A procedure used to diagnose and treat some heart conditions. During cardiac catheterization, a flexible tube called a catheter is inserted into a blood vessel in the arm, groin, upper thigh, or neck. The catheter is then guided through the aorta into the heart using a special x-ray machine. A cardiac catheterization may be done to measure blood pressure and oxygen levels in different parts of the heart, to check how well the heart muscle is working, to take a sample of tissue from the heart, or to look for defects in the valves or chambers of the heart. A dye may be injected through the catheter in order to look at blood flow in the heart and to check for blockages in the arteries leading to the heart.