An implantable medical device is adapted to be implanted at an implantation site such as a native cardiac valve annulus. The implantable medical device includes an expandable frame that is adapted to expand from a collapsed configuration for delivery to an expanded configuration for deployment, one or more valve cusps secured relative to the expandable frame, an inner skirt surrounding a portion of the expandable frame, and an outer skirt surrounding a portion of the expandable frame, the outer skirt attached to the expandable frame via one or more sutures that axially secure the outer skirt relative to the expandable frame yet allow the outer skirt to move radially relative to the expandable frame. The outer skirt may move radially outwards in response to fluid pressure, thereby improving fluid sealing relative to the native cardiac valve annulus.