I know that two years ago, aortic valves were replaced successfully in patients in Kings College in London using just a
catheter through the artery in the leg, just like an
angiogram. The collapsed valve replacement was sent to the correct position and expanded into place using a balloon, squashing the old valve leafs into the heart. As soon as the balloon was deflated, the valve started working immediately. I dont know if this has become common practice now though.