Enteral and Parenteral Nutrition Part 2

Practice of Nutritional Support

General principles of nutritional care

At hospital admission, all patients should be interviewed by a dietitian and have their SGA calculated to determine whether they can be maintained on a normal or modified oral diet (with supplements) in sufficient quantities or whether nutritional support is indicated and, if so, how urgently. In patients requiring nutritional support, the physician and the dietitian should define nutrient intake, route of administration, and goals. The most important objective is maintenance of uninterrupted nutrient intake, to avoid weeks of starvation followed by the urgent institution of parenteral nutrition to an iatrogenically malnourished patient.

Table 1 Procedure for Nasogastric or Nasoenteral Tube Placement

1. Explain the procedure to the patient, to obtain cooperation.

2. Seat the patient comfortably at the edge of the bed, sitting upright.

3. Check nostrils for painful lesions and obstruction.

4. Insert stylet into tube and lubricate.

5. Measure approximate length of tube to be passed by the distance between the tip of the nose to the ear and down to the midepigastrium. Add about 25 cm to this distance.

6. Flex neck slightly.

7. Pass tube through an unobstructed nostril. If the patient finds this very uncomfortable, spray nostril with lidocaine 4% topical solution.

8. Ask the patient to swallow water as the tube is passed.

9. If the patient coughs or chokes, withdraw tube into the pharynx and reinsert.

10. Aspirate gastric contents to confirm position of tube.

11. Air may be injected into the tube while auscultating to determine the intragastric location of the tube.

12. For nasogastric feeding, confirm the tube position by x-ray before infusing.

13. For nasoenteral feeding, place the patient in right lateral position and gradually advance tube. Metoclopramide, 10 mg I.V., may be used to propel the tube.

14. If tube has not passed into the bowel by 24 hours, endoscop-ic or fluoroscopic guidance may be used.

Oral Nutrition

In patients who can eat, close attention to maintenance of oral dietary intake—and use of supplements, where required—should be the standard of care. Enteral nutrition should be considered if it becomes clear that this approach does not permit sufficient intake to meet requirements.

Enteral Nutrition

Enteral nutrition is applicable to all patients, but it should be used with caution in patients with (1) clinically significant gas-troesophageal reflux; (2) intestinal obstruction; (3) GI fistula or recent surgical anastomosis, unless the tube can be inserted distal to the area in question or threaded at operation past the area; and (4) cardiovascular instability with shock. Gastric retention is a relative contraindication. In patients who accumulate secretions in the stomach and then aspirate, it may be possible to pass a feeding tube into the small intestine and aspirate the stomach with a second tube. However, in such cases the relative discomfort of two tubes versus parenteral nutrition should be considered. A recent survey showed that patients preferred parenteral nutrition over enteral nutrition.43

Short-term enteral access Nasogastric or nasoenteric placement of a feeding tube provides short-term enteral access. The tube should be small bore (9 to 12 French) and 105 to 110 cm long [see Table 1]. These tubes are usually made of Silastic or polyurethane. The latter become very slippery when wet, thus aiding insertion. I prefer intestinal placement of the tube, because controlled trials have shown better achievement of nutrient intake44 and, possibly, reduced risk of aspiration when the tube is placed beyond the ligament of Treitz.

Long-term feeding The definition of long-term feeding is arbitrary. Children with Crohn disease have been fed for months by teaching them to pass a nasogastric tube each night, receive a nocturnal feeding, and then remove the tube in the morning before going to school. However, in many instances nasal tubes become uncomfortable, and a gastrostomy tube can be placed endoscopically by a gastroenterologist or an inter-ventional radiologist. This method has been shown to be safer and more cost-effective than a surgically placed gastrostomy. There are two methods of percutaneous endoscopic gastrosto-my (PEG): the pull (Ponsky-Gauderer) method and the push (Russell) method.

Feeding into the small bowel can be performed after the insertion of a percutaneous endoscopic jejunostomy (PEJ). After the tract of the PEG tube is established, a PEJ tube with two arms can replace the tube. One arm remains in the stomach and can be used to drain this organ; the other arm is advanced under endoscopic guidance through the pylorus into the small intestine. In this way, the stomach can be decompressed, and simultaneously, the patient can be fed into the small bowel.

To eliminate the inconvenience of the bulky feeding tube, patients with long-term gastrostomies can be fitted with a so-called button device, which lies flush with the abdominal wall. Between feedings, a valve in the device closes off access to the stomach; during feedings, the feeding tube is inserted past the valve, permitting access to the stomach.

Parenteral Nutrition

The intravenous route is used as a supplement to oral or en-teral nutrition or is used as the sole source of nutrition (TPN) when it becomes clear that the patient is not receiving sufficient nutrients by the other routes. Regular evaluation of SGA should be performed during TPN to ensure that the patient’s nutrient requirements are being met.

Short-term parenteral feeding Short-term infusions are best given through a peripherally inserted central catheter (PICC). These catheters are inserted into an arm or forearm vein and advanced into the superior vena cava. PICCs are comfortable and avoid the risks of subclavian puncture or the difficulties of maintaining sterility of the exit sites of jugular catheters. In addition, full TPN with hypertonic mixtures can be given through these catheters without risk of thrombosis. Despite the designation "short term," these catheters can be used for months.

Long-term parenteral feeding Patients with intestinal failure often require parenteral feeding for years. To permit long-term parenteral feeding, an interventional radiologist advances a specially designed catheter through a subcutaneous tunnel via the jugular vein to the superior vena cava. The tip of this catheter should lie just above the right atrium, to avoid thrombotic complications. Near the exit site, within the subcutaneous tunnel, the catheter is surrounded by a Dacron cuff. Fibroblasts will grow into the cuff, sealing and anchoring the skin exit site.

Nutrient requirements

Protein

Protein requirements are met by giving whole proteins, pep-tides, or amino acids in enteral nutrition and by infusing an amino acid mixture in parenteral nutrition. The goal is to promote nitrogen retention and protein synthesis. Although limiting glucose and lipid (energy) intake will maximize nitrogen retention, dietary protein has an anabolic effect independent of energy intake, and will reduce nitrogen losses when infused alone.45 Thus, the amount of amino acids given appears to be a very important determinant of nitrogen balance.

About 1 to 1.5 g/kg of ideal body weight of protein or amino acids will be sufficient for most patients with normal renal function. Additional amounts should be added for losses from prior depletion or current hypercatabolism. In patients with hepatic failure, protein intake should be restricted to 0.8 to 1.0 g/kg a day.

Glutamine

Glutamine is an amino acid released by muscle and used by immune cells and enteral cells for energy. In malnutrition and after trauma, muscle glutamine and muscle protein synthesis are reduced. The infusion of glutamine normalizes muscle glu-tamine and restores protein synthesis.46 Clinically, bone marrow transplant patients were noted to have fewer episodes of sepsis and a shorter hospital stay if they received a glutamine-supplemented amino acid solution.47 Because glutamine does not have a long shelf-life in solution, dipeptides containing glu-tamine have been used as a substitute. Infusion of solutions containing such dipeptides has been found to increase muscle glutamine and improve protein synthesis.

Immunonutrition

Enteral formulations enriched in arginine, omega-3 fatty acids, and glutamine nucleotides are considered to enhance the immune response; treatment with these formulations is referred to as immunonutrition. These formulations vary in composition, but they are distinguished by high (12 to 15 g/L) or low (4 to 6 g/L) arginine content, presence or absence of gluta-mine and nucleotides, and different concentrations of omega-3 fatty acids. A recent summit on immune-enhancing enteral therapy48 concluded, on the basis of published literature, that immunonutrition should be given to malnourished patients undergoing elective GI surgery and to trauma patients with an injury severity score of 18 or greater or an abdominal trauma index of 20 or greater. Immunonutrition was also recommended, despite lack of evidence, in patients undergoing head and neck surgery or aortic reconstruction, as well as in patients with severe head injury or burns, and in ventilator-dependent nonseptic patients. It was not recommended for patients with splanchnic hypoperfusion or bowel obstruction distal to the access site or after major upper GI hemorrhage.

A systematic review of immunonutrition by Heyland and colleagues49 showed that it reduced septic complications but did not reduce mortality. Their analysis of 22 randomized, controlled trials covering 2,419 critically ill or surgical patients indicated that only high-arginine formulations reduced infectious complications and length of stay. These authors concluded that in patients undergoing elective surgery, immunonutrition may reduce complications and reduce length of stay. Pending further studies, however, immunonutrition was not recommended in patients with critical illness. Because many trauma and septic patients may be critically ill, these authors’ recommendations are at variance with those of the immunonutrition summit (see above). The finding that benefit is seen only with the formulation containing higher amounts of arginine raises the question whether arginine per se or the higher nitrogen intake is responsible for the benefit.

Energy (Glucose and Lipids)

In healthy persons, basal energy expenditure (BEE), or basal metabolic rate (BMR), in kilocalories a day can be predicted with the Harris-Benedict equation:

BEE in males = 66.5 + (13.8 x weight in kg) + (5.0 x height in cm) – (6.8 x age in yr)

BEE in females = 655.1 + (9.6 x weight in kg) + (1.8 x height in cm) – (4.7 x age in yr)

A calculator for determining BEE according to the Harris-Benedict equation can be found on the Internet, at www-users.med.cornell.edu/~spon/picu/calc/beecalc.htm.

For patients substantially on bed rest, about 30% should be added to the BEE to meet their metabolic requirements. In practice, this calculates as a daily expenditure of about 31 kcal/kg. An expert group has suggested a daily intake of 25 kcal/kg in ICU patients.50 Therefore, 25 to 30 kcal/kg/day will meet the needs of most patients, except those with burns. Malnutrition reduces the expected BEE by as much as 35%; injury, sepsis, and, especially, burns increase requirements.51 Baker and col-leagues52 found that in critically sick patients in respiratory failure, the maximal degree of hypermetabolism was about 30%.

Energy requirements during TPN can be met by infusing glucose or lipid emulsions. These nonprotein energy sources enhance nitrogen retention. The most striking increase in nitrogen balance has been found to occur when energy was increased from 0 kcal/kg to 30 kcal/kg of ideal body weight. Increases above that provided only slight improvement. In obese persons, a high-protein formulation with only about 14 kcal/kg/day meets nitrogen requirements53 and is associated with satisfactory wound healing.

Figure 2 Controlling hyperglycemia in intensive care unit patients receiving nutritional support.

Table 2 Daily Electrolyte and Trace Element Requirements for Adults on Total Parenteral Nutrition

Element	Normal	Increased GI Losses	Renal Failure	Comments
Sodium (mmol)	80-120	Meet losses	20-40	Reduce in heart failure
Potassium (mmol)	40—80	80-120	0-20	Correct hypokalemia before starting nutrition
Magnesium (mmol)	5-10	10-20	0-5	Correct hypomagnesemia before starting nutrition
Phosphorus (mmol)	10-15	10-15	0-5	Risk of dangerously low serum levels when feeding patients with severe malnutrition
Zinc (mg)	TPN: 3-4 Enteral: 15-20	TPN: 12-25 Enteral: 50-100	No change	—
Copper (mg)	TPN: 0.25-0.3 Enteral: 2-4	TPN: 0.5-0.7 Enteral: 4-8	No change	Reduce to 0.1 in hepatic failure

Because glucose spares nitrogen in fasting persons, it has been advocated as the main source of energy for parenteral nutrition. However, recent studies have shown that in malnourished patients and septic patients, lipids can promote nitrogen retention and increase total body nitrogen to the same extent as glucose, provided amino acids are given.55 Fats constitute about 30% of total energy in most enteral formulas. Furthermore, glu-cose-lipid mixtures facilitate the control of severe hyper-glycemia in septic patients with insulin resistance.50

Infusion of glucose at rates that exceed energy requirements elevates O2 consumption, CO2 production, resting energy expenditure, and urinary norepinephrine excretion. However, the magnitude of increased CO2 production is small if total calories infused conform to levels recommended for the patient’s clinical situation.51

The exact amount of lipid to include in the parenteral nutrition regimen is controversial. In a randomized, controlled trial of 512 bone marrow transplant patients receiving TPN, sepsis was no more frequent in patients who received 30% of energy as lipid than in those who received only sufficient lipids to meet essential fatty acid (EFA) needs (6% to 8% of energy intake).56 In addition, EFA deficiency developed in some of the latter patients, and in some, this small amount of lipid was insufficient to meet energy requirements without induction of hyperglycemia from the glucose component. These authors recommend giving 25% to 30% of energy as long-chain triglycerides (LCTs). In contrast, a study in 57 trauma patients found that TPN with added lipid increased sepsis and hospital stay.57 It was not clear whether the adverse effect was from the lipid per se or the increased energy intake while on lipid.

Because of their glucose content, both enteral nutrition and TPN enhance the risk of sepsis if the blood glucose level is allowed to rise above 127 mg/dl (7 mmol/L).8 Therefore, insulin should be infused in patients receiving nutritional support to keep them as close to normoglycemia as possible [see Figure 2].

Whereas the major concern with glucose-based formulations is hyperglycemia, the key concern with lipid emulsions is hy-pertriglyceridemia, which may induce pancreatitis. Lipid particles also reduce gas diffusion in the lungs and inhibit the retic-uloendothelial system. Provided that lipid emulsions are infused continuously at a rate that does not exceed 110 mg/kg/hr, hypertriglyceridemia does not occur. When these principles are followed, 30% to 50% of nonprotein calories can be given as fat, especially in glucose-intolerant patients.