Optimising care and reducing risks

Parenteral nutrition (PN) bypasses the normal digestive process and delivers nutrients (glucose, nitrogen, amino acids, triglycerides, vitamins, minerals, electrolytes and water) intravenously (IV).

PN is often referred to as total parenteral nutrition (TPN). The use of the term ‘total’ suggests that all of the patient’s nutritional requirements are provided in one single bag. However, in some circumstances, PN alone may not be meeting ‘total’ requirements because additional IV fluids, electrolytes, and enteral or oral nutrition may be given at the same time, depending on the patient’s clinical condition. It is useful to note that the terms PN and TPN are generally used interchangeably. This causes little difficulty provided healthcare professionals involved in patient care are aware of the difference between the two terms.

The complex process of compounding PN takes place in specialist aseptic units and may involve combining approximately 10–15 different IV nutrition products. This must be carried out without causing any adverse chemical reactions, which could result in patient harm, and also maintaining sterility of the product.

Two types of PN are available:

‘off-the-shelf’ bags and ‘scratch’ bags. The specific type of PN available within individual healthcare settings will depend on local services, contracts and availability of facilities.

Off-the-shelf bags containing pre-determined quantities of glucose, amino acids, triglycerides, water and usually electrolytes are made by a number of commercial companies. Companies tend to make a wide range of off the shelf bags in an attempt to accommodate differing individual patient requirements.

‘Off the shelf PN’ can be divided into those with micronutrients and those without.

With micronutrients: bags of PN that are fully compounded and mixed into a single bag containing micronutrients. As all elements are mixed, these bags must be refrigerated to maintain stability and prevent degradation of vitamins, and have a limited shelf life.

Without micronutrients: usually multichamber bags where elements are separate. When PN is required the bag is manipulated to mix the ingredients. The separation of the elements lends a longer shelf life to these bags and no need to refrigerate.  However, it is recommended that patients be only given PN that is complete and contains micronutrients.(1)

Scratch bags are those that are made according to the patient’s individual requirements. Depending on local services and contracts, scratch bags may need to be ordered 24–48h in advance. Again, these must be stored between 5°C and 8°C in a pharmacy grade refrigerator and they have a limited shelf life.

PN that has been refrigerated should be removed from the refrigerator before administration, for example 2–4h prior, to allow it to reach room temperature. However, the specific length of time will vary according to local policy.

PN is usually given when the patient has a degree of intestinal failure preventing the function of the GI tract in normal digestion, for example, post-operative complications such as anastomotic leaks following GI surgery or prolonged paralytic ileus, or an inaccessible GI tract. Peri-operative PN should also be given to surgical patients who are severely malnourished and cannot meet their nutritional requirements orally or enterally.1 Further examples are:

However, the possibility of meeting nutritional requirements enterally should always be considered before PN is given, due to the risks associated with it. It may be necessary to supplement oral or enteral intake with PN where the GI tract is only partially functioning. Although there is no minimum length of time for which PN should be given,(1) in practice, if PN is likely to be required for less than five days, the risks of therapy are often thought to outweigh the benefits.(2,3)

Consideration must be given to appropriate venous access for the administration of PN. Due to the hyperosmolarity of most PN, it must be administered via a central venous access device (CVAD), where the tip of the device lies within the lower third of the vena cava or the right atrium.(4) The rapid flow of blood within central veins allows for full nutritional and fluid requirements to be met via this route.

PN that is low in calories and volume may have a lower osmolarity and can be administered peripherally. However, it is recommended that peripheral PN should only be given for a limited amount of time and when the PN solution osmolarity does not exceed 85mOsm/l.(5) This type of PN is unlikely to meet the full nutritional requirements of the majority of patients and, therefore, is usually only a short-term option or where PN is being given to supplement enteral nutrition.

The selection of appropriate venous access will depend upon:

There are a number of risks associated with the use of PN and it is therefore recommended that PN be monitored and prescribed by a specialist nutrition support team (NST), having the knowledge and skills to ensure it is given appropriately and safely.(6) The NST should be multidisciplinary and most often includes a consultant, nutrition nurse specialist, dietitian and pharmacist. Some teams may also incorporate expertise from a biochemist, microbiologist and psychologist.

Venous access-related complications must be considered when weighing the risks and benefits of PN for individual patients. Insertion of a CVAD carries the risk of haemothorax, air embolism, infection/sepsis, arterial puncture/haemorrhage, nerve injury, haematoma and surgical emphysema.(7)

However, the most common risk is that of CVAD-related septacaemia. It is suggested that PN is an independent risk factor for developing CVAD-related sepsis(8) and that 1.3–26.2% of patients receiving PN will develop this type of sepsis.(9) Further influencing factors include poor nursing care, malnutrition, immuno-suppression and contamination of CVAD hubs.(9)

Fluid/electrolyte imbalance may occur, as administering nutrition/fluids IV prevents normal homeostatic control.

Approximately 50% of the calories in PN are derived from glucose. Because of changes in the balance of insulin and glucagon during stress and illness, serum levels of glucose may increase, causing hyperglycaemia. Patients with diabetes mellitus who are receiving PN are likely to require changes to their usual management to compensate for this.

Although liver function abnormalities are multifactoral, excessive quantities of calories in PN may lead to higher insulin secretion. This may then lead to the conversion of glucose to fat and cause fatty liver. In addition, the absence of enteral nutrition may cause biliary stasis.(11) In the short-term, PN-induced liver abnormalities generally resolve once PN is stopped.

PN is an IV therapy and must always be administered according to local policy. Common points to consider are:

Close monitoring is essential to ensure any complications are detected quickly and ensure safe use of PN. Local policy will dictate the frequency of the following, but in the UK, this is influenced by the National Institute for Health and Care Excellence. See Table 1 for monitoring guidance.

In summary, maintaining nutrition and fluid intake is essential for health. Wherever possible, this should be attempted via the enteral route, which involves normal digestive processes. However, where complications of the GI tract prevent adequate nutrition being given this way such as entero-cutaneous fistula, PN is an alternative method, providing essential nutrients and fluid intravenously.

There are a number of known risks with the use of PN including electrolyte imbalance, hyperglycaemia and sepsis. In addition, consideration must be given to the type of venous access required to administer PN as this will further influence associated risks and impact on the PN regimen given to the patient. It is recommended that PN be managed by an NST with appropriate skills to ensure it is used safely and to minimise risk.

Close monitoring of patients is imperative to ensure complications are detected quickly; excellent nursing care and knowledge of local policies will further ensure PN is used safely.