A Clinical Evaluation of the HYDRA Self Expanding Transcatheter Aortic Valve

Inclusion Criteria: 1. Subject has given written Informed Consent for study participation prior to procedure. 2. Greater than 55 years of age. 3. Aortic annulus diameter meets the range 18 to 27mm as measured by CT conducted within the past 180 days, or echocardiogram (TEE 3D recommended) if medically contraindicated to CT. 4. Patient has severe degenerative aortic stenosis with echocardiography derived mean gradient \>40mmHg and/or peak velocity greater than 4.0 m/s and/or an initial valve area of \<1.0 cm2. 5. Patient has symptomatic aortic stenosis as demonstrated by NYHA Functional Classification of II or greater. 6. Patient is deemed high operable risk and suitable for TAVI. 7. Patient's predicted operative mortality or serious, irreversible morbidity risk is \<50% at 30 days. 8. Patient has structurally normal cardiac anatomy. 9. Willing and able to comply with all required follow-up evaluation Exclusion Criteria: 1. Patient has a history of a cerebral vascular accident (CVA) or transient ischemic attack (TIA) within the past 6 months. 2. Patient has carotid artery disease requiring intervention. 3. Patient has evidence of a myocardial infarction (MI) within the past 6 months. 4. Patient has hypertrophic cardiomyopathy. 5. Patient has a native aortic valve that is congenitally uni-cuspid, bicuspid, quadricuspid or non-calcified as seen by echocardiography. 6. Patient has mitral or tricuspid valvular regurgitation (≥ grade III) or moderate to severe mitral stenosis. 7. Patient has aortic root angulation \>70 degrees (horizontal aorta). 8. Patient has aortic root diameter of \< 26 mm or \>36 mm. 9. Patient has a pre-existing prosthetic valve or prosthetic ring in any position. 10. Patient refuses blood transfusion or surgical valve replacement. 11. Patient has resting left ventricular ejection fraction (LVEF) \< 20%. 12. Patient has documented, untreated symptomatic coronary artery disease (CAD) requiring revascularization. 13. Patient has severe basal septal hypertrophy. 14. Patient has had a percutaneous interventional or other invasive cardiac or peripheral procedure ≤ 14 days of the index procedure (does not apply for diagnostic angiography or Angio-CT). 15. Patient has a history of or has active endocarditis. 16. Patient has echocardiographic evidence of intracardiac mass, thrombus, or vegetation. 17. Patient has hemodynamic instability (requiring inotropic support or mechanical heart assistance). 18. Patient is in acute pulmonary edema or requiring intravenous diuretic therapy to stabilize heart failure. 19. Patient with significant pulmonary disease (FEV1 \< 30% as predicted). 20. Patient has significant chronic steroid use as determined and documented by the Principal Investigator. 21. Patient has a known hypersensitivity or contraindication to anticoagulant or antiplatelet medication. 22. Patient has renal insufficiency as evidenced by a serum creatinine \> 3.0 mg/dL (265.5μmol/L) or end-stage renal disease requiring chronic dialysis. 23. Patient's iliofemoral arteries have severe calcification, tortuosity (\>two 90 degree bends), diameter \<6mm, or subject has had an aorto-femoral bypass that preclude safe placement of a 18 French sheath. 24. Patient has blood dyscrasia (leukopenia, acute anemia, thrombocytopenia, bleeding diathesis, or coagulopathy). 25. Patient has a current autoimmune disease that, in the opinion of the Principal Investigator precludes the subject from study participation. 26. Patient has significant aortic disease. 27. Patient has a pre-existing endovascular stent graft in the supra- or infrarenal aorta or pre-existing stent grafts in the iliofemoral arteries. 28. Patient has an active peptic ulcer or has had gastrointestinal (GI) bleeding within the past 90 days prior to procedure 29. Patient has a life expectancy \< 12 months. 30. Patient has other medical, social or psychological conditions that, in the opinion of the Principal Investigator, preclude the subject from study participation. 31. Patient has a known allergy to contrast media, nitinol alloys or bovine tissue. 32. Patient has a history of any cognitive or mental health status that would interfere with study participation. 33. Currently participating in another trial.