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MD Consult: Books: Goldman: Cecil Medicine: ORDERING PARENTERAL NUTRITION

Goldman: Cecil Medicine, 23rd ed.

Copyright © 2007 Saunders, An Imprint of Elsevier

ORDERING PARENTERAL NUTRITION

Estimation of Fluid and Calorie Needs

Before parenteral nutrition is ordered, the fluid requirements of the patient should be evaluated. These will vary according to the age of the patient, the disease itself, and the impact of the disease on organ function. Fluid requirements will generally be met with 30 to 40 mL/kg per day or at 1 to 1.5 mL/kcal expended. A more accurate formula for patients with a body weight of 20 kg or more is 1500 mL + 20 mL/kg for each kilogram above 20 kg.

A second step in ordering parenteral nutrition is to estimate the calorie needs of a patient, which remains a difficult task. The total calorie intake should be determined in relation to the energy expenditure. Age, gender, body size, physical activity, fever, and severity of the disease are factors that modulate energy expenditure. Total energy expenditure is the amount of calories burned during 24 hours and can be measured directly by heat loss (direct calorimetry) or indirectly by the relation between oxygen consumption and carbon dioxide production (indirect calorimetry). Calorimetry is viewed as the “gold standard” in measuring energy expenditure. Unfortunately, time limitations and financial constraints preclude this method for everyday clinical practice. Total energy expenditure is the sum of cold- and diet-induced thermogenesis, energy consumed by physical activity, and resting energy expenditure. In hospitalized patients, resting energy expenditure is the most variable component and is highly influenced by body composition and degree of inflammation. The variability for the same disease can be as much as 100%, depending very much on the severity of the disease and the time course. This is why all of the frequently cited equations to predict energy expenditure are often inaccurate in patients with more acute and severe disease and can be associated with a substantial risk for overfeeding or underfeeding.

The Harris-Benedict equation, which was developed in normal individuals and calculates resting energy expenditure, is the most popular of these equations:

Stress and activity factors are added to this calculation. The predicted amount usually falls between 20 and 35 kcal/kg per day. A more practical method, for unstressed subjects, is to calculate calorie needs on the basis of an average value of 1 kcal/kg per hour for men; in women, subtract 5 to 10%. Yet another simple method is given by the following formulas (calculated for ideal body weight):

Forty percent to 50% of Americans are obese, and obesity makes accurate prediction of calorie needs even more difficult. Based on empirical data, a possible approach is to calculate the ideal body weight and to add a percentage (25 to 45%) of the excess body weight to the ideal weight. A formula to calculate ideal body weight is

Formulations

Total parenteral nutrition is the term used to describe a means of feeding whereby all or almost all of the estimated calorie needs are met solely by intravenous administration of nutrients. Supplementary parenteral nutrition refers to a patient who is receiving part of his or her nutritional needs through the gastrointestinal tract and the rest by parenteral infusion. Parenteral nutrition administered through a central vein is referred to as central parenteral nutrition (often also called total parenteral nutrition); when it is given through a peripheral vein, it is referred to as peripheral parenteral nutrition. The main difference is that formulations for peripheral parenteral nutrition need to be isotonic with blood to avoid phlebitis. Hypertonic parenteral formulas are given through a central vein, in which the high flow rapidly dilutes the contents. The main limitation of peripheral parenteral nutrition is that larger volumes are necessary for the nutritional needs of the patient to be met.

The products found in parenteral nutrition formulations typically include macronutrients (protein, carbohydrates, fat emulsions) and micronutrients (vitamins, minerals, and trace elements) ( Table 236-3 ).


TABLE 236-3   — 
MACRONUTRIENTS IN PARENTERAL NUTRITION

Macronutrient Minimum Need Maximum Not to Be Exceeded Total Calories
Carbohydrate 100–150 g/day 4–5 g/kg/day 40–70%
Protein 0.8 g/kg body weight 2 g/kg/day 15–20%
Lipids 2–4% of total calories 2.5 g/kg/day 15–40%
Carbohydrates

Carbohydrates are the main source of calories in almost all parenteral nutrition formulations. In the body, glucose is the main metabolic fuel. In the food industry, the natural form of glucose (d-glucose) is also frequently referred to as dextrose. The brain, renal medulla, leukocytes, erythrocytes, bone marrow, and peripheral nerves use glucose as their main source of oxidative energy. Provision of parenteral glucose has a protein-sparing effect because it decreases the need for skeletal muscle breakdown to provide amino acid precursors for gluconeogenesis. Glucose also stimulates insulin release, thus decreasing lipolysis. Glucose oxidation results in a respiratory quotient of 1 (respiratory quotient = moles of produced carbon dioxide/moles of consumed oxygen), which is higher than the respiratory quotient of 0.7 associated with oxidation of long-chain fatty acids. In patients with limited pulmonary function, the elimination of this extra load of carbon dioxide might be an important additional burden.

The oxidation of 1 g of glucose yields 4 kcal. To meet the needs of the brain, the minimum daily amount of glucose is estimated to be 100 to 150 g. Parenteral nutrition formulations classically contain glucose concentrations ranging from 5 to 70%; the selection depends on the estimated calorie needs and the total volume requirement. In the stressed patient, maximum oxidation rate of glucose is 4 to 7 mg/kg/min (for a 70-kg patient, 400 to 700 g/day). To decrease the risk for metabolic alterations, the maximum rate of glucose infusion should not exceed 5 mg/kg/min.

Protein

In parenteral nutrition, nitrogen is administered as amino acids containing all essential and almost all nonessential amino acids. Amino acids are a source of calories and precursors for the biosynthesis of proteins involved in almost every body function. Protein can be oxidized and yields 4 kcal/g. In steady-state conditions, protein oxidation equals protein intake, and under such conditions, exogenous protein serves as an energy supply. However, in Europe, it is generally assumed that in critically ill patients, the structural or biosynthetic function of amino acids outweighs their function as a metabolic fuel. Therefore, parenteral protein in that part of the world is usually not included in the calculation of calorie needs. The dietary reference intake for healthy adults is 0.8 g per kilogram of body weight per day, assuming that total calorie intake is adequate. In patients with more severe disease, such as the critically ill, 1.2 to 1.5 g/kg may seem more appropriate. Burn patients or patients with important gastrointestinal losses presumably fare better with high-protein regimens (>1.5 g/kg).

The standard formulations for amino acids include solutions containing amino acids only or combinations of amino acids with glucose. Standard amino acid solutions are also available with or without electrolytes. Within the amino acid solutions, different formulations have been tailored to specific disease states. For example, patients with hepatic encephalopathy may benefit from solutions with increased amounts of branched-chain amino acids (valine, leucine, and isoleucine) and reduced amounts of aromatic amino acids. Branched-chain amino acids are oxidized in muscle tissue and therefore may decrease the metabolic burden on the liver. A few clinical studies have shown improved outcome with parenteral glutamine, and current evidence tends to favor its use in subgroups of critically ill patients. There is no convincing evidence for specific amino acid compositions in patients with acute renal failure, who may need as much as 1.5 g/kg/day of protein when undergoing renal replacement therapies. This is in contrast with stable patients and chronic renal failure, for which a moderate protein restriction (0.7 to 1.0 g/kg/day) is recommended.

Lipids

Lipids are fat emulsions used for provision of energy and to supply linoleic acid and linolenic acid, which are essential, polyunsaturated fatty acids. Linoleic acid is the basic substance of the long-chain ω-6 fatty acids, and linolenic acid is the equivalent of the ω-3 fatty acid family. Lipid emulsions are iso-osmolar and contain a core made essentially of triglycerides stabilized by a layer of phospholipids. Phospholipids serve as an emulsifying agent; glycerol is used to make the emulsion isotonic. Fat emulsions are available in concentrations of 10%, 20%, and 30%. Conventional lipid emulsions use soybean oil or safflower oil or a mixture of both containing only long-chain triglycerides. More recent emulsions contain a mixture of both medium- and long-chain triglycerides. Medium-chain triglycerides are derived from coconut oil and contain mainly saturated fatty acids. They have a number of advantages, such as faster clearance from the blood stream, faster oxidation for energy production, no accumulation in liver tissue, and no storage in adipose tissue. However, hepatic metabolism results in ketogenesis and may lead to acidosis. Olive oil has recently been introduced into fat emulsions. Olive oil is rich in monounsaturated oleic acid and, compared with safflower or soybean oil, may have beneficial influences on inflammatory and immunologic mechanisms. Research on the importance of ω-6 and ω-3 fatty acids in the standard Western diet has led to the development of lipid emulsions based on fish oil. Deep-sea fish oil is an important source of the very long chain, polyunsaturated ω-3 fatty acids eicosapentaenoic acid and docosahexaenoic acid. An increased incorporation of ω-3 fatty acids into plasma membranes derived from fish oil, compared with ω-6 fatty acids derived from vegetable oils and animal fats, increases the release of eicosapentaenoic acid rather than arachidonic acid. The lipid mediators derived from these substances differ markedly. Those derived from eicosapentaenoic acid are mainly anti-inflammatory; those derived from arachidonic acid are more pro-inflammatory. The ratio of ω-6 to ω-3 fatty acids in soybean emulsions and in mixtures of long- and medium-chain triglycerides is 7 to 1; in olive oil emulsions, it is 9 to 1. Outside the United States, manufacturers are marketing lipid emulsions with a ratio of ω-6 to ω-3 fatty acids of less than 4 to 1 with the hope of improved nutritional immunomodulation.

Use of lipids as an energy source can have important benefits. Oxidation of 1 g of intravenous fat supplies 9 kcal. It allows a reduced prescription of parenteral glucose and can thereby reduce the risk or severity of hyperglycemia, especially in stressed patients with insulin resistance. Because of a lower respiratory quotient of 0.7, less carbon dioxide is produced during oxidation of 1 g of lipids compared with 1 g of glucose, although substrate oxidation of glucose is probably not decreased. Only 10% of parenteral nutrition is needed as fat to meet essential fatty acid needs. However, most parenteral nutrition formulations now contain 15 to 40% as lipids. Adults usually receive 0.5 to 1.5 g/kg/day. In patients with hypertriglyceridemia (more than 4 to 5 mmol/L or 350 to 400 mg/dL), lipid emulsions should not be started or should be temporarily interrupted.

Micronutrients

Vitamins are essential organic micronutrients, and most of them are used as necessary precursors of coenzymes. Thirteen different vitamins are required in the diet of human beings; nine of these are water soluble and four are fat soluble. The essential trace elements include iron, iodine, copper, manganese, zinc, cobalt, molybdenum, selenium, vanadium, and nickel. These trace elements are needed as enzyme cofactors or prosthetic groups.

Vitamins and trace elements are usually administered as combination products; some are also commercially available as single-item injection. The ideal amount of micronutrients remains poorly defined. The American Medical Association has issued guidelines for the composition of intravenous administration of vitamins and trace elements. These were amended by the Food and Drug Administration in recognition of increased demands in disease states. It is commonly recommended and accepted that all patients should receive daily micronutrient supplementation in standard doses routinely with parenteral nutrition. In critically ill patients, it may be advantageous to increase the dose of vitamins C and E and of selenium because of their role as nonenzymatic reactive oxygen species scavengers. Unless there is clear clinical suggestion of potential deficiency, routine monitoring of serum levels of vitamins or trace elements is not recommended.

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