ALLIANCE Mitral: Safety and Effectiveness of SAPIEN X4 Transcatheter Heart Valve - Mitral

Inclusion Criteria: 1. Failing mitral surgical bioprosthetic valve or a failing native mitral valve with an annuloplasty ring demonstrating ≥ moderate stenosis and/or ≥ moderate insufficiency 2. Bioprosthetic surgical valve or native mitral annulus 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. Interatrial septum or left atrium not suitable for transcatheter transseptal access 3. Failing valve has mild or greater paravalvular regurgitation 4. Failing valve is unstable, rocking, or not structurally intact 5. Annuloplasty ring dehiscence 6. Known severe patient-prosthesis mismatch or bioprosthetic valve with residual mean gradient \> 10 mmHg at the end of the index procedure for implantation of the original valve 7. Annuloplasty ring type not favorable for THV implantation 8. Increased risk of THV embolization 9. Anatomical characteristics that would increase risk of left ventricular outflow tract (LVOT) obstruction 10. Surgical or transcatheter aortic valve extending into LVOT that may impinge on the mitral implant 11. Severe right ventricle (RV) dysfunction 12. Severe regurgitation or stenosis of any other valve 13. Need for aortic, tricuspid, or pulmonic valve intervention within the next 12 months 14. Left ventricular ejection fraction \< 20% 15. Cardiac imaging evidence of intracardiac mass, thrombus, or vegetation 16. Myocardial infarction within 30 days prior to the study procedure 17. Hypertrophic cardiomyopathy with subvalvular obstruction 18. Subjects with planned concomitant ablation for atrial fibrillation 19. Clinically significant coronary artery disease requiring revascularization 20. Any surgical or transcatheter procedure within 30 days prior to the study procedure. Implantation of a permanent pacemaker or implantable cardioverter defibrillator (ICD) is not considered an exclusion. 21. Any planned surgical or transcatheter intervention to be performed within 30 days following the study procedure 22. Endocarditis within 180 days prior to the study procedure 23. Stroke, transient ischemic attack or neurological signs and symptoms attributed to carotid or vertebrobasilar disease within 90 days prior to the study procedure 24. Hemodynamic or respiratory instability requiring inotropic or mechanical support within 30 days prior to the study procedure 25. Renal insufficiency and/or renal replacement therapy 26. Leukopenia, anemia, thrombocytopenia 27. Inability to tolerate or condition precluding treatment with antithrombotic therapy 28. Hypercoagulable state or other condition that increases risk of thrombosis 29. Absolute contraindications or allergy to iodinated contrast that cannot be adequately treated with premedication 30. Subject refuses blood products 31. Body mass index \> 50 kg/m2 32. Estimated life expectancy \< 24 months 33. Female who is pregnant or lactating 34. Active SARS-CoV-2 infection or previously diagnosed with COVID-19 with sequelae that could confound endpoint assessments 35. Participating in another investigational drug or device study that has not reached its primary endpoint 36. Subject considered to be part of a vulnerable population