Inclusion Criteria:  Patients presenting symptomatic AS listed for TAVI Native aortic valve disease Ejection fraction </= 40% Mean trans aortic gradient (echo) of <35 mmHg AVA </= 1cm2 Able to give written informed consent prior to enrolment in the study No contraindication to study procedures CT scan confirmed severe AS *Able and willing to return to the implanting site for the follow-up visits*  Exclusion Criteria:  Unable to provide formal written consent Known pseudo-severe AS prior to TAVI Known allergy to agents required for stress echocardiography Known hypersensitivity or contraindication to aspirin, heparin, clopidogrel, Nitinol, or sensitivity to contrast media, which cannot be adequately premedicated. * Any clinical contraindication to TAVI procedure  This pilot study will be restricted to English speaking subjects.