Cardiac catheterization is a medical procedure where a catheter is inserted into a chamber or blood vessel of the heart. It is used both as a diagnostic and a therapeutic procedure. The procedure is started when a needle is inserted through a small hole made in either the femoral artery or the radial artery after the site is treated with a local anesthetic. A guide wire is then inserted through the hole, and a larger plastic sheath is placed over the wire. The wire is then removed. The sheath contains a valve which prevents blood from flowing out of it, but allows fluids to be inserted through it.