Heart surgery. X-ray of a patient's chest after surgery to replace the aortic valve of the heart. The prosthetic valve (blue) is at lower centre. Stitches can be seen above the valve in the aorta, the largest blood vessel of the body. The aortic valve prevents blood from flowing back into the heart after it has been pumped out into the aorta. Aortic valve replacement is necessary if the valve becomes damaged or is defective. Symptoms include breathlessness and angina. Electrodes (orange) attached to the patient's chest show that the patient's heart is being monitored by an electrocardiograph (ECG) machine.