ALLIANCE AVIV: Safety and Effectiveness of the SAPIEN X4 Transcatheter Heart Valve in Failing Aortic Bioprosthetic Valves

Inclusion Criteria: 1. Failing aortic bioprosthetic valve demonstrating ≥ moderate stenosis and/or ≥ moderate insufficiency 2. Bioprosthetic valve size suitable for SAPIEN X4 THV 3. NYHA functional class ≥ II 4. Heart Team agrees the subject is at high or greater surgical risk 5. The subject has been informed of the nature of the study, agrees to its provisions and has provided written informed consent. Exclusion Criteria: 1. Anatomical characteristics that would preclude safe femoral placement of the introducer sheath or safe passage of the delivery system 2. Failing valve has moderate or severe paravalvular regurgitation 3. Failing valve is unstable, rocking, or not structurally intact 4. Known severe patient-prosthesis mismatch or bioprosthetic valve with residual mean gradient \> 20 mmHg at the end of the index procedure for implantation of the original valve 5. Increased risk of THV embolization 6. Surgical or transcatheter valve in the mitral position 7. Severe mitral regurgitation (\> 3+) or ≥ moderate mitral stenosis 8. Need for mitral, tricuspid or pulmonic valve intervention within the next 12 months 9. Left ventricular ejection fraction \< 20% 10. Cardiac imaging evidence of intracardiac mass, thrombus or vegetation 11. Increased risk of coronary artery obstruction after THV implantation 12. Myocardial infarction within 30 days prior to the study procedure 13. Hypertrophic cardiomyopathy with subvalvular obstruction 14. Subjects with planned concomitant ablation for atrial fibrillation 15. Clinically significant coronary artery disease requiring revascularization 16. Any surgical or transcatheter procedure within 30 days prior to the study procedure. Implantation of a permanent pacemaker or implantable cardioverter defibrillator is not considered an exclusion. 17. Any planned surgical or transcatheter intervention to be performed within 30 days following the study procedure 18. Endocarditis within 180 days prior to the study procedure 19. Stroke, transient ischemic attack or neurological signs and symptoms attributed to carotid or vertebrobasilar disease within 90 days prior to the study procedure 20. Hemodynamic or respiratory instability requiring inotropic or mechanical support within 30 days prior to the study procedure 21. Renal insufficiency and/or renal replacement therapy 22. Leukopenia, anemia, thrombocytopenia 23. Inability to tolerate or condition precluding treatment with antithrombotic therapy 24. Hypercoagulable state or other condition that increases risk of thrombosis 25. Absolute contraindications or allergy to iodinated contrast that cannot be adequately treated with premedication 26. Subject refuses blood products 27. Body mass index \> 50 kg/m2 28. Estimated life expectancy \< 24 months 29. Female who is pregnant or lactating 30. Active SARS-CoV-2 infection or previously diagnosed with COVID-19 with sequelae that could confound endpoint assessments 31. Participating in another investigational drug or device study that has not reached its primary endpoint 32. Subject considered to be part of a vulnerable population