End-stage valvular aortic stenosis obstructs blood flow from the left ventricle into the aorta. Prior to development of the heart-lung machine in 1953, other than a few reports of direct fracture of the stenosed valve on a beating heart or with hypothermic cardiac arrest, there were no effective treatments for relief of outflow obstruction. Starting in the 1960s, the use of mechanical and then bioprosthetic heart valves led to a substantial improvement in survival for patients with severe aortic stenosis. Over the next 60 years, there were progressive improvements in mechanical prostheses (from ball-in-cage to tilting disk to bileaflet valves) and in bioprosthetic valve material and design (from homografts to xenografts to stentless surgical aortic valve prostheses).
