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.
Next, a catheter is placed over the guide wire, and this is threaded back in through the sheath. The catheter is moved into position and the wire is again removed. The catheter allows contrast to be injected through it, or can serve as the conduit for an angioplasty.