MODEL RELEASED. Heart valve surgery. Monitor showing an X-ray of equipment inside a patient's chest cavity during heart valve surgery. A balloon catheter has been inflated in the aorta to prevent blood flowing back to the valve being operated on. Tubes carrying blood to a heart-lung machine are also seen. 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. Defective valves can be replaced with artificial or donor valves.