Gangrenous appendicitis. Illustration of an appendix (lower right) that has developed gangrene in a case of appendicitis. Appendicitis is the acute inflammation of the appendix, a small finger-shaped branch of the large intestine. If untreated the inflamed appendix can burst, causing infection of the lining of the abdomen (peritonitis). The usual treatment for appendicitis is surgery to remove the diseased appendix (appendicectomy). Gangrene is a form of tissue death (necrosis) caused by insufficient blood supply.