Enteral Nutrition

Nutritional support should be considered in any patient that is unable or unwilling to eat enough quantity to fulfill the caloric and nutrient requirements. Nutrient depletion is associated with increased patient morbidity and mortality [1]. Many veterinary patients have oral trauma, ulcers, or mass lesions that preclude them from prehending and swallowing food. In other cases, the patient may voluntarily eat, but has a disease process that causes severe vomiting or diarrhea and, therefore, the nutrients that are ingested do not have adequate time for digestion and absorption [2]. In one study, only 84% of dogs and 68% of hospitalized cats voluntarily consumed their resting energy requirements [3]. In another study, many patients were withheld from food prior to surgery, or had feeding orders during the postoperative period that led to inadequate offering and thus consumption of calories and other nutrients [4].

Inappetence, whether voluntary or not in the situation of stress and illness, can lead to protein calorie malnutrition and negative nitrogen balance. Depending on the patient's overall condition, primary disease process, and anticipated length of time of inappetence, some form of nutritional support should be implemented as soon as possible. In general, if a patient has sustained a 10% decrease in body weight, has increased nutrient losses secondary to vomiting, diarrhea, renal disease, or wound exudates, or if oral intake has been or is anticipated to be diminished or ceased for more than 3 to 5 days, enteral support should be initiated [5-6]. The overall goals of nutritional support in any patient should be to replenish caloric intake and nutrients, reverse negative nitrogen balance, prevent tissue catabolism, decrease the incidence of complications, quicken recovery, and improve overall survival [2]. Enteral nutrition is preferred over parenteral nutrition for a variety of reasons including it being more cost-effective and physiologic, and for maintenance of gastrointestinal health and function, decreased incidence of bacterial translocation, and improved clinical outcome. Unless a specific contraindication exists to prevent some form of enteral nutrition, any portion of the gastrointestinal tract that is functioning should be utilized [7].

Gastrointestinal Health

In addition to its important role in nutrient digestion and absorption, the gastrointestinal tract serves as a physical and immunologic barrier between the enteric bacteria and the relatively sterile environment of the rest of the body [8]. In critically ill and post-surgical patients, normal gastrointestinal protective defense mechanisms can be impaired by the use of glucocorticoids, immunosuppressive drugs, parenteral nutrition, and bowel rest [8]. The presence of luminal nutrients stimulates a complex cascade of events that serves to maintain enterocyte health, gastrointestinal barrier function, gastrointestinal immunity, and overall patient health. The cells lining the intestinal tract, enterocytes, derive approximately half their nutrient requirements from intraluminal substrates [2]. Research has documented that the presence of luminal nutrients is the single most important stimulus for intestinal mucosal growth and maintenance of barrier function [9]. Once ingested, the presence of substrates within the lumen of the gastrointestinal tract stimulates mesenteric blood flow and digestive enzyme and hormone secretion. Enteral nutrients stimulate the secretion of cholecystokinin and biliary flow [10]. Physical contact of fiber with mucosal cells directly stimulates mucosal cell proliferation. Fermentable fiber is metabolized to the short-chain fatty acid butyrate within the large intestine and provides a key substrate fuel for colonocyte health that exerts trophic effects on the gastrointestinal tract [11-12].

As gastrointestinal nutrients are absorbed, mucin secretion is stimulated. Mucin functions to lubricate the ingesta and neutralize bacterial toxins and hydrogen ions [8]. Additionally, luminal nutrients stimulate the excretion of biliary secretory IgA, an important defense mechanism against bacterial adherence and endotoxin binding to the intestinal mucosa [2,8,13]. Without luminal nutrients, even patients receiving parenteral nutritional support undergo intestinal villous atrophy and mucosal atrophy. Luminal nutrition has been shown to maintain villous height and thus improve intestinal absorptive capacity and gastrointestinal barrier function [10].

Benefits of Enteral Nutrition in the Critically Ill Surgical Patient

Within 24 hours of lack of enteral intake, enterocytes undergo atrophy and demonstrate decreased ability for digestion and absorption of nutrients [14-15]. The administration of enteral nutrition early after experimentally induced burn injury and abdominal surgeries has been shown to decrease bacterial translocation and improve clinical outcome [16-17]. In human patients undergoing small intestinal and colonic resection, early enteral nutrition maintained gastrointestinal mucosal integrity, improved wound healing, and had a lower incidence of complications [18-21]. Enteral administration of a liquid diet within 12 hours of admission caused a significant increase in weight gain and earlier return to function in canine patients with parvoviral enteritis [22]. Early enteral nutrition provided to canine and feline patients admitted to a university critical care unit was associated with a significantly increased rate of survival compared with patients that received parenteral nutrition alone [23]. The reported benefits of reduced complication rate, decreased length of hospital stay, and improved overall outcome have now led to the recommendation that enteral nutrition should be provided early in the course of illness, unless specific contraindications exist for using the gastrointestinal tract [18].

Consequences of Inappetence and Parenteral Nutrition

The absence of luminal nutrients during courses of anorexia and administration of parenteral nutrition can result in intestinal villous atrophy, a decrease in enterocyte mass, impaired gastrointestinal barrier function, decreased biliary release of secretory IgA, and suppression of brush border enzyme activity necessary for nutrient breakdown and absorption [24-25]. The consequences of mucosal atrophy are increased gastrointestinal permeability and decreased mesenteric immunity. Bacterial translocation, with stimulation of mesenteric proinflammatory cytokine release, can lead to systemic inflammatory response syndrome, and contribute to multiple organ dysfunction (MODS) in critical patients [26].

Nutritional Assessment

A patient's nutritional status should be documented ideally within 24 hours of admission to the hospital and then carefully evaluated on a regular basis in order to meet nutritional needs and monitor response to therapy. A variety of factors is typically used to determine a patient's nutritional status, and includes overall body condition, the presence or absence of muscle wasting, total protein and serum albumin concentrations, white blood cell count, and serum acute-phase protein concentrations [7]. Factors other than malnutrition such as fluid therapy, hepatic dysfunction, and increased protein loss can alter normal values; for this reason, the use of a specific indicator by itself when determining a patient's overall nutritional status is largely subjective.

Body condition score (BCS) is one method of evaluating a patient's general nutritional status. Various scales are used to determine overall BCS. This author uses a scale of 1 to 5, with 1 being cachexic and 5 being obese. A score of 3 is optimal. Negative nitrogen balance can occur in any patient with injury and illness. For this reason, even a morbidly obese patient should have caloric needs met based on their metabolic body size, not ideal body weight, while recovering from illness. To determine a patient's resting energy expenditure, use the formula: (30 x BWkg) + 70 = Kcal/day, where BWkg is the patient's body weight in kilograms and Kcal/day is the number of kilocalories per day required. In general, if caloric intake is optimal based on metabolic body size, a patient's protein and other nutrient requirements will be met, if veterinary formulas are administered. A more in depth description and discussion of metabolic nutrient requirements are discussed in Metabolism and Nutrition of the Surgical Patient.

Consideration of the individual patient's disease processes and ability to tolerate various forms of enteral feeding is necessary to determine a nutritional plan. First, optimal proportions of specific fuel sources in the form of carbohydrate, lipid, and protein should be determined [27]. Next, a specific diet and proposed means of nutrient delivery should be chosen [27]. Once nutritional support is initiated, the patient's response and tolerance to enteral feeding should be evaluated on a daily basis, at minimum. Changes to the nutritional plan should be made if complications occur or voluntary intake resumes. Once the patient's primary disease process has been treated, a plan should be formulated to transition the patient back to voluntary oral nutrient intake or to some form of indwelling feeding tube after discharge from the hospital.

Enteral Feeding Formulas

The type of enteral formula to administer should be considered based on the patient's clinical disease and ability to tolerate the type and route of feeding, the nutrient profile and specific disease or species requirements, and the individual patient's response to treatment, as well as on the cost and availability of the dietary formulation, its ease of storage and resistance to bacterial contamination and growth [5-7,28]. If a patient is going to require nutritional support for longer than 2 days, an indwelling feeding tube should be considered to minimize stress and potential trauma to the nasopharyngeal region [2,6]. The chosen diet should be balanced to provide the patient's carbohydrate, protein, lipid, and micronutrient requirements, and should be formulated to flow through an appropriately sized feeding tube without difficulty or risk of obstruction [27]. The ideal dietary formulation for tube feeding should be well tolerated, easily digested and absorbed, inexpensive, and have a long shelf life with minimal risk of bacterial contamination [29]. In general, if a feeding tube is less than 14 Fr in size, a liquid diet should be chosen. Blenderized soft food is appropriate for large (> 14 Fr) tubes, whenever possible [30].

Blenderized Diets

Blenderized diets are an appropriate means of feeding through large-bore (esophagostomy and gastrostomy) feeding tubes. Soft food is usually higher in protein and fat, and lower in carbohydrate than liquid diets, and is generally well tolerated by the patient. Hill's P/D can be mixed with water (1/2 cup of P/D with 3/4 cup of water, blenderized then strained) to form a well balanced, calorie-dense diet that can be fed through esophagostomy and gastrostomy tubes. More recently, Hill's Pet Nutrition and the Iams Company have developed A/D and Maximum Calorie diets, respectively. Both diets are extremely palatable and do not require dilution with water or blending. An added benefit of the Maximum Calorie diet is its change in form by heating. The product can be warmed in the microwave, which changes its consistency to a liquid gruel that can be administered through feeding tubes without dilution.

Liquid Diets

A variety of liquid enteral products is available for use in veterinary and human patients (Table 6-1). Liquid enteral formulations are often associated with the development of diarrhea [7]. In some cases, the osmolality of the fluid causes osmotic diarrhea. In other cases, a lack of adequate fiber or fat in the diet results in diarrhea. Supplementing with fiber or fat can slow gastric emptying time and the rate of nutrient passage through the intestines and can improve fecal consistency [7]. In general, the nutrient density of most liquid formulations used for tube feeding approaches 1 Kcal/ml. Approximately 30% of the calories should be provided as protein, 34% of calories as carbohydrate, and 36% of calories provided as lipid [27]. The addition of lipid and carbohydrates to a liquid diet improves feces consistency and decreases the incidence of diarrhea [7]. Many products developed for human use contain less than 20% of calories as protein and, therefore, are insufficient to meet canine and feline requirements [5]. Specific nutrients such as taurine are also not present in adequate quantities in human formulations. For these reasons, products developed specifically for veterinary use are preferred, whenever possible. While the caloric density is important in determining the volume of liquid to administer as continuous or bolus feeding, the type of diet can further contribute to a patient's ability to assimilate and absorb the nutrients provided. Two broad categories of enteral diets are currently available, and are classified based on the predigestion of carbohydrates and proteins present.

Monomeric Diets

Monomeric diets, or elemental diets, contain crystalline amino acids or dipeptides as a nitrogen source, dextrose or other simple sugars or oligosaccharides as a carbohydrate source, and are generally lower in fat. Fats are provided as long- or medium-chain triglycerides that require no digestion by pancreatic or brush border enzymes prior to absorption [14]. Hydrolysates of proteins provide amino acids as mono-, di- and oligopeptides [14]. It is thought that the digestion of di- and tripeptides and crystalline amino acids requires less fuel and energy than that of intact proteins and is thus more desirable in patients with impaired nutrient assimilation [14,31]. Because the nutrients are predigested, monomeric diets have a high osmolality (400 - 700 mOsm/kg) that often results in diarrhea [5]. Dilution of a monomeric diet with water sometimes can decrease the incidence of diarrhea by decreasing the solutions osmolality. However, dilution also decreases the caloric density of the diet, and requires administration of larger volumes to meet the patient's needs. It may not be physically possible to provide adequate caloric and nutrient intake when dilution is necessary to avoid diarrhea.

More recently, evidence suggests that administration of monomeric diets and elemental nutrition may influence gastrointestinal tract integrity. One study that investigated the use of a monomeric diet in cats with experimentally induced enterocolitis demonstrated that monomeric diets may contribute to gastrointestinal atrophy and increased morbidity even in the presence of gut-specific nutrients. Administration of intact nutrients, or a polymeric diet, helped retain gastrointestinal mucosal integrity [5]. For this reason, a polymeric diet is preferred over monomeric diets whenever possible. A monomeric diet should be offered only if a patient fails to tolerate the polymeric diets that are available [31].

Polymeric Diets

Polymeric diets contain intact protein, intact carbohydrate, and lipids. In general, they contain a higher percent of calories as fat than do monomeric diets. Protein is provided as casein, soy, and egg albumin [30]. Proteins are provided primarily intact and require digestion by gastric hydrochloric acid and pancreatic enzymes prior to absorption and nutrient assimilation [14]. Intact lipids provided are usually of vegetable origin, such as corn oil, and are largely composed of long-chain triglycerides [14]. Lipids are digested into chylomicrons by pancreatic and enteric lipase prior to absorption [29]. Most polymeric diets are isoosmolar (300 - 450 mOsm/kg) and are better tolerated with fewer complications of diarrhea compared with monomeric diets [31]. Human products that have a caloric density greater than 1 Kcal/ml can be relatively hyperosmolar (600 - 700 mOsm/kg), and are often associated with diarrhea [5].

Specific Nutrients

Glutamine

Glutamine is a nonessential amino acid that "becomes conditionally essential" during stress and critical illness. During nutrient depletion, intestinal glutaminase activity is upregulated, and glutamine utilization increases. Glutamine is extracted from luminal nutrients and from blood flow to the intestinal basement membrane to provide fuel for the gastrointestinal tract. Glutamine serves as an important substrate for the synthesis of protein and nucleic acids, glutathione, and gastrointestinal mucus [32]. Glutamine also is absolutely necessary for mesenteric immune function, glutathione synthesis, and nitrogen transport [10,32] Glutamine depletion has been associated with gastrointestinal mucosal atrophy, increased permeability, depressed mesenteric immune function, and bacterial translocation. Enteral supplementation with glutamine-enriched formulas has been shown to ameliorate the negative consequences of depletion listed above [32].

Arginine

Arginine is an essential amino acid for dogs and cats [11]. Arginine is known to stimulate the release of various anabolic hormones, including prolactin, insulin, and growth hormone. Arginine is also essential in mediating microvascular function and serving as a precursor for nitric oxide, and is an important vasodilatory substance [10]. Supplementation with arginine is thought to improve nitrogen balance by promoting nitrogen retention and to improve immune function and wound healing [8]. Veterinary nutritional supplements should contain arginine at a minimum of 146 mg arginine/100 Kcal for dogs and 250 mg arginine/100 Kcal for adult cats [31]. Most veterinary products contain arginine in sufficient quantities to meet the needs of critically ill patients, however, they do not meet the requirements necessary to promote improved immune function [11].

Starch

A wide variety of carbohydrate sources is available in enteral formulations. In many products, carbohydrate is provided as corn starch or as mono- or disaccharides in elemental form. Mono- and disaccharides increase the relative osmolality of the preparation and may be associated with diarrhea.

Fiber

The addition of soluble and insoluble fiber to enteral diet formulations serves a dual purpose. Insoluble fiber such as lignin, cellulose, and hemicellulose is present in human formulations (Metamucil Regular or Psyllium fiber 10 - 13 g/100 Kcal) and stimulates enterocyte and goblet cell proliferation when in direct contact within the gut lumen [31]. Fiber can provide a barrier function and limit bacterial adherence and translocation. Additionally, soluble fiber such as pectin is fermented by enteric anaerobic bacteria to the short-chain fatty acid butyrate, acetate, and propionate [14]. Short-chain fatty acids enhance colonocyte health and decrease the incidence of diarrhea. Over-the-counter products that contain psyllium and pectin (1 gram/100 Kcal) can be administered in large-bore feeding tubes in combination with veterinary enteral formulations to decrease the incidence of diarrhea [14]. Oral fiber slows the rate of gastric emptying, and may help improve nutrient assimilation.

Fatty Acids

Enteral diets contain fats as medium- or long-chain triglycerides. Interest has been increased in providing varying proportions of the essential fatty acids linoleic (omega-3) and linolenic (omega-6) when dealing with inflammatory disease states. Omega-3 fatty acids are found in large quantities in flaxseed, canola, and fish oils. Omega-6 fatty acids are present in vegetable oils [11]. Essential long-chain fatty acids are precursors for arachidonic acid synthesis. Dogs can synthesize arachidonic acid from linoleic (omega-6) acid, and can synthesize eicosapentaenoic acid from α-linolenic (omega-3) acid. Cats are deficient in the enzymes necessary for arachidonic acid and eicosapentaenoic acid synthesis, and thus require both in their diets. Arachidonic acid is metabolized by cyclooxygenase enzymes to various pro- and anti-inflammatory mediators, including prostaglandins, thromboxanes, and leukotrienes. Collectively known as eicosanoids, these substances play an important role in mediating immune function, inflammation, platelet function and aggregation, and vascular tone [11]. Increasing the ratio of omega-3 to omega-6 fatty acids in the diet can promote the preferential substitution of linoleic acid into the phospholipid bilayer of cells and result in the production of less inflammatory cytokines. This can help decrease overall inflammation that is thought to contribute to the adverse consequences of systemic inflammatory response syndrome.

Protein

Protein in dietary supplements is typically provided as intact protein, crystalline amino acid, or as hydrolysates of protein. Di- and oligopeptide formulations have a higher osmolality than intact proteins and can cause diarrhea. Intact protein requires digestion by gastric hydrochloric acid and pancreatic enzyme activities. Crystalline amino acids can be absorbed in an energy-consuming active transport process. Di- and oligopeptides, in contrast, are absorbed in passive transport and do not consume energy in the process [14].

Protein requirements for small animal patients differ across species and across disease states. In general, dogs require 3 to 4 grams of protein/100 Kcal, whereas cats require 4 to 6 grams protein/100 Kcal. Lower concentrations of protein are advisable if hepatic or renal dysfunction is present. Higher quantities of protein should be considered in cases of severe protein loss, including burns, excessive wound exudates, or protein-losing enteropathy or nephropathy. The concentration of protein in relation to caloric density should be considered when choosing an enteral formula appropriate for an individual patient. Protein concentration ranges from less than 5 g/100 Kcal to 15 g/100 Kcal [14]. The protein content of many human enteral nutrition products is deficient and should not be used for long-term nutritional support in small animal patients [11]. In general, if a patient's caloric requirements are met with a veterinary enteral product, its protein requirements will be met as well, unless excessive ongoing nitrogen loss occurs [30].

Feeding Options and Considerations

Although enteral nutrition is preferred whenever possible, it is not a suitable mode of nutritional support for all small animal patients. Ideally, it is best to evaluate the patient's status at the time of presentation, or at the latest within 24 hours of admission. The nutritional status should be carefully assessed and the patient's individual needs determined prior to the onset of any complications. The anticipated time of enteral nutritional support and time to resume normal voluntary oral intake should be considered [5]. Animals that are poor anesthetic candidates, have coagulation defects, are recumbent or comatose, and cannot protect their airway will have increased risk of aspiration pneumonia, hemorrhage, or death if an indwelling tube is placed under general anesthesia [5]. Other contraindications to enteral feeding include gastrointestinal obstruction and severe malabsorptive syndromes [8]. Postoperatively, gastric ileus may be present for more than 5 days. In such cases, postgastric feeding in the form of duodenal or jejunal tubes should be considered.

Appetite Stimulation

If an animal is physically able to prehend food and does not have esophageal dysfunction or stricture or gastrointestinal obstruction, oral feeding may be possible if an animal is willing to eat and responds to appetite stimulants. Forcefeeding is extremely stressful for animal and caretaker alike and should be avoided. It is difficult to provide adequate calories to meet an animal's resting energy requirements. In some cases, the animal will associate the mere presence of a caregiver with the stress of forcefeeding and reactively and spontaneously vomit. This reverse Pavlovian response negates the goal of providing enteral nutrition and can lead to aspiration pneumonia, esophagitis, and further debilitation. Appetite stimulants that have been used with some success include benzodiazepene tranquilizers (Diazepam 0.05 - 0.5 mg/kg IV once daily to cats) and serotonin antagonists (2 mg/cat PO bid) [31,33]. In some cases, warming a liquid diet or adding a small amount of spice to it can enhance odor and palatability [33]. Positive reinforcement and encouragement may entice a patient to eat. If voluntary feeding is not successful in meeting the patient's nutritional requirements, some form of involuntary feeding in the form of feeding tube or parenteral administration should be implemented [3].

Nasoesophageal and Nasogastric Tubes

Nasogastric (NG) and nasoesophageal (NE) feeding tubes should be considered for short-term (< 1 week) administration of enteral nutrition.5 Nasoesophageal and nasogastric tubes can be placed efficiently without anesthesia or sedation, in most cases [2]. Contraindications to nasoenteral feeding tube placement include facial or head trauma, impaired neurologic status (moribund or comatose patient without an intact gag reflex), or esophageal disorders including megaesophagus, masses, or stricture [2].

The placement of nasoenteral feeding tubes has been discussed elsewhere [5,34-37]. Both silicone and polyvinylchloride tubes (Argyle Infant Feeding Tubes, Sherwood Medical, Inc; Sovereign Feeding Tube, Monoject,) are available for use. Because polyvinylchloride tubes can harden within days of exposure to hydrochloric acid, silicone tubes are preferable [2]. A 5 to 8 French tube can be placed in cats and small dogs that weigh less than 10 kg, while larger (8 to 10 Fr) tubes can be placed in larger dogs. Early studies documented that patients, in general, tolerated the NE or NG tube with mild to moderate coughing or sneezing as a common complaint. Gastroesophageal reflux can occur if the tip of the tube passes through the lower esophageal sphincter; however, gastric suctioning may be helpful in animals with severe ileus, to decrease gastric distention and vomiting [22]. In one study, 63% of patients demonstrated no complication with the nasogastric tube feeding, and 61% of patients maintained body weight while hospitalized [7]. Some animals were able to eat with the tube in place [27,37]. Administration of liquid enteral diets allowed maintenance of serum albumin, commonly used as a nutritional marker, in hospitalized small animal patients [7]. The most common complications observed were tube dislodgement, vomiting, and diarrhea, which may have been secondary to the patient's primary disease process(es), owing to the enteral feeding tube, or owing to dietary intolerance. In many patients, diarrhea was treated successfully by changing to a different dietary formulation [7]. More serious complications of nasoenteral feeding include epistaxis, rhinitis, vomiting, esophageal stricture, nasopleural intubation with pneumothorax, and aspiration pneumonitis [5]. Mechanical disruption of the tube by clogging is common, because of the small diameter necessary to bypass the nasal meatus. Sometimes, debris clogged within the tube can be dissolved with the use of pancreatic enzyme solutions or carbonated beverage products such as Coca Cola®.

Esophagostomy Tubes

Esophagostomy tubes are an excellent means of providing enteral nutritional support in a patient with full use and function of the gastrointestinal tract [28,38]. Several methods for esophagostomy tube placement have been described [28,34,38-43]. Advantages of esophagostomy tubes include their ease of placement, low risk of complication compared with other indwelling feeding tubes, low cost, lack of need for specialized equipment, immediate use, and positive owner and patient tolerance and use [28]. A recent study investigated the use of esophagostomy versus percutaneous endoscopic gastrostomy tubes, and found that 92% of clients with animals with E-tubes were comfortable and satisfied with their use, versus only 71% of clients whose animals had G-tubes placed [28]. Minor complications associated with E-tube use and placement included vomiting, inadvertent tube removal, patient scratching at the tube, and tube obstruction [28]. More serious complications that have been described, but are uncommon, include hemorrhage during tube placement, cellulitis, and mediastinal placement with pleuritis.

Gastrostomy Tubes

Percutaneous gastrostomy tubes can be placed with or without the assistance of an endoscope. Several techniques have been described [33,44-46]. Gastrostomy tubes are indicated in any patient that has a nonfunctional or injured esophagus, but still has normal gastric function. Gastric tubes are generally well tolerated by clients and animals alike [34]. Complications of gastric tubes include inadvertent tube removal, peritonitis, cellulitis, stoma site infection and pressure necrosis [5,34]. Disadvantages of a gastrostomy tube over esophagostomy tube include the need to wait a minimum of 12 to 24 hours prior to tube use to allow a seal to form, and the need to wait a minimum of 7 to 10 days prior to tube removal.

Jejunostomy Tubes

In general, jejunostomy tubes are well tolerated and provide an effective means of providing nutritional support if only a portion of the gastrointestinal tract is functional. Various methods for placement of jejunostomy tubes have been described [47-50]. Placement of a jejunostomy tube is indicated in patients with upper gastrointestinal obstruction or resection, gastroparesis, pancreatitis, and recurrent aspiration pneumonia [5]. Following placement of the jejunostomy tube, feeding should be initiated after 6 to 12 hours if peristalsis is present [5]. Without a reservoir to allow nutrients to slowly trickle into the gastrointestinal tract, intermittent bolus feeding through a jejunostomy tube is often associated with the development of diarrhea [6]. For this reason, continuous feeding is preferred. Providing a patient's nutrient requirements should be started slowly and then ramped up to full caloric intake over a period of 48 hours. A J-tube should ideally remain in place for a minimum of 7 to 10 days before removal, to decrease the risk of peritonitis. Complications associated with J-tube placement and feeding include orad migration of the tube, abdominal cramping with diarrhea, tube obstruction, focal cellulitis, and tube dislodgement with peritonitis.

Metabolic Complications Associated with Enteral Feeding

Enteral feeding has been associated with a variety of metabolic complications, including hypo- and hyperglycemia, uremia, vitamin and trace mineral deficiencies, and fluid and electrolyte imbalances [5-10]. Overfeeding with resultant vomiting and diarrhea is common. Whenever gastric function is marginal, it is best to aspirate and measure the residual volume in the stomach before the next feeding, to avoid gastric overdistention. If more than one third the volume of the previous feeding is still present, the scheduled feeding should be delayed to allow more time for the stomach to empty. Vomiting can increase the risk of aspiration pneumonia and should be prevented, whenever possible [10].

Initiating Feeding

Enteral nutritional supplementation should be initiated gradually in the inappetent patient. Caution must be exercised when initiating enteral feeding support to a patient that has had inadequate fluid resuscitation. Research has indicated that administration of enteral nutrition to a patient with hypotension and intravascular fluid volume depletion can result in increased gastrointestinal workload and oxygen consumption with impaired oxygen delivery, causing further compromise to the gut barrier [11,51]. If the stomach is functional, bolus meal feeding through an esophagostomy or gastrostomy tube can be administered as 6 small meals in a 24-hour period. Trickle feeding via constant-rate infusion is another method of administering liquid diets through nasoesophageal, nasogastric, and jejunostomy tubes. In some cases, a jejunostomy tube can be placed through a gastrostomy tube until the stomach and duodenum can tolerate feeding. Whether nutritional support is administered as a bolus or as a constant-rate infusion, approximately one quarter to one third of the patient's daily caloric requirements should be fed on the first day. Initially, nutrient boluses should equal 5 to 10 ml/kg until the patient is able to tolerate a larger volume [31]. The volume and caloric density should slowly be increased to full feeding over a period of 48 hours, to avoid oversupplementation and refeeding syndrome. Feeding should cease if clinical signs of salivation, retching, or vomiting occur [31]. The patient's acid-base and electrolyte status should be carefully monitored for hypokalemia, hypophosphatemia, and hyperglycemia during this time.

Refeeding Syndrome

Refeeding syndrome is an uncommon complication that occurs in dogs and cats when overzealous nutritional supplementation is administered to a patient that has been anorexic for a long period. Upon reintroduction of nutrients, insulin release drives potassium and phosphorus intracellularly with glucose [52]. A rapid decrease in serum potassium can result in cardiac dysrhythmias and muscle fasciculations. Hypophosphatemia (< 1 mg/dL) can result in severe red blood cell hemolysis [53-54]. Supplementation with phosphorus as potassium phosphate (0.03 - 0.12 mMol/kg/hour IV CRI or 100 mg/100 Kcal energy/day as oral supplement) and potassium chloride(not to exceed 0.5 mEq/kg/hour) or potassium gluconate (2 - 4 mEq/100 Kcal/day as oral supplement) may be necessary to alleviate clinical signs associated with hypophosphatemia and hypokalemia [55]. If clinical and metabolic signs of refeeding syndrome occur, enteral feeding should be decreased until electrolyte abnormalities have been normalized.

Gastric Atony

Gastric atony is a common complication observed in inappetent and post-surgical patients. Narcotic drugs administered to provide analgesia in the postoperative period often contribute to delayed gastric emptying, ileus, gastroesophageal reflux, and vomiting [56]. Empiric therapy with prokinetic agents can potentially decrease some of the complications associated with gastric atony in some patients [56]. Metoclopramide, a dopaminergic and 5-hydroxytryptamine (5-HT) receptor agonist-antagonist, functions as a central antiemetic and gastric promotility drug. Administration of metoclopramide (1 - 2 mg/kg/day IV CRI or 0.2 - 0.4 mg/kg SQ tid) has been shown to increase lower esophageal tone, increase the force of gastric contractions, and cause pyloric relaxation to promote gastric emptying [56]. The combined effects of metoclopramide serve to decrease vomiting and gastroesophageal reflux.