It is not known how long a critically ill patient can tolerate lack of nutrient intake without adverse consequences, but because critical depletion of lean tissue can occur after 14 days of starvation in severely catabolic patients, it is recommended that nutrition support be instituted in patients who are not expected to resume oral feeding for 7 to 10 days [7]. A recent study conducted by the European Society of Intensive Care Medicine (ESICM) surveyed intensivists from 35 countries using a 49-item questionnaire to determine how they cope with these issues and to assess the current practice of nutritional management in intensive care units (ICUs) [8].

In the ESICM study, 45% of the patients were fed within 24 hours and 47% between 24 and 48 hours of admission to the ICU [8]. The need for nutritional support is determined by the balance between endogenous energy reserves of the body and the severity of stress. The best clinical markers of stress are fever, leukocytosis, hypoalbuminemia, and a negative nitrogen balance.

The purpose of nutritional assessment is to identify the type and degree of malnutrition to devise a rational approach to treatment. Percentage weight loss in the patient’s past 6 months, serum albumin level, and total lymphocyte count are readily available, commonly used measures to assess nutritional status. A 10% or 10-lb weight loss over the previous 12 months is an indicator of protein calorie malnutrition. This results from inadequate caloric intake. Hypoalbuminemic malnutrition or kwashiorkor is due to severe stress or profound malnutrition. Albumin is not a very sensitive indicator of malnutrition in ICU patients because its synthesis is influenced by numerous factors other than nutritional status such as protein losing states, hepatic function, and acute infection or inflammation [9]. Normal concentrations of albumin are unattainable in many critically ill patients because of large fluid shifts and inadequate synthesis to meet demands. Hypoalbuminemia should be viewed as a marker of injury and not as an indicator of impaired nutrition. Most critically ill patents have a combination of the two. The protein calorie malnutrition can be easily treated by supplying adequate caloric intake. The hypoalbuminemic malnutrition is most effectively treated by nutrition support and treatment of the stresses that led to this severe catabolic condition.

Traditionally, weight loss of 10 lb or 10% of usual weight is clinically important, weight loss of 20% to 30% suggests moderate protein calorie malnutrition, and greater than 30%, severe protein calorie malnutrition. Unfortunately, in many critically ill patients, total body weight is often an insensitive parameter because of progressive total body salt and water retention. Anthropometrics (i.e., measurement of triceps skin-fold thickness and midarm muscle circumference) are reasonably accurate even in the presence of excess body water because edema accumulates to a lesser extent in the upper extremities
[10]. However, they are difficult to perform in critically ill patients, time consuming, and not routinely performed. The general appearance of the patient, with emphasis on evidence of temporal, upper body, and upper extremity wasting of skeletal muscle mass, provides a quick, inexpensive, and clinically useful measure of nutritional status. For the reasons above, clinicians have found that body mass index may be a more practical way to assess nutritional status. As presented in Chapter 191, Driscoll suggests that a patient weight less than 85% of the ideal body weight (IBW) or BMI less than 18.5 indicates moderate malnutrition. Severe malnutrition would be considered likely if weight is less than 75% of IBW or BMI is less than 16 kg per m². Thus, a greater sense of urgency to intervene with nutrition support is present under these conditions and should be undertaken within several days of the acute injury.

Malnutrition is closely correlated with alterations in immune response as measured by skin test reactivity and total lymphocyte count. A total lymphocyte count less than 1,000 per mm³ is indicative of altered immune function and is associated with decreased skin test reactivity. Loss of delayed cutaneous hypersensitivity to common antigens is a measure of impaired cellular immunity, which has consistently been found to be associated with malnutrition [9].

Subjective global assessment (SGA) is a method for evaluating nutritional status that uses clinical parameters like history, physical findings, and symptoms [11,12]. The SGA determines whether (a) nutritional assimilation has been restricted because of decreased food intake, maldigestion, or malabsorption, (b) any effects of malnutrition on organ function and body composition have occurred, and (c) the patient’s disease process has influenced nutrient requirements [7].

As stated by the advisory committee convened by the National Institutes of Health, the American Society for Parenteral and Enteral Nutrition, and the American Society for Clinical Nutrition, “there is no ‘gold standard’ for determining nutritional status because (a) there is no universally accepted clinical definition of malnutrition, (b) all current assessment parameters are affected by illness and injury, (c) it is difficult to isolate the effects of malnutrition from the influence of the disease on clinical outcome, and (d) it is not clear which of the commonly used nutrition assessments techniques is the most reliable because of the paucity of comparative data” [7].

According to the ESICM questionnaire, the critical care community appears to most commonly assess nutritional status using the SGA and laboratory parameters [8]. Although there are no data to attest to the reliability of this approach in critically ill patients, serum albumin, stress level, weight loss in excess of 10% of ideal body weight, and SGA have been shown to be reasonable markers of nutritional status in noncritically ill hospitalized patients. Until future studies show otherwise, weight loss, serum albumin, and SGA are likely to be reliable parameters to follow in patients who are not volume overloaded. They are simple to measure, generally accepted, and commonly used.

Body cell mass is the major determinant of the total caloric requirement. Energy needs can be estimated or measured directly using indirect calorimetry. Because estimated energy requirements have been shown to be adequate in most patients, direct measurement is usually reserved for patients in whom estimating energy needs are difficult or when patients do not appear to respond to therapy (e.g., worsening respiratory function, continued weight loss, or a decrease in prealbumin levels, a more sensitive marker of protein synthesis than albumin).

The general principle of macronutrient support is to provide enough energy to promote anabolic functions and avoid caloric overload. Caloric requirements of 25 to 30 kcal per kg should be based on the usual body weight and are adequate for most patients [2,9]. If patients are not responding to therapy as indicated by the parameters listed above, or if they are in a severe catabolic state as occurs in multiple trauma or burns patients, they may need 30 or even 40 kcal per kg.