A patent foramen ovale (PFO) is a type of atrial septal defect. It is a small hole that remains open between left and right atria after birth. During PFO closure surgery, a catheter moves through the right atrium, past the opening and into the left atrium. A metallic closure device is deployed at the end of the catheter, forming one circular disc on the left side of the opening and one circular disc on the right side of the opening. Eventually, the heart tissue will grow around the device.