Carbohydrate in PN is provided by glucose, a cheap energy source available in a range of concentrations, 5–70%. While the inclusion of some carbohydrate is essential for Central Nervous System (CNS) function, the maximum glucose oxidation rate, 4–5 mg/kg /min/day, should not be exceeded as this may result in hyperglycaemia, hepatic steatosis and impaired respiratory function with increased CO ₂ production. Glucose tolerance is impaired in patients with sepsis and concurrent insulin treatment may be necessary to prevent hyperglycaemia.

The nitrogen component of PN is supplied as a mixture of l-amino acids with essential amino acids supplying approximately 40% of the total amino acid nitrogen. Solutions enriched with certain amino acids have become available, e.g. glutamine. Glutamine, although not an essential amino acid, may become essential during metabolic stress. There are problems providing glutamine in PN solutions due to its instability and low solubility in aqueous solutions. At present there is no evidence that supplementation is harmful but further study is required to provide evidence of benefit. Large multicentre trials are ongoing and results expected within the next few years, e.g. Scottish Intensive Care Glutamine or SeleNium Evaluative Trial (SIGNET) and the Scandinavian Critical Care Trials group study. The former is a randomised trial of glutamine-supplemented PN for critically ill patients and is being carried out throughout Scotland involving intensive care and high dependency units. The latter is a large randomised double-blind placebo controlled study of IV glutamine supplementation in intensive care patients in Scandinavia.

Lipid in PN solutions provides non-glucose energy, minimises respiratory and metabolic stress, prevents essential fatty acid deficiency and allows peripheral infusion of nutrients. Lipid can accumulate in the reticuloendothelial system, impairing its ability to remove bacteria and endotoxins and increasing susceptibility to infection; consequently it is recommended that lipid content of PN should not exceed 1.5 g/kg/day. ¹³ Soybean emulsions have been used as the lipid source for more than 30 years. They contain a high proportion (> 60%) of polyunsaturated fatty acids (PUFA), linoleic acid (52–54%) and alpha linolenic acids (7–9%). It has been suggested that soybean oil-based emulsions represent an imbalanced fatty acid supply with an excess of n6 fatty acids, which under conditions of stress may be proinflammatory, and promote platelet aggregation and vasoconstriction. ¹⁴ The ideal lipid emulsion would supply essential fatty acids, provide easily metabolisable energy and have anti-inflammatory properties. Novel lipids promising to modulate inflammatory responses and improve outcomes have been developed. These include emulsions containing MCT, olive oil and fish oil in various combinations as a partial replacement for soybean oil. Supplying n-3 fatty acids may have the opposite effect to fatty acids of the n6 series. Eicosanoids derived from the former tend to promote vasodilation, inhibit platelet aggregation and reduce inflammation. Consequently fat emulsions enriched with n-3 fatty acids would be expected to have a favourable impact on outcome in critically ill patients receiving PN. ¹⁵ n-3 PUFA-containing triglycerides are poorly hydrolysed by lipoprotein lipase; consequently pure fish oil-containing emulsions must be infused at a very low rate to avoid triglyceride accumulation in the circulation. MCT are excellent substrates for lipoprotein lipase-mediated hydrolysis, facilitating plasma triglyceride clearance. ¹⁶ They are not suitable as the sole lipid source as they do not include essential fatty acids and have a tendency to cause metabolic acidosis. These problems have been overcome by combining MCT and fish oil in emulsions. Lipid preparations based on olive oil can also be used to decrease the intake of PUFA. Such novel lipids have been shown to be safe and may offer some advantages over the use of soybean oil alone but there is a lack of sufficient data on immunologic and clinical endpoints. ¹⁷ More work is needed to evaluate these emulsions before recommendations can be made.

The cephalic, basilic or median cubital veins of the forearm are the veins of choice for the peripheral delivery of nutrients (Figure 11.1). Peripheral veins of the lower extremities should be avoided due to higher risks of thrombophlebitis and because of the need to confine the patient to bed. ²⁰ The non-dominant forearm should be used if possible. Frequently in practice, this route of access may not be suitable in very sick patients due to the presence of inflammation from previous cannulation, thrombosis or oedematous limbs making cannulation difficult.

Venous access may be obtained through a Teflon or PVC cannula (18–20 G) or an ultrafine polyurethane catheter (19–23 G). If the former is used, a short extension set should be attached to facilitate feeding. It is recommended that cannulae be covered with a sterile adhesive dressing and re-sited every 1–2 days to delay the onset of PVT. ⁷ This is time consuming for staff and uncomfortable for the patient. In some centres it is common practice to leave the cannula in place until the first sign of thrombophlebitis. An alternative is to use an ultrafine (19–23 G) silicone or polyurethane catheter, 10–15 cm in length that is inserted into a vein in the antecubital fossa. These are often referred to as mid-lines. Placement should only be by a competent practitioner. They have the advantage of being less thrombogenic, longer lasting and more comfortable for patients. There are a number of devices available with different dwell times recommended by the manufacturers. Blood aspirate is necessary to confirm placement. Although they are more expensive initially than cannulae, they may be longer lasting and less labour intensive for staff. Peripheral devices can be placed on the ward with a strict aseptic technique and appropriate skin preparation. Strict hand washing and an aseptic technique are important for all subsequent manipulations of the device.

Hyperosmotic solutions are poorly tolerated by peripheral veins, causing pain, thrombophlebitis and thrombosis. In most nutrient solutions the osmotic components are glucose, amino acids and electrolytes. The inclusion of lipid and an increase in volume of solution will reduce osmolarity. In addition, lipid emulsion-based admixtures may also have a pH better tolerated by small vessels. ⁷ Consideration should be given to electrolyte content, as additions of electrolytes will increase the tonicity and affect the pH. Solutions < 1200 mosm/L have been shown to be tolerated. ²¹ A typical PN regimen for peripheral infusion is outlined in Box 11.2.

Central parenteral nutrition refers to the administration of nutrients via central veins. Because of the risks associated with central vein cannulation, it should be considered only when peripheral access is not feasible or not appropriate (see section on complications of PN overleaf). ⁷ Central vein cannulation should be by well-trained personnel utilising ultrasound guidance and using aseptic technique under strict aseptic conditions.

Central PN is likely to be used in the following patient categories.