Microenteral nutrition (Proceedings)

Nutritional supplementation is vital to the recovery process for most disease processes and after traumatic injury. However, many of these patients do not receive sufficient nutrition during the healing and recovery process. Over time, this lack of proteins, minerals, and energy substrates can lead to a state of general illness, malnutrition, and profound disability, a condition defined as cachexia. A cachexic patient has the potential to develop anemia, reduced cardiac mass and function, decreased pulmonary function and respiratory drive, and altered intestinal morphology and impaired absorptive ability. In human studies, the early use of enteral nutrition has resulted in a decrease in infectious and noninfectious complications, development of SIRS or MODS, length of stay in the ICU and hospital, and mortality. However, some patients are not able to tolerate full nutritional requirements by an enteral route due to intractable vomiting or other significant gastro-intestinal disease states. In these situations, providing small amounts of nutrients to the cells lining the gastro-intestinal tract, complications of disease such as gut derived sepsis may be avoided. Micro-enteral nutrition is the delivery of small amounts of water, electrolytes, and readily absorbed nutrients directly to the gastro-intestinal tract to maintain mucosal cell integrity.

The gastro-intestinal tract receives it's nutrients from intra-luminal absorption. Food within the gastrointestinal tract serves as a direct source of nutrients to the mucosa. There are specific "gut fuels" that are preferentially used as energy substrates by enterocytes, colonocytes, and immune cells that may be present depending upon the composition of the diet. The enteral route of feeding also stimulates mesenteric blood flow, the autonomic nervous system, secretion of various digestive enzymes, hormones, and growth factors, increases GI mucus production, and helps prevent ileus. Finally, the sources and processing of nutrients in a diet can influence the make-up of the gastrointestinal microflora. When the gastro-intestinal tract does not receive sufficient nutrients, several pathologic consequences can occur. These include villous atrophy leading to increased mucosal permeability, decreases in gut-associated lymphoid tissue (GALT), and decreased surface area for absorption of nutrients for systemic use.

When gut barrier failure occurs in critical illness secondary to shock, trauma, or sepsis, this allows translocation of bacterial and endotoxin into the portal and, sometimes, systemic circulations. This gut derived sepsis may lead to a systemic inflammatory response (SIRS). SIRS can alter the function of distant organs, possibly leading to multi-organ failure and death.

Patients that are candidates for micro-enteral nutrition include those patients that are predisposed to or already have developed stress gastric ulceration, patients with oro-facial or cervical injury, patients that are actively vomiting, and those patients that are early in recovery from severe gastro-intestinal disease or gastro-intestinal surgery. Microenteral nutrition is commonly delivered by naso-esophageal or naso-gastric feeding tubes. Naso-esophageal and naso-gastric tubes are among the easiest and least expensive forms of feeding tubes available for use. Argyle infant feeding tubes are ideal, are soft and pliable, and are well tolerated after placement by most patients. Red rubber feeding tubes are a viable alternative, although they can more reactive with the nasal mucosa.

A naso-esophageal feeding tube should be placed in patients with expected or known short-term anorexia that are not vomiting, and have no problems with esophageal motility or strictures. A 3 1/2-5 French tube can be placed in patients that weigh < 10 kg, and an 8 French tube placed in patients that weigh > 10 kg. After the appropriately sized tube is chosen, the patient's nostril and nasal passage are anesthetized with a topical anesthetic agent such as lidocaine or proparacaine. The patient's head should be tilted up to allow the entire nasal passage to be lubricated with the topical anesthetic agent. The tube can be placed in a freezer to make it slightly more rigid for ease of placement. A stay suture should be pre-placed at the lateral aspect of the external nares. The tube should then be measured to the level of the carina to avoid passing it past the lower esophageal sphincter. The tube should be visibly marked from the tip of the tube to the level at the tip of the nose. The distal tip of the tube should be lubricated, and then passed on the ventral floor of the nasal cavity, tilting the nose upwards and passing along the ventro-medial floor of the nasal passage, past the pharynx, until the patient swallows. Once the patient has swallowed, the tube can quickly be passed to the level of the mark. The tube is secured with a friction suture attached to the stay suture. The rest of the tube can be secured between the eyes and on the dorsal nasal planum with sutures or surgical staples. Proper tube placement can be checked by ausculting over the thorax and injecting a small volume of air into the tube and listening carefully for borborygmi. Alternatively, a lateral thoracic radiograph can be performed to check tube placement. The purpose of not allowing the tube to pass through the lower esophageal sphincter is to avoid gastric reflux of hydrochloric acid and possible esophagitis.

Alternatively, a naso-gastric tube can be placed for both enteral nutrition and for gastric decompression in cases of gastric atony, as seen with severe Parvoviral enteritis and pancreatitis or ileus. Placement procedure is identical except the tube is measured to the level of the last rib to ensure it passes the lower esophageal sphincter. All patients with a naso-gastric or naso-esophageal feeding tube should have a firm Elizabethan collar placed to prevent the patient from removing the tube. Topical anesthetic agents such as topical proparacaine or lidocaine can be placed in the nostrils as necessary to help prevent irritation and sneezing. Potential complications of naso-gastric or naso-esophageal feeding include aspiration pneumonia, vomiting, diarrhea, and tube obstruction.

Esophagostomy tubes are an alternative in patients with oro-facial trauma. Gastrostomy tubes can be surgically placed in patients that undergo an abdominal surgical procedure or percutaneously in patients with oro-facial or esophageal injury. These tubes have the advantage of being able to provide for longer term enteral nutritional needs. Their main disadvantage is that placement does require general anesthesia.

Micro-enteral feedings are accomplished at a low, continuous rate infusion. Feedings are started at rates of 0.25-0.5 ml/kg/hr to assess the patient's tolerance for feeding. If tolerated, rates can be increased to 1-2 ml/kg/hr. The patient is monitored for deleterious effects of administering large volumes of fluid/food to the patient, including vomiting and potential aspiration, worsening of pancreatitis, and diarrhea. Gastric decompression of both air and residual fluid, if indicated, is beneficial. The removal of residual air can decrease gastric distention, making the patient more comfortable, decreasing the chance of emesis, and increasing gastric motility. The removal of residual fluid/food will also decrease gastric distention as well as prevent food from stagnating within the stomach if limited motility is present. The volume of fluid should be quantified to help monitor fluid balance (ins/outs) and electrolytes should be monitored since gastric secretions are being removed along with the introduced food/fluid.

The formulation chosen is just as important as supplying nutrition early. Oral formulations are available as electrolyte solutions and in monomeric or polymeric liquid diet formulations. Oral electrolyte solutions (Rebound OES, Virbac AH, Inc, Fort Worth, TX) contain fluid and electrolytes. They can be supplemented with dextrose to provide a carbohydrate source. They are often used initially to ensure some degree of tolerance to enteral feeding prior to changing to a monomeric or polymeric diet. Elemental or monomeric diets are predigested proteins and sugars, and are a mixture of amino acids, di- and tri-peptides, and monosaccharide sugars. Due to their high osmolality, monomeric diets often have the associated complication of causing diarrhea. There are no monomeric diets available specifically formulated for the complete nutritional needs of dogs or cats. Polymeric diets require that polypeptides, proteins, and polysaccharides are digested in the gastrointestinal tract prior to the nutrient absorption. Many polymeric formulations are iso-osmolar and are not associated with as frequent diarrhea as monomeric diets. Clinicare (Abbott Laboratories, North Chicago, IL) and Rebound Liquid Diet (Virbac AH, Inc, Fort Worth, TX) are polymeric liquid diets that contain 1 kcal/ml and 0.85kcal/ml respectively, and are both good choices for enteral feeding through feeding tubes.

Glutamine supplementation is a hot topic in veterinary nutrition. In human nutrition, glutamine is a conditionally essential amino acid that has multiple functions in the sick or injured patient. Glutamine has been shown to be necessary for the production of energy by the liver and kidneys, and is the preferred energy source in certain cells including renal tubular cells, enterocytes, immune cells, and endothelial cells. Glutamine is a "competence factor" necessary for intestinal cell proliferation, intestinal fluid and electrolyte absorption, and mitogenic response to growth factors. Glutamine deprivation produces apoptosis. It is available as a stable powder and is supplemented at a dose of at least 1 gram/kg body weight per day. However, glutamine-rich intact proteins appear to be more effective in increasing mucosal glutamine content than glutamine-enriched solutions. Glutamine has been experimentally shown to decrease the time of post-operative gastro-intestinal ileus in dogs. Further studies regarding the necessity of this amino acid in critically ill under-nourished patients are still pending for our veterinary patients.

The use of early enteral nutrition in small doses is an inexpensive, easy to perform, beneficial procedure that may reduce morbidity. It should be incorporated into the treatment plan of many of our sick or traumatized patients.

References furnished upon request