PARENTERAL NUTRITION SUPPORT FOR ADULTS

Overview
Parenteral nutrition is the provision of nutrients intravenously.  Since the 1960s, major advances have been seen in the technique, delivery, and formulation of parenteral nutrition (1).  The use of guidelines for practice has improved nutritional markers and reduced the rates of complications of patients receiving parenteral nutrition (1,2).  There are two primary types of parenteral nutrition: central parenteral nutrition (CPN) and peripheral parenteral nutrition (PPN).  Central parenteral nutrition (or total parenteral nutrition) provides nutrients through a large-diameter vein, usually the superior vena cava, by access of the subclavian or internal jugular vein.  The CPN formulas are hyperosmolar (1,300 to 1,800 mOsm/L) and consist of glucose (15% to 25%), amino acids, and electrolytes to fully meet the patient’s nutrient needs.  Central parenteral nutrition can be maintained for prolonged periods and adjusted to meet the nutrient and volume needs of patients who require fluid restriction (3).  When venous access for the delivery of nutrients is required for longer than 2 weeks, CPN is indicated (1).

    Peripheral parenteral nutrition uses a peripheral vein for venous access.  This form of parenteral nutrition is similar to CPN except that lower formula concentrations must be used because the peripheral vein can only tolerate solutions that are less than 900 mOsm/L.  Compared with CPN formulas, PPN formulas have lower concentrations of dextrose (5% to 10%) and amino acids (3%).  Because higher concentrations cannot be infused into the peripheral vein, PPN requires larger fluid volumes to provide energy and protein doses comparable to the doses provided by CPN.  The larger fluid volume poses a challenge for patients who require fluid restriction.  The maximum volume of PPN that is usually tolerated is 3 L/day (125 mL/h).  Repletion of nutrient stores is not a goal of PPN, and it should not be used in severely malnourished patients (1). Peripheral parenteral nutrition is indicated only for mildly to moderately malnourished patients who are unable to ingest adequate energy orally or enterally, or for patients in whom CPN is not feasible.  Typically, PPN is used for short periods (5 days to 2 weeks) because of limited tolerance and the vulnerability of peripheral veins (eg, risk of peripheral venous thrombophlebitis) (1).

Nutrition Assessment
A meta-analysis of parenteral nutrition suggests that this route of nutrition support increases infection rates without measurable beneficial outcomes when compared to controls (4).  Therefore, parenteral nutrition should be considered only when the enteral route is contraindicated or when a trial of enteral feeding has failed (3). 

Indications (1-3)
Parenteral nutrition is costly and may result in serious complications if the patient is not monitored closely (5,6).  Parenteral nutrition is beneficial in the following circumstances (7):

• allogeneic bone marrow transplantation, specifically perioperative support of moderately to severely malnourished patients with gastrointestinal cancer acute exacerbations of Crohn disease complicated by fistulas
• gastrointestinal fistulas
• severe, acute necrotizing pancreatitis (if unable to feed enterally at or beyond the Ligament of Treitz)

    Guidelines for the implementation of parenteral nutrition have been developed by the American Society for Parenteral and Enteral Nutrition (ASPEN) (1).  Parenteral nutrition is indicated for patients who are unable to receive adequate nutrients via the enteral route (eg, patients who have a nonfunctional or severely compromised gastrointestinal tract).  These conditions include (1,3,6):

• severe malabsorption cause by massive bowel resection (≥ 70% resected) or severe diarrhea
• intractable vomiting
• severe acute pancreatitis in patients who have a documented history of poor tolerance to enteral nutrition (8,9)
• paralytic ileus
• complete intestinal obstruction
• enterocutaneous fistula (greater than 500 mL/day) (A trial of enteral nutrition can be considered when the output is less than 200 mL/day or when enteral access may be placed posterior to the fistula.) (1)
• severe inflammatory bowel disease
• radiation therapy or bone marrow transplantation

 Contraindications (1,3,7)
Parenteral nutrition is not indicated for patients:

• who have a fully functional and accessible gastrointestinal tract
• for whom venous access cannot be obtained
• whose prognosis does not warrant aggressive nutrition support
• whose need for parenteral nutrition is expected to be less than 5 days
• for whom the risks of parenteral nutrition exceed the potential benefits, such as in cases of severe hyperglycemia (>300 mg/dL), azotemia, encephalopathy, hyperosmolality (>350 mOsm/kg), and severe fluid and electrolyte disturbances (3)

Nutrition Intervention

Parenteral Feeding Formulations
The osmolarity of a parenteral formula depends on the energy substrate mixture, primarily the dextrose, amino acid, and electrolyte content (3).  Dextrose contributes 5 mOsm/g. Amino acids contribute 10 mOsm/g, and electrolytes provide 1 mOsm/mEq of individual additive.  For example, the estimated osmolarity of a parenteral feeding formula that provides 150 g/L of dextrose, 50 g/L of amino acids, and 150 mEq/L of electrolyte additives is 1,400 mOsm/L (3).  The maximum osmolarity tolerated by a peripheral vein is 900 mOsm/L (10,11).  Formulas for peripheral vein administration usually require more fluid and a higher content of fat as an energy source than those for central access, as lipids are isotonic (3).  Midline catheters can be used to improve peripheral vein tolerance to the nutrition infusion because these catheters can access larger veins where the blood flow may dilute the parenteral feeding formulations to a more tolerable concentration (3).

Nutrient Sources and Indications

Carbohydrate sources: The most commonly used source of carbohydrate is dextrose.  Dextrose in its monohydrate form provides 3.4 kcal/g.  Commercial dextrose solutions are available in concentrations ranging from 2.5% to 70% (11).  These solutions are acidic, with a pH ranging from 3.5 to 6.5 (11).  Formulas with final dextrose concentrations greater than 10% are reserved for central venous access (11).  Sugar alcohol glycerol is a less frequently used carbohydrate source, and it provides 4.3 kcal/g.  Parenteral formulas containing sugar alcohol glycerol are protein sparing and induce a smaller insulin response as compared to dextrose-based solutions (12-14). More research is needed to determine the efficacy of the routine use of parenteral formulas that contain sugar alcohol glycerol.

Carbohydrate requirements: The minimum requirements for dextrose are 1 mg/kg per minute (approximately 100 g/day for a 70-kg person).  The maximum amount of carbohydrate tolerated is approximately 5 to 7 mg/kg per minute (1,15). Hyperglycemia, which is caused by various factors including stress, is the most common complication of parenteral nutrition (11).  When carbohydrate is provided in excess, hyperglycemia, hepatic steatosis, and increased CO2 production can occur.  It is recommended that the serum glucose level be maintained at 80 to 110 mg/dL (16).  Maintaining glucose in this range is associated with decreased morbidity and mortality in surgical intensive care patients (16-18).  For patients with hyperglycemia, the parenteral-nutrition dextrose should be started conservatively and gradually increased to the patient’s individualized goal rate.  It is recommended that the goal rate should not exceed 4 to 5 mg/kg per minute or 20 to 25 kcal/kg per day (19).  Capillary blood glucose should be measured at least every 6 hours (19). Regular insulin can be provided subcutaneously or added directly to the parenteral solution (18,19).  An insulin drip provides more consistent and safe glucose control (19).  Blood glucose concentrations can be controlled by an initial regimen of 0.05 to 0.1 units of insulin per gram of dextrose in the parenteral solution or by 0.15 to 0.2 units of insulin per gram of dextrose in hyperglycemic patients (19).  When added to the solution, it is recommended that two-thirds of the total amount of sliding-scale insulin required over 24 hours be added to the next day’s parenteral formula (19).  A proportional increase in fat may be necessary to increase the energy provided to patients whose blood glucose levels are difficult to control (19).  Rarely, hyperglycemia is caused by a chromium deficiency; however, if insulin is ineffective or a chromium deficiency is confirmed, increasing the chromium contained in the parenteral formula may be appropriate (20).  Refer to Metabolic Complications of Parenteral Nutrition.

    Patients at risk of developing refeeding syndrome should be monitored closely.  Refeeding syndrome refers to metabolic and physiologic shifts of electrolytes and minerals (eg, phosphorus, magnesium, and potassium) caused by aggressive nutrition support (1,21).  Carbohydrate delivery stimulates insulin secretion, which causes an intracellular shift of these electrolytes and minerals with the potential for severe hypophosphatemia, hypomagnesemia, and hypokalemia (19,21).  Symptoms of refeeding syndrome include fatigue, lethargy, muscle weakness, edema, cardiac arrhythmia, and hemolysis (19,21).  The patients at greatest risk for refeeding syndrome are those with marginal nutrient stores secondary to a disease or medical therapy; these patients should initially receive 15 to 20 kcal/kg of formula per day, and then the amount of parenteral formula should be slowly increased (19,21). Refer to Metabolic Complications of Parenteral Nutrition.

Protein sources: Crystalline amino acids, which provide 4 kcal/g, are the most common source of protein in parenteral formulas.  Standard or balanced amino acid products are mixtures of essential and nonessential amino acids ranging in concentrations from 3% to 20%; although 8.5% and 10% are most frequently used for parenteral nutrition compounding (11).  Most commercially available amino acid formulations also contain electrolytes and/or buffers.  Modified or special amino acid products have been formulated for certain disease states and conditions in which conventional amino acid solutions may not be well tolerated (eg, renal failure and hepatic failure) (11).  The contribution of these formulas to an improvement in overall clinical outcome is debatable.  These formulas usually cost much more than conventional amino acid solutions. Therefore, the clinician should evaluate the cost in light of the potential benefit to the patient.  The 2002 ASPEN guidelines for nutrition support are the  basis for the following  discussion of specialized amino acid formulas (1):

• Formulations for liver disease: Formulas that contain high levels of branched-chain amino acids (BCAA) and low levels of aromatic aminos acid are designed to correct abnormal amino acid profiles associated with hepatic encephalopathy, as BCAA are metabolized independently of liver function.  The 2002 ASPEN guidelines recommend that BCAA formulas only be used for patients with chronic hepatic encephalopathy and patients with hepatic encephalopathy that worsens and does not respond to pharmacotherapy (1).
• Formulations for metabolic stress: Solutions that contain high levels of BCAA have been designed to support the increased muscle proteolysis associated with metabolic stress, which occurs in septic patients and trauma patients as well as patients with thermal injuries or patients who are in a hypermetabolic state.  While the use of BCAA-enriched formulations slightly improves nitrogen balance in certain patient groups, clinical evidence does not support improved outcomes (1).  According to the ASPEN guidelines, BCAA have not been shown to favorably influence the clinical outcomes of critically ill patients (1).
• Formulations for renal disease: Formulations designed for patients with renal disease primarily contain the essential amino acids; this formula design is based on the theory that nonessential amino acids can be physiologically recycled from urea, while the essential amino acids must be provided by the diet (11).  These formulas have no statistically significant advantages when compared to standard amino acid formulas (22).  Therefore, standard formulations in reduced amounts are recommended for patients with renal disease (1).
• Concentrated amino acid formulations: Highly concentrated (15% to 20%) amino acid formulations are available for use when fluid restriction is required.  These formulations are similar in composition to standard amino acid formulas, except that they may contain larger amounts of chloride and acetate (11).
• Pediatric formulations: Pediatric formulas have been designed to meet the unique amino acid requirements of neonates, infants, and children in order to promote weight gain, nitrogen balance, and growth and to avoid an excess of certain amino acids (eg, phenylalanine and methionine) (1).  These products generally contain taurine, tyrosine, histidine, arginine, and L-cysteine (semi-essential to neonates) (1).
• Glutamine: Glutamine is a conditionally essential amino acid during times of metabolic stress.  There has been considerable interest in the potential effects of glutamine.  Currently, crystalline amino acid formulations do not contain glutamine.  Due to stability and compatibility issues, there is no intravenous form of glutamine that is commercially available for admixture in parenteral solutions (11).  Studies of glutamine-supplemented parenteral formulas have used extemporaneous preparations of powdered L-glutamine sterilized by filtration (23,24).

Protein requirements: Protein requirements should be based on the patient’s individual needs and disease process.   Critical illness and hypermetabolism are associated with increased protein turnover, protein catabolism, and negative nitrogen balance (25).  During a critical illness, protein requirements can double to approximately 15% to 20% of total energy.  Protein requirements for the critically ill patient are at least 1.5 g/kg per day; adequate energy should be provided to meet the estimated needs as determined by indirect calorimetry or prediction equations (25).  Protein sparing does not typically improve with protein intakes greater than 1.5 g/kg per day, except in severely burned patients (25).   

Lipid sources: Isotonic lipid emulsions provide energy and essential fatty acids.  Lipid sources are derived from either soybean oil or a 50:50 mixture of soybean and safflower oils (11).  Lipid sources are emulsified with egg yolk phospholipid; therefore, their use may be contraindicated in patients with egg allergies.  Each gram of lipid provides 9 kcal.  Lipid emulsions are available in 10% (1.1 kcal/mL), 20% (2 kcal/mL), and 30% (3 kcal/mL) concentrations (11).  The 30% lipid formulation is approved only for compounding of the total nutrient admixture, not for direct intravenous administration (11).

    Investigational intravenous fat emulsion (IVFE) products include physical mixtures of medium-chain triglycerides and long-chain triglycerides.  These formulations may be useful for patients who are intolerant to the long-chain triglyceride products during critical illness and metabolic stress and also in patients with carnitine deficiency (11).  Structured lipid formulas containing linoleic acid, medium-chain fatty acids, and very long–chain omega-3 fatty acid are being investigated to determine if they produce less inflammatory and nonthrombogenic prostaglandins than standard lipid emulsions (11).  These formulas have been studied in patients with sepsis, atopic dermatitis, severe ulcerative colitis, and those undergoing elective surgery (26,27). These formulas are under investigational study and are not available for intravenous use in the United States (11).

    Because of the enhanced microbial growth potential with infusion of IVFE from dextrose and amino acid formulations, the Centers for Disease Control and Prevention recommends a 12-hour hang-time limit for IVFE (28).  The United States Pharmacopeial has also endorsed the use of IVFE products within 12 hours of opening the original manufacturer’s container if the IVFE products are infused as separate preparations from dextrose and amino acids (29).  However, because of the lower pH (5.6 to 6.0) of a total nutrient admixture that contains IVFE, dextrose, and amino acids in the same container, the fat emulsion may be administered over 24 hours (11).  Whether infused separately or as a total nutrient admixture, the infusion rate of IVFEs should not exceed 0.11g/kg per hour to reduce side effects such as hypertriglyceridemia and infectious complications (11,30).  

Lipid requirements: Lipids provide an important source of essential fatty acids.  Two to four percent of daily energy needs should be supplied as linoleic acid (1% to 2% of linoleic acid and 0.5% of alpha-linolenic acid) (1) or 25 to 100 mg/kg of essential fatty acids (1,31,32).  A minimum of 500 mL of 10% lipid stock solution or 250 mL of 20% stock solution administered over 8 to 10 hours, two to three times per week, is sufficient to prevent a deficiency of essential fatty acids.  Alternately, 500 mL of a 20% fat emulsion can be given once a week (33).  Hyperlipidemia can occur with excess amounts or rapid infusion rates of intravenous lipids (19).  Serum triglycerides levels should be evaluated before infusion of intravenous lipids.  Acceptable serum triglycerides levels are less than 250 mg/dL 4 hours after lipid infusion for piggybacked lipids and less than 400 mg/dL for continuous lipid infusion (1).  The infusion of intravenous lipids has been associated with impaired immune responses and vascular integrity (19,31,34).  Infusion rates of greater than 110 mg/kg per hour may result in reduced lipid clearance and impaired reticuloendothelial function and pulmonary exchange (31).  It is recommended that fat intake be restricted to less than 25% to 30% of total energy, or 1 g/kg per day, and provided slowly over 8 to 10 hours if administered as an intravenous supplement (31,33).  No more than 2.5 g/kg per 24 hours should be provided to adult patients (1).  The recommendations for lipids given to critically ill patients requiring parenteral nutrition are more conservative; the data support <1.0 g/kg per day (1,33). Carnitine deficiency can lead to fat deposition in the liver and muscle, impaired ketogenesis, and neurologic symptoms.  Parenteral nutrition solutions do not contain carnitine, and the supplemental use of carnitine in adults has not been studied.  However, carnitine supplementation has been suggested for neonates who receive parenteral nutrition for more than 2 weeks (35).

Parenteral Intravenous Vitamins and Requirements
In 2000, the Food and Drug Administration (FDA) modified the requirements for adult intravenous multivitamin products.  The required amounts of ascorbic acid, thiamin, pyridoxine, and folic acid were increased, and a requirement for vitamin K was added (36)  (see Table B-1).  With the addition of vitamin K to these formulations, the prothrombin time and international normalized ratio of patients who receive anticoagulant therapy should be closely monitored, and anticoagulant medication should be adjusted as needed (19).  Two formulas without vitamin K are available for patients whose prothrombin time or international normalized ratio levels are difficult to manage (37).   The vitamin and mineral requirements for parenteral nutrition should be based on age and gender-specific Dietary Reference Intakes (37).  Clinicians should consider the deleterious impact of exceeding the FDA-recommended levels of parenteral vitamins, and the potential harm that excessive vitamin intake has on other micronutrients, trace elements, and immune status (37). 

Table B-1: FDA Requirements for Parenteral Multivitamin Products (36)

Parenteral Intravenous Trace Minerals and Requirements
The ASPEN recommendations for daily parenteral intake of the trace elements zinc, copper, chromium, and manganese were updated in 2004 (Table B-2) (16).  These updated recommendations include the addition of selenium supplementation (20 to 60 mcg/day) (16). The trace minerals are available as single-entity products in various combinations and concentrations for adults, pediatrics, and neonates; the combination of trace minerals with electrolytes is also available (11,37).  Other elements that may be supplemented on an individualized basis include molybdenum, iodine, and iron.  Iron supplementation is generally not required for short-term parenteral nutrition, unless the patient is anemic.  Oral iron supplementation is the preferred route. However, if oral supplementation is infeasible, iron dextran can be intravenously administered (37).  The addition of iron to total nutrient admixtures is not recommended because of compatibility problems (38).  Patients with intestinal fluid losses (eg, ostomy) may require additional supplementation of zinc and chromium (see Table B-2).  Recommendations for patients with intestinal losses are 12 mg of additional zinc per liter of output from the small bowel and 17 mg/L for stool or ileostomy output (16,39).  Chromium requirements may also increase to 20 mcg/day with gastrointestinal losses in adults (16).

Table B-2: Daily Parenteral Trace Element Supplementation for Adults (Dose per Day)

a Add 2 mg/day for  hypermetabolic patients.
bAdd 12 mg/L for small bowel losses and 17 mg/L for stool or ileostomy losses.

Adapted from Clark SF. Vitamins and Trace Elements. In: Gottschlich MM, ed. The A.S.P.E.N. Nutrition Support Core Curriculum: A Case-Based Approach—The Adult Patient.  Silver Spring, MD: American Society of Enteral and Parenteral Nutrition; 2007:148 (Table 8-5) from A.S.P.E.N. Board of Directors.  Clinical Pathways and Algorithms for Delivery of Parenteral and Enteral Nutrition Support in Adults. Silver Spring, MD: A.S.P.E.N.;5 with permission from the American Society for Parenteral and Enteral Nutrition (A.S.P.E.N.) A.S.P.E.N. does not endorse the use of this material in any form other than its entirety.   

Parenteral Electrolytes and Requirements
Maintenance of therapeutic amounts of various electrolytes are added to parenteral formulations depending on the patient’s requirements (11).  Electrolytes are available in salt forms, including sodium and potassium as chloride acetate, or phosphate; calcium as chloride, gluconate, or gluceptate; and magnesium as sulfate or chloride.  Calcium gluconate and magnesium sulfate are the preferred forms of these cations because they produce fewer physiochemical incompatibilities (11).  Standard daily electrolyte ranges for adults include the following ranges with adjustments as indicated based on the clinical situation (Table B-3) (11):

Acetate and chloride do not have specific ranges for intake; rather, they are adjusted as needed to maintain the acid-base balance (10,11).

Total Nutrient Admixture Parenteral Solutions
Total nutrient admixture parenteral solutions, also known as three-in-one or all-in-one solutions, are composed of a mixture of amino acids, dextrose, lipids, vitamins, trace elements, and electrolytes in one container.  This method of nutrient delivery differs from the conventional method (two-in-one) of providing CPN, in which lipids are in a separate container and “piggybacked in” with the amino acid–dextrose solution.  Both types of parenteral formulation systems are in use today (11).  By definition, the total nutrient admixture includes the lipid emulsion on a daily basis, providing an additional energy source (11).  Total nutrient admixtures have decreased the cost of CPN solutions because of decreased administrative and equipment costs associated with CPN preparation and decreased nursing time (38).  Total nutrient admixtures may also help prevent excessive dextrose administration in critically ill patients (38,40).  Also, lipids are administered over a 24-hour period, which may promote better patient tolerance (40).

    One disadvantage of total nutrient admixtures is that they provide a better bacterial growth medium than the conventional system.  Also, most particulate matter in the admixture cannot be visually inspected (38,40,41).  Conventional solutions use a 0.22-mm in-line filter; however, total nutrient admixtures require a larger in-line filter (a 1.2-mm filter) because they contain lipids.  This larger filter is sufficient for trapping solution particulates, precipitates, and Candida albicans, but it does not protect against infusion contaminates such as Staphylococcus epidermidis, Escherichia coli, and bacterial endotoxins (11,38,40,41).  Refer to the previous discussion on Lipid Sources for hang-time and infusion guidelines. 

Stability and Compatibility
The concentrations of calcium and phosphate ions are directly related to the risk of precipitation, which can result in serious injury and death (42).  As the concentration of either micronutrient rises, the likelihood of precipitation increases (11).   Product-specific solubility curves that depict solubility limits for calcium and phosphate salts have been published (42).  The verification of large calcium doses (more than two times the Dietary Reference Intake) can help minimize the risk of precipitation (11).

Transitional Feeding (6)

Cyclic CPN: The infusion of CPN over a limited amount of time (usually 12- to 18-hour periods) is called cyclic CPN.  Cyclic CPN is indicated for patients who are metabolically stable and for patients who require long-term CPN, such as patients who receive CPN at home.  One advantage of cyclic CPN is that feedings more closely resemble physiologic (discontinuous) feedings, which may reduce the hepatic toxicity associated with continuous feedings.  Another advantage is improved quality of life, because the patient is free from CPN equipment during the day. 

Parenteral to enteral: When the patient is transitioned from parenteral support to enteral support, the tube feeding can be initiated at full strength at 10 to 50 mL/h.  As the rate of tube feeding is increased, the rate of parenteral nutrition is proportionately decreased.  Tapering of parenteral formula can begin when enteral tube feedings are providing 33% to 50% of nutrient requirements.  Once enteral tube feedings are well tolerated and provide more than 60% of energy requirements and 100% of fluid requirements, parenteral nutrition can be discontinued (6).

Parenteral to oral: When patients are transitioned from parenteral support to an oral diet, oral intake is started as clear liquids and then progressed in a stepwise fashion to an appropriate diet as tolerated.  Nutrient intake studies should document the adequacy of oral intake.  The total parenteral nutrition should be tapered to half of the original rate when the patient is eating 50% of the total estimated energy needs.  The CPN should be discontinued when oral intake consistently meets 60% of estimated nutrient needs and 100% of fluid needs.  If oral intake does not progress to adequate amounts, tube feeding should be considered in lieu of PPN or CPN (6).

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Bibliography
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Position of the American Dietetic Association: ethical and legal issues in nutrition, hydration, and feeding.  J Am Diet Assoc.  2002;102:716-725.

Manual of Clinical Nutrition Management                                                     
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