Engager Direct Aortic Clinical Study

To confirm the safety and performance of the Medtronic Engager™ Transcatheter Aortic Valve Implantation System via direct aortic approach.

To confirm the safety and performance of the Medtronic Engager™ Transcatheter Aortic Valve Implantation System via direct aortic approach in patients with symptomatic severe aortic valve stenosis at high or extreme risk for surgical aortic valve replacement for which treatment via direct aortic access is preferred.

Eligibility Criteria:
Inclusion Criteria:

* Severe aortic stenosis as defined by Doppler echocardiography: aortic valve area ≤ 0.8 cm2 (or AVA index ≤ 0.5 cm2/m2), or mean gradient ≥ 40 mmHg, or max velocity ≥ 4m/s
* NYHA Functional Class II or greater;
* Logistic EuroSCORE I ≥ 20%, or comorbidity judged by the heart team to pose an absolute or relative contraindication for conventional aortic valve replacement;
* Non-calcified ascending aortic site ≥ 50 mm from the aortic valve basal plane;
* Age ≥ 18 years;
* The patient has been informed of the nature of the study and has consented to participate, authorizing the collection and release of his/her medical information by signing a consent form ("Patient Informed Consent Form"). Patient should understand the implications of participating in the study and should be legally competent to provide informed consent.

Exclusion Criteria:

* Unicuspid or bicuspid aortic valve;
* Echocardiographic evidence of intracardiac mass, thrombus, or vegetation;
* Left ventricular ejection fraction \< 25%;
* Left ventricular outflow obstruction, such as Hypertrophic Obstructive Cardiomyopathy and severe septal hypertrophy;
* Mitral or tricuspid regurgitation greater than 2+ by angiography or moderate by echocardiography;
* Patients with life expectancy less than 12 months due to an underlying non-cardiac co-morbid disease;
* Known hypersensitivity or contraindication to aspirin, heparin, ticlopidine, clopidogrel, Nitinol or contrast medium that cannot be adequately controlled with pre-medication;
* Sepsis or acute endocarditis;
* Blood dyscrasia such as acute anemia, leucopenia, or thrombocytopenia; bleeding diathesis, or coagulopathy;
* Renal insufficiency assessed by creatinine clearance \< 20 ml/min and/or end-stage renal disease requiring chronic dialysis;
* Active peptic ulcer or GI bleeding within 3 months from the planned index procedure;
* Untreated clinically significant coronary artery disease requiring revascularization;
* Cardiogenic shock, suspected cardiogenic shock, or hemodynamic instability requiring inotropic support or mechanical heart assistance;
* Significant aortic disease, including abdominal and thoracic aortic aneurysm, defined as maximal luminal diameter of 5 cm or greater;
* Need for emergency surgery, cardiac or non-cardiac;
* History of myocardial infarction in the last 6 weeks;
* History of TIA or CVA in the last 6 months;
* Therapeutic invasive cardiac procedure, with the exception of BAV, performed within 30 days of the planned date of valve implantation, or 6 months in the case of drug-eluting stents;
* Pre-existing prosthetic heart valve or prosthetic ring in any position;
* Patent RIMA or pre-existing patent RIMA graft at direct aortic access site and/or trajectory;
* Patient refuses a blood transfusion;
* Pregnant or breastfeeding women;
* Patient is currently enrolled in another investigational device or drug trial that may influence the outcome of this trial.