Reperfusion injury and low-flow states in which hypoperfusion of the GI tract is suspected must be considered when initiating enteral feedings. Certain clinical conditions frequently observed in critically ill patients such as hypovolemia, hypotension, and hemorrhagic and septic shock pose a risk for low splanchnic blood flow that can lead to GI dysmotility, increased mucosal permeability, endotoxemia, and multiple system organ failure (26, 27, 28). Disproportionate vasoconstriction can occur in response to an insult sustained during critical illness, raising concern that initiation of EN may be poorly tolerated by an underperfused intestine and intestinal ischemia is a potential but rare complication of EN (29, 30, 31, 32, 33, 34).

Despite these concerns, evidence indicates that with appropriate patient selection, careful initiation and close monitoring, EN can be used successfully in critically ill patients (30, 32, 33, 34, 35). EN may be provided guardedly to patients who are receiving inotropes or pressor agents, while observing for signs of intolerance or gut ischemia (25, 31, 33, 34). EN should be conservative until the patient is stabilized in some situations. For patients requiring significant hemodynamic support (including high-dose vasopressor use, alone or in combination with large volume fluid or blood product resuscitation to maintain cellular perfusion), EN should be withheld or delivered at a very low rate of infusion until the patient is fully resuscitated and stable (24, 34).

Patients who have had GI anatomy altered either by surgical resection, reconstruction, or replacement with intestinal transplantation can be fed enterally. The choice of enteral feeding will be largely dependent on the GI anatomy and health of the mucosa. An intact stomach, pancreas, and liver will enhance digestion and absorption. For the patient with SBS, several factors influence tolerance to EN (34, 36, 37, 38, 39, 40, 41). In general, adults with more than 100 cm of small bowel ending in stoma or more than 60 cm of small bowel anastomosed to the colon can tolerate EN (via oral food or tube feedings) and achieve adequate nutrition with close monitoring and individualization of the diet. The site and extent of the surgical resection will also impact the patient’s ability to tolerate EN. Digestion and absorption of
most nutrients occurs in the duodenum and proximal jejunum, whereas the distal 100 cm of ileum is responsible for absorption of vitamin B₁₂ and bile salts. Patients with jejunal resections generally tolerate EN unless more than 75% has been resected. The transit time is usually normal and they retain the ability to absorb vitamin B₁₂ and bile salts. Resections of the ileum are associated with greater malabsorption. Malabsorption of bile salts can cause fat malabsorption, steatorrhea, and loss of fat-soluble vitamins (42). Since the terminal ileum is the site of vitamin B₁₂ absorption, the patient will require supplemental parenteral or nasal vitamin B₁₂ for life if the terminal ileum is resected. Patients may also experience rapid intestinal transit and small bowel bacterial overgrowth leading to intolerance of enteral formula. Loss of the ileocecal valve may result in decreased transit time through the proximal gut and loss of fluid and nutrients. Without the ileocecal valve, colonic bacteria can reflux and colonize the small bowel (bacterial overgrowth), which inhibits digestive enzyme activity and worsens chronic diarrhea and GI nutrient loss. For the patient with SBS, bacteria in the colon metabolize undigested carbohydrate and soluble fiber into short-chain fatty acids which provide a source of energy, aid in fluid and electrolyte absorption, and stimulate intestinal adaptation.

A standard, polymeric formula with complex nutrients (intact proteins, typically casein or whey, as well as glucose polymers, and a mixture of long- and mediumchain triglycerides) can be used for most patients with compromised intestinal function. The addition of soluble fiber is especially useful in patients with an intact colon to improve absorptive function and serve as a source of energy via generation of short-chain fatty acids (acetate, propionate, and butyrate) from bacterial metabolism of malabsorbed fiber in the colon (35).

Although the length of the remaining intestine is critical for successful EN in SBS, the health of the intestinal mucosa is also an important consideration. If the mucosa of the remaining bowel is diseased (i.e., Crohn disease, radiation enteritis), absorption will be impaired. For these patients, the use of a predigested EN liquid formula may enhance nutrient absorption and reduce output.

The ability to tolerate EN following surgical resection is also dependent on the amount of adaptation that has occurred (43). Medications (e.g., antidiarrheals, pancreatic enzymes, bile acid sequestrants, antibiotics, and probiotics) can be used during EN therapy to enhance absorption and reduce GI symptoms (44). EN support is also used following intestinal and multivisceral transplantation in those individuals with permanent intestinal failure (45). A jejunostomy tube is placed directly into the allograft at the time of surgery and EN commences within 2 weeks following transplantation (46). As EN is advanced, PN is reduced and oral nutrition is initiated. A standard polymeric formula that is high in protein and low in potassium is used because there are no data to suggest significant malabsorption of the intestinal allograft.