8 NUTRITIONAL ASSESSMENT AND ENTERAL NUTRITION
Margaret M. McQuiggan M.S., R.D., CNSD, Frederick A. Moore M.D.

NUTRITIONAL ASSESSMENT

1. What does a nutritional assessment include? Show answer

1. The medical and surgical history is used to establish preexisting (comorbid) conditions, metabolic stress, and alterations in organ function.
2. The physical examination focuses on the muscle mass, adipose stores, skin integrity, and hydrational state.
3. Laboratory data include the chemistry profile (Na, K, CO2, Cl, BUN, creatinine, glucose), ionized Ca, serum PO4, and Mg, complete blood count (CBC) with differential, arterial blood gases (ABGs; to assess acid-base status and CO2 retention), albumin, transferrin, prealbumin, and urinary nitrogen.
4. The drug profile can reveal agents that affect the metabolism of nutrients (insulin, levothyroxine, corticosteroids) or alter energy expenditure (beta-blockers, Diprivan).
5. Anthropometric data include height and weight; skinfold testing with calipers is only useful once edema has resolved but is rarely used in the acute care setting. Although information on adipose reserve, body cell mass, intra- and extracellular water, and third space fluid may be elucidated, standards for bioelectrical impedance analysis (BIA) have yet to be determined.
6. A nutrition history reveals preexisting nutritional practices.
7. The social history explores economic data or substance abuse behaviors and may predict the likelihood of adequate home care for the patient upon discharge.

2. What are primary and secondary malnutrition?

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Primary malnutrition is the consumption of inadequate kilocalories, protein, vitamins, or minerals. It may occur because of poor food choices, anorexia, poverty, alcoholism, suboptimal support regimens, or after bariatric surgery. Secondary malnutrition may occur even when adequate food is infused or consumed. It results from organ dysfunction (hypoalbuminemia with cirrhosis), malabsorption (Crohn’s disease), immobility (muscle wasting), drug therapy (insulin resistance with corticosteroids), or the inflammatory response (reprioritization of hepatic synthesis of acute phase instead of constitutive proteins).

3. What is the significance of serum proteins in nutritional assessment?

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Table 8-1. SERUM PROTEINS

TIBC = total iron-binding capacity.

KEY POINTS: HALF-LIVES OF SERUM PROTEINS USED AS NUTRITIONAL MARKERS

1. Pre-albumin: 2-4 days
2. Transferrin: 8-10 days
3. Albumin: 20-21 days

The most readily available proteins for nutritional assessment are albumin, transferrin, and prealbumin, which are all constitutively produced in the liver. Their half-lives are 20-21 days, 10-12 days, and 2-4 days, respectively. The level of all three plummets shortly after injury or surgery as the liver reprioritizes the production of acute phase proteins. Then, as inflammation, infection, and stress begin to resolve, the liver resumes production of constitutive proteins. Adequate kilocalories and protein facilitate this process. Because of their shorter half-lives, prealbumin and transferrin are most useful in the intensive care unit (ICU) setting and should be limited to patients with creatinine clearance > 50 mL/min. Levels of both proteins may be depleted in patients with hepatic failure or cirrhosis because of decreased synthetic function. Prealbumin travels in the circulation bound to retinol-binding protein (RBP) and vitamin A. Levels of prealbumin may be elevated in renal failure despite nutritional compromise, because of decreased catabolism and decreased excretion of RBP. Transferrin is elevated with iron depletion, independent of the effects of nutrition. (See Table 8-1.)

4. What is the significance of urinary nitrogen in nutritional assessment?

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Total urinary nitrogen (TUN) is the most reliable indicator of nitrogen utilization and excretion in surgical ICU patients. However, urinary urea nitrogen (UUN) is more readily available in most hospital laboratories. Although TUN and UUN are nearly equal in healthy ambulatory patients, critically ill patients exhibit a poor correlation between the two. Optimal nutrition support should place a patient in 13 to 15 nitrogen balance. One may estimate the protein needs of the patient by adding:

[24 h UUN (g) + 2 g N insensible losses + 3] x 6.25 = required amount of protein (g)

The total in brackets is multiplied by 6.25 to convert nitrogen grams to protein grams. Thus, if the laboratory reported a 13-g UUN/24 hours and a 2-g N insensible loss (skin, hair, feces) + 3 g for optimal anabolism, the patient would require 18 g N × 6.25 = 112.5 g of protein for anabolism. Urinary nitrogen is not useful as a guide for nutritional prescription in hepatic failure, renal dysfunction (< 50 mL/min creatinine clearance), or recent spinal cord injury.

5. How are protein requirements determined?

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Protein need is determined based on the weight of the patient, current stress factors, extraordinary skin losses, and organ function. Although the recommended daily allowance (RDA) for protein for healthy individuals is only 0.8 g protein/kg body weight, the following guidelines may be used in surgical patients. (See Table 8-2.)
Table 8-2. PROTEIN REQUIREMENTS IN RELATION TO INJURY LEVEL

Injury Level Protein Requirement
Mild stress or injury 1.2-1.4 g/kg
Moderate stress or injury 1.5-1.7 g/kg
Severe stress or injury 1.8-2.5 g/kg

6. Should protein be severely restricted in surgical patients with hepatic failure or renal failure?

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Protein should be restricted to 0.7 g/kg in patients with encephalopathy, only if the hepatic encephalopathy produces significant clinical consequences. Only 10% of chronic liver disease patients are protein sensitive; thus, other causes of encephalopathy such as infection, constipation, and electrolyte disturbance should be explored. Otherwise, a more typical postsurgical protein load may be adopted (1.4 g/kg). In injured patients with renal failure, one must balance the need for increased protein with the need for increased dialysis. One should provide the amount of protein required and dialyze more frequently.
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7. How are kilocalorie needs determined?

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There are several methods for setting kilocalorie targets in the surgical patient: standard prediction equations, kilocalorie per kilogram estimations, and indirect calorimetry. One common prediction equation, the Harris Benedict (HBE), was developed in 1919 for use on ambulatory, fasted, healthy people. Basal energy expenditure (BEE), the number of kilocalories required at rest daily, is calculated using the following equations:

Female BEE = 655 + 9.6 (kg) + 1.8 (cm) -4.7 (age)
Male BEE = 67 + 14 (kg) + 5 (cm) – 6.7 (age)

Subsequently, the above sums are multiplied by stress factors to determine total kilocalorie goals. (See Table 8-3).
Many clinicians use a total kcal/kg goal as shown in Table 8-4.
Table 8-3. KCAL NEEDS IN RELATION TO STRESS LEVELS

Stress Level Example Kcal Needs
Mild Closed fracture, pneumonia, or splenic laceration BEE × 1.2
Moderate Bowel resection, hepatorrhaphy, or thoracotomy BEE × 1.4
Severe Major bowel perforation with resection, major open wounds,
intraabdominal abscess BEE × 1.6

Table 8-4. KCAL / KG GOALS

Patient Feeding Level (kcal/kg) Level by Indirect Calorimetry
Normal weight patients 25-30 REE† × 1.0
Underweight patients 35-40 REE × 1.2
Obese patients 20-25* REE × 0.85
Morbidly obese 10-20* REE × 0.75

†Resting energy expenditure (REE) is the measure of energy expenditure in a fed state and is generally 10% higher than BEE.
*Use adjusted weight.

8. What is indirect calorimetry?

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It is a bedside test in which the patient’s production of carbon dioxide and consumption of oxygen are measured for approximately 30 minutes until steady state is achieved. Results are inserted into the modified Weir equation:

REE = [(3.796 x VO2) + (1.214 x VCO2)] x 1440 min/day

where REE = resting energy expenditure (kcal/day), VO2 = oxygen consumption (L/min), and VCO2 = CO2 exhaled (L/min).

The chart reports the number of kilocalories the patient is predicted to consume in 24 hours and the respiratory quotient (RQ). RQ = VCO2/VO2 and provides information on the type of substrate being used. The RQs for the metabolism of fat, protein, and carbohydrate are 0.7, 0.83, and 1.0, respectively. Overfeeding results in an RQ > 1.0.
KEY POINTS: RESPIRATORY QUOTIENT

1. Defined as ratio of CO2 produced to O2 consumed
2. Easy to perform on mechanically ventilated patients
3. Identifies principal metabolic substrate used by the patient
4. Ratio for fat (0.7), protein (0.83), and carbohydrates (1.0)
5. 5. Ratio < 1 indicates starvation or underfeeding; ratio > 1 indicates overfeeding, lipogenic status
6. Increased CO2 production linked to difficulty with ventilator weaning and impaired immune response

9. When is indirect calorimetry useful?

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The test may be performed on mechanically ventilated patients as soon as they are relatively stable, with a fractional concentration of oxygen in inspired gas (FiO2) < 60% and peak end-expiratory pressure (PEEP) < 10. Studies are helpful:

* When overfeeding (e.g., in diabetes mellitus, chronic obstructive pulmonary disease [COPD]) would be undesirable
* When underfeeding (e.g., renal failure, large wounds) would be especially detrimental
* In patients whose physical or clinical factors promote energy expenditure deviant from normal
* When drugs are used that might alter energy expenditure (e.g., paralytic agents, beta blockers)
* In patients who do not respond as expected to calculated regimens