Gastrointestinal disease, including Crohn’s disease, ulcerative colitis, pancreatitis and malabsorption syndrome

 Major trauma including severe burns, severe septicaemia

 Major abdominal surgery; severely malnourished patients may benefit from early peri and postoperative parenteral nutrition if surgery has resulted in a non-functioning gastrointestinal tract

 Malignancy of the small bowel

 Radiation enteritis, when TPN is considered if enteritis is severe after treatment of a primary malignancy

 High-dose chemotherapy, radiotherapy and bone marrow transplantation. Patients are often ill for a limited time (3–6 weeks) and are unable to eat. TPN can be administered during this period to ensure that the patient’s nutritional requirements are adequately met.

 Consultant

 Senior registrar/registrar

 Pharmacist

 Clinical psychologist

 Nutrition nurse(s)

 Dietitian(s)

 Biochemist(s).

 Training nursing staff in the techniques required for IV administration of TPN fluids

 Helping with patient training for HPN

 Monitoring of patients in HPN clinics

 Liaising with the staff from the home-care company

 Advising on the patient’s drug therapy

 Liaising with the patient’s community pharmacist.

 Water

 Protein source – measured in grams of nitrogen

 Energy source – carbohydrate and fat

 Electrolytes

 Trace elements

 Vitamins and minerals.

Baseline water requirements

Water accounts for over 50% of body weight. To prevent patients becoming dehydrated, daily water losses and gains must be carefully considered. Water can be lost through urine and faeces and through ‘insensible losses’, i.e. through the skin and lungs. Patients with burns and gastrointestinal losses will require increased volume. Patients with renal and cardiac failure should be given reduced volumes.

Several methods are available for estimating daily fluid requirements, but most take into consideration body weight and measured urine output, and an allowance is made for insensible losses. The average adult requires between 1500 and 4000 mL of fluid per day. A TPN regimen will need to provide this volume of fluid on a daily basis.

Protein source

Protein requirements vary from one patient to another and are highly dependent on the metabolic status of the patient. Undernourished patients requiring parenteral nutrition are, generally, said to have a negative nitrogen balance. This means that the amount of nitrogen excreted in urine and faeces is greater than the amount of nitrogen administered.

Lack of nitrogen in the body can result in poor wound healing and interference with the body’s defence mechanisms. To overcome this problem, a utilizable source of nitrogen must be administered to the patient. This is achieved by administering amino acid solutions in a TPN formulation. Nitrogen requirements can be estimated from a 24-hour urine collection. This is done by analysing the total amount of urea excreted and by considering the individual patient’s body weight and clinical ‘type’.

Energy sources

Carbohydrates and fats are chosen to provide optimal energy sources for TPN patients. The relative proportions of each will be dependent on the clinical requirements of the patient and formulation considerations. The carbohydrate of choice is normally dextrose and is available in solution with concentrations ranging from 5% to 70% weight in volume (w/v). Like amino acid solutions, dextrose solutions are hypertonic and have a low pH (3–5). If high concentrations of dextrose are added to the TPN bags, they must only be given centrally.

The fat component in a TPN formulation is administered in the form of an oil-in-water emulsion. Fat emulsions are isotonic with plasma, have neutral pH and provide a high calorie source in a low volume. As a result, they are often used in combination with dextrose to provide the necessary calorie content, thereby avoiding the potential problems encountered with excessive dextrose administration.

Fat emulsions provide the patient with essential fatty acids and also act as a vehicle for fat-soluble vitamins which are required in the TPN formulation. Fat is not required in every TPN formulation, but fat deficiency can occur in patients who do not receive fat components for periods longer than 1 month.

Commercially available preparations are based on soya bean oils and are composed of varying combinations of long and medium chain triglycerides. Newer fat solutions have been developed, incorporating olive oil and fish oils, which are claimed to protect patients on long-term parenteral nutrition from complications. Larger and longer trials are required to prove these claims. The energy content of commercially available solutions for both carbohydrates and fats is expressed in kcal/L, e.g. Intralipid 10% provides 550 kcal/500 mL; dextrose 5% provides 210 kcal/500 mL.

Electrolytes

The main electrolytes of clinical significance in a TPN formulation include sodium, potassium, magnesium, calcium, phosphate and chloride. The requirement for electrolytes can be met in the form of injectable solutions of varying percentage content. Electrolyte content of each is expressed in terms of mmol/L. The individual role of each electrolyte in a TPN formulation is given in Table 44.1.

Trace elements

Trace elements act as metabolic cofactors and are said to be essential for the proper functioning of several enzyme systems in the body. Despite being termed essential, they are only required in very small quantities, expressed in micromoles. The main trace elements required in a TPN formulation are zinc, copper, manganese and chromium.

Vitamins and minerals

Vitamin requirements fall into two categories: fat soluble and water soluble. Four fat-soluble vitamins (vitamins A, D, E and K) and nine water-soluble vitamins (vitamins B₁, B₂, B₃, B₅, B₆, B₁₂, C, folic acid and biotin) are said to be essential.

Vitamins and minerals are normally included in foods taken in orally and must therefore be included in TPN formulations for patients on long-term parenteral nutrition. The NICE guidelines published in 2006 recommend that patients must receive vitamins and trace elements daily in their TPN bags.

Preparation and training

For patients in hospital, a suitable TPN regimen will be prescribed. On receipt of the prescription, the pharmacist checks the suitability and compatibility of the formulation, the required volume of each component is calculated and details are transferred to a worksheet. Patient details are entered into a computer and labels generated for the worksheet and the final product. In the preparation area, items required for the compounding process are collected together in an appropriate tray ready for transfer to the clean room facility. Batch numbers and expiry dates for each product used are recorded on the worksheet. The pharmacist checks all details, including calculations, before the compounding procedure begins.

Compounding of a TPN formulation is carried out under strict aseptic conditions (in a Grade A environment) using a laminar airflow (LAF) cabinet within a clean room facility. Chapter 40 gives details regarding clean room facilities, gowning-up procedures for entry to clean rooms and working procedures for using LAF cabinets. SOPs should be available for all staff carrying out aseptic dispensing procedures.

TPN/HPN bags

The components of a TPN formulation are sterile and are prepared under sterile conditions as the formulation is eventually infused directly into the bloodstream of the patient. It is therefore essential that the bags used to hold the TPN formulation are also sterile. In the past, only polyvinyl chloride (PVC) bags were used for TPN formulations. However, because of the problems of leaching of plasticisers from PVC bags containing a fat component, ethylvinyl acetate (EVA) bags (which contain no plasticizers) are now recommended. However, EVA bags have been shown to be permeable to oxygen; hence multilayer EVA bags are now available for formulations requiring prolonged storage.

Bags are usually supplied with a pre-mounted sterile filling set attached. The filling set consists of a number of hollow plastic tubes (up to six) with a plastic spike attached to the end of each. The spikes are used to pierce the rubber septum of the bottles and bags of amino acids, glucose and fat emulsion to enable filling of the components into the TPN bag. Clamps fitted with air vents are attached to each filling tube to clamp off the source bottles and bags when they are empty. Filling sets are used for compounding purposes only and are disconnected and replaced with a sterile hub before being sent out to the patient. Every HPN bag is supplied with a sterile giving set which allows the bag to be infused into the patient.

TPN bags vary in size, ranging from small 250 mL bags used for neonatal TPN up to 4 L bags for adult TPN. Figure 44.1 shows a TPN bag with filling set attached.