Intracardiac rupture of a dissecting aortic aneurysm is a rare event. Although previous cases of rupture into the right atrium (RA) have been reported, we are unaware of any other report of communication into the superior vena cava (SVC). We recently had the opportunity to study and localize the site of fistula by continuous fiberoptic measurement of Sv02. This was confirmed by aortic angiogram and surgery.