Irregular heart rate. Looks like they are hoping the pellet scars over quickly---so it won't move around anymore and bounce up against the heart muscle/wall.
http://www.c-n.com/apps/pbcs.dll/article?AID=/20060215/NEWS/602150312<snip>
One of the pellets from the 28-gauge shotgun that Cheney had fired had migrated to the heart,
either touching or embedding into the heart muscle near its top chambers, called the atria. That irritated those chambers to cause the irregular heartbeat known as atrial fibrillation.But doctors also spotted inflammation -- which always occurs when something foreign invades the body -- that was causing a temporary block in blood flow, by touching or pushing the heart, explained Dr. David Blanchard, chief of emergency care at Christus Spohn Hospital Corpus Christi-Memorial.
A pellet striking the heart can cause those problems, but it's not normally thought of as a heart attack, said Dr. Samin Sharma, chief of interventional cardiology at New York's Mt. Sinai Medical Center. "What probably happened is the pellet hit the heart and the heart released some enzymes" associated with a heart attack, he said.
As for the atrial fibrillation, it's not immediately dangerous but must be treated because if left uncontrolled, it can spur blood clots. Most cases can be corrected with medication.