Cardiac catheterization is a procedure in which a heart specialist inserts a small tube (catheter) through a large blood vessel in the arm or leg, and then passes the tube into the heart. Once inside the heart, doctors use the catheter to evaluate how the heart is working by measuring pressure and oxygen levels within the heart's chambers. Through the catheter, doctors inject a special dye that provides an X-ray image of the heart's internal structure and blood flow patterns. In some patients, the X-ray dye also is injected into the coronary arteries to identify areas that have become narrow. This procedure is called coronary angiography.