An ngt inserted for the purpose of introducing food for oral feeding cannot be tolerated

Introduction

Enteral nutrition involves direct access to the gastrointestinal tract with a tube—that is, there is bodily invasion. Although usually considered equivalent to eating, enteral nutrition differs from simple eating:

Enteral nutrition bypasses normal eating mechanisms of smell, taste, chewing, and swallowing as well as the cephalic phase of digestion.

Enteral formulas are usually fixed, defined, and unvaried, as opposed to the wide variety of nutrients in food.

Enteral feeding is involuntary, or nonvolitional, and often delivered continuously, as opposed to the intermittent and voluntary intake of oral food.

Enteral nutrition is associated with risks and complications.

This article will focus on tubes and access to the gastrointestinal tract, types of enteral formulas, indications, complications, and drug–formula interactions.

Potential Risks Associated With Enteral Feeding

Enteral nutrition typically is delivered into the stomach with a nasogastric tube,27–29 and in many cases this leads to satisfactory delivery of nutrition. However, gastric motility (particularly gastric emptying) and absorption are impaired in critical illness,30–32 and this may lead to enteral feeding intolerance.33

Enteral feeding intolerance has been reported to occur in 31% to 46% of patients with gastric feeding33,34 and usually is manifest by large gastric residual volumes and vomiting.27,33,35 It leads to a large number of patients not achieving their expected energy delivery requirements36 and appears to place patients at a higher risk of pneumonia and possibly mortality.33 This often is exacerbated in critically ill patients when their enteral nutrition is withheld for diagnostic and therapeutic procedures.

Managing potential enteral feeding intolerance by delaying the initiation of nasogastric feeding is illogical, because it will reduce the chance of the patient's meeting energy requirements, may worsen intestinal permeability,37 and may lead to increased infectious complications and hospital length of stay.4 There is therefore a much greater rationale for more proactive strategies such as the use of an evidence-based feeding guideline that includes the use of promotility drugs and small bowel feeding when patients develop features of intolerance.5

Metabolic Complications

Enteral nutrition results in far fewer metabolic complications than does parenteral nutrition. The most common metabolic complication of enteral feeding is probably hyperglycemia, particularly in patients with pre-existing glucose intolerance. This can be handled through tighter diabetes control. Hyperglycemia rarely necessitates reduction of the feeding.

Tube-feeding formulas with high energy density or protein content do not contain enough water for some patients to handle the renal solute load. Patients receiving these formulas who are unable to regulate their fluid needs voluntarily and do not have adequate intravenous fluid intake may become dehydrated, hyperosmolar, and hyperglycemic. The osmolality of the formula is not related to these problems because carbohydrates (the major osmotic component) are metabolized and do not contribute to the renal solute load except when glycosuria occurs. The fluid status and blood glucose levels of patients at risk should be closely monitored.

Selecting the Formula Category to Meet Qualitative Requirements

Enteral nutrition support in its simplest form is merely a delivery system to overcome limitations of oral intake. This implies that all other gastrointestinal functions, that is, digestion, absorption, motility, and elimination, are intact and will function normally when nutrients are delivered into the stomach. In fact, derangements of any or all of the listed functions are extremely common in children requiring enteral nutrition support, and these abnormalities may have implications for formula selection. In addition, allergy or other adverse reaction may require that formula be selected to eliminate the suspected trigger. The most significant differences between formulas suitable for enteral administration are among the forms in which macronutrients are provided.

Formulation

The EN can be administered intermittently or continuously. Selection of pathway for EN administration and the type of infusion to be adopted will influence its formulation design. This also involves determining the total period for diet administration, the volume to be infused, infusion rate, if gravity drip will be used, and in which form it will be provided (infusion pump or by bolus). Table 2 outlines the programming of EN according to feeding tube positioning in pre- or postpyloric location.

Table 2. Programming of EN according to feeding tube positioning

Enteral formulations should be nutritionally complete when used as exclusive nutrition or as a supplement to patients with normal oral ingestion; or nutritionally incomplete when used only as a supplement nutrition. The evaluation of the digestive and absorptive capacity of the patient should be performed for better enteral formula selection (Scheme 2).

Several enteral formulations are based on fresh food, processed food, or both fresh and processed food. Therefore, nutrients comprising EN are generally the same constituents of a normal diet, consumed by the oral route, including carbohydrate (40–60% total energy needs), protein (14–20% total energy needs), fat (15–30% energy needs), and fiber (40–20 g l− 1). Different factors should be considered to facilitate the choice of the most appropriate enteral formulation for patients with EN indication, such as caloric density, osmolarity and osmolality, administration pathway, source and complexity of nutrients, and disease.

The EN caloric density (kcal ml− 1) should be based on the patient’s total calorie needs versus the volume of enteral diets to be administered per day. Enteral diets with higher energy density have a lower amount of water, which can range from 690 to 860 ml l− 1 diet. The categorization of enteral formulas, according to its energy density, is shown in Table 3.

Table 3. Categorization of enteral formulas according to its energy density

Vitamin and mineral supply varies according to the specific needs of the patients and their disease. In the specific nutritional needs, you should evaluate the indication of additional micronutrient supplementation, even when the formulation, per se, achieves those values recommended by the Recommended Dietary Allowance (RDA). Clinical nutritional patient evaluation should include objective and/or subjective indicators to identify, as early as possible, any risk of specific micronutrient deficiency for it to be immediately corrected and/or prevented.

Some specialized and very specific formulations to particular clinical situation (e.g., renal failure) are insufficient in some vitamin and mineral supply. Therefore, EN dietary planning attends to the need for supplementation or not of these micronutrients. For the long-term use of incomplete enteral feeding, the supplemental vitamins and minerals should be indicated.

In patients with malabsorption syndromes, investigate the possible fat soluble vitamins (A, D, E, and K) deficiency to correct it shortly. There is a lack of specific vitamin and mineral recommendations for critically ill patients. However, in such a condition, the needs of antioxidant nutrients are increased due the oxidative stress, and it is recommended to supplement vitamins A, C, and E, zinc, and selenium.

EN osmolality (mmol l− 1 solution) and osmolality (mOsm kg− 1 water) are associated with its digestive tolerance. Although the stomach tolerates diets with higher osmolality, more distal portions of the gastrointestinal tract respond better to isosmolares formulations. Therefore, hyperosmolar diets infused by gastrostomy or nasogastric feeding tube have better digestive tolerance when compared with administration by postpyloric or jejunal probes.

The nutrients that most affect the osmolality of a solution are simple carbohydrates (mono- and disaccharides), which have greater osmotic effect than the higher molecular weight carbohydrates (starch); minerals and electrolytes, due the property of its dissociation into smaller particles (e.g., sodium, potassium, and chloride); hydrolyzed proteins; crystalline amino acids; as well as medium-chain triglycerides, because they are more soluble than long-chain triglycerides. The more hydrolysates components contains the formulation, the higher its osmolality.

Enteral diets should not exceed the value of the renal solute load tolerated by the kidneys (800–1200 mOsm, in normal situation). Renal solute load can be calculated by adding 1 mOsm for each mEq of sodium/potassium/chloride, and 5.7 mOsm (adults) or 4 mOsm (children) for each gram of protein from its formula. Special attention should be given to critical clinical situations, such as sepsis, postoperative, polytrauma, and severe burn, where the urine becomes very dense, with high osmolality (around 500–1000 mOsm kg− 1), even under appropriate hydration.

Importantly, the influence of the medication osmolality is usually neglected. The mean osmolality of liquid medications administered orally or by feeding tube ranges from 450 to 10 950 mOsm kg− 1 water. Certain manifestations of gastrointestinal intolerance may be related to the medication, although it is often attributed to enteral formulation.

In specific clinical situations, there may be demands for change in the types of nutrients used; the quantity and/or form these should be presented. In such cases, nutritional therapy becomes more specialized. These adaptations involve changes from simple source of nutrients used until its physicochemical and structural modifications. Thus, specialized formulations for enteral use may provide different sources of vitamins, minerals, carbohydrates, lipids, and proteins, and these nutrients may be presented in their entirety or hydrolyzed (wholly or partly) structure.

Some specialized EN formulations are part of immunonutrition. The immunonutrition is a nutritional intervention that explores the particular activity of various nutrients in alleviating inflammation and modulating the immune system, in which are included the omega-3 fatty acids, arginine, glutamine, nucleotides, and antioxidants. There is a current consensus that perioperative immunonutrition can beneficiate elective surgical patients, especially those malnourished patients submitted to major gastrointestinal surgery. In these patients, administration of enteral diets containing n-3 PUFA, nucleotides and arginine contributes to decrease postoperative infectious and noninfectious complications and must be initiated 5–7 days preop (500–1000 ml day− 1) and maintained in the postoperative period.

Although the benefit of using this enteral formula combining different nutrients with immunomodulatory functions is well established in surgical patients, data are lacking to confirm or guide the effective and safe use of enteral diets containing isolated immunonutrients in different clinical populations, including arginine and glutamine. In hemodynamically stable condition, arginine may offer immunologic and metabolic benefits, but its participation in the synthesis of nitric oxide may constitute a potential risk for septic patients. Enteral glutamine should be considered to treat burn patients and trauma victims, but there is not sufficient evidence for its use in critically ill patients with failure of multiple systems.

Other nutrients that may compose specialized EN formulations include the branched chain amino acids (BCAAs). BCAAs provide primary fuel for skeletal muscle during stress and sepsis. Therefore, leucine, isoleucine, and valine may be added to specialized EN formulas as supplemental metabolic sources to attend the metabolic needs of skeletal muscle during hypermetabolic conditions.

Gastrostomy feeding

Patients who require long-term or permanent enteral feeding should be considered for gastrostomy feeding tube or gastrostomy button (a smaller appliance which is almost flush with the skin) placement. The technique of percutaneous endoscopic gastrostomy (PEG) is a well-established procedure for providing enteral nutrition support. Although the feeding gastrostomy may be placed surgically, PEGs compare favorably, with a lower morbidity and cost; the surgical gastrostomy requires a laparotomy and a general anesthetic, whereas the PEG is performed under local anesthesia using a fiber-optic endoscope. A range of PEG kits are available for infants, children, and adults. A feeding jejunostomy may be considered as the means of providing early postoperative nutrition support in selected patients and may be used for short- or long-term support.

Systematic reviews

In a meta-analysis of 30 randomized controlled trials, enteral nutrition was compared with early parenteral nutrition in hospital in-patients [1]. Early nutrition was defined as the initiation of nutrition support within 96 hours of hospital admission, ICU admission, or surgery. Only studies in which hospital mortality, length of stay, and/or complications had been reported were included. The analysis excluded all non-English language studies and those associated with “immunonutrition”, including arginine, nucleotides, omega-3 fatty acids, and glutamine; this poorly reflects recent practice in intensive care units, in which glutamine-containing parenteral nutrition solutions are being increasingly used. Of the 30 studies, 10 were in medical patients, 11 in surgical patients, and 9 in trauma patients. Parenteral nutrition was associated with a 7.9% increase in infective complications, a 3.5% increase in catheter-related blood-stream infections, a 4.9% increase in non-infective complications, and an increase in the length of stay by 1.2 days. There was no effect of nutrition type on mortality or on technical complications, but enteral nutrition was associated with an increased risk of diarrhea. The authors acknowledged that as nutritional support has become more interventional, the numbers of complications have increased, particularly in the more interventional forms of enteral nutrition. They also noted that enteral nutrition is generally associated with lower nutritional intake overall.

4.1 Typical Exclusive Enteral Nutrition Protocol

EEN involves the use of an exclusive period of a liquid diet for a period of up to 8 weeks, with exclusion of normal diet during this period.15,35 The potential role of EEN arose from descriptive studies more than 4 decades ago: these case reports and case series illustrated improved inflammatory status in adult patients managed with nutritional intervention.36–40 A randomized controlled study compared EEN and corticosteroids (CS) in adults with active CD: in this Irish study EEN and CS performed similarly.41 Although numerous subsequent studies have shown benefits with EEN in adults, many more recent reports have shown that EEN is more efficacious in children.18,42

5 When should EN be initiated in critically ill patients?

Early EN is usually defined as initiating enteral feedings within 48 hours of intensive care unit (ICU) admission. Many RCTs have compared early EN versus delayed nutrient intake in critically ill patients receiving mechanical ventilation, and, when these results were aggregated, early EN was associated with a trend toward mortality reduction and a significant reduction in infectious complications. Starting EN early does not seem to affect the duration of mechanical ventilation or ICU length of stay. The presence of bowel sounds and the passage of flatus are not necessary before the institution of EN.

Definition

Enteral feeding is a method of providing nutrients directly into the gastrointestinal (GI) tract when a person cannot receive food orally. It is used in patients who have an adequate functional GI tract and can digest and absorb food but in whom oral intake is inadequate to maintain or restore optimal nutritional status. Also known as tube feeding, enteral nutrition (EN) delivers nutrients directly to the stomach or intestines through a thin flexible tube. It is administered through a nasogastric tube placed via the nose, or a percutaneous tube placed into the stomach (gastrostomy) or the small intestine (jejunostomy). EN is generally considered safer and the preferred method of delivering nutritional support over parenteral nutrition. In this article, the use of enteral feeding is reviewed.