Injuries to blood vessels, especially at the catheter introduction site, are among the most common problems seen after cardiac catheterization. These problems include: blood clots, damage to the blood vessel, ongoing bleeding, a collection of blood within the tissues under the skin or in the artery itself and abnormal connections that form between the artery and a vein. Most of the time these are temporary problems. For example, a collection of blood below the skin can develop following cardiac catheterization. Sometimes these tender masses can be the size of a baseball or softball. The mass will usually disappear in 1 to 2 weeks as the blood gradually spreads and is reabsorbed by the body. In rare cases, the injuries can require surgery to fix the problem.