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Nutrition support of critically ill patients Thomas E. Edes MD To cite this article: Thomas E. Edes MD (1991) Nutrition support of critically ill patients, Postgraduate Medicine, 89:5, 193-200, DOI: 10.1080/00325481.1991.11700903 To link to this article:

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-@CME credit article

Nutrition support of critically ill patients Guidelines for optimal management

Thomas E. Edes, MD

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Preview Management of critically ill patients often involves nutrition support in the form of enteral or parenteral feeding. Knowing when to initiate feeding and how to calculate a patient's protein and calorie requirements is key to obtaining optimal results. Dr Edes offers guidelines for providing proper nutrition support, discusses the use of indirect calorimetry, and explains the metabolic processes of starvation and catabolism.

Optimal management of critically ill patients often involves nutrition support. It is now well accepted that nutrition can alter the outcome and survival ofhospitalized patients. 1 For nutrition support to be beneficial, guidelines should be followed and precautionary measures taken (table 1). The overall effect of nutrition support depends on the patient's status, the clinical situation, and knowledgeable management. This article discusses the crucial components of nutrition management, namely, assessment of the patient's status and needs, determination of the optimal feeding route and regimen, use of indirect calorimetry, and avoidance of the refeeding syndrome.

Patient status and nutritional

needs Before nutrition support is initiated, the patient's metabolic and nutritional status should be assessed. The presence of sepsis or injury affects metabolism and nutrient requirements. Therefore, an understanding

of the differences between the metabolism of starvation and the catabolic state is important (see box on page 194). Nutrition support must be somewhat individualized. The following categories are helpful in determining specific nutritional needs. • Well-nourished patients without sepsis or injury. Nutrition support is not needed for short-term problems. If normal eating is likely to resume within 5 days, no specialized feeding should be initiated. The risks of nutrition support outweigh any potential benefit. During observation or diagnostic testing, providing water and electrolytes is sufficient. • Malnourished patients without sepsis or injury. In this state of simple starvation, protein and calorie requirements are reduced. Increased nutrient intake is necessary for repletion, but caution is needed to avoid the refeeding syndrome' (see later discussion). • Well-nourished or malnourished patients with sepsis or injury. An altered metabolic state exists initially


and is characterized by hypermetabolism, increased endogenous protein breakdown, and a tendency for lipid oxidation and insulin resistance. Unless adequate calories and protein are provided, profound muscle catabolism and weight loss may result. Well-nourished patients may do well without feeding for a few days, but malnourished patients should receive prompt, consistent nutrition support.

Feeding route Voluntary oral intake is the best feeding method in most situations. However, adequate oral intake is not feasible in many critically ill patients. When nutrition support is indicated, the enteral or parenteral route must be selected. The enteral route is favored in patients with a functioning gastrointestinal tract because it is less expensive, is more physiologic, has fewer metabolic complications, and has fewer serious complications. Early enteral feeding has significant benefits. In patients receiving nothing by mouth, absence of gut stimulation results in atrophy of the intestinal mucosa within 3 days. Even if adequate quantities of essential nutrients are provided by parenteral feeding, significant loss of villus height, total cell mass, mucosal thickness, and absorptive capacity occurs. In contrast, enteral feeding maintains gut physiology, cell mass, and the integrity of tight intercellular continued


Protein requirements are reduced in starvation but increased in the catabolic states of sepsis and injury.

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Starvation versus the catabolic state In simple starvation, fat stores provide sufficient calories for days or weeks. Glycogen stores may provide glucose initially but become depleted within 36 to 48 hours. After that, the body utilizes protein to meet glucose needs, since fat cannot be converted into glucose. The body has no analogous protein stores, so the protein breakdown for gluconeogenesis, in addition to routine protein turnover, would likely lead to death by simple starvation within 2 weeks. Fortunately, an adaptive response to starvation allows humans to survive for much longer periods with inadequate food intake. The response involves a decrease in basal metabolic rate, an increase in the recycling oflactate and pyruvate back into glucose by the Cori cycle, and the utilization by nerve cells of ketone bodies rather than glucose.' The overall impact of these changes is a decrease in protein requirements to about one third of that in the nonadapted state. In contrast, the catabolic state in sepsis and injury induces an increase in protein requirements. Protein catabolism, as represented by urine urea nitrogen, may increase by 50% in patients with sepsis and may

junctions and may prevent translocation of bacteria or endotoxin into portal circulation.' The addition of glutamine to parenteral formulations may reduce the extent of intestinal atrophy when the gut is not used. 4 However, the gut response is less than that with enteral feeding. Tube feeding is generally well tolerated when initiated with a full-


nearly double in those with severe trauma or burns.2 This increase in protein catabolism occurs without an adequate compensatory increase in protein synthesis.3 Mediators of this catabolic response include glucocorticoids, catecholamines, glucagon, and perhaps interleukin-1. These substances induce an increase in lipid mobilization and oxidation, skeletal muscle catabolism, and hepatic gluconeogenesis, as well as a state of insulin resistance. The septic state is highly catabolic, with net catabolism even in the presence of abundant protein and calories. Since providing adequate substrate may not be sufficient, what can be done to overcome the catabolic response? The potential benefits of growth hormone and branched-chain amino acids in septic patients have been investigated. Manson and Wilinore4 studied the effect of growth hormone treatment on nitrogen balance during hypocaloric feeding. As expected, nitrogen balance was persistently negative during the control period, when no growth hormone was administered. Surprisingly, nitrogen balance was positive for days 2 to 6 of growth hormone treatment. Although these

strength isotonic formula delivered continuously at a rate of 30 mUhr and increased each day by 20 mUhr until the goal rate is reached. Gastric residual should be checked every 4 hours for a few days, and feedings should be withheld if the residual exceeds 150 mL. The head of the patient's bed should be elevated to at least 20°. In patients with absent gag reflex, altered consciousness, or a gas-

tric emptying problem, the tip of the feeding tube should be placed beyond the pylorus to reduce the risk of aspiration. Parenteral feeding can usually be managed with use of standard solutions. A mixture of 1,000 mL of 8.5% amino acids plus 1,000 rnL of 50% to 70% dextrose given daily through a central line is usually sufficient. Weekly administration of in-


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Hemodynamic status takes precedence over nutritional status in the initial management of critically ill patients.

results are intriguing, the study was conducted with healthy subjects in a nonstressed state. Thus, it is premature to assume that growth hormone treatment has clinical application in critically ill patients. Plasma levels of branched-chain amino acids fall in patients with severe liver disease, sepsis, or trauma. Bower and associates5 studied the effect of supplementation with branched-chain amino acids in such patients. Within 24 hours of onset of injury or sepsis, patients received parenteral nutrition with either a standard amino acid formula (25% branched-chain amino acids) or a branched-chain amino acid formula (45% branched-chain amino acids). A small positive trend in nitrogen balance was noted on 3 of the 10 days in patients receiving a branched-chain amino acid formula; however, no significant effect on protein breakdown or total nitrogen balance was noted. Overall, treatment with branched-chain amino acids appears to have marginal benefits in overcoming the catabolic response. The potential for benefit is restricted to a few patients, such as those with major trauma, who have profound metabolic stress and extremely negative nitrogen balance. Because of this

travenous lipid prevents essential fatty acid deficiency, bur more frequent administration may be necessary to provide extra calories or to replace carbohydrate calories in patients with respiratory failure or insulin resistance. The insulin resistance of stress may resolve unpredictably, leading to profound hypoglycemia if insulin has been used to maintain tight blood glucose con-

questionable clinical impact, use of these formulas does not appear warranted for routine nutrition support of septic patients. An important point is that the stress of sepsis induces an obligatory protein loss, regardless of the provision of nutrients. Initially, nutrition support may have little impact. This is not to say that nutrition support is unimportant but rather that stabilization of the patient is much more important in the critical first 24 hours or so. Therefore, hemodynamic status takes precedence over nutritional status in the initial management of critically ill patients. References 1. Cahill GF Jr. Srarvation in man. N EngiJ Med 1970;282(12):668-75 2. Blackbum GL, Bisttian BR. Maini BS, et al. Nutritional and metabolic assessment of the hospitalized patient. JPEN J Parenter Enteral Nutr 1977;1(1):11-22

3. Oowes GH Jr, George BC, Vtllee CAJr, et al. Muscle proteolysis induced by a circulating peptide in patients with sepsis or trauma. N Engl J Med 1983;308(10):545-52 4. Manson JM, Wdmore DW. Positive nitrogen balance with human growth hormone and hypocaloric intravenous feeding. Surgery 1986; 100(2):188-97

5. Bower RH, Muggia-Sullam M, Vallgren S, et al. Branched chain amino acid-enriched solutions in the septic patient: a randomized, prospective rrial. Ann Surg 1986;203(1):13-20

trol. Electrolyte dosage may be adjusted as needed. The daily dose of amino acids may be decreased to 500 mL (42 g of protein) if awtemia occurs, or may be increased, depending on the patient's requirements. A recent article' provides an excellent overview of total parenteral nutrition and guidelines for preventing potential problems. Central hyperalimentation is preferred, because


peripheral parenteral nutrition is limited in calorie content, usually requires continuous lipid infusion, and is often inadequate for patients in catabolic states.

Protein and calorie requirements Before feeding is initiated in a critically ill patient, a reasonable assessment of protein and calorie requirements must be made.

continued 195

A nitrogen balance study is useful for determining protein needs for patients with a protracted or critical illness.

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Table 1. Guidelines for initiating nutrition support in critically ill patients

Assess need Evaluate clinical situation Defer nutrition support in hemodynamically unstable patients Give prompt support if patient is catabolic or malnourished Give only water and electrolytes if less than 5 days of parenteral support is anticipated Determine route Provide enteral feeding if patient has functional gastrbintestinal tract Administer total parenteral nutrition through a central line for a catabolic or malnourished patient who is expected to receive nothing by mouth for at least 5 days Estimate requirements Protein 1. Estimate at 0.8 to 2 g of protein per kilogram of body weight (0.6 g for patients in renal failure) 2. Measure actual needs with nitrogen balance study 3. Assess adequacy by weekly measurement of serum transferrin levels Calories 1. Estimate with Harris-Benedict equation in routine cases 2. Use indirect calorimetry for more accurate estimate when respiratory failure is imminent or when weaning from a ventilator is difficult

PROTEIN-Initial protein requirements can be calculated on the basis of body weight and range from 0.8 glkg for noncatabolic adult patients to 2 glkg for highly catabolic patients. A more specific estimate may be needed for patients with a protracted or critical illness. A nitrogen balance study (ie, measurement of nitrogen intake minus nitrogen loss) is useful for determining actual protein needs (see box on page 197). A calorie count (including protein intake) and a 24-


hour urine urea nitrogen level are obtained concurrently. Because protein is 16% nitrogen by weight, protein intake is multiplied by .16 to obtain the equivalent nitrogen intake. A factor of 4 is added to the urine urea nitrogen value to compensate for unmeasured loss of nitrogen (ie, estimated urine nonurea nitrogen, such as creatinine and uric acid, and loss through stool and skin). A positive nitrogen balance is desired. When the balance is negative,

the deficit (expressed in grams of nitrogen) is multiplied by 6.25 to estimate the additional grams of protein needed daily to achieve a positive nitrogen balance. For example, a patient who weighs 70 kg (154 lb) is given 60 g of protein a day through tube feeding. The 24-hour urine urea nitrogen value is 8 g. The nitrogen intake (60 g X .16) minus the nitrogen loss (8 g urine urea nitrogen + 4 gunmeasured loss) is 9.6 g minus 12 g, or -2.4 g. To obtain a positive nitrogen balance, tube feeding should be increased to provide at least an additional 15 g (2.4 g X 6.25) of protein per day. CAIDRIFS-The Harris-Benedict equation is useful for estimating calorie needs on the basis of sex, weight, height, and age (see box on page 197). The computed resting energy expenditure is the approximate number of calories needed daily for the resting, nonstressed state. This value can be multiplied by a stress or activity factor, generally 1.1 for mild stress to 1.4 for severe catabolic stress, for a more accurate estimate. The ability to determine a patient's specific calorie needs is helpful because both underfeeding and overfeeding can have a significant adverse effect. The problems of underfeeding are well recognized. Prolonged underfeeding can impair the immune response, decrease respiratory drive/' and reduce the chance of sue-


Both underfeeding and overfeeding can potentially cause complications in critically ill patients.

cess in weaning a patient from mechanical ventilation. The potential complications of overfeeding tend to be overlooked. Providing excess calories can cause hepatic fat infiltration and fatty metamorphosis. It can also increase carbon dioxide production and interfere with the weaning process. 8 In most cases the Harris-Benedict equation provides an adequate estimate of calorie needs. However, a more individualized and accurate estimate is needed in some clinical situations (eg, impending respiratory failure, active weaning from mechanical ventilation). Indirect calorimetry is useful in these cases. Also, if the clinical situation changes (eg, resolution of sepsis, removal from a ventilator, change in activity, fluctuation in body temperature), calorie requirements will also change.

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Indirect calorimetry Indirect calorimetry is a method of determining calorie expenditure in a given metabolic situation. The carbon dioxide and oxygen levels in inspired air and expired air are measured and compared. The average of several measurements taken over a 10- to 20-minute period gives a close approximation of calorie expenditure. Indirect calorimetry also provides the respiratory quotient (RQ), which reflects the number of carbon dioxide molecules produced per molecule of oxygen consumed. The RQ

Fonnulas for estimating protein and calorie requirements Nitrogen balance (nitrogen intake minus nitrogen loss)

Nirrogen balance= (protein intake X .16)- (24-hr urine urea nirrogen + 4)

If balance is negative, multiply deficit by 6.25 to estimate the increase in daily protein intake (in grams) needed to achieve positive balance.

The Harris-Benedict equation REE (men) = 66 + (13.7 X wt) + (5 X ht) - (6.8 X age) REE (women) = 655 + (9.6 X wt) + (1.8 X ht) - (4.7 X age) Multiply resting energy expenditure (REE) value by stress or activity factor (ranging from 1.1 to 1.4) to obtain daily calorie requirements. Wt, weight (kg); ht, height {cm}; age (yr}.

varies, depending on the metabolic substrate. For example, the RQ is 1 for pure carbohydrate metabolism, 0.8 for protein, and 0.7 for fat. Because respiratory effort may decrease as carbon dioxide production drops, patients with respiratory distress may benefit from a reduction in carbon dioxide production achieved by lowering the RQ. 9 Lowering the RQ involves two principles: avoidance of excess calories and substitution of fat for carbohydrate. A common error is to assume that providing additional fat calories will reduce the RQ. On the contrary, excess calories may cause


the RQ to increase above 1, possibly to as high as 4, depending on the amount of excess calories. 10 Proper use of indirect calorimetry and the RQ requires recognition of the following: • Excess calories tend to greatly increase the RQ and production of carbon dioxide. • When the RQ is greater than 1, total calories should be decreased initially to approximate the calorie requirement; indirect calorimetry may then be repeated. • When the RQ is less than 1 bur greater than 0.85 and further reduction of the RQ is deemed beneficial, continued


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Lowering the respiratory quotient involves two principles: avoidance of excess calories and substitution of fat for carbohydrate.

Thomas E. Edes, MD Dr Edes is director of nutrition support service, associate professor of medicine, and associate chief of staff for education, Harry S Truman Memorial Veterans Hospital and University of Missouri-Columbia. A general internist with a graduate degree in nutrition, Dr Edes has major research interests in primary care nutrition and the role of nutrition in reducing cancer risk.

fat calories may be substituted for carbohydrate calories. Simply adding fat calories is counterproductive. • Lowering the RQ is beneficial only when respiratory failure is impending or when weaning from mechanical ventilation is difficult. Patients on long-term ventilatory suppon or patients who will not undergo weaning for several days do not benefit from high-fat feedings or a reduction in RQ. Although indirect calorimetry provides useful information in selected patients, its cost ($200 to $300) precludes routine use. Iflipid substitution is desired, a dietitian can be helpful in making appropriate changes in enteral feeding or in adjusting glucose calories and substituting lipid in parenteral feedings. Although lipid substitution can be beneficial, studies have shown that intravenous administration of lipid has potentially adverse ef-

fects. 11. 12 Despite these reports, intravenous lipid feedings of 50 g! day or less have no definitive adverse effects. This amount is present in a standard 500-mL unit of 10% intravenous lipid and provides 450 kcal.

Refeeding syndrome Patients who are malnourished, commonly because of alcoholism or chronic insufficient protein and calorie intake, are at risk for the refeeding syndrome. When these patients receive carbohydrate or protein, the activation of anabolic enzymes and pathways may rapidly deplete any available cofactors. Notably, profound hypophosphatemia and hypomagnesemta may occur, requmng prompt repletion. Cognizance of this phenomenon; daily assessment of phosphate, magnesium, and other electrolytes during the first few days of feeding; and provision of adequate but not excessive calories



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should prevent the potentially dire consequences of the refeeding syndrome.Ll

Summary and conclusion

Nutrition support is an important component of the management of critically ill patients. Before support is initiated, realistic goals should be established and risks and benefits considered. Initially, hemodynamic status should take precedence over nutritional status. Enteral feeding is the preferred route when the gastrointestinal tract is functioning. In most cases calorie requirements can be estimated adequately with use of the Harris-Benedict

References 1. Meakins JL, Pietscb JB, Bubenick 0, et al. Delayed hypersensitivity: indicator of acquired fu.ilure of host defenses in sepsis and trauma. Ann Surg 1977; 186(3):241-50 2. Weinsier RL, Krumdieck CL Death resulting from overzealous total parenteral nutrition: the refeeding syndrome revisited. Am J Clin Nutr 1980; 34(3):393-9 3. Mochizuki H, Trocki 0, Dominioni L, et al. Mechanism of prevention of postburn hypermetabolism and catabolism by early enteral feeding. Ann Surg 1984;200(3):297-310 4. Hwang TL, O'Dwyer ST, Smith RJ, et al. Preservation of small bowel mucosa using glutamine-enriched parenteral nutrition. Surg Forum 1986;37:56-8 5. McClave SA, Short AF, Mattingly DB, et al. Total parenteral nutrition: conquering the complexities. Postgrad Med 1990;88(1):235-48 6. Doekel RC Jr, Zwillich CW, Scoggin CH, et al. Clinical semi-starvation: depression of hypoxic ventilatory response. N Engl JMed 1976;295(7): 358-61

equation. Indirect calorimetry provides valuable infonnation in patients with impending respiratory failure; withdrawing excess calories and substituting lipid calories for carbohydrate calories, if necessary, may be beneficial. Oinical response can be assessed by nitrogen balance studies and weekly measurement of weight and serum

transferrin levels. IQII


Earn credit on this article. See CME Quiz.

Address for correspondence: Thomas E. Edes, MD (14A), Harry S Truman Memorial Veterans Hospital, Columbia,


7. Bassili HR, Deitel M. Effect of nutritional support on weaning patients off mechanical ventilators. JPEN J Parenter Enteral Nutr 1981;5(2): 161-3 8. Dark OS, Pingleton SK, Kerby GR. Hypercapnia during weaning: a complication of nutritional support. Chest 1985;88(1):141-3 9. AskanaziJ, NordensttomJ, Rosenbawn SH, et al. Nutrition for the patient with respiratory fu.ilure: glucose vs. fat. Anesthesiology 1981 ;54(5): 373-7 10. Elia M, Uvesey G. Theory and validity of indirect calorimetry during net lipid synthesis. Am J Clin Nurr 1988;47(4):591-607 11. Fucher Gw, Hunter KW; Wdson SR, et al. Diminished bacterial defences with intralipid. Lancer 1980;2(8199):819-20 12. Allardyce DB. Cholestasis caused by lipid emulsions. Surg Gynecol Obstet 1982; 154(5): 641-7 13. Solomon SM, Kirby OF. The refeeding syndrome: a review. ]PEN J Parenter Enteral Nutr 1990;14(1):90-7




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Nutrition support of critically ill patients. Guidelines for optimal management.

Nutrition support is an important component of the management of critically ill patients. Before support is initiated, realistic goals should be estab...
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