Nutrition in Critical Care

By James Collier BSc (Hons) - Consultant in Nutrition and Moderator of

Patients in the critical care setting are at high risk of malnutrition, due to the nature of their illness and their hypermetabolic state. Their immune system is compromised, so they are at increased risk of infection and septicaemia. Delayed healing and infections contribute to prolonged Intensive Care stay, increased mortality and morbidity and higher treatment costs. Correct nutrition from the onset of admission is imperative.

Choices of feeding in critical care are enteral tube feeding (ETF) or parenteral nutrition (PN). Enteral feeding literally means using the gastrointestinal tract for the delivery of nutrients, which includes eating food, consuming oral supplements and all types of tube feeding. The route of ETF most often used in critical care is naso-gastric tubes (NGT). Patients who need feeding post pyloric sphincter are given naso-jejunal tubes (NJT) which are being used more frequently these days. Some patients may already have percutaneous endoscopic gastrostomy (PEG) tubes in situ, if it is known they will not be able to feed for sometime after surgery.

As a rule PN should only be administered if there is a clinical reason that the gastro-intestinal tract should not be used, as there is more risk of infection and it provokes the acute-phase inflammatory response. It has been shown that early feeding (within 24hours of admission) reduces the risk of sepsis and improves clinical outcome (Heyland 2000), but also the choice of feed may affect speed of recovery.

Types of feed
A number of enteral feeds are available form a variety of different manufactures in this highly competitive industry. Standard feed (generally 1kcal/ml) are most often used, although there are higher energy alternatives (1.2 or 1.5kcal/ml) available for patients who need more calories in a shorter period of time, or who do not tolerate large volumes. New EC guidelines have called for all enteral feeds to be nutritionally complete (i.e. provides sufficient amounts of all nutrients to meet average requirements) in 1500kcal (EC 1999). There are also fibre-containing feeds which help both diarrhoea and constipation; some have specific blends of insoluble and soluble fibres, shown to maintain gut function integrity and flora (Khalil et al 1998).

Respiratory failure is the most common organ failure on an intensive care unit (ICU), and feeding is of utmost importance to the ventilated patient (Collins 2000). Certain enteral feeds exist which are both high in calories and designed for the ventilated patient; made up of a higher fat : carbohydrate ratio, as fat metabolism produces less carbon dioxide than carbohydrate metabolism, supposedly helping to wean the patient off the ventilator.

Some patients may not tolerate the above whole protein feeds and may require elemental or semi-elemental feeds. Elemental feeds are nutritionally complete, but contain fully digested forms of the macronutrients, i.e. amino acids, monosaccharides and fatty acids. Semi-elemental feeds are partially digested containing oligopeptides and amino acids, oligosaccharides and monosaccharides, and medium, short and free fatty acids.

Immunonutrition is a relatively new concept in critical care feeding to which there is a growing body of evidence reporting benefits. Such feeds may contain arginine, omega-3 fatty acids, nucleotides and glutamine. Arginine is an amino acid shown to improve immune response to bacteria, viruses and tumour cells, promote wound healing and increase protein turnover (Seifter et al, 1978; Efron & Barbul 2000). Omega-3 fats enhance immune function by boosting neutrophil activity, and reduce inflammation (Wigmore et al 1997). Nucleotides are essential for maintaining cellular integrity and enhancing production of repair cells. Glutamine is the most abundant amino acid in the body and becomes essential during severe physical stress. Evidence as to which elements are more beneficial in immunonutrition is conflicting.

Impact (Novartis Nutrition) is an immunonutrition feed containing arginine, omega-3 fats and nucleotides and has been shown in many studies to improve immune parameters and clinical outcome. Oxepa (Abbott Nutrition) contains omega-3 and omega-6 fatty acids, Stresson (Nutricia Clinical Care) contains glutamine, omega-3s and omega-6s, and Recovan (Fresenius-Kabi) contains glutamine, arginine, omega-3s and omega-6s. Of the above, Impact has by far the most research and has been available and used for much longer, and there is little evidence to date of the benefits of the other three to ICU patients.

Many ICUs now have protocols for the use of immunonutrition feeds, often where all patients admitted to ICU for more than 24hours will commence on such a feed. Such ICUs have demonstrated a significant reduction in overall therapeutic interventions, including infectious complications and reducing length of stay on ICU and hospital (Hibbert et al 2001). The associated reduction in costs, more than compensates for the higher cost of feeding all patients with immunonutrition feeds (Senkal et al 1999).

Problems in tolerating enteral tube feeding
A number of problems have been noted in respect of tolerating ETF, most commonly diarrhoea or constipation. Before the type of feed or regimen is altered, it is important to rule out side effects of any medication or pathogenic cause. Stool samples may highlight any bacterial infections. If no other cause can be identified, the dietitian may suggest a different type of feed, e.g. fibre-containing or semi-elemental.

Large aspirates, abdominal bloating and vomiting can be common side effects, in which case the rate or method of administration need to be reviewed. Some patients fail to tolerate large volumes at one time, in which case the pump rate may need to be slowed, a higher energy feed used or frequent rest periods may be advised to allow gastric emptying.

There are a variety of routes of nutrition and types of feed available to patients in critical care, and evidence points to the effectiveness of immunonutrition feeds. Trials do indicate benefits to therapeutic parameters and in reduction of length of stay. Further trails should be directed towards evaluation of cost-effectiveness of immunonutrition (Hibbert et al 2001), and to look at longer-term follow-up to see if benefits extend to after the patient’s discharge from hospital.

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