Metabolic Stress and Critical Illness

Starvation and metabolic stress result in deviations from normal nutrient metabolism that alters individual nutrient needs. A chronically starved patient with mild metabolic stress experiences a reduction in resting metabolic rate of somewhere between 10 to 30 percent (Heimburger 2015). This reduction in resting metabolic rate is the body’s response to lowered energy intake that leads to a decrease in overall energy needs. In starvation, with the absence of adequate glucose from carbohydrate, the body makes alterations to utilize fat as a primary source of fuel. Lipolysis—the process of breaking down of fat for fuel—occurs as the body is adapting to conserve lean muscle mass and to prevent valuable protein loss in the absence of adequate intake. Unless this process is interrupted with the gradual resumption of normal nutrition, the chronically starved individual will eventually develop anorexia and/or cachexia.

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Risk for malnutrition increases when patients experience two or more of the following conditions: inadequate calorie intake, weight loss, changes to body composition such as fat and muscle loss, accumulation of fluid or reduced grip strength (AND, 2015a). In the metabolically stressed population, malnutrition occurs when infection, traumatic injury, sepsis, or chronic inflammatory illness increase nutrient needs beyond the individual’s ability to meet those needs. In metabolic stress, nutrient needs are so high that they are often unable to be met with oral intake alone, and in these cases, nutrition support becomes an integral component of treatment.

Determining Energy Needs

Energy needs are significantly elevated in metabolic stress. As compared to the healthy state, resting energy expenditure (REE) in sepsis may increase by 50 to 80 (and include urinary nitrogen excretion loads of up to 30 grams per day due to muscle catabolism and impaired protein synthesis) (Barbour, Barbour and Hermann 2015). Indirect calorimetry is the preferred method for determining energy needs for a critically ill patient. Indirect calorimetry has been shown to be more accurate than energy estimations made from predictive equations. Critically ill patients should be allowed to rest for 30 minutes prior to the measurement of resting metabolic rate. Although indirect calorimetry is the gold standard in determining energy needs, when unavailable, predictive equations may suffice for the critically ill. Predictive equations tend to be less reliable for obese patients. In the case of critical illness and obesity (BMI >30), The American Society for Parenteral and Enteral Nutrition (A.S.P.E.N.) recommends permissive underfeeding (hypocaloric feeding) with enteral nutrition. Permissive underfeeding may be associated with lower mortality rates than targeted feedings (Arabi et al. 2011). In permissive underfeeding, calories should not exceed 60 to 70 percent of target energy requirements, or 11 to 14 kcal/kg actual body weight per day (22 to 25 kcal/kg ideal body weight per day) (McClave et al. 2009). Table 3.1 contains predictive equations for the critically ill patient.

Predictive equations for the critically ill. Abbreviations: PSU, Penn State University; RMR, resting metabolic rate; VE, minute ventilation in L/min; Tmax, maximum daily temperature in Celsius (AND, 2015a)

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