An Angioplasty procedure takes place in a cardiac catheterization labortary. During the procedure, a balloon catheter - a long, thin flexible tube with a small uninflated balloon at its tip - is threaded through the guiding catheter to where the artery is narrowed. A guidewire - a tiny, thin wire of about .014 inches' diameter - is then passed across the narrowed segment. It serves as a guide for positioning the tiny balloon across the blockage. Once in position, the balloon is inflated with water and x-ray dye.