Parenteral nutrition: current status and concepts.
Parenteral nutrition is regarded as a form of nutrition in some countries and as an extension of intravenous fluid therapy in others. The optimum clinical application of parenteral nutrition as a form of therapy requires detailed knowledge of the nutrient solutions themselves, including the commonly used solutions such as dextrose, soybean oil emulsion, synthetic crystalline L-amino acid solutions; older solutions such as xylitol, protein hydrolysates; and newer solutions such as glycerides and special purpose amino acid solutions. Additionally, information has accumulated over the past 10 years, leading to the rational use of vitamins and trace elements in parenteral nutrition. Metabolism of the substrates has been correlated with known pathways of intermediary metabolism in normal, starved and stressed subjects. Several new concepts have arisen: a) Infusion of excessive quantities of dextrose results in lipogenesis and increased carbon dioxide production. Hyperalimentation of this type is being replaced by infusion of lesser quantities of dextrose, supplemented by intravenous infusion of lipid as a calorie source. b) Protein hydrolysates and racemic synthetic crystalline amino acid solutions have been replaced by synthetic crystalline L-amino acid solutions. c) A new fat emulsion based on safflower oil is competing successfully with the traditional soybean oil emulsion. d) Newer substrates are being explored. These include branched chain amino acids, keto analogues of amino acids, synthetic glycerides and maltose. e) Deficiencies of essential fatty acids, trace elements and vitamins have been studied in patients on long term parenteral nutrition and their mechanisms elucidated. Official recommendations for intravenous administration of these nutrients have been made. f) Several techniques have been applied in several circumstances, including protein sparing therapy, cyclic nutrition, home therapy, and parenteral nutrition in liver and renal failure. Parenteral nutrition is now used extensively, not only in major hospitals where the resources of a team approach with physician, nurse, pharmacist and dietitian are available, but also in smaller hospitals where all of these facilities may not be at hand. However, whatever the setting, the principles behind the clinical application of parenteral nutrition should be well understood by those involved, including current approaches to safe preparation and infusion of parenteral nutrition solutions.