Indications for parenteral nutrition

Patients who are unable to eat or absorb food adequately from the gastrointestinal tract should be considered for parenteral nutrition. The circumstances where this occurs include:

Route of administration

Parenteral nutrition may be given in the following ways:

Although most recipients of TPN are inpatients, many individuals who require long-term TPN have successfully managed to administer TPN in the home. These patients have permanent central catheters through which prepackaged nutrition fluids are administered, usually at night.

Components of TPN

TPN should, as its name suggests, provide complete artificial nutrition. An appropriate volume of fluid will contain a source of calories, amino acids, vitamins and trace elements (Fig 54.2). The calorie source is a mixture of glucose and lipid. Many patients who receive TPN are given standard proprietary regimens and prepackaged solutions. These have made TPN much easier, but as with any such approach in medicine there are some patients who require more tailored regimens.

Complications

Total parenteral nutrition is the most extreme form of nutritional support and can give rise to considerable difficulties. In order to pre-empt these, consistent careful nursing care and biochemical monitoring are required.

Catheter site sepsis is a constant fear in these patients. The nutrient-containing infusion fluids are, of course, also excellent bacterial and fungal growth media, and risk of infection is further heightened by the presence of a foreign body, the catheter. Strict attention to aseptic technique both in the siting of a catheter and in its maintenance will serve to avoid many of these problems.

Misplacement of a catheter and infusion of nutrient solutions extravascularly can be very serious. Central catheters should be placed under X-ray control. The possibility of embolism, either thrombotic or air, should be easily avoided as long as their potential is recognized.

The most common metabolic complication is that of hyperglycaemia. Against a background of increased stress hormones, especially if there is infection, there may be marked insulin resistance and consequently an increased glucose level. The use of insulin to correct these metabolic effects is best avoided. The composition of the IV regimen should be adjusted if metabolic disorders occur. Many other biochemical abnormalities have been reported in association with TPN. These include:

An acute form of metabolic disturbance is the re-feeding syndrome. Patients who have been malnourished for a significant period of time before initiation of TPN are most at risk, especially those with chronic alcoholism. In the undernourished state the metabolic and cellular processes are slowed down. As soon as nutrition is supplied, there is activation of these processes and increased utilization of minerals and micronutrients. This may result in dangerously low plasma levels of some minerals, trace elements and vitamins, especially inorganic phosphate, magnesium and vitamin B1. Re-feeding syndrome is preventable by prescribing TPN at half or even a quarter of total calculated requirements over the initial few days and gradually increasing it as a metabolically stable state is achieved.

With proper patient assessment and biochemical and clinical monitoring these complications can be minimized.

Monitoring patients on TPN

In addition to baseline assessment of patients receiving TPN, there should also be a strict policy for careful clinical and biochemical monitoring of these patients (Fig 54.3). This is especially important if the TPN is medium to long term. The tests described on pages 104–105 have particular relevance here.

Special attention must be paid to the micronutrients in long-term TPN patients as any imbalance here may result in a single nutrient deficiency state. Such situations are increasingly rare except in those patients relying solely on artificial diets for their nutrients.

Because biochemical changes may precede the development of any clinical manifestation of a nutritional deficiency, careful laboratory monitoring should be instituted.

However, measurement of trace elements and vitamins is often affected by the acute phase response and care needs to be taken in interpretation.