How to enterally refeed a critically ill patient

Small bowel atrophy begins within three days of nothing per os (NPO), but is necessary in the management of some patients.

NPO is a common procedure for patients with vomiting and/or diarrhea, pancreatitis, parvoenteritis, megaesophagus, inflammatory bowel/lymphoma, intestinal surgery/short bowel syndrome.

Clinical consequences of an atrophied small bowel are reduced local synthesis and repair, and decreased local and systemic immunocompetence. This patient is more susceptible to systemic infections with the translocation of bacteria and toxins from the bowel.

A septic patient is more likely to be anorectic which results in the continued downward spiral of protein-calorie deficiencies unless there is interventional feeding.

• Normal GI morphology

Food nutrients and dietary ingredients maintain normal GI morphology and function via chemical and mechanical stimulants.

The type and quantity of nutrients alter mucosal cell mass by affecting the rate of stem cell division. Physical presence of food in the intestines stimulates the release of endogenous and hormonal secretions. Gastric, duodenal and pancreatiobiliary secretions promote mucosal structure and function.

• Small bowel atrophy

Intestinal integrity is maintained by trophic hormones, blood flow, neurologic input, enteral nutrients and dietary ingredients. What nutrient and dietary characteristics would we want in a product used to begin refeeding an atrophied small bowel?

Small bowel atrophy is characterized by decreased villus height and crypt depth, decreased surface area and motility, decreased brush border enzymes for digestion, decreased secretions and immunity with increased translocation of bacteria, endotoxin and cytokines.

During NPO, there is an absence of positive intraluminal nutrient and dietary factors which lead to decreased hormonal and neurologic stimuli and blood flow.

The small bowel has lost 22 percent of its total weight, 28 percent of its mucosal weight, 35 percent of its mucosal protein, and 25 percent of its mucosal DNA by day seven of NPO. When refeeding these patients, what would logically be the most desirable characteristics of the first food product fed to a patient with an atrophied small bowel?

Protein: Protein digestion is begun in the gastric mucosa with pepsin, but completed by pancreatic enzymes secreted into the intestine via the common bile duct and mucosal brush border enzymes (endopeptidases and exopeptidases). Metabolism of dietary protein is dependent upon the complete digestion and then absorption of single, di- and tripeptides using a sodium-dependent gradient active transport system.

With NPO treatments, gastric and pancreatic protein digestion may remain functional. However, with an atrophied mucosal lining, the complete digestion and absorption of dietary amino acids has to be questioned.

Currently, the recommendation is to provide a hydrolyzed protein source such that dietary di- and tripeptides are fed. These are the most readily absorbable forms of protein in the absence of digestion.

The intestinal uptake of glutamine by the small bowel is known to increase with surgery and trauma to the intestines.

Glutamine is the preferred fuel source for enterocytes. In 80 percent of the published animal data, there is a positive effect with glutamine- enriched feedings over those not containing glutamine. Specifically in dogs, there was an increased intestinal requirement for glutamine during the immediate postoperative phase (<7 d) of intestinal surgery. Intestinal uptake rates returned to normal after day 10 postoperative. Glutamine is a not an essential amino acid, according to AAFCO and is not found in all pet food products. Glutamine is considered a conditionally essential nutrient, probably needed only during periods of physiologic stress to stimulate DNA synthesis and increase mucosal mass early in recovery.

Although glutamine may stimulate stem cells, all essential nutrients will be required for the enterocyte to complete division and maturation. Therefore, a complete and balanced food product specific for the dog or cat containing approximately 2 percent glutamine is recommended.

Fat: Macro molecules of dietary fat are digested with gastric lipase to triglycerides which are then emulsified by hepatic bile acids with pancreatic lipase enzymes into smaller free fatty acids and monoacylglycerides. These smaller forms are absorbed into the enterocytes and repackaged into either chylomicrons exported into the lymphatics or medium chain triglycerides exported to blood bound to albumin. The functionality of fat digestion in a patient with bowel atrophy is generally intact because it is not dependent on mucosal enzymes; however, with an atrophied mucosal lining, the absorption of fat may be a problem.

Currently, the recommendation is to provide a fat source in the form of long and medium chain triglycerides. A source of fat in the diet is necessary to increase the caloric density and provide essential fatty acids, primarily linoleic acid.

Carbohydrate: Intraluminal digestion of starch digestion occurs with enzymes from the pancreas, however the final stage of carbohydrate digestion and absorption occurs with the mucosal brush border enzymes (e.g., lactase, sucrase, isomaltase and maltase). With decreased mucosal mass, there will be a loss of brush border disaccharidase enzyme activity. Most disaccharide enzymes are "diet inducible," i.e., they will return with refeeding as the mucosal mass regenerates.

However, initially when readily available carbohydrates are not digested in the small bowel, starches and sugars enter the large bowel where microbial action produces changes in osmolarity, water flux and gas, resulting in diarrhea.

It would appear however, that brush border maltase activity remains functional and is not affected by intestinal atrophy. Therefore, initial enteral feedings using a product containing maltodextrin carbohydrate source is advisable when refeeding.

Fiber: Fiber is generally thought to be indigestible and not often considered when refeeding a patient but should be a component of the refeeding diet. Fiber modulates intestinal motility, i.e., slows a hypermotile state and stimulates a hypomotile state.

Fiber in the diet provides intestinal bulk because it is indigestible which then provides that intraluminal physical stimuli the entire length of the small bowel to re-establish normal peristaltic action and transit time. Fiber incorporated into initial refeeding formulas helps to normalize intestinal water content and buffer intraluminal toxins, and hence controls diarrhea.

Therefore, moderate amounts of dietary fiber, although contrary to 'feeding a highly digestible diet', is desirable in the first feedings to a patient with an atrophied small bowel. It is possible to have both a highly digestible and indigestible fraction of a diet with each performing different functions.

Diets not requiring ingredient digestion and with nutrients in a readily absorbable form are called monomeric.

Diets requiring digestion of ingredients to derived nutrients are called polymeric.

Other desirable characteristics that will improve tolerance of the first food fed to a patient with an atrophied small bowel are lactose-free and isotonic with blood (~300 mOsm/l).

An ideal diet would be a homogenized monomeric liquid nutritionally complete and balanced for the dog or cat (with arginine, taurine and carnitine) containing approximately 1 kcal/ml that could be fed per os or through a feeding tube (3-22 Fr.)

Does a single product with all the above characteristics exist for use in veterinary patients?

No, but there are two product types readily available to general practitioners fed in combination that do provide these attributes. There are two specialized liquid monomeric formulas (Peptamen by Nestlé Purina or Perative by Abbott Labs). Combining one of these liquid diets with a moderate fiber canned diet (Hill's w/d or Purina OM) has been a very successful formula for us when refeeding parvoenteritis, pancreatitis and short bowel patients. In these patients, often the first stool, if not normal, will be a soft-formed stool which is an indication of improving bowel function.

If tube feeding a patient, the fiber component will have to be omitted until per os feedings can resume. When refeeding by tube feedings only, we recommend using initially one of the monomeric liquid products for the first 24 to 48 hours followed by using a complete and balanced polymeric product (CliniCare by Abbott Labs).

Diarrhea is rare on this feeding plan as well because the CliniCare product has been properly reformulated.

The average lifetime of an enterocyte (from stem cell to exfoliation off the villus tip) is three to four days, therefore a four-day refeeding plan is recommended (Table 1).

Table 1. Per Os Feeding Recommendations

• Mucosal maintenance

For intestinal mucosal maintenance, attempts should be made in every case to feed enterally as soon as possible unless vomiting prohibits it. Relative to meeting the patient's total daily caloric need, mucosal maintenance requires a relatively small amount of enteral feeding. Small bowel atrophy has been significantly minimized with as little as 0.7 ml/kg BW/hr in people, 4-5 ml/kg BW/hr in infants or 10 percent of daily energy requirement in piglets.

Most of our patients can tolerate similar small volumes of food within 24 hours of surgery or when vomiting has ceased.

Dr. Remillard is staff nutritionist at MSPCA Angell Animal Medical Center in Boston. She received her DVM degree from Tufts University in 1987 and her diplomate certification from the American College of Veterinary Nutrition in 1991. She received specialty training as a nutrition resident at the Virginia Polytechnic Institute and did a research fellowship with the Johns Hopkins School of Medicine.

Disclosure: Consulting and research for Hill's Pet Nutrition, NestléPurina and Abbott Laboratories.