Total Parenteral Nutrition (TPN)
Parenteral nutrition is by definition given IV.
Partial parenteral nutrition supplies only part of daily nutritional requirements, supplementing oral intake. Many hospitalized patients are given dextrose or amino acid solutions by this method.
Total parenteral nutrition (TPN) supplies all daily nutritional requirements. TPN can be used in the hospital or at home. Because TPN solutions are concentrated and can cause thrombosis of peripheral veins, a central venous catheter is usually required.
Parenteral nutrition should not be used routinely in patients with an intact gastrointestinal (GI) tract. Compared with enteral nutrition, it has the following disadvantages:
(See also Overview of Nutritional Support.)
TPN may be the only feasible option for patients who do not have a functioning GI tract or who have disorders requiring complete bowel rest, such as the following:
TPN requires water (30 to 40 mL/kg/day), energy (30 to 35 kcal/kg/day, depending on energy expenditure; up to 45 kcal/kg/day for critically ill patients), amino acids (1.0 to 2.0 g/kg/day, depending on the degree of catabolism), essential fatty acids, vitamins, and minerals (see table Basic Adult Daily Requirements for Total Parenteral Nutrition).
Children who need TPN may have different fluid requirements and need more energy (up to 120 kcal/kg/day) and amino acids (up to 2.5 or 3.5 g/kg/day).
Basic Adult Daily Requirements for Total Parenteral Nutrition
Basic TPN solutions are prepared using sterile techniques, usually in liter batches according to standard formulas. Normally, 2 L/day of the standard solution is needed. Solutions may be modified based on laboratory results, underlying disorders, hypermetabolism, or other factors.
Most calories are supplied as carbohydrate. Typically, about 4 to 5 mg/kg/minute of dextrose is given. Standard solutions contain up to about 25% dextrose, but the amount and concentration depend on other factors, such as metabolic needs and the proportion of caloric needs that are supplied by lipids.
Commercially available lipid emulsions are often added to supply essential fatty acids and triglycerides; 20 to 30% of total calories are usually supplied as lipids. However, withholding lipids and their calories may help obese patients mobilize endogenous fat stores, increasing insulin sensitivity.
Many TPN solutions are commonly used. Electrolytes can be added to meet the patient’s needs.
TPN solutions vary depending on other disorders present and patient age, as for the following:
For renal insufficiency not being treated with dialysis or for liver failure: Reduced protein content and a high percentage of essential amino acids
For heart or kidney failure: Limited volume (liquid) intake
For respiratory failure: A lipid emulsion that provides most of nonprotein calories to minimize carbon dioxide production by carbohydrate metabolism
For neonates: Lower dextrose concentrations (17 to 18%)
Because the central venous catheter needs to remain in place for a long time, strict sterile technique must be used during insertion and maintenance of the TPN line. The TPN line should not be used for any other purpose. External tubing should be changed every 24 hours with the first bag of the day. In-line filters have not been shown to decrease complications. Dressings should be kept sterile and are usually changed every 48 hours using strict sterile techniques.
If TPN is given outside the hospital, patients must be taught to recognize symptoms of infection, and qualified home nursing must be arranged.
The solution is started slowly at 50% of the calculated requirements, using 5% dextrose to make up the balance of fluid requirements. Energy and nitrogen should be given simultaneously. The amount of regular insulin given (added directly to the TPN solution) depends on the plasma glucose level; if the level is normal and the final solution contains 25% dextrose, the usual starting dose is 5 to 10 units of regular insulin/L of TPN fluid.
Progress of patients with a TPN line should be followed on a flowchart. An interdisciplinary nutrition team, if available, should monitor patients. Weight, complete blood count, electrolytes, and blood urea nitrogen should be monitored often (eg, daily for inpatients). Plasma glucose should be monitored every 6 hours until patients and glucose levels become stable. Fluid intake and output should be monitored continuously. When patients become stable, blood tests can be done much less often.
Liver tests should be done. Plasma proteins (eg, serum albumin, possibly transthyretin or retinol-binding protein), prothrombin time, plasma and urine osmolality, and calcium, magnesium, and phosphate should be measured twice/week. Changes in transthyretin and retinol-binding protein reflect overall clinical status rather than nutritional status alone. If possible, blood tests should not be done during glucose infusion.
About 5 to 10% of patients with a TPN line have complications related to central venous access.
Catheter-related sepsis rates have decreased since the introduction of guidelines that emphasize sterile techniques for catheter insertion and skin care around the insertion site. The increasing use of dedicated teams of physicians and nurses who specialize in various procedures including catheter insertion also has accounted for a decrease in catheter-related infection rates.
Glucose abnormalities (hyperglycemia or hypoglycemia) or liver dysfunction occurs in > 90% of patients.
Glucose abnormalities are common. Hyperglycemia can be avoided by monitoring plasma glucose often, adjusting the insulin dose in the TPN solution, and giving subcutaneous insulin as needed. Hypoglycemia can be precipitated by suddenly stopping constant concentrated dextrose infusions. Treatment depends on the degree of hypoglycemia. Short-term hypoglycemia may be reversed with 50% dextrose IV; more prolonged hypoglycemia may require infusion of 5 or 10% dextrose for 24 hours before resuming TPN via the central venous catheter.
Hepatic complications include liver dysfunction, painful hepatomegaly, and hyperammonemia. They can develop at any age but are most common among infants, particularly premature ones (whose liver is immature).
Liver dysfunction may be transient, evidenced by increased transaminases, bilirubin, and alkaline phosphatase; it commonly occurs when TPN is started. Delayed or persistent elevations may result from excess amino acids. Pathogenesis is unknown, but cholestasis and inflammation may contribute. Progressive fibrosis occasionally develops. Reducing protein delivery may help.
Painful hepatomegaly suggests fat accumulation; carbohydrate delivery should be reduced.
Hyperammonemia can develop in infants, causing lethargy, twitching, and generalized seizures. Arginine supplementation at 0.5 to 1.0 mmol/kg/day can correct it.
If infants develop any hepatic complication, limiting amino acids to 1.0 g/kg/day may be necessary.
Abnormalities of serum electrolytes and minerals should be corrected by modifying subsequent infusions or, if correction is urgently required, by beginning appropriate peripheral vein infusions. Vitamin and mineral deficiencies are rare when solutions are given correctly. Elevated blood urea nitrogen may reflect dehydration, which can be corrected by giving free water as 5% dextrose via a peripheral vein.
Volume overload (suggested by > 1 kg/day weight gain) may occur when patients have high daily energy requirements and thus require large fluid volumes.
Metabolic bone disease, or bone demineralization (osteoporosis or osteomalacia), develops in some patients given TPN for > 3 months. The mechanism is unknown. Advanced disease can cause severe periarticular, lower-extremity, and back pain.
Adverse reactions to lipid emulsions (eg, dyspnea, cutaneous allergic reactions, nausea, headache, back pain, sweating, dizziness) are uncommon but may occur early, particularly if lipids are given at > 1.0 kcal/kg/hour. Temporary hyperlipidemia may occur, particularly in patients with kidney or liver failure; treatment is usually not required. Delayed adverse reactions to lipid emulsions include hepatomegaly, mild elevation of liver enzymes, splenomegaly, thrombocytopenia, leukopenia, and, especially in premature infants with respiratory distress syndrome, pulmonary function abnormalities. Temporarily or permanently slowing or stopping lipid emulsion infusion may prevent or minimize these adverse reactions.
Gallbladder complications include cholelithiasis, gallbladder sludge, and cholecystitis. These complications can be caused or worsened by prolonged gallbladder stasis. Stimulating contraction by providing about 20 to 30% of calories as fat and stopping glucose infusion several hours a day is helpful. Oral or enteral intake also helps. Treatment with metronidazole, ursodeoxycholic acid, phenobarbital, or cholecystokinin helps some patients with cholestasis.
Consider parenteral nutrition for patients who do not have a functioning gastrointestinal tract or who have disorders requiring complete bowel rest.
Calculate requirements for water (30 to 40 mL/kg/day), energy (30 to 35 kcal/kg/day, depending on energy expenditure; up to 45 kcal/kg/day for critically ill patients), amino acids (1.0 to 2.0 g/kg/day, depending on the degree of catabolism), essential fatty acids, vitamins, and minerals.
Choose a solution based on patient age and organ function status; different solutions are required for neonates and for patients who have compromised heart, kidney, or lung function.
Use a central venous catheter, with strict sterile technique for insertion and maintenance.
Monitor patients closely for complications (eg, related to central venous access; abnormal glucose, electrolyte, mineral levels; hepatic or gallbladder effects; reactions to lipid emulsions, and volume overload or dehydration).