Parenteral nutrition

Feed composition

Carbohydrate is normally provided as concentrated glucose. 30–40% of total calories are usually given as lipid (e.g. soya bean emulsion). The nitrogen source is synthetic, crystalline L-amino acids which should contain appropriate quantities of all essential and most non-essential amino acids. Carbohydrate, lipid, and nitrogen sources are usually mixed into a large bag in a sterile pharmacy unit. Vitamins, trace elements, and appropriate electrolyte concentrations can be achieved in a single infusion, thus avoiding multiple connections. Volume, protein and calorie content of the feed should be determined on a daily basis in conjunction with the dietician.

Choice of parenteral feeding route

Central venous

A dedicated catheter (or lumen of a multi-lumen catheter) is placed under sterile conditions. For long-term feeding, a subcutaneous tunnel is often used to separate skin and vein entry sites. This probably reduces the risk of infection and clearly identifies the special purpose of the catheter. Ideally, blood samples should not be taken nor other injections or infusions given via the feeding lumen. The central venous route allows infusion of hyperosmolar solutions, providing adequate energy intake in reduced volume.

Peripheral venous

Parenteral nutrition via the peripheral route requires a solution with osmolality <800mOsm/kg. Either the volume must be increased or the energy content (particularly from carbohydrate) reduced. Peripheral cannulae sites must be changed frequently.

Complications

Catheter-related		Misplacement Infection Thromboembolism
Fluid excess
Hyperosmolar, hyperglycaemic state
Electrolyte imbalance
Hypophosphataemia
Metabolic acidosis		Hyperchloraemia Metabolism of cationic amino acids
Rebound hypoglycaemia (from high endogenous insulin levels)
Vitamin deficiency	Folate (pancytopaenia) Thiamine (encephalopathy, neuropathy, heart failure) Vitamin K (hypoprothrombinaemia)
Vitamin excess	Vitamin A (dermatitis) Vitamin D (hypercalcaemia)
Fatty liver