Nutritional support is now recognized as being necessary in many hospitalized patients. The pathophysiology and hallmarks of malnutrition have been described earlier; here the forms of nutritional support that are available are discussed.
Some form of nutritional supplementation is required in those patients who cannot eat, should not eat, will not eat or cannot eat enough. It is necessary to provide nutritional support for:
• All severely malnourished patients on admission to hospital
•Moderately malnourished patients who, because of their physical illness, are not expected to eat for 3- 5 days
• Normally nourished patients not expected to eat for 7- 10 days Enteral rather than parenteral nutrition should always be used if the gastrointestinal tract is functioning normally
Feeds can be given by:
• Fine-bore nasogastric tube (commonest method).
• Percutaneous endoscopic gastrostomy; this is useful for patients who need enteral nutrition for a prolonged period, e.g. following a head injury with swallowing problems. A catheter is placed percutaneously into the stomach, which has been dilated with air via a gastroscope.
• Needle catheter jejunostomy. A fine catheter is inserted into the jejunum at laparotomy and brought out through the abdominal wall.
A polymeric diet with whole-protein and fat can be used except in patients with severely impaired gastrointestinal function who may require predigested, i.e. elemental diet. In these patients, the nitrogen source is purified low molecular weight peptides or amino acid mixtures with sometimes the fat being given partly as medium chain triglycerides.
The aim of any regimen is to achieve a positive nitrogen balance, which can usually be obtained by giving 3-5 g of nitrogen in excess of output. Nitrogen loss can be calculated using the formula:
Nz loss (g per 24 hours) = urinary urea (mmol per 24 hours) – 0.028 + 2 (the 2 representing non-urinary nitrogen excretion). Daily amounts of diet vary between 2 and 2.5 litres and the full amount can be started immediately. Hypercatabolic patients require a high supply of nitrogen (15 g per day) and often will not achieve positive nitrogen balance until the primary injury is resolved.
The success of enteral feeding depends on careful supervision of the patient with monitoring of weight, biochemistry and diet charts.
TOTAL PARENTERAL NUTRITION (TPN)
There are two approaches:
PLACEMENT OF CENTRAL VENOUS CATHETER. This has been the standard approach for many years because of the high incidence of thrombophlebitis in peripheral vems.
PERIPHERAL PARENTERAL NUTRITION. This is now being used with the realization that lower total energy requirements are adequate, i.e. 2000 kcal in 24 hours. Specially formulated mixtures for peripheral use are now available with a low osmolality and containing lipid emulsions. Heparin and corticosteroids are added to the infusion and local application of glyceryl trinitrate patches reduces the occurrence of thrombophlebitis and prolong catheter usage. Initially, peripheral parenteral nutrition is used (each catheter will last for about 5 days) allowing more time to consider the necessity for having to insert a central venous catheter.
TPN is much more complicated and potentially more dangerous than enteral nutrition. It should therefore not be used unless absolutely necessary. It is seldom necessary for periods of less than 10 days.