A surgical re-constructive procedure that involves connecting a piece of proximal small bowel usually sideways into a more distal small bowel.
Bypass of Distal Stomach or Proximal Duodenum
A perforated pyloric ulcer, distal gastric cancer or proximal duodenal ulcer may require removal of the stomach antrum and proximal duodenum.
The role of Roux-en-Y following resection of the distal stomach or proximal duodenum can be best understood by considering the alternatives. If the amount of stomach removed is modest, the duodenal stump and stomach can be anastomosed end-to-end (Bilroth I operation). If a larger amount of stomach is removed, the end-to-end anastomosis performed in a Bilroth I procedure is more difficult. Instead, a gastro-enterostomy is performed, i.e. the gastric remanant is anastomosed to a more distal part of the bowel. The free end of the duodenum is oversewn. This is the Bilroth II procedure. The disadvantage of this is that bile can flow into the stomach, causing dyspepsia.
In the Roux-en-Y procedure, the small bowel is divided, giving two ends: the bilio-pancreatic limb and Roux limb (also known as the Roux loop). The duodenal stump and the jejunum that follows is forms the bilio-pancreatic limb. The bilio-pancreatic limb is anastomosed onto the Roux loop as an entero-enterostomy to form the common limb. The Roux loop is formed from the remaining small bowel and the free edge is brought up to the stomach and anastomosed to the gastric remnant as a gastroenterostomy. If there is no stomach remnant, as in a total gastrectomy, the jejunum can be directly anastomosed to the oesophagus.
In this situation there is nothing wrong with the distal stomach or duodenum. The stomach is divided into a much smaller pouch (which forms the new stomach) and a larger part which is bypassed. The smaller stomach induced early satiety and reduces appetite.
The larger, bypassed stomach, along with the duodenum and jejunem form the bilio-pancreatic loop. This is connected some distance down a long Roux loop. Bile and pancreatic enzymes only mix where the loop joins the Roux loop. In a long Roux loop, the effective absorptive area is significantly reduced as mixing takes place with little remaining small bowel to absorb food, causing an iatrogenic malabsorption that leads to weight loss.
See animation at https://www.svch.net/bariatric/surgicalpro/open-gastric-bypass.htm