Enteral Nutrition (EN) : Pathophysiology and Treatments
Enteral nutrition (EN) is the delivery of nutrients by tube or mouth into the gastrointestinal (GI) tract. This chapter focuses on delivery through a feeding tube.
- Digestion and absorption are the GI processes that generate usable fuels for the body. Understanding the mechanisms of these processes can enhance rational use of EN support.
- Digestion is the stepwise conversion of complex chemical and physical nutrients via mechanical, enzymatic, and physicochemical processes into molecular forms that can be absorbed from the GI tract.
- Nutrients are absorbed across the brush border membrane of intestinal cells and reach the systemic circulation through the portal venous or splanchnic lymphatic systems, provided the GI or biliary tract does not excrete them.
- Many factors can alter these stepwise processes and interfere with digestion and absorption, such as functional immaturity of the neonatal gut.
CLINICAL PRESENTATION AND INDICATIONS
- EN is indicated for the patient who cannot or will not eat enough to meet nutritional requirements and who has a functioning GI tract. Additionally, a method of enteral access must be possible. Potential indications include neoplastic disease, organ failure, hypermetabolic states, GI disease, and neurologic impairment.
- The only absolute contraindications are mechanical obstruction and necrotizing enterocolitis. Conditions that challenge the success of EN include severe diarrhea, protracted vomiting, enteric fistulae, severe GI hemorrhage, and intestinal dysmotility.
- EN has replaced parenteral nutrition (PN)as the preferred method of specialized nutrition support. Advantages of EN over PN include maintaining GI tract structure and function; fewer metabolic, infectious, and technical complications; and lower costs.
- The optimal time to initiate EN is controversial. Early initiation within 24 to 48 hours of hospitalization is recommended for critically ill patients because this approach appears to decrease infectious complications and reduce mortality. If patients are only mildly to moderately stressed and well nourished, initiation can be delayed until oral intake is (or expected to be) inadequate for 7 to 14 days.
The goals of EN are to reverse protein-calorie malnutrition, maintain adequate nutritional state, promote growth and development of infants and children, and reduce disease-related morbidity and mortality.
- EN can be administered through four routes, which have different indications, tube placement options, advantages, and disadvantages .
- The choice depends on the anticipated duration of use and the feeding site (i.e., stomach versus small bowel).
- Short-term access is generally easier, less invasive, and less costly than long-term access. Feeding tubes used for short-term access are not sui for long-term use owing to patient discomfort, long-term complications, and mechanical failure.
- The most frequently used short-term routes are accessed by inserting a tube through the nose and threading it into the stomach (nasogastric), duodenum (nasoduodenal), or jejunum (nasojejunal).
- The stomach is generally the least expensive and least labor-intensive access site; however, patients who have impaired gastric emptying are at risk for aspiration and pneumonia.
- Greater skill is required to place the feeding tube beyond the pylorus. Techniques to facilitate manual placement include using a stylet or weighted tube, or administering metoclopramide or erythromycin.
- Long-term access should be considered when EN is anticipated for more than 4 to 6 weeks. The most popular option is gastrostomy followed by jejunos- tomy.
- The gastrostomy exit site requires general stoma care to prevent inflammation and infection. The gastrostomy tube is easily removed after it is no longer needed. If the fistula does not close spontaneously, histamine2-antagonist therapy and silver nitrate cautery usually prevent further leaking.
- EN can be administered by continuous, continuous cyclic, bolus, and intermittent methods. The choice depends on the feeding tube location, patient’s clinical condition, intestinal function, residence environment, and tolerance to tube feeding.
- Continuous EN is preferred for initiation, for critically ill patients, and for patients with limited absorption capacity because of rapid GI transit time or severely impaired digestion. Continuous EN has the advantage of being well tolerated. It has the disadvantages of cost and inconvenience owing to pump and administration sets.
- Cyclic EN has the advantage of allowing breaks from the infusion system, thereby facilitating activities of daily living, especially if EN is administered nocturnally.
- Bolus EN is most commonly used in long-term care residents who have a gastrostomy. This method has the advantage of requiring little administration time (e.g., 5 to 10 minutes) and minimal equipment (e.g., a syringe). Bolus EN has the potential disadvantages of causing cramping, nausea, vomiting, aspiration, and diarrhea.
- Intermittent EN is similar to bolus EN except that the feeding is administered over 20 to 60 minutes, which improves tolerability but requires more equipment (e.g., reservoir bag and infusion pump). Like bolus EN, intermittent EN mimics normal eating patterns and is sui only for the patient with a gas- trostomy.
- Historically, EN formulations were created to provide essential nutrients including macronutrients (e.g., carbohydrates, fats, and proteins) and micronutrients (e.g., electrolytes, trace elements, and vitamins).
- Over time, formulations have been enhanced to improve tolerance and meet specific patient needs. For example, nutraceuticals or pharmaconutrients are added to modify the disease process or improve clinical outcome; however, these health claims are not regulated by the Food and Drug Administration (FDA).
- Fiber, in the form of soy polysaccharides, has been added to several EN formulations. In addition to providing an excellent energy source, potential benefits include trophic effects on colonic mucosa, promotion of sodium and water absorption, and regulation of bowel function.
- Osmolality is a function of the size and quantity of ionic and molecular particles primarily related to protein, carbohydrate, electrolyte, and mineral content. Osmolality is commonly thought to affect GI tolerability, but there is a lack of supporting evidence.
- EN formulations are classified by their composition and intended patient population . Most formularies should contain no more than one product per category. Some categories can be omitted depending on the patient population.
- Most EN products are ready-to-use prepackaged liquids, which have the advantages of convenience and lower susceptibility to microbiologic contamination. The major disadvantage is storage space. Closed-system containers provide a prefilled, sterile 1- to 1.5-L supply of EN formula; they can be cost effective when water, electrolytes, or drugs must be added.
- Polymeric formulations contain intact macronutrients and are similar to food.
- Standard polymeric formulations have a well-proportioned mix of macronutrients, with or without fiber. These formulations are best suited for tube feeding because, to maintain isotonicity (300 mOsm/L), they are not sweetened and not pala. Standard formulations have a nonprotein calorieâ€“nitrogen ratio of 125:1 to 150:1.
- High-protein formulations have a nonprotein calorieâ€“nitrogen ratio of less than 125:1. Candidates for these formulations require more than 1.5 g of protein/kg/day and are generally critically ill because of trauma, burns, pressure sores, surgical wounds, or high fistula output.
- High caloric density formulations are indicated for patients requiring restriction of fluids, electrolytes, or both, such as patients with renal insufficiency or congestive heart failure.
- Elemental or peptide-based formulations have partially hydrolyzed protein or fat components. Peptide-based formulations replace some of the protein with dipeptides and tripeptides, thereby optimizing absorption. Although indications are not clearly established, they probably include patients who do not tolerate standard formulations because of malabsorption and patients who benefit from medium-chain triglycerides (e.g., patients with pancreatic insuf- ficiency).
- Disease state-specific formulations are designed to meet specific nutrient requirements and to manage metabolic abnormalities. Unfortunately, scientific and clinical research supporting their efficacy is minimal, except for low carbohydrate formulations supplemented with specific fatty acids for patients with acute respiratory distress syndrome (ARDS).
- Oral supplements are not intended for tube feeding. They are sweetened to improve taste and are therefore hypertonic.
- A module is a powder or liquid that can be added to a commercially available product. Alternatively, a modular product can be mixed to concentrate nutrients in less volume.
- Hydration formulations are used to maintain hydration or treat dehydration. They can be administered by mouth or feeding tube. The glucose content of these formulations can decrease fecal water loss and generate a positive electrolyte balance.
- Conditionally essential amino acids (e.g., glutamine and arginine) are added to some formulations for critically ill patients, but their benefit is questionable.
INITIATION AND TITRATION OF ENTERAL NUTRITION REGIMENS
- Schedules for progression from initial to target rates should be individualized. The need to reach nutrient goal should be balanced with the need for tolerance.
- In adults, continuous EN feedings are typically started at 20 to 50 mL/h and advanced by 10 to 25 mL/h every 4 to 8 hours until the goal is achieved. Intermittent EN feedings are started at 120 mL every 4 hours and advanced by 30 to 60 mL/h every 8 to 12 hours.
- In children, EN feedings are typically started at 1 to 2 mL/kg/h for continuous feeding or 20 to 25 mL/kg per bolus and advanced by similar volumes every 4 to 12 hours.
- In premature infants, feedings are started at lower rates or volumes, usually 10 to 20 mL/kg/day.
- The practice of diluting hyperosmolar EN formulations should be avoided unless necessary to increase fluid intake.
- Patients should be monitored for metabolic, GI, and mechanical complications
- Metabolic complications associated with EN are analogous to those of PN, but the occurrence is lower.
- Gastric residual volume is thought to increase the risk of vomiting and aspiration. Residual volume is measured by aspirating the stomach contents into a syringe attached to the open end of the feeding tube. Although the definition is controversial, residual is probably excessive if it is greater than 200 to 500 mL in adults, or if it is twice the bolus volume or hourly infusion rate in children. The determination should be based on a trend rather than an isolated finding and should be made in conjunction with the presence of symptoms.
- The stepwise approach for managing excessive gastric residual volume with GI symptoms is slowing, not stopping, the tube feeding; initiating metoclopramide; considering a transpyloric feeding tube; trying a proton pump inhibitor or histamine2-receptor antagonist; and minimizing use of narcotics, sedatives, and other agents that delay gastric emptying.
- In addition to avoiding excessive gastric residuals, methods for preventing aspiration pneumonia include keeping the head of the bed at 30Â° to 45Â° during feeding and for 30 to 60 minutes after intermittent infusions, changing from bolus to intermittent or continuous administration, providing good oral care, and monitoring tube placement for proper positioning.
- Management of diarrhea should be directed at identifying and correcting the cause. The most common causes are sorbitol, drug therapy, infection, malabsorption, and factors related to tube feeding (e.g., rapid delivery or advancement, intolerance to composition, large volume administered into small bowel, and formula contamination). Switching to a fiber-containing, lower fat, peptide-based, or lactose-free formulation can be beneficial. As a last resort, pharmacologic intervention (e.g., opiates, diphenoxylate, or loperamide) can be used to control severe diarrhea.
- Techniques for clearing occluded tubes include pancreatic enzymes in sodium bicarbonate and using a declogging device. Techniques for maintaining patency include flushing with at least 30 mL of water before and after medication administration, at least every 8 hours during continuous feeding, and after each intermittent feed.