Parenteral nutrition

Parenteral nutrition (PN) is feeding a person intravenously, bypassing the usual process of eating and digestion. The person receives nutritional formulae that contain nutrients such as glucose, amino acids, lipids and added vitamins and dietary minerals. It is called total parenteral nutrition (TPN) or total nutrient admixture (TNA) when no significant nutrition is obtained by other routes. It may be called or total peripheral nutrition  (also TPN) when administered through vein access in a limb, rather than through a central port in body.

Mechanism
A mechanical pump under computer control is used to dispense the TPN fluid. Pumps are available that allow TPN administration at home, usually with the preparation and attachment by a family member. These pumps operate on an external dispensing line, part of a single-use dispensing cassette. Connection of the dispensing line to the patient is via a valve on a semi-permanant attached venous port whose closure is displaced by a connection on the dispensing line. Preparation, attachment, and valve replacement require care in sanitation and sterile techniques at specific locations. The use of a rechargeable battery and a portable component pack allows a convenient household mobility for many patients during administration periods, these being typically from twelve to sixteen hours a day.

Indications
Total parenteral nutrition (TPN) is provided when the gastrointestinal tract is nonfunctional because of an interruption in its continuity (it is blocked, or has a leak - a fistula) or because its absorptive capacity is impaired. It has been used for comatose patients, although enteral feeding is usually preferable, and less prone to complications. Parenteral nutrition is used to prevent malnutrition in patients who are unable to obtain adequate nutrients by oral or enteral routes.

Gastrointestinal disorders
TPN may be the only feasible option for nutrition patients who do not have a functioning gastrointestinal tract or who have disorders requiring complete bowel rest, including bowel obstruction, short bowel syndrome, prolonged diarrhea regardless of its cause, high-output fistula, very severe Crohn's disease or ulcerative colitis, and certain pediatric GI disorders including congenital GI anomalies.

Use in cancer
The benefit of TPN to cancer patients is largely debated, and studies to date have generally showed minimal long term benefit. There is no evidence to support the idea that intravenous nutrition 'feeds the cancer, not the patient', but weight loss with advanced disease is significantly more complicated than simply replacing calories as cancer produces a multitude of chemicals that also lead to weight loss, and giving extra nutrition does not prevent this.

Duration
Short-term PN may be used if a person's digestive system has shut down (for instance by peritonitis), and they are at a low enough weight to cause concerns about nutrition during an extended hospital stay. Long-term PN is occasionally used to treat people suffering the extended consequences of an accident, surgery, or digestive disorder. PN has extended the life of children born with nonexistent or severely deformed organs.

Complications
TPN is an artificial method of feeding, fully by-passing the GI tract. This unnatural way of feeding the body is far from perfect and comes with several significant complications

Infection
TPN requires a chronic IV access for the solution to run though, and the most common complication is infection of this catheter. Infection is a common cause of death in these patients, with a mortality rate of approximately 15% per infection, and death usually results from septic shock.

Blood clots
Chronic IV access leaves a foreign body in the vascular system, and blood clots on this IV line are common. Death can result from a clot that starts on the IV line but breaks off and goes into the lungs. This process is called a pulmonary embolism

Patients under long-term TPN will typically receive a periodic heparin flush to dissolve such clots before they become dangerous.

Fatty liver and liver failure
Fatty liver is usually a more long term complication of TPN, though over a long enough course it is fairly common. The pathogenesis is still unknown.

Other complications
Total parenteral nutrition increases the risk of acute cholecystitis due to complete disuse of gastrointestinal tract, which may result in bile stasis in the gallbladder. Other potential hepatobiliary dysfunctions include steatosis, steatohepatitis, cholestasis, and cholelithiasis. Six percent of patients on TPN longer than 3 weeks and 100% of patients on TPN longer than 13 weeks develop biliary sludge. The formation of sludge is the result of stasis due to lack of enteric stimulation and is not due to changes in bile composition. Gallbladder sludge disappears after 4 weeks of normal oral diet. Administration of exogenous cholecystokinin (CCK) or stimulation of endogenous CCK by periodic pulse of large amounts of amino acids have been shown to help prevent sludge formation. These therapies are not routinely recommended. Such complications are suggested to be the main reason for mortality in people requiring long-term total parenteral nutrition, such as in short bowel syndrome. In newborn infants with short bowel syndrome with less than 10% of expected intestinal length, thereby being dependent upon total parenteral nutrition, 5 year survival is approximately 20%.

Complications are either related to catheter insertion, or metabolic, including refeeding syndrome. Catheter complications include pneumothorax, accidental arterial puncture, and catheter-related sepsis. The complication rate at the time of insertion should be less than 5%. Catheter-related infections may be minimised by appropriate choice of catheter and insertion technique. Metabolic complications include the refeeding syndrome characterised by hypokalemia, hypophosphatemia and hypomagnesemia. Hyperglycemia is common at the start of therapy, but can be treated with insulin added to the TPN solution. Hypoglycaemia is likely to occur with abrupt cessation of TPN. Liver dysfunction can be limited to a reversible cholestatic jaundice and to fatty infiltration (demonstrated by elevated transaminases). Severe hepatic dysfunction is a rare complication. Overall, patients receiving TPN have a higher rate of infectious complications. This can be related to hyperglycemia.

Solutions
The nutrient solution consists of water and electrolytes; glucose, amino acids, and lipids; essential vitamins, minerals and trace elements are added or given separately. Previously lipid emulsions were given separately but it is becoming more common for a "three-in-one" solution of glucose, proteins, and lipids to be administered.

Ideally each patient is assessed individually before commencing on parenteral nutrition, and a team consisting of specialised doctors, nurses, clinical pharmacists and Registered Dietitians evaluate the patient's individual data and decide what PN formula to use and at what infusion rate.

For energy only, intravenous sugar solutions with dextrose or glucose are generally used. This is not considered to be parenteral nutrition as it does not prevent malnutrition when used on its own.

Total parenteral nutrition
Solutions for total parenteral nutrition may be customized to individual patient requirements, or standardized solutions may be used. The use of standardized parenteral nutrition solutions is cost effective and may provide better control of serum electrolytes.

Standardized solutions may also differ between developers. Following are some examples of what compositions they may have. The solution for normal patients may be given both centrally and peripherally.

Individual components
Individual nutrient components may be added to more precisely adjust the body contents of it. That individual nutrient may, if possible, be infused individually, or it may be injected into a bag of nutrient solution or intravenous fluids (volume expander solution) that is given to the patient.

Administration of individual components may be more hazardous than administration of pre-mixed solutions such as those used in total parenteral nutrition, because the latter are generally already balanced in regard to e.g. osmolarity and ability to infuse peripherally. For example, incorrect IV administration of concentrated potassium can be lethal, but this is not a danger if the potassium is mixed in TPN solution and diluted.

Vitamins may be added to a bulk premixed nutrient immediately before administration, typically in two doses, one fat soluble, the other water soluble, this since the additional vitamins can promote spoilage of stored product.