Chapter 072. Malnutrition and Nutritional Assessment (Part 7)

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Assessment of Circulating (Visceral) Proteins The serum proteins most used to assess nutritional status include albumin, total iron-binding capacity (or transferrin), thyroxine-binding prealbumin (or transthyretin), and retinol-binding protein. Because they have differing synthesis rates and half-lives—the half-life of serum albumin is about 21 days whereas those of prealbumin and retinol-binding protein are about 2 days and 12 h, respectively—some of these proteins reflect changes in nutritional status more quickly than others. However, rapid fluctuations can also make shorter-half-life proteins less reliable. Levels of circulating proteins are influenced by their rates of synthesis and catabolism, "third spacing" (loss into interstitial spaces), and,...

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Chapter 072. Malnutrition and Nutritional Assessment (Part 7) Assessment of Circulating (Visceral) Proteins The serum proteins most used to assess nutritional status include albumin, total iron-binding capacity (or transferrin), thyroxine-binding prealbumin (or transthyretin), and retinol-binding protein. Because they have differing synthesis rates and half-lives—the half-life of serum albumin is about 21 days whereas those of prealbumin and retinol-binding protein are about 2 days and 12 h, respectively—some of these proteins reflect changes in nutritional status more quickly than others. However, rapid fluctuations can also make shorter-half-life proteins less reliable.
Levels of circulating proteins are influenced by their rates of synthesis and catabolism, "third spacing" (loss into interstitial spaces), and, in some cases, external loss. Although an adequate intake of calories and protein is necessary to achieve optimal circulating protein levels, serum protein levels generally do not reflect protein intake. For example, a drop in the serum level of albumin or transferrin often accompanies significant physiologic stress (e.g., from infection or injury) and is not necessarily an indication of malnutrition or poor intake. A low serum albumin level in a burned patient with both hypermetabolism and increased dermal losses of protein may not indicate malnutrition. On the other hand, adequate nutritional support of the patient's calorie and protein needs is critical for returning circulating proteins to normal levels as stress resolves. Thus low values by themselves do not define malnutrition, but they often point to increased risk of malnutrition because of the hypermetabolic stress state. As long as significant physiologic stress persists, serum protein levels remain low, even with aggressive nutritional support. However, if the levels do not rise after the underlying illness improves, the patient's protein and calorie needs should be reassessed to ensure that intake is sufficient. Assessment of Vitamin and Mineral Status The use of laboratory tests to confirm suspected micronutrient deficiencies is desirable because the physical findings for these are often equivocal or
nonspecific. Low blood micronutrient levels can predate more serious clinical manifestations and may also indicate drug-nutrient interactions. Estimating Energy and Protein Requirements A patient's basal energy expenditures (BEE, measured in kilocalories per day) can be estimated from height, weight, age, and gender using the Harris- Benedict equations: where W is weight in kg; H is height in cm, and A is age in years. After solving these equations, total energy requirements are estimated by multiplying the BEE by a factor that accounts for the stress of illness. Multiplying by 1.1–1.4 yields a range 10–40% above basal that estimates the 24-h energy expenditure of the majority of patients. The lower value (1.1) is used for patients without evidence of significant physiologic stress; the higher value (1.4) is appropriate for patients with marked stress such as sepsis or trauma. The result is used as a 24-h energy goal for feeding. When it is important to have a more accurate assessment of energy expenditure, it can be measured at the bedside using indirect calorimetry. This technique is useful in patients who are believed to be hypermetabolic from sepsis or trauma and whose body weights cannot be obtained accurately. Indirect
calorimetry can also be useful in patients having difficulty weaning from a ventilator, as their energy needs should not be exceeded to avoid excessive CO 2 production. Patients at the extremes of weight (e.g., obese persons) and/or age are good candidates as well, because the Harris-Benedict equations were developed from measurements in adults with roughly normal body weights. Because urea is a major byproduct of protein catabolism, the amount of urea nitrogen excreted each day can be used to estimate the rate of protein catabolism and to determine if protein intake is adequate to offset it. Total protein loss and protein balance can be calculated from the urinary urea nitrogen (UUN) as follows: The value of 4 g added to the UUN represents a liberal estimate of the unmeasured nitrogen lost in the urine (e.g., creatinine and uric acid), sweat, hair, skin, and feces. When protein intake is low (e.g., less than about 20 g/d), the equation indicates both the patient's protein requirement and the severity of the catabolic state (Table 72-5). More substantial protein intakes can raise the UUN because some of the ingested (or infused) protein is catabolized and converted to UUN. Thus at lower protein intakes the equation is useful for estimating requirements, and at higher protein intakes it is useful for assessing protein balance.
Further Readings American Society for Parenteral and Enteral Nutrition: The science and practice of nutrition support: A case-based core curriculum. Dubuque, Kendall/Hunt, 2001. Available online at: www.nutritioncare.org Baker H: Nutrition in the elderly: Hypovitaminosis and its implications. Geriatrics 62(8):22, 2007 Chapman IM: Nutritional disorders in the elderly. Med Clin North Am 90(5):887, 2006 Heimburger DC, Ard JD (eds): Handbook of Clinical Nutrition, 4th ed. Philadelphia, Mosby Elsevier, 2006 Shils ME et al (eds): Modern Nutrition in Health and Disease, 10th ed. Baltimore, Lippincott Williams & Wilkins, 2005