Cardiopulmonary bypass surgery, also known as open heart surgery, is a type of surgery that uses a heart-lung machine to temporarily take over the function of the heart and lungs during surgery. The heart-lung machine pumps oxygenated blood through the body while the heart is stopped, allowing the surgeon to perform complex procedures on the heart. The surgery is used to treat a variety of heart conditions, including coronary artery disease, valve disease, and congenital heart defects.