Surgery Of The Small Intestine
M. Joseph Bojrab, D.V.M., M.S., Ph.D.
The small intestine is composed of four layers of tissue, the tunica mucosa, tunica submucosa (provides the main holding power in the intestine), tunica muscularis (the inner circular layer and outer longitudinal layer), and tunica serosa, it is important to be able to differentiate between the small and large intestine. The intestinal size is not a valid criterion since the small intestine may be distended and the large intestine may be small and contracted. The large intestine is usually lighter in color and the longitudinal muscular layer is usually more prominent. Also, the vasa recti of the colon are usually larger than those of the small intestine.
In general, the proximal jejunum is thicker walled and more prominent than the small intestine. Vessels coursing along the antimesenteric border readily identify the distal ileum. The cecum is located at the junction of the ileum and the colon. The duodenocolic ligament runs from the caudal flexure of the duodenum to the descending left colon (sometimes mistaken for an adhesion).
Mechanical obstructions are the most common intestinal surgical problems encountered in companion animals. The extent of the symptoms and metabolic changes depend on the site and degree of obstruction, Simple obstruction is when the lumen of the bowel is compromised without interference to its blood supply. Impairment of intramural circulation may ensue from increasing distension, especially just proximal to the obstruction. Where there is severe interference with blood supply to a segment of intestine and venous return is interfered with, strangulation obstruction is encountered. This type of obstruction is commonly seen in intussusceptions, hernias, and torsions.
A simple obstruction can be either a high or a low obstruction. A high obstruction refers to the region of the pylorus, duodenum and proximal jejunum. A low obstruction refers to the distal jejunum, ileum, colon and rectum.
A high obstruction is indicated by vomiting (the predominant sign), due in part to the high amplitude of contractions in the proximal portion of the intestinal tract. If fluid loss is predominantly gastric juice, then more chloride is lost than sodium, and alkalosis occurs as plasma bicarbonate builds up in extracellular fluid. Dehydration proceeds to the point of oliguria and decreased cardiac output, manifested by lack of skin pliability and markedly elevated PCV. High obstruction will cause the death of the patient in 3-4 days, whereas obstructions in the ileum will allow the animal to live 5-7 days. If the descending colon is obstructed, the animal can survive for a long time and starvation may be the apparent cause of death. If obstruction is below the bile and pancreatic ducts, then the loss is similar to plasma with sodium and potassium lost as well as chlorine.
Low obstructions cause an increase in intraluminal content of fluid and gas from ingestion. This will cause a slight distension and subsequent and violent contractions. Gas present is mostly swallowed air. Eventually, further distension and paralysis of the proximal intestinal segment occurs, Intestinal distension actually promotes increased secretion and decreased fluid absorption. As distension progresses, intestinal venous congestion occurs and fluid will be taken up by the bowel wall and exude from the serosal surface. Toxic peritoneal fluid will eventually occur as the intestinal wall loses its viability.
A strangulation obstruction is an embarrassment of venous supply of the bowel wall that promotes additional movement of flow (including blood and plasma), into the obstructed area. Ischemia of the bowel wall enables bacteria and their toxins to enter the peritoneal cavity. Luminal fluid, plasma, and blood shift into the peritoneal cavity and severe hypovolemic changes commence. A complex series of events follow: constriction of hepatic veins and visceral pooling, capillary obstruction from sludging and cardiac changes & profound tissue hypoxia, and severe metabolic acidosis.
Another important event occurring in intestinal obstruction is the rapid proliferation of bacteria. During small intestinal stasis, whatever the cause, bacteria proliferate rapidly. Bacteria and bacterial toxins probably don't cross normal intestinal mucosa associated with a simple mechanical upper small bowel obstruction. They play a significant role in the pathogenesis of a strangulation obstruction. Here the mucosa plus intestinal wall undergo pressure necrosis caused by a sustained increase of intraluminal pressure, and eventually allow lethal factors to pass through.
Pre-operative antibiotics are considered important to minimize potential contamination of the peritoneal cavity during surgery and post-op.
When performing an intestinal resection and anatomists, one shouldn't be too hasty to resect intestine following relief of obstruction. Bathing the involved segment in a warm tissue sponge may return color and peristalsis. if tissue color remains blue- black and dull after five minutes, resection should be performed. The most likely place for foreign bodies to lodge is the jejunal flexures or the ileocecolic junction. The determination of viability of an intestinal sequence is based on pulse, pink color, and presence of peristalsis.
Intestinal resection surgery exteriorizes the involved segment of intestine from the abdominal cavity in a manner that prevents abdominal soilage and does not interfere with the blood supply to the intestine. The remaining intestine is packed off with warm moistened laparotomy pads. Intestinal segments are handled as little as possible. They should not be grasped with thumb forceps. The ends of intestine to be anastomosed should be supported with warm moistened sponges rather than intestinal clamps in a manner that prevents gross soilage, eases suturing, and minimizes intestinal trauma.
The arcadial vessels to the diseased segment are ligated with 3-O synthetic absorbable suture material three times. Adequate mesentary is left to adjacent vessels to facilitate later closure of the mesenteric defect. Next the communicating branches of the vasa recta vessels along the borders of each healthy segment are ligated. The segment to be removed should include 4 cm of healthy intestine at each end. Intestinal contents near the transection site are milked away and the assistant's fingers are applied across healthy intestine 5 cm from proposed transection site. Using a #10 surgical blade, or a Radio Surgery Unit (Ellman) a 30-degree angled transection that shortens the antimesenteric border is made. This improves the blood supply to the anastomotic edges. If the intestinal end apertures are of different sizes, the smaller section may be cut at a more acute angle to increase its lumen size.
A simple interrupted pattern with 3-0 synthetic absorbable suture material on a curved taper needle is used to close the two free ends of intestine. The first suture is placed at the mesenteric intestinal junction, 4-5 mm from the edge. It should enter the serosal surface, pass through all layers into the lumen of the first segment, and exit from the 2nd segment by penetrating the mucosa out through the serosa 4-5 mm from its cut edge. The submucosal layer is the strongest and its penetration is necessary for suture security. The first throw is put in the suture and is pulled very firmly so that the suture material cuts through the serosa and mucosa down to the tough submucosa. The crushing suture is tied with 3 more half hitches. The ends of the suture are cut short. The simple interrupted suture apposed the submucosa of the 2 edges, thereby causing accurate approximation of the serosal and mucosal layers of each side there is no inversion. Or eversion of tissue and the lumen is not decreased in diameter. Because the suture has been cut through the soft tissue, the pressure is released on the microcirculation of the serosal, muscular, and mucosalsurfaces; and greater capillary blood supply is re-established to the cut edges. This procedure is labeled the crushing technique.
The second suture is placed 180 degrees away from the first suture in the antimesenteric border of the intestine. Place sutures 4-5mm from the cut ends, penetrating all layers of bowel and tie in a manner that apposes the tissue firmly. The remaining borders are approximated. The same simple interrupted suture pattern is used placing sutures 3-5 mm apart along the incision line. After the entire circumference has been anastomosed, it is inspected for potential leaks and if they are found, additional sutures should be placed. The optimum number of sutures depends on the size of the intestinal opening.
The defect in the mesentery should be tacked together, being careful to preserve the blood supply to the intestine. 3-0 or 4-0 absorbable suture is recommended. Rinse the area of the anastomosis and all the other soiled abdominal areas clear of debris with warm lactated Ringers solution and place the intestine back into the abdomen.
Post-operatively the patient should be watched for signs of infection or ileus. Post-op oral and systemic antibiotics should be continued or 7-1O days. Feeding can begin as soon as the animal completely recovers from anesthesia.