• Primary safety endpoint is all-cause mortality [ Time Frame: at 30 days ]
• Primary effectiveness endpoint is moderate or greater paravalvular leak [ Time Frame: at 30 days ]

• Primary safety endpoint is all-cause mortality [ Time Frame: at 30 days ]
• Primary effectiveness endpoint is freedom from moderate or greater paravalvular leak [ Time Frame: at 30 days ]

The Portico NG Approval study will be conducted as a prospective, multi-center, international, single-arm investigational study.

Up to 189 (includes up to 20 roll-ins) high or extreme risk patients with symptomatic, severe native aortic stenosis who are determined by an independent subject selection committee to meet eligibility criteria for Portico NG Transcatheter Aortic Heart Valve implantation will undergo Portico™ NG Valve implantation via a transfemoral or alternative access approach according to the site's anesthesia protocol for TAVR (transcatheter aortic valve replacement) procedure.

Subjects participating in the clinical study will be followed for a total of 12 months with data collected at screening, baseline, procedure, prior to hospital discharge, and follow-up at 30 days, 12 months and annually at 2, 3, 4 and 5 years.

Inclusion Criteria:

• Subjects must have a Society of Thoracic Surgeons (STS) score of ≥7% OR documented heart team agreement of high or extreme risk for surgical aortic valve replacement due to frailty or co-morbidities not captured by the STS score.
• Subject has symptomatic aortic stenosis as demonstrated by NYHA (New York Heart Association) Functional Classification of II, III, or IV.
• Subject has senile degenerative aortic valve stenosis with echo-derived criteria, defined as: aortic valve area (AVA) of ≤ 1.0 cm2 (or indexed EOA (effective orifice area) ≤ 0.6 cm2/m2) AND mean gradient ≥40 mmHg or peak jet velocity ≥ 4.0 m/s or doppler velocity index (DVI) ≤0.25. (Qualifying AVA baseline measurement must be within 90 days prior to informed consent).
• Aortic annulus diameter of 19-27mm and ascending aorta diameter of 26-42 mm for the specified valve size listed in the IFU (Instructions For Use), as measured by CT (Computed Tomography) conducted within 12 months prior to informed consent. (If a CT is contraindicated and/or not possible to be obtained for certain subjects, a 3D echocardiogram and non-contrast CT of chest and abdomen/pelvis may be accepted).

Exclusion Criteria:

• Evidence of an acute myocardial infarction (defined as: ST Segment Elevation as evidenced on 12 Lead ECG) within 30 days prior to index procedure.
• Any percutaneous coronary or peripheral interventional procedure performed within 30 days prior to index procedure.
• Blood dyscrasias as defined: leukopenia (WBC<3000 mm3), acute anemia (Hb < 9 g/dL), thrombocytopenia (platelet count <50,000 cells/mm³). History of bleeding diathesis or coagulopathy
• Active peptic ulcer or upper GI bleeding within 3 months prior to index procedure that would preclude anticoagulation
• Recent (within 6 months prior to index procedure date) cerebrovascular accident (CVA) or a transient ischemic attack (TIA).
• Mixed aortic valve disease (aortic stenosis and aortic regurgitation with predominant aortic regurgitation 3-4+).
• Pre-existing prosthetic heart valve or other implant in any valve position, prosthetic ring, severe circumferential mitral annular calcification (MAC) which is continuous with calcium in the left ventricular outflow tract (LVOT), severe (greater than or equal to 3+) mitral insufficiency, or severe mitral stenosis with pulmonary compromise.
• Minimum access vessel diameter of <5.0mm for small FlexNav™ Delivery System and <5.5 mm for large FlexNav™ Delivery System