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* From the Nutrition Support Service, Department of Surgery, Beth Israel Deaconess Medical Center/Harvard Medical School, Boston, MA.
Correspondence to: George L. Blackburn, MD, PhD, Nutrition Support Service, Harvard Medical School, One Deaconess Road-West Campus, Boston, MA 02215; e-mail: gblackbu{at}caregroup.harvard.edu
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Introduction |
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Assessment of malnutrition in critically ill patients begins with obtaining any history of recent, involuntary weight loss (exceeding 5% within 1 month or 10% over 6 months), although fluid overload usually prevents the accurate determination of dry weight in the ICU.2 Physical examination should focus on signs of protein–calorie deficiency (such as temporal wasting), signs of specific micronutrient deficiency (such as anemia, glossitis, or rash), hydration state, and edema. Dry weight and height are used to calculate the ideal body weight, the percentage of ideal body weight, and the body mass index (BMI). Ideal weight can be calculated as follows:
Men = 106 lb for 5 feet in height plus 6 lb for each additional inch.
Women = 100 lb for 5 feet in height plus 5 lb for each additional inch.
If an individual’s frame is small, the estimated ideal body weight may be reduced by 10%; conversely, for a large frame, 10% may be added. BMI is defined as the weight in kilograms divided by the square of the height in meters. Normal BMI ranges from 19 to 25. Survival at a BMI below 14 is very unusual.
Anthropometric data (skinfold thickness and arm muscle circumference), as well as creatinine height index (the urinary creatinine level according to height), while useful in ambulatory patients, are significantly less accurate measures of malnutrition in the critically ill patient, particularly in those who have fluid overload or renal dysfunction.3
Albumin is the most common laboratory measurement of visceral protein status. Contrary to popular thinking, hypoalbuminemia is rarely present in cases of isolated calorie malnutrition.4 Hypoalbuminemia is more commonly a marker of the systemic inflammatory response and, as such, has prognostic importance. It has been associated with increased morbidity and mortality among hospitalized patients.5 The daily hepatic synthesis rate for albumin is 120 to 170 mg/kg of body weight.6 Albumin is distributed between the intravascular and extravascular spaces. During injury, the liver increases production of acute-phase proteins and reduces albumin synthesis. The decrease in albumin coupled with extravasation and enhanced catabolism (both mediated by cytokines) culminates in hypoalbuminemia. Therefore, serum albumin concentration is a poor index of nutritional status but rather serves as a marker of injury and metabolic stress during injury response.7
The goals of nutrition support in ICU patients as summarized by a consensus statement from the American College of Chest Physicians are as follows8 :
1. To provide nutrition support consistent with the patient’s medical condition and the available route of nutrient administration.
2. To prevent and treat macronutrient and micronutrient deficiencies.
3. To provide doses of nutrients compatible with the existing metabolism.
4. To avoid complications related to the technique of dietary delivery.
5. To improve patient outcomes such as those affecting resource utilization, medical morbidities and mortalities, and subsequent patient performance.
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Total Parenteral Nutrition in ICU Patients |
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TPN plays an important role in patients in whom the gut cannot be used. Administration of 25 kcal/kg of usual body weight is adequate for most patients with normal BMI.8 In most patients this goal approximates the one calculated from the Harris Benedict equation. With BMI < 19, overfeeding may result in a refeeding syndrome characterized by electrolyte abnormalities (hypophosphatemia, hypokalemia, and hypomagnesemia), volume overload, and congestive heart failure.11 Refeeding syndrome is less likely if TPN is introduced gradually. Start with no more than 100 to 150 g dextrose and low concentrations of sodium chloride, and implement stringent monitoring of electrolytes (daily for the first 2 to 3 days) and blood sugars (every 6 h until persistently euglycemic with dextrose at goal).
The amount of CO2 produced when fuel is burned may be clinically important in patients for whom ventilator weaning is problematic. Indirect calorimetric techniques are used to determine the respiratory quotient [RQ] (the ratio of CO2 produced to O2 consumed), as well as the resting energy expenditure.12 Overfeeding carbohydrates results in an RQ close to 1.0, whereas consumption of fuels that are predominantly fat-based yields RQs closer to 0.7 (mixed fuels, 0.8 to 0.9).13 Composition of the TPN can be altered to prevent overfeeding in general, and overfeeding carbohydrates in particular, thus, calorimetry may be an important tool in certain patients.
The protein (amino acid) goal in TPN ranges from 1.2 to 1.5 g/kg/d and should be adjusted with periodic monitoring to promote nitrogen retention and to support protein synthesis.8 However, in critically ill persons, it is usually impossible to effect a positive nitrogen balance, as the cytokine and catabolic hormone cascade prevent anabolism. The administration of protein in higher quantities is unlikely to promote lean mass accrual. Azotemia can be aggravated by a high protein load, and thus, BUN values > 100 mg/dL might be an indication to decrease nitrogen intake, although this is not well validated in the acute illness setting. A more usual issue in feeding the patient with acute renal failure is that volume restrictions limit the quantity of feeding. In persons with chronic renal insufficiency, 0.8 g/kg/d of protein is sufficient. Another possible indication for limiting protein consumption in TPN occurs in persons in whom hepatic encephalopathy is a major clinical problem. Reducing the amino acid load or using a high quantity of branched-chain amino acids (BCAAs) have been shown to improve mental status.14
The lipid component of TPN consists of omega-6-polyunsaturated fatty acids that may be administered separately from the dextrose/protein or as part of a three-in-one solution. Theoretical concerns with overfeeding of lipids include injury to the reticuloendothelial system, which might lead to immunosuppression and can negate the beneficial effect of nutrition support.15 However, limiting fat calories to 30% of total calories is unlikely to lead to this complication, especially when the fat is infused slowly as with the three-in-one solution. Triglyceride levels > 400 mg/dL are a relative contraindication to adding lipids.
Carbohydrates should constitute the remainder of the total calories at between 3 and 5 g/kg/d,8 however, the specific amount should be adjusted appropriately to maintain a blood glucose level < 220 mg/dL. Many patients require coinfusion of regular insulin (usually as a component of the TPN) with supplemental subcutaneous administration of sliding-scale regular insulin if necessary. Postoperative hyperglycemia (blood glucose level > 220 mg/dL) has been shown to increase the risk of nosocomial infection to a degree that nullifies the benefits of nutritional repletion.16 Severe stress (eg, postoperative patients) is accompanied by rising plasma levels of the counterregulatory hormones glucagon, epinephrine, and cortisol, and thus, postoperative patients are most at risk from TPN-induced hyperglycemia.
Fluid restriction is often vital in cardiac, pulmonary, postoperative, and renal patients in the ICU. For such patients, TPN can be restricted to 1 L. Maximally concentrating nutrients allows the provision of 1,000 kcal and 70 g of protein per liter, which is often a substantial percentage of the weight-based feeding goal. Vitamins and trace elements are usually administered as components of the TPN. In addition, a number of medications, such as histamine-2 receptor antagonists and metaclopramide, can be mixed in with the TPN solution.
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Enteral Nutrition Support in ICU Patients |
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Enteral feeding is usually started with an elemental formula with reduced fat content at low rates until tolerance is determined. Rates may be advanced toward the goal every 8 h, as tolerated, as long as the gastric residual is low, and abdominal distension and pain are absent. Multiple vitamins need to be ordered separately. Caloric requirements are calculated as for TPN. The main difference is that many disease-specific enteral formulas exist.
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Disease-Specific Formulations |
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Pulmonary
Pulmonary formulas are designed to be high in fat (50%) and low
in carbohydrates to reduce CO2 production,
thereby reducing ventilatory demand. In preclinical studies, a tailored
pulmonary formula reduced pulmonary neutrophil accumulation and
inflammatory cytokines and improved cardiopulmonary hemodynamics and
gas exchange.19
This disease-specific pulmonary
formulation contains eicosapentaenoic acid and 
-linolenic acid
(which modify production of proinflammatory cytokines) and antioxidants
(vitamin E, vitamin C, and beta-carotene), and is a calorically dense
formula, suitable in particular for fluid-restricted patients with
ARDS.
Hepatic
Hepatic enteral formulas contain relative large amounts of the
BCAAs valine, leucine, and isoleucine, with low quantities of aromatic
amino acids. These products are tailored for patients with hepatic
encephalopathy.20
21
22
The rationale is that infusion of
BCAA corrects the imbalance between aromatic amino acids and BCAAs in
plasma and the CNS that might contribute to the mental disturbances
that are common. The use of BCAA-enriched formulas for short periods
may be beneficial because they improve nitrogen balance and lessen
encephalopathy, but their use for longer periods becomes expensive and
may limit protein synthesis, resulting in an inadequate nitrogen
balance.23
Renal
Specific renal formulas are usually low in protein or contain
variable proportions of BCAA. The solutions are usually calorically
dense and contain up to 2 kcal/mL. To achieve this density, some
formulas may contain significant amounts of fat, the ingestion of which
may result in bloating and delayed gastric emptying. Potassium,
phosphorus, and magnesium are present in substantially lower amounts
than is the case for typical enteral feeds. Renal patients are
also at increased risk of certain micronutrient toxicities. However, it
is important to feed patients adequately to avoid body cell mass
catabolism and malnutrition. For critically ill patients, it is best to
use dialysis to clear nitrogen and fluid and to feed them an adequate
protein diet than to underfeed protein.8
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Novel Pharmaconutrients on the Horizon |
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References |
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