When full nutritional support is not possible in the alimentary canal, the intravenous route is available for nutrient delivery (1,2). This chapter describes the basic features of intravenous nutritional support, and demonstrates how to create a parenteral nutrition regimen to meet the needs of individual patients.
I. Substrate Solutions
A. Dextrose Solutions
Carbohydrates are the main source of nonprotein calories in parenteral nutrition (PN), and dextrose (glucose) is the carbohydrate source in PN. The available dextrose solutions are shown in Table 38.1.
Table 38.1 Intravenous Dextrose Solutions
* Based on an oxidative energy yield of 3.4 kcal/g for dextrose.
Because the energy yield from dextrose is relatively low (3.4 kcal/g), the dextrose solutions must be concentrated to provide enough calories to satisfy daily requirements. (The standard solution is 50% dextrose, or D50.) The solutions used in PN are hyperosmolar, and must be infused through large central veins.
B. Amino Acid Solutions
Protein is provided as amino acid solutions that contain varying mixtures of essential (N = 9), semiessential (N = 4), and nonessential (N = 10) amino acids. These solutions are mixed with dextrose solutions in a 1:1 volume ratio. Ex-amples of standard and specialized amino acid solutions are shown in Table 38.2.
Table 38.2 Standard & Specialty Amino Acid Solutions
1. Standard Solutions
Standard amino acid solutions (e.g., Aminosyn in Table 38.2) are balanced mixtures of 50% essential amino acids and 50% nonessential and semiessential amino acids. Available concentrations range from 3.5% up to 10%, but 7% solutions (70 g/L) are used most often.
2. Specialty Solutions
Specially-designed amino acid solutions are available for patients with severe metabolic stress (e.g., multisystem trauma or burns), and for patients with renal or liver failure.
Solutions designed for metabolic stress (e.g., Aminosyn-HBC in Table 38.2) are enriched with branched chain amino acids (isoleucine, leucine, and valine), which are preferred fuels in skeletal muscle when metabolic demands are high.
Renal failure solutions (e.g., Aminosyn RF in Table 38.2) are rich in essential amino acids, because the nitrogen in essential amino acids is partially recycled to produce nonessential amino acids, which results in smaller increments in blood urea nitrogen (BUN) than seen with breakdown of nonessential amino acids.
Solutions designed for hepatic failure (e.g., HepaticAid) are enriched with branched chain amino acids, because these amino acids block the transport of aromatic amino acids across the blood-brain barrier (which are implicated in hepatic encephalopathy).
It is important to emphasize that none of these specialized formulas have improved outcomes in the disorders for which they are designed (3).
Glutamine is the principal metabolic fuel for rapidly-dividing cells like intestinal epithelial cells and vascular endothelial cells (4). However, based on a meta-analysis of 5 multicenter trials that showed increased mortality in patients receiving intravenous glutamine (1), the recent guidelines on nutrition support do NOT recommend IV glutamine for PN regimens (1).
C. Lipid Emulsions
Lipids are provided as emulsions composed of cholesterol, phospholipids, and triglycerides (5). The triglycerides are derived from vegetable oils (safflower or soybean oils) and are rich in linoleic acid, an essential fatty acid (6).
Lipids are used to provide 30% of daily calorie requirements, and 4% of the daily calories should be provided as linoleic acid to prevent essential fatty acid deficiency (7).
Table 38.3 Intravenous Lipid Emulsions for Clinical Use
% calories as EFA (Linoleic acid)
Unit Volumes (mL)
EFA = essential fatty acid.