A. Central versus Peripheral Parenteral Nutrition—The route of parenteral nutrition should be secondary to the principle of meeting the individual patient’s calorie and pro- tein goals. Peripheral parenteral nutrition (ie, given through a peripheral vein) can be used in patients who can tolerate the daily 3-L fluid requirement necessary to obtain adequate calorie administration or in patients in the early phase of enteral alimentation as a supplement. Currently, the permis- sible concentrations of glucose, amino acids, and other nutri- ents delivered via peripheral vein alimentation are limited by phlebitis caused by the high osmolality of the alimentation solution. Advances in catheter technology may allow for peripheral administration of solutions of greater than 600 mOsm/L without damage to the vein. A solution of 900 mOsm/L may be well tolerated and could reduce the volume of peripheral alimentation fluid to 2 L/day. Even with this new technology, patients requiring severe fluid restric- tion should receive central parenteral nutrition (via a central venous catheter) using one of several fluid-restricted formu- las (Table 6–7).
B. Placement of Catheters for Total Parenteral Nutrition—Central and peripheral venous catheters are composed of scarified polyvinylchloride, standard polyvinylchloride, polyethylene, silicone, hydromer-coated polyurethane, standard polyurethane, fluoroethylene,
propylene, or Teflon. The lowest rate of thrombogenicity is seen with the hydromer-coated polyurethane. The rate of thrombophlebitis is relatively low when catheters are used in a central vein owing to the rapid rate of dilution of the hyperosmolal solution. Peripheral venous access is associated with a higher rate of thrombophlebitis, which is secondary to the high-osmolality solution infused into a small vein. The size of the peripheral catheter is important, with the larger catheters having a more frequent rate of thrombophlebitis. Recent data would suggest that the use of a small silicone- coated catheter may increase the life span from 2–5 days when infusing a fluid of very high osmolality through a peripheral vein. Osmolality above 900 mOsm/kg is not rec- ommended for peripheral infusion.
Traditional aseptic technique is required for placement of central venous catheters. The subclavian vein is the most commonly used site, followed by the internal jugular vein. Central venous access also can be obtained by the use of a long venous catheter placed in the upper arm vein and passed up near but not into the right atrium. Central catheters also can lead to thrombosis as a result of improper placement in the subclavian vein. The tip of the catheter should be positioned at the entry of the right atrium.
Heparin (1000 units/L) or hydrocortisone (5 mg/L) added to the TPN solution can reduce the occurrence of thrombophlebitis resulting from peripheral administration of hyperosmolar solutions. A nitroglycerin patch on the skin (5 mg) acts as a local vasodilator and also has been associated
Name1 Amino Acids (g/L) Dextrose (g/L) Calories (kcal/L) Osmolarity (mosm/L) BCAA2(%)
Central A5% D15% 50 150 710 1250 19
Peripheral A3.5% D5% 35 50 310 760 19
Peripheral high BCAA 3.5% D5% L3% 35 50 310 800 41
Fluid-restricted (central) A10% D21% 100 210 1114 2108 19
Severe fluid restriction (central) A12% D15%3
120 150 910 1950 19
High branched-chain amino acids (central) A3.5% D20%
35 200 820 1476 46
Renal failure (central) A2.7% D35% 27 350 1292 2426 39
Key: Dextrose (D) = 3.4 kcal/g, amino acids (A) = 4.0 kcal/g
Note: All formulas can have 3% lipid added to them to provide 30 g of lipid per liter; 270 additional calories. Each 1 g amino acids = 10 mosm; each 1 g dextrose = 5 mosm
Each 1% amino acids = 100 mosm; each 1% dextrose = 50 mosm
1Lipids are included in many of these formulas at 10–60% of total calories; these formulations are called “3 in 1;” 3% is 3 g lipid per 100 mL 2Branched-chain amino acids
3Contraindicated in renal failure and hepatic encephalopathy
with a reduction in thrombophlebitis. Subcutaneous tunnel- ing may help to reduce the rate of catheter infection, but the best precaution is optimal nursing care and the use of chlorhexidine as an antiseptic for skin preparation.
Catheter-related infection is a major concern. The two most likely causes for catheter-related infections are migration of bacteria down the catheter sheath and trapping and growth of bacteria that accumulates on the fibrin tip at the distal end of the catheter. Replacement of the catheter involves either exchange over a guidewire or selection of a new site. If obvious infection is present at the original site, a new site must be selected. If there is no obvious infection at the catheter site, the catheter may be exchanged aseptically over a guidewire. The removed catheter tip should be sent for culture, and if bacteria grow over the next 24–72 hours, the exchanged catheter should be discontinued and a new site selected. Central line placement has a 3–5% likelihood of causing pneumothorax or some other serious complication. Changes of catheter sites reserved solely for TPN usage are not needed on a regular basis but only when there is evidence of local or systemic infection or other complication of the catheter.
The most common complication of TPN is catheter- related infection. In a pediatric setting, 15% of patients may develop bacteremia or candidemia. Patients at highest risk are those with diabetes mellitus. It has been estimated that catheter-related infections occur in 3% of nondiabetic adults and in 17% of diabetic adults. The most serious infections are due to Candida species, with mortality rates as high as 34% despite antifungal treatment.
C. Carbohydrate and Protein—Since the intravenous route is not the natural route for nutritional substrate administra- tion, it is important to provide adequate but not excessive amounts of protein, carbohydrate, and fat on a daily basis. Most critically ill patients need 1.5–2.5 g/kg per day of pro- tein. The ideal body weight value should be used in calculat- ing the daily protein requirements. Dextrose administration to most critically ill patients should not exceed 3.0 mg/kg per minute (4.3 g/kg per day). This generally translates into about 300 g dextrose, or 2 L of 15% dextrose, in a 70-kg adult. Administration of greater amounts of dextrose can result in glucose intolerance, abnormal liver function tests, and fatty infiltration of the liver.
D. Lipid—Currently available intravenous fat emulsion prod- ucts are derived from soybean or a mixture of soybean and safflower oil. The products vary slightly in the amount of linoleic, linolenic, and oleic acids. Each product is available in 10% and 20% concentrations, but the 20% product is the best choice because of its caloric density and the lack of imbalance in the phospholipid-to-lipid ratio. Intravenous lipid can be administered as a separate 20% concentration over 20–24 hours or—more commonly—as part of the TPN called “3 in 1” with dextrose and amino acids. Maximum fat administration can be estimated at 2 g/kg per day or 140 g/day (1260 kcal).
The use of intravenous fat administration in critically ill patients initially was very controversial. Some of the early stud- ies did not demonstrate any improvement in nitrogen reten- tion when glucose calories were exchanged for fat calories.
The septic patient has a reduced ability to use calories provided as dextrose, so any amount of dextrose in excess of 300 g/day (1020 kcal) may not be used as energy and could contribute to the development of fatty liver infiltration and mild elevations in liver function tests. Because septic patients have an approximately threefold increase in fat oxi- dation rate, fat calories may be readily used in these patients. As a precaution, however, and because excessive amounts of intravenous lipids in animals contribute to an increased incidence of sepsis and associated morbidity, a maximum of 60% of total calories as intravenous fat is acceptable in most critically ill patients.
There is some interest in the use of peripheral adminis- tration of lipid, amino acids, and dextrose in a single 3-L bag via a very small catheter. In theory, the catheter floats in the vein, causing less luminal damage. An option used by some is to administer the peripheral infusion of lipid emulsion for 18 of the 24 hours and to run in 5% dextrose over the 6-hour resting period. This makes physiologic sense because fasting will permit clearance of very low-density lipoprotein (VLDL) particles and allow for adaptation to the nonfed state.
Essential fatty acid requirements are estimated to be approximately 1–4% of total energy requirements and should be in the form of linoleic acid. An elevation of the eicosatrienoic acid (triene) to arachidonic acid (tetrane) ratio to 0.4 is indicative of essential fatty acid deficiency. Treatment of essential fatty acid deficiency requires approximately 10–20% of total energy to be in the form of linoleic acid.
E. Parenteral Nutrition Solutions—Some standard par- enteral nutritional formulas and those containing higher amounts of branched-chain-enriched amino acid formulas are listed in Table 6–7. Most formulas provide approximately 1 kcal/mL of TPN. Standard parenteral nutrition solutions do not contain glutamine owing to the instability of this amino acid in solution. Standard parenteral formulas also do not contain large amounts of arginine. Both glutamine and arginine can be added to the parenteral formulas before administration, but there is no convincing evidence that added arginine is helpful. Recent data suggest that glutamine may be a preferred fuel for enterocytes and lymphocytes. The use of glutamine-enriched formulas can prevent postinjury expansion of the extracellular water compartment in bone marrow transplant patients. There also may be a slight reduc- tion in the incidence of infection.
F. Recommendations for Ordering Central Parenteral Nutrition—Each hospital should have standard formulas for parenteral nutrition. Consider using a central parenteral nutrition formula with 15% dextrose, 5% amino acids, and
5% lipid containing 1160 kcal/L with osmolarity of 1250 mosm/L (see Table 6–7). Fluid-restricted formulas are often required in critically ill patients. These solutions con- tain more concentrated mixtures of amino acids. Special for- mulas may be useful in patients with hepatic and/or renal failure.
G. Recommendations for Peripheral Parenteral Nutrition—A standard solution is 3–5% amino acid and 5% dextrose for peripheral vein administration, for example, 3.5% amino acid and 5% dextrose. Each milliliter provides approximately 0.3 kcal. Therefore, 3 L of this solution pro- vide 105 g protein (amino acids), 150 g dextrose, and about 900 kcal.
Using a microcatheter that allows for a higher-osmolarity solution to be infused safely, more calories can be given via a peripheral vein by adding 20% lipid. For ICU patients, a solution of 5% amino acid, 5% dextrose, and 5% lipid has 900 mOsm/L. Two liters of this formula provides 100 g pro- tein and 1640 kcal (55% of calories from lipid).