What should I monitor for total parenteral nutrition?

A multidisciplinary approach to the management of patients receiving TPN is recommended to reduce complications, writes Deirdre McCormack

Total parenteral nutrition is the aseptic delivery of nutrients into the circulatory system via a central venous catheter or the peripheral veins. TPN is used when the gut is not functioning and there are several indications that may suggest its use (see Table 1).

Access routes and duration
Establishing and maintaining suitable access to the circulation is essential for the successful management of TPN. The route used is dependent on the anticipated duration of feeding and the osmolarity of the solution.

Central vein cannulation is the most commonly used route, however peripheral feeding is an acceptable alternative for short-term feeding. A single dedicated lumen should be used for administration of TPN.

Nutritional assessment
Prior to commencement of TPN the patients nutritional requirements are assessed. Energy and nitrogen requirements can vary, depending on age, sex, body composition, clinical status and activity.

A baseline biochemical assessment should identify any abnormal plasma electrolyte levels, liver function tests, renal function tests, glucose levels or lipid screen.

Nutritional requirements

Fluid
TPN may be the sole source of fluid or used in combination with other sources such as iv fluids, iv antibiotics and blood products. The fluid balance chart is an informative summary of determining adequacy of input versus output.

Acute changes in fluid can also be determined by monitoring acute changes in biochemical parameters such as albumin, haemoglobin, mean cell volume, urea and sodium. To maintain fluid levels, the following is required:

• 18-60 years old 35ml/kg/day
• > 60 years old 30ml/kg/day

Plus replacement of ongoing fluid losses, eg. pyrexia, urine, drains, excess wound exudates, stomas and high GI losses.

Energy
Energy requirements are most commonly estimated using predictive equations.1 These are based on population data, taking into account activity and stress factors.

It is imperative that these equations are not used in isolation and that monitoring of the patient occurs to assess efficacy. Indirect calorimetry may be used. However, this is not practical in the acute setting.

Nutritional requirements are also affected by medical conditions, thus alternate evidence based predictive equations exist for certain conditions such as liver disease, renal failure and the critically ill obese.

Nitrogen
Patients do not have a requirement for nitrogen per se, but for amINO acids, which are the substrates needed for protein synthesis (1g nitrogen = 6.25g protein).

The aim of nutritional support is to achieve a state of nitrogen balance using nitrogen balance data where available or general recommendations.2 The nitrogen in parenteral nutrition is provided in the form of an amINO acid solution.

Electrolytes
The normal daily electrolyte requirements are:

• Sodium 1-1.5 mmol/kg
• Potassium 1-1.5mmol/kg
• Calcium 0.1-0.15mmol/kg
• Magnesium 0.1-0.2 mmol/kg
• Phosphate 0.5-0.7mmol/kg.

Vitamins and trace elements

• Water soluble (Pabrinex), fat soluble (Vitlipid) and combined preparations (Cernevit Multibionta) of vitamins are available.
• Additrace contains trace elements, eg. iron, zinc, manganese, copper, chromium, selenium, molybdenum, fluoride and iodine.

Monitoring
Regular monitoring is essential to detect and minimise complications and determine response to nutritional support.

Patients receiving TPN should have their nutritional requirements reviewed regularly, taking into account clinical condition, treatments (eg. dialysis), drug therapy, nutritional status, response to TPN and supporting laboratory data.

Clinical assessment of the patient can reveal ascites, oedema, impaired wound healing or loss of muscle mass that may not be evident from monitoring weight and biochemical indices (see Table 2).

Complications
Mechanical, infectious and nutritional complications can arise, including:

• Mechanical pneumothorax, malposition, embolism
• Infectious sepsis, thrombophlebitis
• Nutritional complications including
  • fluid overload/dehydration
  • electrolyte imbalance
  • hyperglycaemia/hypoglycaemia
  • over feeding
  • re-feeding syndrome
  • nutrient deficiency
  • hepatobiliary dysfunction.

Standard versus tailored regimens
TPN can be provided by a standard or patient specific prescription. Many factors influence this decision such as frequency of use, patient types, local compounding facilities and cost.

Patients on TPN that are metabolically stable can tolerate slight under or over provision of nutrients, fluid or electrolytes with no complications.

It is the careful assessment that will identify those patients who are likely to be substrate intolerant and require frequent manipulations or specifically tailored regimens.

Albumin
A low plasma albumin level is occasionally used as a reason for nutrition support. Albumin is a negative acute phase protein, levels dropping within six hours of an acute injury, decreasing by up to 50% in severe cases. This is due to the increased transcapillary escape rate and a reduced return via the lymphatic system. Albumin is not a useful indicator of nutritional status, but is a useful prognostic marker.

Novel substrates
The principal novel substrate with the most clinical evidence is glutamine. It should be reserved for critically ill and surgical patients with anticipated prolonged length of stay.

After injury/hypercatabolic conditions, profound intracellular glutamine depletion has been found, thus is regarded as a conditionally essential amINO acid. Studies have shown improvements in the clinical outcome of hypermetabolic patients.

Optimal management
Parenteral nutrition is the most complex and expensive form of artificial nutrition support. A multidisciplinary approach to the management of  these patients can optimise this therapy and reduce complications.

Deirdre Mc Cormack is a clinical nutritionist at St. James's Hospital Dublin

References on request.

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