Heart surgery. Image 9 of 9. Surgeon using a defibrillator to restart a heart during an operation to repair an aortic aneurysm. A powerful electric current passes through the heart from one of the defibrillator's paddles to the other. The patient's heart was intentionally stopped during the operation to allow surgery on the heart. An aortic aneurysm is a weakening of the wall of the aorta, the largest blood vessel in the body. In this case, the section of aorta nearest the heart has been replaced with a prosthetic blood vessel. Without surgery, an aneurysm can rupture with fatal consequences. For a sequence showing the operation, see images M560/461 to M560/469.