22
Nutrition
22-1. Key Points
■ Malnutrition can present as either undernutrition or obesity.
■ The components of a nutritional assessment include a history and physical exam, anthropometric measurements, and biochemical tests.
■ An increase in energy expenditure (energy needs) is defined as hypermetabolism, and an increase in nitrogen excretion (protein needs) is defined as hypercatabolism.
■ Most patients receiving specialized nutrition support (parenteral or enteral nutrition) require 25–30 kcal/kg/day and 1–2 g protein/kg/day.
■ Critically ill, obese patients (body mass index [BMI] > 30) should receive 11–14 kcal/kg actual body weight per day (22–25 kcal/kg/day ideal body weight) and 2 g/kg ideal body weight per day of protein if BMI = 30–40 or 2.5 g/kg ideal body weight per day if BMI > 40. This approach applies only in patients with normal renal function.
■ The water requirement for most adult patients without substantial extrarenal losses is 30–40 mL/kg/day.
■ Parenteral nutrition (PN) should be reserved for patients whose gastrointestinal tracts are not functional or accessible (e.g., severe acute pancreatitis, severe short bowel syndrome).
■ Total nutrient admixtures (TNAs) contain dextrose, amino acids, lipid emulsion, electrolytes, vitamins, and trace elements in one container.
■ The advantages of TNAs include decreased nursing administration time, decreased potential for touch contamination, and reduced expense (the patient needs only one pump and one intravenous administration set).
■ The advantages of central vein PN over peripheral vein PN include the ability to concentrate the formulation, administer adequate calories and protein, and use the catheter for long-term administration.
■ For PN calculations: 1 g hydrated dextrose = 3.4 kcal, 1 g amino acids = 4 kcal, and 1 g lipid = 9 kcal. (Intravenous fat emulsion actually provides 10 kcal/g because it includes calories provided as glycerol and phospholipid.)
■ All PN formulations should be filtered during administration (0.22-micron filter for two-in-one PN formulations and 1.2-micron filter for TNAs).
■ Enteral nutrition support is generally used in patients who cannot or will not eat but have a functional and accessible gastrointestinal tract.
■ Enteral tube feeding can be provided by one of the following methods: nasogastric, nasoduodenal, nasojejunal, gastrostomy, or jejunostomy.
■ Diarrhea associated with enteral tube feeding is often caused by pharmacotherapy (e.g., sorbitol in liquid drug preparations as a vehicle).
■ Patients receiving phenytoin or warfarin concurrently with enteral tube feeding should have the tube feeding held at least 1 hour before and after each dose.
22-2. Study Guide Checklist
The following topics may guide your study of this subject area:
■ General understanding of macronutrient and micronutrient requirements
■ Factors incorporated into a nutritional assessment
■ Determination of nutritional requirements based on patient-specific variables
■ Methods of administering nutrition support
■ Patient factors that help to choose the nutrition support route
■ Assessment of nutrition support tolerance and efficacy
■ Management of electrolytes and trace elements
■ Identification of nutrition-support complications
■ Major drug–nutrient interactions
22-3. Overview
General Nutrition
U.S. dietary guidelines (MyPlate)
■ Maintain a healthy weight.
■ Maintain a low-fat diet (< 30% of total calories).
■ Eat plenty of fruits, vegetables, and grain products.
■ Use salt, sugar, and alcohol in moderation.
Malnutrition
■ Causes of undernutrition (protein-calorie malnutrition):
• Depressed intake of nutrients (starvation, semistarvation)
• Alteration in nutrient metabolism (trauma, major infection)
■ Causes of obesity:
• Excessive caloric intake (especially carbohydrate and fat)
• Alteration in nutrient metabolism (genetic predisposition)
• Sedentary lifestyle
Dietary reference intakes (DRIs) of selected nutrients
■ Fiber: 20–35 g/day (many people find this goal unpalatable)
■ Calcium: 1,200–1,500 mg/day in adolescents and young adults; 1,000 mg/day in men until age 65 and women until age 50; 1,200–1,500 mg/day for life thereafter (usually requires supplements)
■ Multivitamins: Helpful to meet daily requirements of pregnant and lactating women, elderly persons, and those who eat vegetarian or low-calorie diets
Nutritional assessment components
History and physical examination
■ Dietary intake (anorexia, bulimia, hyperphagia, taste alterations)
■ Underlying pathology affecting nutrition (cancer, burns)
■ End-organ effects (diarrhea, constipation)
■ Gastrointestinal surgery (bariatric surgery, gastrectomy, bowel resection)
■ General appearance (edema, cachexia)
■ Skin appearance (scaling skin, decubitus ulcers)
■ Musculoskeletal effects (depressed muscle mass, growth retardation)
■ Neurologic effects (depressed sensorium, encephalopathy)
■ Hepatic effects (jaundice, hepatomegaly)
Anthropometrics
■ Skinfold measurements for assessment of fat (triceps, calf)
■ Arm muscle circumference for assessment of skeletal muscle
■ Waist circumference for assessment of abdominal fat (metabolic syndrome)
■ Weight for height to determine undernutrition or obesity
• Change in weight over time is one of the best indicators of nutritional status.
■ Head circumference in infants to document appropriate growth
■ Percentage of ideal body weight (IBW) after calculation of IBW for patient:
• IBW of males (kg) = 50 + (2.3 × height in inches over 5 feet)
• IBW of females (kg) = 45.5 + (2.3 × height in inches over 5 feet)
■ Body mass index (BMI) for assessment of undernutrition or obesity calculated from body weight (kg) and height (m): BMI = weight (kg)/height2 (m2)
Biochemical assessment
Serum albumin concentration
■ Good prognostic indicator and good for assessment of long-term nutritional status
■ Poor for repletion marker because of long half-life (21 days) and large body pool
Serum prealbumin concentration
■ Good for short-term assessment of nutrition support because of short half-life (2 days) and small body pool
■ Possible increase in serum prealbumin concentrations in patients with kidney disease because of impaired excretion
Serum transferrin concentration
■ Good for short-term assessment of nutrition support because of short half-life (7 days) and small body pool
■ Elevated in iron-deficiency anemia
Creatinine height index
■ Requires 24-hour urine collection to determine levels of creatinine
Other methods of nutritional assessment
■ Muscle strength testing
■ Bioelectrical impedance (i.e., a low-grade electrical current runs through the body to identify body protein stores and fat stores)
Types of malnutrition
■ Starvation-related malnutrition: Features depleted fat and muscle stores, normal biochemical measurements (e.g., classic starvation without inflammation)
■ Chronic disease-related malnutrition: Features mild to moderate inflammation (e.g., congestive heart failure, chronic kidney disease)
■ Acute disease or injury-related malnutrition: Features severe and acute inflammation (e.g., major infection, trauma)
■ Obesity: Demonstrated as elevated body weight to at least 120% of IBW or BMI > 27.8 (male) or > 27.3 (female)
• Class I obesity: BMI > 30 and < 35
• Class II obesity: BMI > 35 and < 40
• Class III obesity: BMI > 40
Selected definitions
■ Hypermetabolism: An increase in energy expenditure above normal (usually > 10% above normal)
■ Hypercatabolism: An increase in protein losses above normal (usually via urinary excretion of urea nitrogen)
■ Nutrition support therapy: Parenteral nutrition (PN) or enteral nutrition (EN)
■ Basal energy expenditure (BEE): A calculation of the normal energy needs of healthy adult men or women using sex, age, height, and weight
■ Harris–Benedict equations for BEE:
• Male (kcal/day) = 66 + 13.7 (weight in kg) + 5 (height in cm) − 6.8 (age in yr)
• Female (kcal/day) = 655 + 9.6 (weight in kg) + 1.8 (height in cm) − 4.7 (age in yr)
■ Mifflin equations for energy expenditure:
• Male (kcal/day) = 10 (weight in kg) + 6.25 (height in cm) − 4.9 (age in yr) + 5
• Female (kcal/day) = 10 (weight in kg) + 6.25 (height in cm) − 4.9 (age in yr) − 161
■ Resting energy expenditure (REE): A measured value of energy expenditure (generally ~10% above BEE or Mifflin in health, but can be 100% above BEE in severe burns)
■ Respiratory quotient (RQ): The value that results when carbon dioxide production (VCO2) is divided by oxygen consumption (VO2)
• RQ for carbohydrate oxidation = 1
• RQ for fat oxidation = 0.7
• RQ for protein oxidation = 0.8
• RQ for fat synthesis = 8
■ Indirect calorimetry: The most accurate method to determine energy requirements. It is a noninvasive procedure that measures VO2 and VCO2, and resting energy requirements are then calculated using the Weir equation.
■ Body cell mass: Lean, metabolically active tissue (skeletal muscle, body organs)
■ Lean body mass: Body cell mass, extracellular fluid, and extracellular solids (bone, serum proteins)
■ RDI: Recommended daily intake
22-4. Nutritional Requirements
Calorie Requirements
■ Most clinicians dose nutrition support therapies in total calories (i.e., using carbohydrate, fat, and protein-calorie contributions to obtain the desired dose).
• 25 kcal/kg/day for adults with little stress (e.g., elective surgery)
• 30 kcal/kg/day for patients with infections and skeletal trauma
• 35 kcal/kg/day for patients with major trauma (head injury, long-bone fractures)
• 40 kcal/kg/day for patients with major thermal injury (> 50% total body surface area burn)
• Critically ill, obese patients (BMI > 30) should receive 11–14 kcal/kg actual body weight per day (22–25 kcal/kg/day ideal body weight). This is known as hypocaloric feeding.
■ Multiply BEE by the stress factor to determine calorie requirements:
• 1 × BEE for patients with little stress
• 1.3 × BEE for patients with infections and/or skeletal trauma
• 1.5 × BEE for patients with major trauma
• 2 × BEE for patients with severe thermal injury
■ Measure the REE via indirect calorimetry for calorie requirements.
Caloric Contribution of the Major Macronutrients
■ Glucose: 3.4 kcal/g because hydrated glucose is used in PN (glucose powder would be 4 kcal/g)
■ Fat: 9 kcal/g
■ Protein: 4 kcal/g
• Protein requirements are usually dosed in grams per kilogram per day.
• 0.8 g/kg/day is the adult recommended daily allowance (RDA) for protein in the United States.
• 1 g/kg/day is the adult RDA for patients with minor stress (elective operations).
• 1.5 g/kg/day is the adult RDA for patients with major trauma or infection.
• 2 g/kg/day is the adult RDA for patients with severe head injury, sepsis, or severe thermal injury.
• For critically ill, obese patients, provide 2 g/kg/day IBW when the patient’s BMI = 30–40 and 2.5 g/kg/day when > 40.
Measurement of Nutritional Efficacy Using Nitrogen Balance (NB)
NB = nitrogen in − nitrogen out
■ Nitrogen in (grams) is determined by dividing the grams of protein taken in on the day of balance by 6.25.
■ Nitrogen out (grams) is determined by measuring the grams of urea nitrogen excreted during a 24-hour urine collection and then adding a factor of 2 or 4 g for insensible nitrogen loss or stool loss.
■ Positive NB can be used to document adequacy of nutritional support:
• In undernourished patients, +4 to +6 g/day is desired.
• Nitrogen equilibrium (−2 to +2 g/day) is usually adequate in critically ill patients.
Other Requirements during Nutrition Support
Water
■ Up to 35 mL/kg/day for average-sized adults
■ 40 mL/kg/day for smaller adults and adolescents
■ > 40 mL/kg/day for patients with extrarenal losses (e.g., gastrointestinal drains)
Electrolytes
■ Sodium requirements:
• 60–100 mEq/day in adults
• 2–6 mEq/kg/day in children
■ Chloride requirements:
• 60–100 mEq/day in adults
• 2–6 mEq/kg/day in children
■ Potassium requirements:
• 60–100 mEq/day in adults
• 2–5 mEq/kg/day in children
■ Calcium requirements:
• 5–15 mEq/day in adults
• 2–3 mEq/kg/day in children
■ Phosphorus requirements:
• 20–45 mmol/day in adults
• 1–2 mmol/kg/day in children
■ Magnesium requirements:
• 10–20 mEq/day in adults
• 0.25–1 mEq/kg/day in children
Vitamins
■ Vitamins are provided daily in both PN (added) and EN (endogenous).
■ Most enteral formulations provide the DRI for vitamins in a volume of 1,000–1,500 mL.
■ For parenteral vitamin products:
• Adult products contain 12 (M.V.I.-12) or 13 vitamins (Infuvite Adult, M.V.I. Adult); vitamin K is added separately when the product with 12 vitamins is used.
• Pediatric products (M.V.I. Pediatric, Infuvite Pediatric) contain all 13 vitamins.
Trace elements
■ Zinc: 3–5 mg/day in adults with PN; 50–250 mcg/kg/day in children with PN
■ Copper: 0.5–1.2 mg/day in adults with PN; 20 mcg/kg/day in children with PN (maximum of 300 mcg/day)
■ Chromium: 10–15 mcg/day in adults with PN; monitored but not given to children
■ Manganese: 50–100 mcg/day in adults with PN; monitored but not given to children
■ Selenium: 40–80 mcg/day in adults with PN; 1.5–3 mcg/kg/day in children with PN
22-5. Nutrition Support Therapies
Parenteral Nutrition
Indications
PN is generally used for patients who cannot be fed via the gastrointestinal tract. PN should begin after 5–7 days of lack of bowel function.
Severe acute pancreatitis
■ Oral or tube feeding will usually exacerbate this condition unless given via the jejunum (e.g., nasojejunal feeding tube or jejunostomy).
Short bowel syndrome
■ This condition requires PN from a few weeks to lifelong, as needed.
Ileus
■ This condition is secondary to lack of bowel function (e.g., postoperative after gastrointestinal surgery).
Other indications
■ Bowel obstruction
■ Bowel ischemia
■ Neonates who cannot eat in the first day of life
■ Preoperatively for undernourished patients who are undergoing an elective operation and for whom there is no direct access to the gastrointestinal tract (e.g., partial small bowel obstruction from cancer)
■ Pregnancy with severe hyperemesis gravidarum (i.e., there is an inability to tolerate oral or enteral nutrition)
■ Gastrointestinal fistulae where oral or EN should be restricted
Components of parenteral nutrition
Protein
■ Protein should be included in all PN formulations.
■ Standard amino acids from 10%, 15%, or 20% stock solutions can be used for most patients.
■ Final concentrations in the PN formulation vary from 2% to 7%.
Fat
■ Fat is provided as intravenous fat emulsion either as a separate infusion or admixed with the rest of the PN formulation, making a total nutrient admixture (TNA).
■ Products are manufactured as 10%, 20%, and 30% fat emulsions. (In the United States, 30% can be used only for TNAs, not for direct infusion.)
■ Fat provides essential fatty acids to the patient who is most likely not eating by mouth.
■ Fat provides nonprotein calories other than glucose.
■ Common doses used in adults are ~1 g/kg/day (9–10 kcal/kg/day) but should not exceed 2.5 g/kg/day.
■ Intravenous fat emulsions contain phospholipid to emulsify the product and glycerol to make the emulsion isotonic. (Both of these components provide modest calories that result in supplying 10 kcal/g equivalent of fat.)
Dextrose
■ Common doses of dextrose in critically ill patients are 3–4 mg/kg/min (~15–20 kcal/kg/day).
■ Dextrose is in all PN formulations for obligate needs (central nervous system, renal medulla, white blood cells, red blood cells, wound healing).
■ PN formulations are usually made from 50% to 70% dextrose in water.
■ Final concentrations in the PN formulation vary from D10W (10% dextrose in water) to D35W (35% dextrose in water).
■ Dextrose should never exceed a dose of 5 mg/kg/min (~25 kcal/kg/day).
Sodium
■ Sodium can be provided as chloride, acetate, or phosphate salts in PN.
■ After phosphate addition, the remaining anions are added on the basis of acid–base status. They are split between chloride and acetate with a normal pH, predominantly acetate with metabolic acidosis, and predominantly chloride with metabolic alkalosis.
■ Requirements can be increased when the patient has extrarenal losses from nasogastric suction, abdominal drains, or ostomy losses.
Potassium
■ Potassium can be provided as chloride, acetate, or phosphate salts in PN.
■ Requirements can be increased with administration of potassium-wasting drugs (diuretics, steroids) or in severe undernutrition.
■ Like sodium, the remaining potassium can be added as acetate or chloride on the basis of acid–base status after the proper dose of phosphate is determined.
Calcium
■ Most practitioners add calcium as the gluconate salt.
■ Higher doses of calcium (~20–25 mEq/day) are needed in patients receiving long-term PN to help prevent metabolic bone disease.
■ Addition of calcium is limited in PN formulations because of the potential to precipitate with phosphate salts, which ultimately results in insoluble calcium phosphate.
Phosphate
■ Phosphate is added as the sodium or potassium salt.
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