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MODEL RELEASED. Heart valve surgery. Surgeons attaching a patient to a heart-lung machine during a heart valve replacement operation. Tubes for the patient's blood are seen at lower right. A heart- lung machine takes over the function of the heart and lungs, allowing the heart to be stopped during surgery. Heart valves prevent blood from flowing back into the heart chamber from which it has just been pumped. Valve failure may be life- threatening. This is keyhole surgery, where the operation is performed through a small incision in the chest, rather than by opening the chest as in open heart surgery. This reduces the risk of infection and shortens healing time. (KEYSTONE/SCIENCE PHOTO LIBRARY/ANTONIA REEVE)