Heart valve surgery. Artificial heart valve being stitched, ready to be pulled into position in the heart during heart valve replacement surgery. Heart valves prevent blood from flowing back into the heart chamber after the blood is pumped out by the heart. Valve failure may be life-threatening. This is open heart surgery, where the chest cavity is opened up to access the heart during surgery. The metal bar (lower left) is one of the chest retractors holding the rib-cage open. The heart is in the space at lower right, into which the sutures are disappearing.