prosthetic aortic valve stenosissevere aortic valve stenosisbicuspid aortic valve stenosisaortic valve stenosis murmuraortic valve symptomsmild aortic valve stenosisaortic stenosis signs and symptomscalcific aortic valve diseasedegenerative aortic valve diseasecalcified aortic valveaortic valve disorderbicuspid aortic valve symptoms