A silicone catheter is placed into a central vein, usually using the infraclavicular approach to the subclavian vein. The skin-entry site should be dressed carefully and not disturbed unless there is a suggestion of catheter-related sepsis.
Complications of catheter placement include central vein thrombosis, pneumothorax and embolism, but the major problem is catheter-related sepsis. Organisms, mainly staphylococci, enter along the side of the catheter, leading to septicaemia. Sepsis can be prevented by careful and sterile placement of the catheter, by not removing the dressing over the catheter entry site, and by not giving other substances (e.g. blood products, antibiotics) via the central vein catheter.
Sepsis should be suspected if the patient develops fever and leucocytosis. In two-thirds of cases, organisms can be grown from the catheter tip. Treatment involves removal of the catheter and appropriate systemic antibiotics.
With TPN it is possible to provide sufficient nitrogen for protein synthesis and calories to meet energy requirements. Electrolytes, vitamins and trace elements are also necessary. All of these substances are infused simultaneously.
NITROGEN SOURCE. Synthetic L-amino acid solutions are used, which contain between 9 and 17 g of nitrogen per litre. Most patients require at least 14 g of nitrogen per day.
ENERGY SOURCE. This is mainly provided by glucose with additional calories provided by a fat emulsion. Fat infusions provide a greater number of calories in a smaller volume than can be provided by carbohydrate. They are not hypertonic and they also prevent essential fatty acid deficiency.
Essential fatty acid deficiency has been reported in long-term parenteral nutritional regimens without fat emulsions. It causes a scaly skin, hair loss and a delay in healing.
The calorie-to-nitrogen ratio should be approximately (kcal : g) 150:1 (0.6 MJ per gram of protein).
ELECTROLYTES AND TRACE ELEMENTS
The electrolyte status should be monitored on a daily basis and electrolyte solutions given as appropriate. Water-soluble vitamins can be given daily but fat-soluble vitamins should be given weekly, as overdose can occur. A trace-metal solution is available for patients on longterm parenteral nutrition, but if the patient requires blood transfusions trace-metal supplements are not needed.
CENTRAL VENOUS TPN REGIMEN. Most hospitals now use 3-litre bags with the constituents being premixed under sterile conditions by the pharmacy. A standard parenteral nutrition regimen is given.
PERIPHERAL PARENTERAL NUTRITION. This is administered via 5-litre bags over 48 hours.
shows the composition which provides 12 g of nitrogen and 1500 non-protein calories in 24 hours. Complications
• Catheter-related (see above)
• Metabolic, e.g. hyperglycaemia-insulin therapy is usually necessary
• Electrolyte disturbances
• Liver dysfunction
MONITORING OF PATIENTS ON PARENTERAL NUTRITION.
Essential monitoring includes daily plasma electrolytes and weekly assessments of nutritional status (weight and skinfold thickness). Nitrogen balance should also be measured on a weekly basis. Home parenteral nutrition is occasionally required for patients with virtually no small bowel.