Investigation of the NVT ALLEGRA Plus THV System in Patients With Severe Aortic Stenosis or Failed Surgical Aortic Bioprosthesis

Inclusion Criteria: 1. Symptomatic severe calcific stenosis of a native aortic valve with an AVA ≤1.0 cm2 (or AVA index ≤0.6 cm2/m2), AND mean aortic pressure gradient ≥ 40mmHg OR maximal transaortic velocity ≥4.0m/s OR Doppler velocity index ≤0.25 on site-reported echocardiography OR symptomatic patients with degeneration of a surgical bioprosthetic valve (stenosis +/- insufficiency) on site-reported echocardiography 2. Local multi-disciplinary Heart Team and Central Screening Committee (CSC) agree on indication and eligibility for TAVI 3. Age ≥18 years 4. Patient has signed the Patient Informed Consent Form 5. Patient is willing and able to comply with requirements of the study, including all follow-up visits Exclusion Criteria: Patient will not be included if ANY one of the following conditions exists: General: 1. Mean aortic annulus diameter as measured by pre-procedural CT or internal diameter of the bioprosthesis is \<16.5 mm or \>27 mm 2. Echocardiographic evidence of intracardiac thrombus or vegetation (site-reported) 3. Significant disease of the aorta that would preclude safe advancement of the ALLEGRA Plus THV System 4. Severe ilio-femoral vessel disease that would preclude safe placement of an 18 Fr introducer sheath or make endovascular access impossible 5. Porcelain aorta 6. Severe left ventricular dysfunction with ejection fraction (EF) \<20% (site-reported) 7. Evidence of active endocarditis or other acute infections 8. Renal failure requiring continuous renal replacement therapy 9. Untreated clinically significant coronary artery disease requiring revascularization 10. Any percutaneous interventional procedure (e.g. PCI with stenting) within 14 days prior of the index procedure 11. Acute MI ≤30 days prior to the index procedure 12. Symptomatic carotid or vertebral artery disease requiring intervention or carotid/vertebral intervention within the preceding 45 days 13. Cerebrovascular accident (CVA) or transient ischemic attack (TIA) ≤6 months or prior CVA with moderate or severe disability (e.g. modified Rankin scale score \>2) 14. History of bleeding diathesis or coagulopathy; acute blood dyscrasias as defined: thrombocytopenia (platelets \<80,000/µl), acute anemia (hemoglobin \<10 g/dl), leukopenia (WBC \<3000/ µl) 15. Active peptic ulcer or gastrointestinal (GI) bleeding ≤3 months 16. Severe (greater than 3+) mitral insufficiency (site-reported) 17. Uncontrolled atrial fibrillation 18. Required emergency surgery for any reason 19. Known hypersensitivity to contrast media, which cannot be adequately pre-medicated or contraindication to anticoagulant or anti-platelet medication or to nitinol alloy or to bovine tissue 20. Life expectancy ≤12 months due to other medical illness 21. Currently participating in another investigational drug or device study 22. Hypertrophic obstructive cardiomyopathy 23. Pregnancy or intend to become pregnant during study participation Specific exclusions in patients with native aortic valve disease (site-reported): 24. Unicuspid or bicuspid aortic valve 25. Non-calcified aortic stenosis 26. Identified risk of coronary occlusion due to Sinus of Valsalva anatomy and/or bulky calcified aortic valve leaflets in close proximity to coronary ostia Specific exclusions in patients with degenerated surgical bioprosthetic aortic valves (valve-in-valve) (site-reported): 27. High risk of coronary occlusion 28. Partially detached leaflets that may obstruct a coronary ostium