ENTERAL NUTRITION SUPPORT FOR ADULTS

Definition
Enteral nutrition support is the provision of nutrients to the gastrointestinal tract via a feeding tube, catheter, or stoma to maintain or replete the patient’s nutritional reserves (1,2). Enteral nutrition is the preferred route for the provision of nutrition for patients who cannot meet their needs through voluntary oral intake (1,2). This section pertains to nutrition support via enteral tube feeding.

Nutrition Assessment

Indications (1-5)
Enteral nutrition support via tube feeding should be considered for patients who are unable to ingest adequate amounts of nutrients orally and have an adequately functioning gastrointestinal tract.  The advantages of enteral feeding over parenteral feeding include:

• much lower cost (Grade II)* (4) and shorter length of hospital stay (6-8)
• avoidance of complications associated with parenteral feedings (eg, infectious complications (Grade I) (4), pneumothorax, catheterembolism, and cholecystitis) (4,5,9,10)
• maintenance of gastrointestinal mucosal integrity and prevention of bacterial translocation (5,10)

    Early enteral nutrition is well tolerated by intensive care unit (ICU) patients (4).  Evidence-based guidelines for critically ill patients recommend initiating enteral nutrition 24 to 48 hours after injury or admission to the ICU if the patient is hemodynamically stable, has a functioning gastrointestinal tract, and is adequately fluid resuscitated (Grade I) (4). When initiated early appropriate enteral tube feeding may prevent bacterial translocation, which is the passage of bacteria across the intestinal wall due to atrophy of intestinal villi (10).  Maintaining gastrointestinal integrity by enteral feedings is theorized to prevent translocation, which leads to fewer infectious complications (5,10-12). 

Contraindications (1-5)

Enteral nutrition support should be avoided in patients who do not have an adequately functioning gastrointestinal tract.  Specific contraindications include:

• intractable vomiting
• severe diarrhea
• high-output enterocutaneous fistula (greater than 500 mL/day)
• conditions warranting total bowel rest, such as severe acute pancreatitis (unless jejunal enteral feeding can be provided beyond the Ligament of Treitz) (1-3)
• severe inflammatory bowel disease
• upper gastrointestinal hemorrhage (caused by esophageal varices, portal hypertension, or cirrhosis) (5)
• short-bowel syndrome (less than 100 cm of small bowel remaining)
• intestinal obstruction (depending on location)
• a prognosis that does not warrant aggressive nutrition support

    The absence of bowel sounds does not preclude safe enteral nutrition (5,10,11,13).  Although paralytic ileus (absence of bowel sounds or flatus) was once considered a contraindication to enteral feedings, it is now known that an ileus has different effects on different areas of the intestine; for example, postoperative ileus appears to affect colonic and stomach function to a greater extent than small-bowel function (13).  The clinical condition of the patient is an important consideration in the decision to initiate enteral nutrition.  A soft, nontender abdomen, adequate perfusion, and hemodynamic stability are indicators of the potential for the safe administration of enteral nutrition (4,5).   For most patients, lower gastrointestinal bleeding does not affect the administration of enteral support (5,14).   

*The American Dietetic Association has assigned grades, ranging from Grade I (good/strong) to Grade V (insufficient evidence), to evidence and conclusion statements. The grading system is described in Clinical Nutrition Management A Reference Guide.

Nutrition Intervention
Enteral feedings can be nutritionally adequate if an appropriate formula is selected with consideration of each patient’s individual estimated requirements.  Tube feedings may be used as the sole source of nutrients or as a supplement to inadequate oral nutrition.  Enteral nutrition should be initiated within 48 hours of injury or admission to the ICU, and the average intake delivered within the first week should be at least 60% to 70% of total the estimated energy requirements, as determined by the nutrition assessment (Grade II) (4).  Provision of enteral nutrition within this time frame and at this intake level is associated with a shorter hospital stay, fewer days on mechanical ventilation, and fewer infectious complications (Grade II) (4).  The delivery of 14 to 18 kcal/kg per day, or 60% to 70% of the enteral feeding goal, is associated with improved outcomes (Grade II) (4).  Initial evidence suggests that ICU patients and obese surgical patients who are provided greater than 70% of their enteral feeding goal may have outcomes that are more detrimental when compared to patients who received 60% to 70% of their enteral feeding goal (Grade III) (4).

How to Order the Diet
The physician in collaboration with the dietitian determines the appropriate prescription for the tube-feeding regimen, including the route and type of formula to be utilized.  A dietitian should facilitate the selection of the formula type and goal rate for tube feeding.  A nutrient intake study may be beneficial to verify that the patient’s total nutrient intake (oral feeding plus tube feeding) is adequate once the tube-feeding goal rate is reached.

The order specifies:

• product, either by name or as “Standard Tube Feeding,” according to hospital protocol
• volume, rate, and timing, including the initial volume and rate, as well as the progression and goal volume and rate (At a standard dilution of 1.0 kcal/mL, the volume will be roughly equal to the number of kilocalories specified.)
• administration and monitoring, following either the facility’s standard procedures or individualized orders, including the administration of extra water to flush the tube or meet fluid requirements

• Orogastric:   The feeding tube is inserted through the mouth, with the tip resting in the stomach.
• Nasogastric:  The feeding tube is inserted through the nose, with the tip resting in the stomach.
• Nasoduodenal: The feeding tube is inserted through the nose, with the tip resting in the duodenum.
• Nasojejunal:  Feeding tube is inserted through the nose, with the tip resting in the jejunum.
• Esophagostomy:  Feeding tube is inserted through a surgical opening in the neck and passed through the esophagus, with the tip resting in the stomach.
• Gastrostomy:  Feeding tube is inserted through the abdominal wall into the stomach via percutaneous endoscopic guidance or surgical placement (surgical “open” gastrostomy).
• Jejunostomy:  Feeding tube is inserted through the abdominal wall into the jejunum via percutaneous endoscopic guidance or surgical placement (surgical “open” jejunostomy).

Enteral Formula: Categories and Selection
Choosing the most appropriate tube-feeding formula is critical for achieving nutritional goals.  Formulas should be selected based on digestibility/availability of nutrients, nutritional adequacy, viscosity, osmolality, ease of use, and cost (5).  In addition, the nutritional status of the patient, including electrolyte balance, digestive and absorptive capacity, disease state, renal function, medical or drug therapy, and possible routes of enteral infusion should be considered (5).  Enteral formulations are considered medical foods by the Food and Drug Administration (FDA); therefore, their labels can make “structure and function” claims without the approval of the FDA (5).  Limited evidence is available regarding the efficacy and outcomes associated with the use of specialized enteral formulations (5). 

    Enteral formulas can be classified as standard (or polymeric), elemental, or semi-elemental. Standard formulas include synthetic formulas and blenderized formulas.  Specialized enteral formulas include disease-specific formulas and nutrient-modified formulas. Additionally, individual modular components that can supplement the formula are available (15).  Most enteral formulations provide adequate amounts of vitamins and minerals to meet the Reference Daily Intakes when provided in volumes of 1,000 mL to 1,500 mL daily (16).  Enteral formulas contain a large amount of water; their water content generally ranges from 70% to 85% (15).   It is important to be aware of patients with potential food allergies when prescribing an enteral formula.  Enteral formulations may contain milk, soy, corn, or egg products, all of which are common allergens (15).  Most enteral formulations are lactose free and gluten free (15).

Standard or polymeric formulas: Standard or polymeric formulas must be digested into dipeptides and tripeptides, free amino acids, and simple sugars in the small bowel.  Polymeric formulas require adequate digestive and absorptive capability and are indicated in patients with normal or near-normal gastrointestinal function.  There are two basic types of polymeric formulas, synthetic formulas and blenderized formulas; however, synthetic formulas are the most commonly used formulas due to safety and feasibility in an institutional setting (5).

    Synthetic formulas are used for standard tube feedings.  Their energy content ranges from 1.0 to 2.0 kcal/mL.  The protein content provides 12% to 20% of total energy and consists of intact protein, generally casein or soy protein isolate (15).  Lactalbumin, whey, and egg albumin are also sources of intact proteins.  Formulas that contain intact proteins require normal levels of pancreatic enzymes for digestion and absorption (15).  Carbohydrate sources include corn syrup solids, hydrolyzed cornstarch, maltodextrin, sucrose, fructose, and sugar alcohols. Carbohydrates provide 40% to 90% of total energy (15).  The fat content ranges from less than 10% to more than 50% of total energy. Common fat sources are corn oil and soybean oil; however, safflower, canola, and fish oils are also used in enteral formulas (15).  The osmolality of synthetic formulas ranges from 270 to 700 mOsm/kg.  Because a high incidence of lactase deficiency in illness is presumed, lactose is not present in most synthetic enteral formulas (15).

Elemental or semi-elemental formulas: These formulas consist of hydrolyzed macronutrients.  Protein is present either as free amino acids (monomeric) or as bound amino acids in dipeptides or tripeptides (oligomeric).  Carbohydrate sources consist of oligosaccharides, sucrose, or both.  Most monomeric formulas have a low-fat content or contain a large percentage of medium-chain triglycerides (MCT) oil.  These formulas are low-residue, hyperosmolar, and usually lactose-free.  They are indicated for patients with compromised gastrointestinal function, such as patients who have acute pancreatitis (15).  Formulas with predigested nutrients should not be used for patients with normal digestion and absorption, because they are unnecessary for these patients and cost more than standard intact (polymeric) nutrient formulas

Modular components: These products are individually packaged components that may be combined in varying amounts to meet the patient’s individual nutritional needs.  Examples include protein powders, carbohydrate powders, MCT oil, fiber, and specific amino acids (eg, glutamine and arginine).  Protein powders are the most commonly used modular additives, and they provide 7 to 15 g of protein per serving (15).  Modular components may also be added to premixed formulas to enhance the intake of one or more macronutrients. If modular components are added to premixed formulas, the preparation should follow the organization’s Hazard Analysis and Critical Control Point Enteral Nutrition Plan (7).  

Nutrient-Modified and Disease-Specific Formulas
Nutrient-modified and disease-specific formulas are available for a variety of conditions.  These formulas have been altered in one or more nutrients in an attempt to optimize nutrition support without exacerbating the metabolic disturbances associated with various diseases.  Limited evidence is available regarding the efficacy and outcomes associated with the use of most disease-specific enteral formulations (5). Standard enteral formulas are appropriate for most critically ill patients (4).   

    Disease-specific formulas are more expensive than standard enteral formula, and a dietitian should carefully evaluate their potential benefit for an individual patient before recommending them.  If such formulas are used, the patient should be monitored and advanced to a standard formula as soon as possible (4).  Nutrient-modified formulas include:

• Formulas containing fiber: Fiber is added to enteral formulas for a variety of potential health benefits (17).  Soluble fibers, such as guar gum, oat fiber, and pectin, are fermented to short-chain fatty acids, which are easily absorbed by the gastrointestinal mucosa.  Short-chain fatty acids, the preferred fuel for colonocytes, help to increase mucosal growth and promote sodium and water absorption (17).  It is important to consider the type of fiber supplemented in the enteral formula  Most formulations contain a combination of soluble and insoluble fiber (15,17).  Soluble fiber has been shown to reduce the incidence of diarrhea (18,19). However, studies of formulations supplemented with insoluble fiber have not yielded the same results (15,20).  Enteral formulas containing fiber generally provide 5 to 14 g of fiber per liter, which is less than the recommended 20 to 35 g of fiber per day.  When a fiber formula is used, adequate free-water needs should be maintained. The use of fiber-supplemented enteral formulations should be monitored closely, particularly in critically ill patients or patients who are hypotensive and at risk for bowel ischemia (15).  Cases of bowel obstruction from the use of these formulations have been reported (21,22).  Fiber-containing formulas are indicated when there are no contraindications for their use and the goal is to maintain the health and function of the normally functioning gastrointestinal tract and defecation pattern (23).
• Formulas containing omega-3 fatty acids: The type and amount of fat in enteral formulas may affect immune function (24-27).  Fish oils, a rich source of omega-3 fatty acids, provide eicosapentaenoic acid.  Unlike omega-6 fatty acids, which are found in more common fat sources (eg, corn oil and soybean oil) and have a greater immunosuppressive effect, eicosapentaenoic acid metabolizes into less immunosuppressive prostaglandins and leukotrienes.  A study has shown the beneficial effects of omega-3 fatty acids on the length of hospital stay and the infection rate following burn injury (26).  In another study, fewer gastrointestinal and infectious complications occurred in patients who received a formula rich in fish oils when compared to patients who received a standard polymeric formula (27). Enteral supplementation with omega-3 fatty acids has beneficial effects in the treatment of acute respiratory distress syndrome (ARDS).  In a study of 146 patients with ARDS, patients who received enteral formula supplemented with omega-3 fatty acids (eicosapentaenoic acid) and gamma-linolenic acid (an omega-6 fatty acid) had significant improvements in oxygenation, lower ventilation variables, fewer days of ventilator support and shorter stays in the ICU when compared to controls (28). 
• Formulas containing arginine: Arginine is a conditionally essential amino acid during stress due to greater utilization of the urea cycle.  Arginine is important in immune function and wound healing and is commonly found in immune-enhancing and wound-healing enteral formulas.  The immune-enhancing properties of arginine include effects on the production and maturation of T lymphocytes and natural killer cells (29,30). Arginine may be useful in treating inflammatory diseases and acquired immunodeficiency syndrome, and it enhances collagen formation in wound healing (31,32).  Although studies have demonstrated positive outcomes of arginine supplementation, more research is needed to determine the optimal dose and timing of administration (4).
• Formulas containing glutamine: Classified as a nonessential amino acid, glutamine is the primary oxidative fuel for rapidly dividing cells, such as enterocytes and leukocytes.  During stress or injury, the metabolic demand for glutamine can exceed the capacity of skeletal muscle to release it (33).  A fall in glutamine concentration is associated with atrophy of the intestinal mucosa, impaired immune function, and decreased protein synthesis (29).  Substantial research has been completed in the past decade on the effects of glutamine in enhancing small-intestine growth and repair from injury.  Glutamine is not present in standard parenteral formulas, and it is present in only small amounts in enteral formulas (16).  Formulas that contain intact proteins contain some bound glutamine.  Although most research has involved parenteral glutamine, one study demonstrated that patients who received glutamine-rich enteral formula had significantly reduced hospital costs, caused by the prevention of secondary infections, when compared to patients who received isoenergetic control formula (34).  However, other studies have demonstrated no effect with glutamine supplementation (35,36).  The optimal amount and form of oral glutamine required to achieve beneficial results is still unknown (4).  Dosage and safety studies have found that 20 to 40 g/day of glutamine supplementation is safe and well tolerated by adults (37).  Glutamine supplementation is contraindicated in patients with hyperammonemia, hepatic failure, or renal failure due to excess ammonia production (5).
• Formulas containing branched-chain amino acids (BCAA): Formulas supplemented with BCAA have primarily been used for patients with hepatic failure in an attempt to improve the ratio of BCAA to aromatic amino acids and prevent or improve hepatic encephalopathy.  The use of BCAA-supplemented enteral formulas has not been validated because studies have provided mixed results.  A randomized study found that BCAA supplementation reduced hospital admissions and improved nutritional status in patients with advanced liver disease when compared to a standard formulation; however, there was no difference in encephalopathy scores between the two groups (38).  The routine use of BCAA-enriched formulas is not recommended (15).  However, these formulations may be appropriate for patients who exhibit chronic encephalopathy and are unresponsive to pharmacotherapy in conjunction with standard enteral formulations (2,39). The American Society for Parenteral and Enteral Nutrition (ASPEN) guidelines for nutrition therapy in liver disease restrict the use of BCAA-enriched formulas to patients with refractory (chronic) encephalopathy that is unresponsive to pharmacotherapy (2). 

    Disease-specific formulas include:

• Formulas for renal disease:Renal formulas currently contain whole proteins, are energy dense, and have modified electrolyte content (eg, sodium, potassium, phosphorus, and magnesium).  These formulas may be indicated in renal patients whose serum electrolyte levels are difficult to manage or for whom renal dialysis is delayed (15).  Earlier renal formulas contained only essential amino acids along with modified electrolyte content.  These early formulas were intended to prevent the buildup of nitrogenous waste products.  However, the clinical outcomes with these formulas were disappointing, leading to their very limited use (for example, in rare cases where dialysis is not possible) (2,15).

    The 2002 ASPEN guidelines state: “On the basis of current evidence, providing specialized nutrition support to acute renal failure patients should be accomplished with an intake containing a balanced mixture of both essential and nonessential amino acids.”(2)  

    Most renal formulas are dense in energy, so that volume can be restricted if needed.  The protein content ranges from less than 40 g to more than 70 g in 2,000 kcal.  These formulas meet the Dietary Reference Intakes with the exception of select vitamins, minerals, and electrolytes that are normally restricted in renal insufficiency (eg, potassium, sodium, phosphorus, and magnesium).  If dialysis is delayed, an energy-dense, reduced-protein formula is appropriate (40).  However, long-term use of these formulas requires close monitoring of the patient’s nutritional status (15).  Patients who receive renal replacement therapy have higher protein needs.  Their needs can often be met with a standard enteral formula (41).  Patients on renal replacement therapy with persistent hyperkalemia or hyperphosphatemia may benefit from renal formulations with reduced electrolyte content (41).  There is insufficient data to determine if renal formulas produce different outcomes than standardized formulas (42).  Patients who receive continuous renal replacement have higher protein needs; therefore, standard high-protein enteral formulations are appropriate for these patients (15).  The nutrition goals of patients with renal failure should include adequate protein and energy intake, with modifications in fluid volume and electrolyte content that are individualized based on the patient’s clinical condition.

• Formulas for hepatic disease:Special enteral formulas have been designed for patients with hepatic failure.  These formulas are modified in fluid, protein, and mineral content and may not meet the Dietary Reference Intakes for various nutrients.  These formulas contain increased levels of BCAA along with decreased levels of aromatic amino acids in an attempt to treat or prevent hepatic encephalopathy.  The total protein content varies among formulas and is often low.  Studies of these formulas are inconclusive (39).  Hepatic formulas are only recommended for patients with chronic hepatic encephalopathy and patients whose hepatic encephalopathy worsens and does not respond to pharmacotherapy (2,39).  Patients with cirrhosis who do not have a history of encephalopathy usually tolerate standard enteral formulas (2,15).
• Formulas for pulmonary disease and ARDS:Pulmonary disease formulas are low in carbohydrate and high in fat to decrease carbon dioxide (CO2) production in patients with compromised pulmonary function.  Formulas that contain omega-3 fatty acids as a fat source may be beneficial in patients with early ARDS (2).  Although a few studies have shown decreased CO2 levels in hypercapnic patients receiving these formulas (28,43), more data regarding the effectiveness of these formulas are needed.  Talpers et al found that an excess of total energy was as deleterious to CO2 production as carbohydrate intake (43).  An excess of total energy should be avoided; and energy intake should be equal to or less than the energy needs of patients with pulmonary disease and CO2 retention (2,15).  The high lipid content of these formulas may cause delayed gastric emptying (44).  These formulas tend to contain intact nutrients.  They are usually low in fluid and nutritionally complete.  If pulmonary formulas are used, the patient’s ventilatory status and CO2 production should be monitored, and overfeeding should be avoided (2,15).
• Formulas for diabetes mellitus: Patients with diabetes mellitus who follow a diet with increased intake of monounsaturated fatty acids along with lower carbohydrate intake may have improved blood glucose control (15).  Therefore, enteral formulas with modified amounts and types of carbohydrate and fat have been developed.  These formulas contain less carbohydrate (34% to 40%), more modified fat (40% to 49%), and 10 to 15 g/L of fiber (15).  Outcomes data regarding these formulas are limited in their context and their applicability to persons with diabetes mellitus (15).  A prospective study that compared a diabetic formula to a standard fiber-containing formula in long-term care patients did not demonstrate any clinically significant benefit from the diabetic formula, with the exception of improved high-density lipoprotein cholesterol levels (45).  A study of diabetes patients in a hospital setting concluded that diabetic formulations resulted in a neutral effect on glycemic control (46).  According to the American Dietetic Association Evidence Analysis Project for Diabetes there is insufficient evidence to determine whether the nutrient composition of enteral formulations has an impact on medical costs, mortality rates, infectious complications, and length of hospital stay in patients with diabetes (Grade V) (47).  Diabetic formulas may be appropriately used in patients with blood glucose levels that have proved difficult to control with traditional methods (15).  The American Diabetes Association suggests that either a standard (50% carbohydrate) or a lower carbohydrate content (33% to 40%) formula be used for tube-fed patients (48).  It is generally recommended that patients be started on a standard formula with close monitoring of blood glucose levels and the use of insulin as needed for glycemic control (15,49). 
• Formulas for immune enhancement: Formulas have been designed with increased amounts of immune-enhancing nutrients (eg, arginine, glutamine, and omega-3 fatty acids) along with increased amounts of antioxidant vitamins and minerals.  It is theorized that these formulas will enhance the immune response and prevent infection.  Immune-enhancing enteral nutrition (an enteral formula that contains pharmacological doses of nutrients intended to impact the immune system) is not recommended for routine use in trauma patients and critically ill patients in the ICU (Grade II) (4).  Immune-enhancing enteral nutrition is not associated with reductions in infectious complications, length of hospital stay, cost of medical care, days on mechanical ventilation, or mortality in moderately to less severely ill ICU patients (Grade II) (4).  These formulas may be associated with increased mortality in severely ill ICU patients, although adequately powered trials evaluating this finding have not been conducted (4). 

Water/Fluid Requirements
The National Research Council recommends 1 mL of fluid per 1 kcal of energy expenditure for adults with average energy expenditure who live under average environmental conditions (50). Medical conditions that may reduce fluid requirements include congestive heart failure, renal failure, ascites, syndrome of inappropriate antidiuretic hormone, and malignant hypertension.  Fluid requirements may be increased for pregnant patients; patients with fever, burns, diarrhea, vomiting, or high-output fistulas or ostomies; and patients receiving ventilatory support (51). Patients with pressure ulcers and patients medically managed on air-fluidized beds also have additional fluid needs.  Refer to Nutrition Management of Fluid Requirements.

    There are several methods to determine fluid requirements (50).  There is no evidence that compares the effectiveness of these methods for estimating the fluid needs of adults (Grade V) (52).  The methods include (52):

aHolliday MA, Segar WE.  The maintenance need for water in parenteral fluid therapy.  Pediatrics.  1957;19:823-832.
bInstitute of Medicine.  Dietary Reference Intakes:  Water, Potassium, Sodium, Chloride, and Sulfate.  Washington, DC: National Academy Press; 2004.
cNutrition assessment.  In: Manual of Clinical Dietetics. 6th ed.  Chicago, Ill: American Dietetic Association; 2000:33.

Criteria for Formula Selection (15)
There are a variety of enteral nutrition products on the market, many of which have only subtle differences in composition.  The following criteria should be considered when selecting a formula:

• Energy density: An energy density of 1 kcal/mL is considered standard.  Additional free water is usually necessary to meet fluid requirements.  Higher energy concentrations may be indicated when fluid must be restricted or when feeding volumes sufficient to meet energy requirements cannot be tolerated.
• Osmolality: Products are available at isotonic osmolalities (300 mOsm/kg), moderate osmolalities (400 mOsm/kg), and high osmolalities (700 mOsm/kg).  The main contributors to osmolality are sugars, free amino acids, and electrolytes.  High-carbohydrate, amino acid–based, or peptide-based formulas have a moderate to high osmolality. Formula osmolality has not been conclusively found to be a direct cause of diarrhea (15).
• Protein: Protein sources are intact proteins, peptides, or amino acids.  Generally, protein contributes 9% to 24% of total energy.  High-nitrogen formulas may not be well tolerated in patients with certain renal or hepatic disorders.  High- nitrogen concentrations can result in a higher renal solute load and can predispose elderly patients to dehydration.  One gram of nitrogen requires 40 to 60 mL of water for excretion.
• Fat: Fat sources are long-chain triglycerides (LCT) and MCT.  The fat content usually ranges from 3% to 35% of energy for amino acid–based or peptide-based formulas and from 25% to 55% of energy for standard formulas.  Fats do not contribute to osmolality.  Inclusion of MCT may be beneficial when there is fat malabsorption or maldigestion, since MCT do not require pancreatic lipase for absorption, and intraluminal hydrolysis is more rapid and complete than with LCT (15).  MCT do not supply essential fatty acids.  They may cause complications for cirrhotic patients who have a limited ability to oxidize MCT.  The administration of MCT along with LCT increases the total intestinal absorption of both types of fats, compared with the absorption when one or the other is administered alone.
• Carbohydrate: Carbohydrate is the most easily digested and absorbed nutrient.  Enzyme digestion is very efficient, as surface digestion is not rate limiting (except with lactose).  The transport process is the slowest part of carbohydrate metabolism.  Carbohydrate sources are monosaccharides, oligosaccharides, and lactose.  The carbohydrate content of formulas ranges from 35% to 90% of energy.  Longer carbohydrate molecules exert less osmotic pressure, taste less sweet, and require more digestion than do shorter ones.  Glucose polymers are better absorbed than free glucose and enhance absorption of calcium, zinc, and magnesium in the jejunum.  Some specialty formulas include fiber, fructose, and/or fructo-oligosaccharides.  Fructo-oligosaccharides occur naturally in a variety of fruits and vegetables and provide sweetening at a lower cost than does sucrose.  Fructo-oligosaccharides are poorly absorbed by the small intestine, fermented in the colon, and promote the growth of healthy species of colonic bacteria (15).
• Lactose: Lactose is present in milk-based formulas and some blenderized formulas.  Most commercial formulas are lactose free.  Due to the presumed high incidence of secondary lactase deficiency in illness, lactose is not present in most enteral formulas (15).
• Residue: Milk-based formulas and other formulas with intact nutrients are generally low residue.  Blenderized and fiber-supplemented formulas leave a moderate to high residue.
• Fiber: See the previous discussion of formulas containing fiber in the nutrient-modified formulas list.
• Sodium and potassium: Select formula according to the patient’s nutrition prescription and laboratory profile.
• Renal solute load: The main contributors to renal solute load are protein, sodium, potassium, and chloride.  A high–renal solute load in sensitive patients can result in clinical dehydration.
• Safety: A ready-to-use closed bag is recommended, as it is more sterile and provides less risk for contamination than canned or powdered products.  Formulas that are made in a blender in the facility are discouraged because they carry a greater risk of infection, require careful handling, tend to clog tubes, and need a high volume to meet nutrient needs.  If formulas are mixed, follow the organization’s Hazard Analysis and Critical Control Point Enteral Nutrition Guidelines to ensure safety.
• Viscosity: Blenderized, high-fiber, and high-density formulas should not be administered through tubes with a diameter smaller than 10 French unless a pump is used.  Formulas may flow through an 8-French diameter tube when a pump is used (15).
• Vitamin K:  Patients who are receiving enteral nutrition support while on anticoagulant therapy should be monitored closely.  Significant vitamin K intake from enteral formulas can antagonize the effect of the anticoagulant drug warfarin and result in treatment failure (53).  Most enteral formulations contain modest amounts of vitamin K and provide daily vitamin K intake similar to the average dietary intake from foods (53).  Consistent intake of an enteral formulation containing less than 100 mcg of vitamin K per 1,000 kcal is not expected to cause warfarin resistance (53). Refer to Anticoagulant Therapy for specific guidelines for enteral nutrition regimens in this group of patients.
• Cost: Amino acid– and peptide-based formulas are usually more expensive than synthetic formulas containing intact nutrients.

Enteral Feeding Administration (54)

Continuous feeding/delivery: Continuous feedingsrequire that the enteral formula be administered at a controlled rate with a pump over a 24-hour period.  Continuous feedings are indicated for unstable critically ill patients, patients unable to tolerate high-volume feedings, patients with malabsorption, and patients at increased risk for aspiration.  Feedings may be initiated at full strength in the stomach, or at an isotonic strength in the small bowel, at a rate of 10 to 50 mL/h; then, the rate may be gradually increased as tolerated in increments of 10 to 25 mL/h to the goal rate.  Strength and volume should not be increased simultaneously.

Intermittent or cyclic feeding/delivery: Intermittent or cyclic feedings are administered over an 8- to 20-hour period by using a pump to control the rate of delivery.  This method of tube feeding is most beneficial for patients who are progressing from complete tube feeding support to oral feedings (as discontinuation of feedings during the day may help to stimulate appetite) and in ambulatory home-care patients who are unable to tolerate bolus feedings (allows freedom from the administration pump and equipment). Since this method of delivery usually requires a higher infusion rate, monitoring for formula and delivery tolerance is necessary.  Formula and delivery intolerance can be avoided by a gradual transitioning of the patient from continuous feeding to an intermittent feeding schedule.

Formula delivery not requiring a pump: The syringe bolus-feeding method involves the delivery of 250 to 500 mL of formula via a feeding tube over a 20- to 30-minute period, three to four times a day, to meet estimated nutritional requirements.  This method is usually restricted to gastric feedings and may be contraindicated in patients who have a high risk of aspiration, disorders of glucose metabolism, or fluid management issues. 

Enteral Feeding Formula and Equipment Maintenance Guidelines

Formula:

• Bring formula to room temperature before feeding.
• Hang time should be less than 8 hours or as specified by the manufacturer, unless ready-to-hang, prefilled containers are used (which may hang for up to 24 hours at room temperature).  Discard any formula remaining in the container after hang time has expired.
• Opened, unused formula should be kept refrigerated for no longer than the manufacturer’s specifications (usually 24 to 48 hours).
• Refer to organization-specific interdisciplinary enteral nutrition monitoring protocol and policy as needed.

Formula delivery guidelines:

• Irrigate the tube every 4 hours with 20 to 30 mL of warm sterile water to ensure patency for continuous feeding.  Also, irrigate the tube before and after each intermittent feeding or medication administration (2).
• To reduce bacterial contamination, flush water through the bag and tube every 8 hours before adding new formula when an open system is in place.
• Avoid putting food and beverages into the tube (eg, juice, milk, and soda).
• Flush tube with water before and immediately after administration of medicines to avoid clogging the tube.
• To reduce the risk of contamination and infection, the feeding bag and tubing should be changed every 24 hours or as specified by the manufacturer.
• Refer to organization-specific interdisciplinary enteral nutrition monitoring protocol and policy as needed.

Patient Monitoring Guidelines

Refer to the American Dietetic Association’s Critical Illness Evidence-Based Nutrition Practice Guideline and the Enteral Nutrition Support MNT Protocol (4,55,56).  Also refer to organization-specific interdisciplinary enteral nutrition monitoring protocols as needed.  Guidelines for patient monitoring and avoidance of complications associated with the delivery of enteral nutrition are described below.

Patients with nasoenteric tubes:

• Provide mouth and nose care every 8 hours to prevent parotitis and skin breakdown around the nose.
• Verify the placement of a nasoenteric tube prior to feeding initiation and every 4 to 8 hours thereafter, or as specified by the organization’s protocol.

Avoidance of intestinal hypoxia and bowel necrosis (4,15,56):

• Assess bowel sounds every 8 hours.
• Feed into the small bowel (postpyloric position).
• Administer feeding in patients who are adequately fluid resuscitated and have a sustained mean arterial pressure of at least 70 mm Hg.
• Use iso-osmolar formulations and advance the feedings when tolerance is demonstrated.
• Assess the need to hold feedings if the patient experiences a sudden period of hypotension or if there are increases in the dosages of pressor agents (eg, dobutamine, norepinephrine, and epinephrine) or an increased need for ventilatory support (56).
• Assess the need to hold feedings if the patient develops increased nasogastric output, abdominal distention, or abdominal pain.

 Avoidance of gastrointestinal intolerance and aspiration:

• Recommendations from the North American Summit on Aspiration in the Critically Ill state that feeding ideally should be into the small bowel with the tube tip at or below the Ligament of Treitz when two or more risk factors for aspiration are present (5,57).  These risk factors include: prior aspiration, decreased level of consciousness, neuromuscular disease, structural abnormalities of the aerodigestive tract, endotracheal intubation, vomiting, persistently high gastric residual volumes (GRVs), and the need for a supine position (57).
• Use a 50- to 60-mL syringe to check GRVs in nasogastric-fed or gastrostomy-fed patients before each intermittent or bolus feeding.  The GRV should be checked before bolus feedings or during continuous pump feedings, intermittently, or when there are signs of feeding intolerance (eg, abdominal distention or vomiting). Enteral nutrition should be held when a GRV greater than or equal to 250 mL is documented on two or more consecutive assessments (4).   Holding enteral nutrition when the GRV is less than 250 mL can result in patients not meeting their nutritional requirements (Grade IV) (4).  Consider a small-bowel tube placement in patients who have more than 250 mL GRV or formula reflux in two consecutive measures (Grade II) (4).  Small bowel tube placement (postpyloric position) is associated with reduced GRV (Grade I) (4).  Adequately powered studies have not been conducted to evaluate the impact of GRV on aspiration pneumonia (4). Patients with high GRVs may benefit from medications that stimulate gastric motility (eg, metoclopramide) (Grade II) (4).  Patients with consistently high GRVs must be evaluated to exclude medical problems (eg, ileus, bowel impaction, gastroparesis, or pancreatitis) that may cause feeding intolerance.  Feeding tubes with a diameter smaller than 10 French may be unreliable in determining residuals (58,59).
• Blue dye (FD&C Blue #1) should not be added to enteral formulas to detect aspiration.  The risk of using blue dye outweighs any perceived benefit.  The presence of blue dye in tracheal secretions is not a sensitive indicator for aspirations (Grade III) (4,60). 
• If not contraindicated, maintain the head of the patient’s bed at a 45° angle during feedings to reduce the risk of aspiration pneumonia (Grade II) and reflux of gastric contents into the esophagus and pharynx (Grade I) (4).  If bolus feeding, keep the head of the bed in this position for 30 to 60 minutes after feeding.
• If the patient has a history of gastroparesis or repeated high GRVs, then consider the use of a promotility agent in critically ill ICU patients, if there are no contraindications (Grade II) (4).  Promotility agents (eg, metoclopramide) have been associated with increased gastrointestinal transit, improved feeding tolerance, improved enteral nutrition delivery, and possibly a reduced risk of aspiration (Grade II) (4).

Metabolic/laboratory data monitoring guidelines: Blood glucose levels less than 140 mg/dL are associated with decreased mortality, shorter hospital stays, and fewer infectious complications in critically ill patients (Grade I) (4).  Strict glycemic control (80 to 110 mg/dL) reduces the amount of time on mechanical ventilation in critically ill ICU patients (Grade I) (4).  Dietitians should promote the attainment of these levels for blood glucose control as closely as possible without placing the patient at risk for hypoglycemia (4).  Suggested laboratory monitoring guidelines include: 

• baseline sodium, potassium, chloride, CO2, blood urea nitrogen, creatinine, and glucose daily until stable, then biweekly to weekly 
• baseline prealbumin, then biweekly to weekly during visceral protein repletion (consider impact of inflammatory status on reliability of negative acute-phase proteins)
• baseline albumin and liver function tests
• baseline calcium, magnesium, and phosphorus daily until stable, then weekly
• baseline complete blood cell count, then as needed
• 24-hour urine analysis for urine urea nitrogen (or total urea nitrogen) once the goal rate of tube feeding is attained, then weekly until stable

Other patient monitoring guidelines:

• Weigh the patient at least once per week.
• Maintain daily records of intake, output, and bowel movements.

 Transitional feedings, enteral to oral: Depending on the swallowing function of the patient, oral intake should begin with liquids and advance to appropriate foods as tolerated.  When oral intake reaches 500 kcal or more, the dosage of tube feedings may be proportionately tapered.  Switching the patient from a continuous tube feeding to night tube feeding only or discontinuing tube feeding 1 to 2 hours before meals will often stimulate appetite and speed transition to adequate oral intake.  When oral intake consistently meets or exceeds 60% of the patient’s energy requirements and 100% of the fluid requirements, discontinuation of tube feedings should be considered (3).

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Bibliography
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