Factors Influencing Energy Expenditure

Increasing Energy Expenditure

• Tachycardia

• Ventilator weaning versus assisted ventilation

• Fever, infection

• Increased with trauma, injury, and total body surface area burn

• Reduced ambient temperature in burn-injured patients Reducing Energy Expenditure

• Analgesia for pain control, sedation, anesthetics

• Neuromuscular blockade

• Obesity (reduced caloric requirements per actual body weight)

• Warmer ambient temperature in burn-injured patients catabolism occurred. Pruitt and colleagues [67] reported in a study of 44 burn injured adults that energy expenditure increased significantly when the ambient temperature was reduced to 22°C due to increased heat loss through the burn wound and nonshiv-ering thermogenesis. In contrast to burn injured patients, Bardutzky [68] reported that in ten patients with acute ischemic stroke, basal energy expenditure decreased by 30% with moderate hypothermia, from a mean daily total energy expenditure of 1549 kcal/d with a body temperature of 37°C to 1099 kcal/d with 33°C. Thus, the presence of fever or hypothermia and ambient temperatures are important factors influencing energy expenditure, emphasizing the need to measure actual energy expenditure.

Methods for Determining Energy Requirements

It is important to measure REE by indirect calorimetry, because many of the equations used to predict energy requirements, such as the Harris-Benedict equation or the Curreri formula, can overestimate energy needs by as much as 44% and can lead to overfeeding [60]. Indirect calorimetry is especially important in underweight and obese patients, where estimation of energy expenditure is difficult [69]. An indirect calorimeter is used to measure heat and energy production indirectly through measurement of oxygen consumption and carbon dioxide production and calculation of REE using the Weir equation. The recommended practice for performing indirect calorimetry is to measure REE while patients are receiving continuous enteral feeding in a thermoneutral environment. This accounts for the thermic effect of feeding and any shivering/nonshivering themogenesis, with the remainder being stress or activity. The usual practice in critically ill patients is to add up to 10% to the measured REE to account for the activity/stress of patient care and in burn patients, 20 to 30% of REE. Swinamer reported in 1987 [70] that in critically ill patients activities such as weighing, repositioning, and chest physiotherapy increased energy expenditure above resting levels by 20 to 30%, but the actual contribution of these activities to total energy expenditure was small (1 to 3.6%) resulting in a mean total 24 h energy expenditure only 6.9% above the measured resting energy expenditure, and that an activity factor of no greater than 10% above resting energy expenditure is appropriate. However, McClave [71] reported that the addition of 10% to the measured REE in critically ill patients reduces the accuracy by which the "snapshot" REE correlates to the 24 h total energy expenditure (TEE) and should not be done. Hart [72] indicated that overfeeding burn patients with calories greater than 20% beyond measured REE is not beneficial and results in increased carbon dioxide production as well as increased fat deposition rather than lean body mass.

In the past, the respiratory quotient (RQ) value obtained from indirect calori-metry was used to assess substrate utilization, since theoretically metabolism of fat should result in a RQ of 0.7, protein 0.8, and 1.00 for carbohydrate. However, the body's ability to use individual nutrient substrates may be altered by the stress response and overall disease process [71]. McClave also cautions against the use of RQ to diagnose overfeeding (RQ is increased) or underfeeding (RQ is reduced), because the specificity and sensitivity are low, and the measured RQ does not correlate to under- or overfeeding in all patients. However, the RQ value is useful to confirm that results are within the physiologic range of 0.67 to 1.3.

Although measurement of energy expenditure via indirect calorimetry is the most accurate method of determining energy requirements, indirect calorimetry may not be available or appropriate in many circumstances. It should not be performed when FiO2 is greater than 60% in ventilated patients, in patients with air leaks from chest tubes or bronchopleural fistulas, during hemodialysis or continuous renal replacement therapy, within 24 h following general anesthetic, and with cases where it has not been possible to achieve "steady state" (where VO2 and VCO2 change by less than 10). In critically ill patients, it is recommended that patients with a BMI < 20 kg/m2 receive approximately 37 kcal/kg actual body weight (ABW); patients with a BMI between 20 to 30 kg/m2 25 to 30 kcal/kg; and for obese patients with a BMI > 30 kg/m2 approximately 20 kcal/kg [69]. In patients with burns, there are over 40 published predictive equations for assessing energy requirements; however, the Harris-Benedict equation multiplied by a stress factor 1.5 to 2.0, depending on burn size, is commonly used. In obese patients, adjusting the actual weight by 50% has shown to improve correlation of Harris-Benedict from 0.39 to 0.42 with measured REE [73].

Underweight and overweight critically ill patients may be at increased risk. Severely obese (BMI > 40 kg/m2) critically ill patients are at increased risk of morbidities, such as prolonged ventilator dependence, increased incidence of multiorgan failure, and intensive care unit (ICU) length of stay, and depressed left ventricular ejection fraction [74]. Gottschlich and colleagues in a prospective study of 15 obese patients matched to nonobese patients reported that obese burn patients are at increased risk of morbidities, such as of infection, sepsis, longer ventilation, and greater requirements for exogenous insulin [75]. Anesthetists have shown in patients undergoing abdominal surgery that intraoperative subcutaneous tissue oxygen tension is significantly less in obese patients (BMI > 30 kg/m2) even with supplemental oxygen administration, predisposing to a significantly increased risk of infection [76]. In over 500 patients undergoing colorectal surgery randomly assigned to received 30 or 80% oxygen during and for 2 h after surgery, the arterial and subcutaneous partial pressure of oxygen was higher and the incidence of wound infection was halved in the high oxygen group [77]. However, in regards to mortality, in a retrospective analysis of 63,646 patient data sets from a multi-institutional ICU database, Tremblay [78] reported increased mortality only in underweight patients (BMI < 20 kg/m2), not in overweight, obese, or severely obese patients.

Risks of Underfeeding

In critically ill patients, underfeeding can result in depressed ventilatory drive, decreased respiratory muscle function, impaired immune function, and increased infection. Recently, Rubinson and colleagues [79] reported that in a prospective cohort analysis of 138 patients, patients who were underfed (6 kcal/kg) had significantly increased risk of developing nosocomial bloodstream infections. Burn patients are at even greater risk of underfeeding due to their hypermetabolism and requirements for wound healing.

Providing nutrition support after underfeeding can result in refeeding syndrome. Refeeding syndrome includes the metabolic and physiologic changes in glucose and electrolytes occurring from providing nutrients after a period of starvation or low caloric intake. The presence of nutrients stimulates insulin and anabolism resulting in the intracellular uptake of potassium, phosphorus, and magnesium, expansion of extracellular fluid space, and subsequent reduction in circulating levels of potassium, phosphorus, and magnesium to deleterious levels, leading to cardiac arrhythmias and congestive heart failure.

Risks of Overfeeding

Overfeeding can be associated with increased carbon dioxide production, requiring increased ventilation, and can lead to hyperglycemia, hypertriglyceridemia, hepatic steatosis, refeeding syndrome, azotemia, hypertonic dehydration, and metabolic acidosis [80]. Overfeeding complications were more common in the past when critically ill patients were receiving hyperalimentation and excessive glucose calories by total parenteral nutrition (TPN) [81]. Jeejeebhoy and colleagues [82] showed that in an animal model of sepsis (TNF-a treated rats), increasing energy intake was not beneficial due to increased stabilization and prolongation of the effects of TNF activity with increased morbidity; however, increasing protein was beneficial [83].

Krishnan and colleagues report in a prospective cohort study of 187 critically ill adults (excluding trauma and burns) with a mean BMI of 25 kg/m2 that feeding more than 66% of goal intake (18 kcal/kg) was associated with increased morbidity and mortality [84]. In particular, obese patients may be at increased risk of overfeeding, and Dickerson and colleagues [85] indicated that in a retrospective analysis of 40 critically ill, overweight patients (> 125% of ideal body weight), those who received hypocaloric enteral nutrition (< 20 kcal/kg) had a shorter ICU stay and no difference in prealbumin or nitrogen balance. Thus in critically ill patients, permissive underfeeding may be helpful in overweight patients.

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