets. The device is available on the market in the following sizes: 20 mm, 23 mm, 26 mm, and 29 mm (A). The Commander Delivery System is 14 F expandable introducer sheath compatible with 20–26 mm valves and 16 F expandable introducer sheath compatible with 29 mm valves (B,C). (D–G) Self-expandable THV. The bioprosthesis is manufactured by suturing 3 valve leaflets and a skirt, made from a single layer of the porcine pericardium, onto a self-expanding, multi-level, radiopaque frame made of Nitinol (D–F). CoreValve (D), Evolut R (E), and Evolut PRO (F) in the following sizes: 20 mm, 23 mm, 26 mm, and 29 mm. ©. The loading system. The outer diameter of the catheter is 15 Fr (AccuTrak™ stability layer) and 12 Fr, and the outer diameter of the valve capsule is 18 Fr. The catheter can be used for femoral, subclavian/axillary, or ascending aortic (direct aortic) access sites. (H) The Portico re-sheathable transcatheter aortic valve system (Abbott Structural Heart, St Paul, MN, USA). (I) The ACURATE neo (Boston Scientific, Marlborough, MA, USA) self-expanding THV. (L) Lotus mechanically expanded valve (Lotus Valve System (MEV; Boston Scientific Corp., Natick, MA, USA) (H): Portico valve is designed with large, open cells and intra-annular leaflet placement to preserve flow and access to the coronary arteries after deployment The Portico valve was delivered by a flexible, first-generation Portico Delivery system, which had an 18 F outer diameter for the small valves (23 and 25 mm) and a 19 F outer diameter for the larger valves (27 and 29 mm). (I): the ACURATE neo bioprosthesis consists of a self-expanding nitinol frame with three porcine pericardial leaflets and a stent body with an outer and inner pericardial skirt. (L): The MEV is constituted by 3 bovine pericardial tissue valve leaflets and a braided nitinol frame with a polycarbonate-based urethane adaptive seal. λ From Willson AB et al., transcatheter aortic valve replacement with the St. Jude Medical Portico valve: first-inhuman experience. J. Am. Coll. Cardiol. 2012; 60: 581–86; † From Mollmann H, EuroIntervention 2013; 9 (suppl): S107–10. from Meredith IT et al. Boston Scientific Lotus valve. EuroIntervention. 2012; 8 (suppl Q): Q70–Q74. Abbreviation. MEV = mechanically expanded valve. THV = transcatheter heart valve.

Figure 7

FEA simulations of TAVI in two investigated patients who showed post-operative thrombosis. The positioning (a,a′) and reopening (b,b′) of CoreValve and SAPIEN devices (left and right sides, respectively). Abbreviations in other figures.

Figure 8

Depict preoperative (A) and postoperative (B) 3 D CT scan with TAVI thrombosis. (C) Biomodelling of a Sapien XT reveals an incomplete deployment (red arrow) of the device with PAVR and thrombotic formation (red arrow). (D) The yellow arrow disclose a distortion of the stent and reduced mobility of the leaflet of the bioprosthesis in correspondence of the PAVR. Abbreviations; PAVR paravalvular aortic regurgitation. Other abbreviations in previous figures.

Figure 9

(A): Early SVD with calcifications (white arrow) of TAVI in patient receiving self-expanded first-generation CoreValve 26 mm (CoreValve, Minneapolis, Minnesota). (B): Classification of SVR based on recommendations of the VARC-2 for stent/stentless xenograft. The useful elements to define SVD as valve-related dysfunction were the mean aortic gradient ≥20 mm Hg, the effective orifice area ≤0.9–1.1 cm², a dimensionless valve index <0.35 m/s, and moderate or severe prosthetic regurgitation. Phase 0 displays the absence of morphological leaflet anomaly and absence of hemodynamic alteration. Phase 1 discloses early morphological changes without hemodynamic compromise. The morphological alterations typical of stage 1 are also referable to prostheses where the degenerative process is controlled using antithrombotic drugs that reduce the thickening of the leaflet. Phase 2 reveals morphological abnormalities of valve leaflets of SVD associated with hemodynamic dysfunction. The bioprosthesis in this phase can manifest as stenosis or regurgitation. The thrombosis is a factor favoring phase 2, leading to stenosis or paravalvular leakage and regurgitation. Phase 2 includes two subcategories, phase 2S and phase 2R. In the evolutive stage of 2S degeneration, an increase in the mean transvalvular gradient (≥10 mm Hg) and decrease in the valvular area without leaflet thickening occur. SVD may occur in the 2RS form including moderate stenosis and moderate regurgitation. Phase 3 of SVD highlights severe stenosis or severe regurgitation with severe hemodynamic change. Abbreviations: R, regurgitation; SVR, structural valve degeneration; S, stenosis; VARC, Valve Academy Research Consortium.