Simple obstruction refers to intestinal obstruction where there is no evidence that there is a great risk of perforation. This may occur for instance in an open loop large bowel obstruction, where the ileo-caecal valve does not impede reflux into the small intestine and the bowel may decompress back through the stomach and by a naso-gastric tube or vomiting. Another example would be a band adhesion across the small intestine which similarly decompresses.
A complex obstruction refers to obstruction associated with a bowel that is ischaemic or perforated. This may occur in closed loops, or when there is necrosis of a part of the bowel wall, eg. pressure necrosis where it is stuck against the neck of a hernia or a band adhesion.
Clinical evidence to support a diagnosis of complicated obstruction, would be; an ill patient, abnormal vital signs, clinical evidence of peritonitis, leucocytosis and abnormal imaging eg. pneumo-peritoneum.