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Nutrition in critical illness

Created: 6/2/2005

 

Patient nutrition is an often neglected aspect of the overall management of patients, but 50% of surgical patients suffer protein energy malnutrition. Sepsis, injury and starvation are the main contributors to postoperative morbidity and mortality. Following the initial resuscitation of the critically ill patient, the nutritional status should be assessed and a plan of nutritional management made. It is best to use the enteral route if possible, however adequate calorie intake is often difficult to achieve. The currently available intravenous feeding regimens present a confusing array of mixtures of fat, carbohydrate, amino acids, vitamins and minerals and there are several steps to follow to initiate treatment safely. Many hospitals have a nutrition team, which assists with the management of enteral feed or total parenteral nutrition and helps with monitoring during treatment, but the clinician should have a working knowledge of patient nutrition to initiate a good management plan.

Nutritional requirements

The average 70 kg man requires about 2000 kcal/day. Without food, the body uses glycogen stores and then muscle protein, to provide energy (see Anaesthesia and Intensive Care Medicine 3:8: 274). In the healthy person, the metabolic rate is reduced and stores are conserved. In illness or after surgery, burns or trauma, the following sequence occurs.

  • Energy requirements are increased by up to 30%.
  • Metabolism is affected by altered levels of catecholamines and cortisol.
  • Blood sugar control becomes deranged as patients develop an apparent increased resistance to insulin.
  • There are major fluid and electrolyte losses from diarrhoea, vomiting or nasogastric losses, excessive sweating, stoma losses and surgical drains.
  • Fluid shifts, caused by leaky membranes or fluid moving into the third space, create difficulties in assessing fluid balance.

Assessing nutritional status

The following identify patients at risk of protein energy malnutrition:

  • clinical history (e.g. nausea, vomiting, diarrhoea, abdominal distension, previous surgery, weight fluctuations)
  • dietary history (types and amounts of food taken, dysphagia)
  • physical examination (weight: height, body mass index (BMI), general appearance).
  • The following tests can be used to establish the severity of protein energy malnutrition and the response to nutritional intervention:
  • anthropometric (skin fold thickness)
  • biochemical (albumin, transferrin and pre-albumin)
  • immunological (lymphocyte count).

However, they are unspecific, and similar abnormalities may be found in other conditions. In particular, albumin can fall rapidly with sepsis.

Enteral feeding

If the patient has any functional bowel, the enteral route should be used if possible. The best nutrition comes from a balanced diet that is chewed, swallowed and digested. Gut motility is influenced by hormones released during mastication. Stomach emptying controls the delivery of food to the jejunum to maximize absorption. The gut mucosa is more likely to retain its normal function if it is bathed in the correct nutrients. The large bowel requires an adequate amount of fibre to ensure regular soft bowel actions. As soon as any of these aspects of feeding are defective, the absorption of nutrients is affected. Factors that reduce the chances of successful enteral feeding are shown in Figure 1. Problems associated with enteral feeding are listed in Figure 2.

Figure 1



Figure 2


Routes of enteral feeding

Nasogastric feeding is the most commonly used route, but it relies on adequate gastric emptying. Sometimes it is impossible to pass the nasogastric tube or maintain its position. Feed is usually started at about 30 ml/hour via a continuous infusion pump and increased slowly, provided the gastric residue after 4 hours does not exceed 200 ml. Regimens aim for a maximum calorie intake that is dependent on weight and general condition.

Nasojejunal feeding avoids the pylorus and is used in patients with gastric stasis. Nasojejunal tubes are more difficult to pass and often require insertion under direct fibre-optic observation. Feeds are started at about 30 ml/hour. Absorption is assessed by checking for abdominal discomfort and distension.

Percutaneous gastric feeding – using an endoscopic technique, a tube is passed percutaneously into the stomach to avoid the discomfort of a nasogastric tube. These can be used for continuous or bolus feeds. They are commonly used in patients requiring long-term enteral feeding, such as those with cerebral palsy, after head injury and following major maxillofacial surgery.

Percutaneous jejunal feeding – the percutaneously placed tube passes from the stomach into the jejunum. This route is sometimes used for postoperative feeding if gastric stasis is likely and a nasojejunal tube cannot be passed.

Type of enteral feed

Most enteral feeds are a mixture of fat, carbohydrate, protein, water, electrolytes, minerals and fibre (Figure 3). Standard feeds have about 1 kcal/ml, but more concentrated versions are available with about 1.5 kcal/ml. Energy is delivered as fat and carbohydrate.

Figure 3



Some specialist feeds (e.g.
Pulmocare) provide a greater percentage of calories as fat (by providing fewer calories derived from carbohydrate, the amount of carbon dioxide produced is reduced, thereby assisting ventilatory weaning in some patients). Other specialized feeds (e.g. elemental solutions of amino acids or peptides) are available for patients with short-bowel syndrome, malabsorption or severe inflammatory bowel disease. Extra food supplements can also be used to provide extra fat or carbohydrate.

Weaning off enteral feed

Many patients return to general wards on established enteral feed. Weaning back to a normal diet can be best achieved by its introduction during the day and continuing with enteral feed at night, reducing the night feeds as the daytime feed increases. The normal daytime diet can be supplemented by sip feeds comprising sachets of easy-to-digest, milk-based, flavoured feed. Ward nursing staff should be encouraged to keep a clear record of nutritional intake, remembering that nutrition stops for surgery and investigations. Intake is also affected if patients do not like the food offered, are unable to open the packaging or incapable of feeding without assistance. Ward dietitians have an important role in advising on nutritional requirements and can be helpful in finding suitable food supplements to suit an individual patient’s needs.

Parenteral feeding

When the bowel is not functioning, unable to be used or all attempts at feeding using this route have failed, parenteral nutrition is indicated. A multidisciplinary nutrition team including doctors, nurses, pharmacists and dietitians can improve the efficacy of total parenteral nutrition (TPN). Insertion of a feeding line is usually undertaken by doctors and cared for by nurses; the dietitian can advise on nutritional requirements and the pharmacist can recommend nutritional preparations and additives. The whole team has a role to play in the continuing care of the feeding line, monitoring of response to therapy and audit of complications.

Patient selection

Common reasons for requiring TPN are:

  • post surgery – if bowel function is likely to be disturbed
  • short-bowel syndrome
  • gastrointestinal fistulae
  • prolonged paralytic ileus
  • inflammatory bowel disease
  • preoperatively – in malnourished patients with ineffective bowel function.

Sepsis, severe burns and pancreatitis were also considered reasons for using the parenteral route, but enteral nutrition is now recognized as more appropriate. In some cases, a combination of parenteral nutrition and enteral feed may be used.

Venous access

Ideally, TPN should be given via a tunnelled subclavian vein central line. The incidence of catheter-associated infection can be reduced by meticulous care during insertion, using a full aseptic technique. A post-insertion, chest radiograph should be taken to exclude a pneumothorax and to check that the catheter tip lies in the inferior vena cava (such positioning reduces the risk of thrombosis). To reduce the risk of catheter-related sepsis, the feeding line should be used for TPN only, and no other drugs or fluids should be given through the catheter nor should the line be used for blood sampling. Some mixtures of TPN are specially formulated to be given peripherally for short-term treatment, but intravenous lines used for such a purpose generally have a short life.

Choice of mixture

TPN solutions should contain a balanced mix of protein, carbohydrate and lipid, together with water, vitamins, electrolytes and minerals. Generally TPN is produced in large bags containing all the requirements for 24 hours. Protein is provided as a balanced solution of essential (40%) and non-essential (60%) amino acids.

Lipid emulsions are used because it is possible to supply a large amount of energy in a small volume (9 kcal/g), which is non-irritant to veins. Glucose (4 kcal/g) is used as the main source of carbohydrate, but requires close control of blood sugar. Feeds with a high proportion of glucose may delay weaning from ventilation in some patients, because they sometimes cause excess carbon dioxide production. Other micronutrients must be added to mixtures to avoid deficiency after a few days. These include vitamins and trace elements, which are usually added to the bags in the pharmacy. Electrolytes can be added if required. Phosphate supplements may be required in considerable amounts in patients with malnutrition.

Monitoring

when the intravenous route is chosen for feeding, it is important that the correct nutrients are given in the correct proportions to avoid serious derangement of electrolytes, blood sugar and fat metabolism. Figure 4 lists the routine investigations that should be undertaken.

Figure 4


Recent advances

The constituents of TPN have been modified and simplified over the past few years. The use of a single bag every 24 hours reduces the infection risk by reducing the need to handle the line connections. Several companies have special bags that allow mixing of the constituents immediately before use without the risk of contamination, this increases the shelf-life of the product and enables the pharmacy to supply TPN almost on demand. The vitamins and minerals required are available in single-day vials for addition immediately before use.

Recent research on immunotherapy suggests that the addition of substances such as glutamine, arginine and the omega-3 fatty acids, may enhance the immune response during critical illness. Recent work suggests that blood sugar control in a tight range reduces mortality in critical care patients.

FURTHER READING

Anderson I D. Care of the Critically Ill Surgical Patient. London: Arnold, 1999.

British National Formulary. Appendix 7 Borderline Substances.

Goldhill D R, Withington P S. Textbook of Intensive Care Part 4. Nutrition. London: Chapman & Hall, 1997.

© 2003 The Medicine Publishing Company Ltd


ArticleDate:20050206
SiteSection: Article
 
   
    
                                            
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