Body Weight

Although seemingly a straightforward measurement, body weights in ICU patients are often distorted by efforts at volume resuscitation and changing fluid distribution among various body compartments. Consequently, the patient’s current weight should be compared with his or her “usual” weight as well as predicted body weight (PBW) (see formulas for PBW in Table 15.1), also referred to as lean or ideal body weight, with attention paid to estimating the “dry” weight of volume-overloaded patients.

Significant weight loss (> 10%) during the 6 to 12 months before the critical illness episode should signify a patient at high risk for clinically significant malnutrition. Despite its limitations, current weight, when compared with predicted body weight, can be used to estimate how the patient’s fat calorie stores compare with normal and can guide the appropriate caloric prescription.

Serum Proteins

The profile of serum albumin, transferrin, and prealbumin, which normally gives a balanced composite view of the patient’s serum protein status, is less reliable during periods of critical illness and vigorous intravascular fluid resuscitation. The reprioritization of the liver synthetic pathways and the increased catabolism of several of these proteins change them from good measures of nutritional status to prognostic indicators of severity of illness, systemic inflammation, and clinical outcomes (e.g., mortality).

Time Lines for Reaching Adequate Daily Nutrition

An equally important factor in deciding which patients should receive nutritional support is the anticipated clinical course of the current illness. When will the patient again be ingesting an adequate oral intake? In well-nourished patients, nutrition support should be started if no oral intake is anticipated for greater than 5 days. In the face of preexisting malnutrition, however, anticipation of receiving nothing by mouth for more than 3 days should trigger nutritional intervention. For ventilated patients, studies suggest that nutritional therapy should start within 24 to 48 hours of ICU admission unless contraindicated. Consensus guidelines recommend that efforts to provide > 50% to 65% of goal calories be made over the first week in order to achieve the clinical benefit of enteral nutrition (EN). However, more recent data from the EDEN randomized trial of patients with acute lung injury on ventilators showed that providing trophic EN (i.e., providing ~25% of recommended daily nutritional goals) for the first 6 days of ventilation resulted in similar clinical outcomes as in those who received 80% of full goal.

Caloric Goal Delineation

Delineation of realistic clinical goals is important in order to prescribe appropriate cost-conscious prescriptions for total parenteral nutrition (TPN) or total enteral nutrition (TEN). To make a rational caloric prescription, one needs to know the individual patient’s total energy expenditure (TEE) as well as the patient’s caloric tissue goals. TEE defines the severity of hypermetabolism. TEE can be estimated by indirect calorimetry in many ICU patients but not those on ventilators with high inspired oxygen concentrations (FiO₂ > 0.6) and positive end-expiratory pressure (PEEP) of 7.5 to 10 cm H₂O since the measurement depends on the patient being able to tolerate a brief interruption of ventilation. In the ICU patient on bed rest, resting energy expenditure (REE) measured over 30 minutes generally approximates TEE. After determining TEE, the caloric (or “nonprotein” energy) prescription is guided by the clinician’s goal for the patient’s fat stores (Table 15.1).

If indirect calorimetry is not feasible, REE can be estimated by using the Harris Benedict or the Penn State equations with “multipliers” to take into account the hypermetabolic effects of critical illness (see online Table 15.E1). Some intensivists use an even simpler “one-size-fits-all” rule, such as providing 25 kcal/kg PBW. Even with the adjustments, these rules can still under- or overestimate actual TEE in hypermetabolic states, emphasizing the desirability of indirect calorimetry measurements if possible and the need for regular monitoring of the patient’s response to nutritional therapy as discussed further on.

Protein Goal Delineation

During critical illness, protein is mobilized from many body tissues, such as the intestinal tract, skeletal muscle, albumin mass, and the skin. This provides precursors for crucial protein synthesis (e.g., acute-phase plasma proteins and immunoglobulins), wound healing, and for energy if other substrates are not readily available. The magnitude of this mobilization and redistribution can be impressive, with urinary nitrogen losses of 30 to 50 g/day typically observed in patients with multiple trauma or severe sepsis or after bone marrow transplantation. This represents a loss of greater than 1 kg of lean tissue each day (with loss of 30 g of lean tissue being roughly equivalent to loss of 1 g of nitrogen). Endogenous production of glutamine from skeletal muscle may become insufficient, making this generally nonessential amino acid conditionally essential.

Although the catabolic process that is induced by inflammatory mediators cannot be reversed by providing exogenous protein (i.e., the latter cannot per se make the patient’s nitrogen balance positive), the magnitude of the protein loss can be diminished by providing protein and energy substrates during periods of critical illness. Some chronically critically ill patients who have evidence of inadequate growth hormone action have responded favorably to brief courses of growth hormone. However, the Food and Drug Administration recommends against the use of growth hormone based on an increased risk of mortality, nosocomial infection, and organ dysfunction in critically ill patients supplemented with growth hormone.

Determining how much protein to provide by nutritional therapy should, in general, be independent of one’s total (nonprotein) caloric prescription (described earlier). How much protein to give to a critically ill ICU patient initially remains more or less an empirical decision. One acceptable approach for these circumstances is to give 2 g of protein/kg current body weight (assuming normal functioning hepatic and renal disposal systems [Table 15.2]). For sick but not critically ill ICU patients, daily protein administration on the order of 1.5 g/kg/day would be an appropriate starting point. In comparison, recommended daily protein intake for healthy, well-nourished adults is only on the order of 0.8 to 1 g/kg/day.

The degree of protein catabolism and effects of the daily protein prescription on this loss can be monitored by serial measurements of the patient’s nitrogen balance. When calculating nitrogen balance, one should account for changes in blood urea nitrogen (BUN) and fecal nitrogen loss as shown in the equation:

where BUN (in g) = [0.6 x weight(f) x BUN(f)] – [0.6 x weight(i) x BUN(i)], BUN(i) and BUN(f) are the initial and final values of blood urea nitrogen (BUN expressed in g/L–not the laboratory output of mg/dL) and weight(i) and weight(f) are the initial and final weights (in kg) during the measurement period, respectively. Urinary nitrogen (in g) is measured as the concentration of urea in urine multiplied by the volume of a 24-hour urine collection.

For the purposes of the equation, 6.25 g of protein is assumed to be equivalent to 1 g of nitrogen so that nitrogen intake equals protein intake (in grams) divided by 6.25. The “4” in the equation represents normal fecal (and other nonurinary) nitrogen loss (4 g/24 hours). This may be, on the one hand, much less (close to zero) if the patient is on TPN and has no stools or, on the other hand, much more (double or triple) if the patient has diarrhea, an enterocutaneous fistula, or a wound vacuum manager. Wound vacuum drainage is equivalent to 2 g of nitrogen per liter.