Nutrition in Critically ill Patients

1 Why is nutrition therapy in critical illness important?

Critical illness is most often accompanied by a catabolic stress state in which patients demonstrate a systemic inflammatory response, hypermetabolism, multiple organ dysfunction, infectious complications, and malnutrition. Malnutrition is associated with impaired immunologic function and increased morbidity and mortality in acutely ill patients. Therefore nutrition therapy is important to attempt to improve patient outcomes.

2 What are the goals of nutritional therapy in critically ill patients?

Over the past decade, there has been a shift away from the concept of nutrition support, where nutrition was provided as a fuel to support patients during a time of critical illness, toward the concept of nutrition therapy, where nutritional interventions are focused on modulating the immunologic and inflammatory response of critical illness. Therefore the generally accepted goals of nutritional delivery in critically ill patients are to

3 How should the nutritional status of critically ill patients be assessed?

Nutritional status assessment in critically ill patients is difficult. Traditionally, albumin, prealbumin, and anthropometric measurements have been used to assess nutritional status. However, these are inaccurate in critical illness because of fluid resuscitation and the acute phase response. Therefore one must rely on information such as recent weight loss, comorbid illnesses, and recent gastrointestinal (GI) dysfunction.

4 What mode of feeding (enteral or parenteral) should be initiated in critically ill patients?

Unless an absolute contraindication to enteral nutrition (EN) exists (such as ischemic bowel or bowel obstruction), EN should be initiated preferentially over parenteral nutrition (PN). Several randomized controlled trials (RCTs) have compared EN with PN in critically ill patients with an intact GI tract. When these studies were aggregated in a meta-analysis, no difference in survival was seen. However, EN is associated with a significant reduction in infectious complications, and it is less expensive than PN. Evidence also suggests that lack of use of the GI tract rapidly results in atrophy of gut luminal mucosa, which may lead to bacterial translocation across the gut wall and into the systemic circulation. Even small amounts of, or trophic, EN increase blood flow to the gut, preserve GI epithelial structures, and maintain villous height. EN also improves immune function by supporting gut-associated lymphoid tissue. EN is therefore recommended over PN unless the patient has an absolute contraindication to enteral feeding (discussed later).

5 When should EN be initiated in critically ill patients?

Early EN is usually defined as initiating enteral feedings within 48 hours of intensive care unit (ICU) admission. Many RCTs have compared early EN versus delayed nutrient intake in critically ill patients receiving mechanical ventilation, and, when these results were aggregated, early EN was associated with a trend toward mortality reduction and a significant reduction in infectious complications. Starting EN early does not seem to affect the duration of mechanical ventilation or ICU length of stay. The presence of bowel sounds and the passage of flatus are not necessary before the institution of EN.

6 How many calories should critically ill patients receive?

Energy expenditure varies with age, sex, body mass, and type and severity of illness. During critical illness, total energy expenditure (TEE) can be measured with indirect calorimetry. However, in clinical practice, resting energy expenditure (REE) is usually estimated by using a variety of available equations and is then multiplied by a stress factor of 1.0 to 2.0 to estimate TEE (and therefore caloric requirements). Roughly 25 kcal/kg ideal body weight is often the standard practice, and other equations, such as Harris-Benedict, Ireton-Jones, and Weir, are commonly used (Table 8-1). Unfortunately, predictive equations tend to be inaccurate. The optimal amount of calories to provide critically ill patients is unclear given the paucity of existing data, but studies do suggest that providing an amount of calories closer to goal calories is associated with improved clinical outcomes.

^{Table 8-1} Examples of Predictive Equations for Ree in Critical Illness

ActBW, Actual body weight = weight on admission (kilograms); AdjBW, adjusted body weight = ideal body weight + 0.4 (actual body weight − ideal body weight); BMI, body mass index; Ht, height (centimeters); IBW, ideal body weight = 50 + 2.3 per inch > 60 inches (men), 45.5 + 2.3 per inch > 60 inches (women).

7 What should be the composition of EN in critically ill patients?

Few data are available to inform the macronutrient composition of enteral feedings. In general, critically ill patients should receive an amount of protein daily between 1.5 and 2.0 g/kg of ideal body weight. The use of whole protein, or polymeric, formulas is recommended because insufficient data exist to support the routine use of peptide-based formulas in most patients. In most enteral formulas, approximately 25% to 30% of calories are from fat. Similar to the situation with protein, evidence in the literature is insufficient to support the routine use of high-fat or low-fat enteral formulas. In some ICU populations, specific enteral formulas are recommended (discussed later). For example, formulas containing arginine are often considered in patients who have had elective surgery, trauma, or burns but should generally not be used in patients with sepsis. Specific formulas designed for patients with renal failure are also available.

8 Should critically ill patients in shock and/or receiving vasopressors receive EN?

Ischemic bowel is a very rare complication of EN but has been reported in critically ill patients and can be fatal. Therefore the general recommendation is that EN be avoided in patients who are in shock and in those patients in whom resuscitation is active, vasopressors are being initiated, or vasopressor doses are increasing. Once patients are resuscitated and hemodynamically stable, EN may be initiated, even if they are receiving stable lower doses of vasopressors. However, special attention should be paid to signs of enteral feeding intolerance such as abdominal distention or increasing gastric residual volumes (GRVs).

9 Should gastric or small-bowel EN be used?