Mitral Valve Prolapse
Mitral Valve Prolapse (MVP, or Barlow's Syndrome) is a condition where a person's mitral valve is a little looser than it should be. As a result, there can be some leakage of blood backwards, from the lower to upper chamber of the heart. On the whole heart function is normal, and the problem generally stays fairly stable over time.
Some people with this fairly common disease are at heightened risk of infection of the mitral valve. While the odds of infection setting in are slim, if a piercing becomes infected and this infection enters the bloodstream, it is a problem that can become very serious very quickly, and is very difficult to treat.
If you have this heart condition (or any other condition that increases your chance of cardiac infection), you should take antibiotics before any open wounds are formed on you, and this most definitely applies to piercings and other body modifications.
If you have a condition like this, and you don't want to die from your piercing (yes, it has happened), it is essential that you consult your doctor before any piercing.