Enteral Nutrition
World Small Animal Veterinary Association World Congress Proceedings, 2005
Michael Willard
Texas A&M University
College Station, TX, USA

There are many possible options for someone wishing to supply nutritional support to a patient that either will not or cannot eat. Intravenous nutrition is a very sure way to supply calories and protein, and usually only requires a dedicated IV line and some level of expertise. However, intravenous nutrition is time and labor intensive as well as very expensive, and has the potential to cause serious metabolic complications as well as have associated infections. Enteral nutrition is usually more involved from the standpoint of needing to place feeding tubes in various parts of the alimentary canal. But, enteral nutrition is much easier to administer, has less risk associated with it, is less expensive, and supplies nutrients to the intestinal mucosa (something which parenteral nutrition does not do). When considering enteral nutrition, there are two main questions: a) what will you feed? and b) how will you feed it?

For dogs and cats with severe intestinal disease, including inflammatory bowel disease (IBD), elemental diets are a consideration. While the pathogenesis of IBD is unclear, it is reasonable to hypothesize that some initial insult causes increased mucosal permeability (i.e., paracellular permeability) that allows the inflammation to become self-perpetuating. Large molecular weight nutrients that gain access to the mucosal tissues via paracellular pathways can elicit substantial inflammation and thereby maintain the inflammatory disease. It becomes a positive feed back situation in which mucosal inflammation of any cause leads to increased mucosal permeability which leads to macronutrients gaining access to the intestinal submucosa which causes more mucosal inflammation which continues to increase mucosal permeability which leads to more nutrients gaining access to the submucosa which. Therefore, feeding very small molecular weight nutrients can be helpful if the molecular weight is so small that the nutrients do not elicit an immune response.

Oligomeric elemental diets are composed of amino acids, simply sugars, and perhaps medium chain triglycerides. These are usually human products (e.g., Vivonex) which come as a powder that is reconstituted. Because they are formulated for people, they are often protein deficient for cats and some dogs. Therefore, we typically mix one packet of Vivonex HN with 350 ml of water and 250 of 8.5% amino acids. While this is a very high quality diet, it is relatively expensive and often causes watery diarrhea in patients with severe intestinal disease (which are the patients that you would be giving this diet to in the first place) due to its high water content.

Polymeric elemental diets are composed of larger nutrients than are found in oligomeric elemental diets. There are some commercial partially hydrolyzed diets that can be used as polymeric elemental diets. While often marketed as hypoallergenic diets, the author prefers to use them to maximize nutrient absorption in dogs and cats with significant small intestinal disease that is causing weight loss.

Hypoallergenic diets are commonly used in animals with gastrointestinal disease. While elemental diet would generally qualify as elemental diet, most elimination diets are not elemental but rather are a novel protein source combined with a carbohydrate that is unlikely to elicit a hyperimmune response. Homemade hypoallergenic diets, although more trouble to prepare than commercial diets, are often more restrictive than any commercial diet. If a homemade hypoallergenic diet works, then it has been established that diet can be a successful therapy, and it is reasonable to embark upon a protracted search for an effective commercial hypoallergenic diet that works as well as the home made diet. The homemade hypoallergenic diet does not usually have to be completely balanced if it is going to be used for a relatively short time period (i.e., < 6-9 months) in a sexually mature dog or cat. One part of a protein source (e.g., white turkey meat without the skin or white fish meat) plus two parts of a carbohydrate source (e.g., potato) is often sufficient.

If the patient will not eat and enteral nutrition is desired, then there are five basic tubes that can be placed. Each tube is named for the site at which it enters the alimentary canal. There are nasopharyngeal tubes, pharyngostomy tubes, esophagostomy tubes, gastrostomy tubes, and enterostomy tubes. The choice of which tube to use is dependent upon what you wish to bypass. Nasopharyngeal, pharyngostomy and esophagostomy tubes just bypass the mouth, so they are primarily indicated with dealing with oral injuries or with animals that simply refuses to eat. Gastrostomy tubes can be used for the same situations as well as for animals with esophageal diseases. Enterostomy tubes are generally reserved for patients with gastric or pancreatic diseases. One should usually choose the easiest tube to place and maintain that will accomplish the desired goal.

Nasoesophageal tubes are inserted into the nostril and advanced through the nostril and into the esophagus until the thoracic inlet or the base of the heart is reached. They require almost no equipment besides the tubing, but they are usually of relatively small diameter (as would be expected in tubes that have to go through the nostrils), meaning that commercial liquid diets are required. They are especially useful in critically ill animals in which one is desirous of beginning microalimentation, but cannot afford to anesthetize for tube placement. It is difficult to position them incorrectly, but it can be done. It is important to recognize that not all dogs and cats with a tube inserted into the trachea will cough and act like the tube is misplaced. One should always instill water or saline through the tube the first time, to be sure that it does not elicit a cough (meaning that the tube has gone down the trachea instead of the esophagus). Not only are nasoesophageal tubes relatively easy to place, but they may be removed anytime after they have been placed.

In nasoesophageal tubes as well as pharyngostomy and esophagostomy tubes, one must be sure that the tube does not extend across the lower esophageal sphincter into the stomach. If the tube goes across the lower esophageal sphincter into the stomach, gastroesophageal reflux may be the result and cause esophagitis (which can be severe).

The following four tubes are all placed through an incision in the skin. Therefore, some degree of infection is unavoidable. However, by using sterile technique as much as possible, infection can be kept to a minium. The ostomy site should be lightly bandaged to keep it clean, and the site should be inspected every 1-3 days, depending upon the patient. If a patient is having problems with excessive infection, then one may apply disinfectants such as topical antibacterial ointments. However, it is generally advantageous to keep the ostomy site as dry as possible.

Pharyngostomy tubes allow home made gruels to be administered, but should be avoided unless one is familiar with and adept at placing them. It is easy to misplace the tube, resulting in constant gagging of the patient. Proper placement will allow one to use them successfully, but the smaller the patient, the easier it is to misplace them. The end of the tube is near the head, which can be disadvantageous in animals with bad dispositions. Like the nasoesophageal tube, they can be removed at any time after they are placed.

Esophagostomy tubes are the most desirable as they are easy to place, require minimal equipment, allow use of home made gruels (thereby diminishing the client's expense) and are the safest to place and use. It is important to make sure that the tube is straight in the esophagus and does not have a coil in it that will cause obstruction. Like the nasoesophageal tube, they can be removed anytime after they are placed. Very rarely, a stricture can result where the tube perforates the esophagus. Like pharyngostomy tubes, the end of the tube is near the head, which can be disadvantageous in animals that bite. These tubes can be used for weeks to months.

Gastrostomy tubes can be placed surgically, endoscopically, or with a dedicated device (e.g., Eld device). They allow home made gruels to be fed, and are not near the head (which can be advantageous in biting animals). However, the peritoneal cavity must be transversed when placing them, meaning that pancreatitis can result if leakage occurs. Such leakage can be more likely than some people realize, especially if the tube placement is a little bit off and results in a bit more tension on the stomach than desired. In particular, use of dedicated devices for placement can easily result in misplacement (i.e., either in the distal esophagus or too far back in the stomach). Once the tube is placed endoscopically or with a device, it generally is left in place for 12-14 days to allow a good seal to occur between the stomach and body well so that no leakage occurs after the tube is pulled out. In large breed dogs (which are more likely to have some difficulty with movement of the stomach and lack of an adhesion between the stomach and the abdominal wall), one may make small 1 cm incisions on either side of the tube and close them, incorporating the gastric wall into the wall closure. This will help insure a good seal between the stomach and abdominal wall, and thus no peritonitis. If long term use of a PEG tube is desired (i.e., months to years), one may replace the tubes as they wear out by inserting low profile replacement tubes.

Enterostomy tubes are placed surgically during abdominal exploratories, although they can be place with laparoscopes. Their use requires the clinician to give a near constant infusion of commercially prepared liquid nutrients.


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Speaker Information
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Michael Willard, DVM