Richard G. Barton, MD
University of Utah, School of Medicine
Salt Lake City, Utah USA

Nutrition support, source control (e.g., treatment of infection, grafting of burns and stabilization of fractures) and restoration of oxygen transport (i.e., resuscitation from shock) have long been thought to be the cornerstones in the management of critically ill and injured patients. Despite current interest in specific immunity-enhancing nutrients, nutritional pharmacology and route of nutrition support, the basis of nutrition support in the critically ill patient continues to be the provision of sufficient protein and calories to minimize the loss of lean body mass and to support organ function, immune function and reparative processes. While data proving that short-term nutrition support is beneficial in improving clinically important outcomes are limited,1 it has long been known, and continues to be demonstrated, that malnutrition adversely affects outcomes.2-4 Similarly, data proving a relationship between energy expenditure, caloric intake and clinical outcomes are limited.

Inadequate caloric intake may be harmful in critically ill patients. In a randomized, controlled trial comparing enteral and parenteral nutrition in patients with severe head injuries, enterally fed patients had a worse neurological outcome, which was thought secondary to the fact that the enterally fed patients demonstrated feeding intolerance and received less than 10 kcal/kg/day.5 In a subsequent study by the same authors, the outcome differences were eliminated when isocaloric parenteral and post-pyloric enteral feeding were compared.6 Underfeeding has been shown to decrease the regeneration of respiratory epithelium and cause respiratory muscle weakness,7 creating the potential for prolonged mechanical ventilation. Several studies suggest that appropriate nutrition support improves weaning from mechanical ventilation and may reduce the length of hospital stay.8-12 More recently, patients receiving hypocaloric feedings (less than 90% of resting energy expenditure [REE]) measured by indirect calorimetry were found to have decreases in several anthropometric measurements after seven days but no differences in ICU length of stay (LOS), hospital LOS or ventilator days when compared to patients receiving appropriate (90% to 110% of REE) or excessive (greater than 110% of REE) caloric intake.13 Other studies have demonstrated an association between energy deficit and complication rates14 and length of intensive care unit stay.15

Excess caloric intake can cause increased carbon dioxide (CO2) production and ventilator dependence, hypertriglyceridemia and hepatic steatosis16 and hyperglycemia with its attendant complications. When overfeeding occurs, the excess calories are stored as fat and may not prevent erosion of the lean body mass in critically ill patients.17,18 Increased CO2 production may result in delayed ventilator weaning, prolonged mechanical ventilation and increased hospital LOS.19-21 Overfeeding has been associated with increased septic complications in trauma patients22 and hyperglycemia with increased infectious complications in cardiac surgery patients.23,24 In a recent large, randomized, controlled trial, intensive glycemic control was associated with decreased septic complications, lower incidence of renal failure requiring dialysis and decreased mortality.25 While it can be argued that intensive insulin therapy, with its attendant anabolic effects, might have been responsible for the improved outcome, multivariate analysis of the data from the original study suggested that glycemic control was primarily responsible for the beneficial effects.26 The importance of aggressive insulin use notwithstanding, avoidance of overfeeding and particularly carbohydrate overfeeding, is the first step in the control of hyperglycemia.

The measurement of energy expenditure in critically ill patients poses a number of problems, and no currently available method is ideal. Continuous whole body calorimetry provides the most accurate means of assessing energy balance,27-29 but it is not practical in patients confined to the intensive care unit. The doubly-labeled-water technique has been used to assess energy expenditure in healthy individuals over extended periods, but it depends upon steady-state CO2 and water turnover and a stable body water pool during the study period.27,30 Critically ill patients are poor candidates for this method of energy expenditure measurement due to large changes in body water and CO2 production.27,31,32

Magnetic resonance spectroscopy using31 P can be used to assess relative ATP, creatine phosphate and inorganic phosphate content in muscle and other tissues and has been proven useful in assessing and monitoring skeletal muscle energetics in animals and humans33-35 and has been reviewed elsewhere.36 While this technology offers great potential for the assessment and monitoring of metabolism, it is expensive and currently not practical for routine clinical use. Indirect calorimetry is used to determine the heat produced by oxidative processes by measuring oxygen consumption and carbon dioxide production, which are then used to calculate REE using the abbreviated Weir equation.36,37

Indirect calorimetry is widely used at the bedside. It is convenient, relatively inexpensive and accurate in estimating REE when compared to standard predictive formulae. In septic and injured patients, measured REE has been shown to exceed predicted REE by 20% to 40%38-42 and in head-injured patients by as much as 70% to 80%.43 On the other hand, in critically ill children, predicted energy expenditure based on the Recommended Daily Allowance, the Harris Benedict equation, the World Health Organization equation and the Schofield equation, all with stressrelated corrections, was grossly overestimated when measured by indirect calorimetry.44 Perhaps the biggest weakness of indirect calorimetry is that it is performed on patients at rest, so it does not account for energy expenditure during periods of activity. While this may not be a problem in sedated, mechanically ventilated patients, many clinicians will increase caloric input by 20% to 25% above measured REE to account for physical activity, particularly in patients who are agitated, ambulatory or involved in intense physical therapy.45-47 In summary, inadequate or excessive caloric intake is associated with poor outcomes in critically ill patients.

Current recommendations are to provide 25 to 30 kcal/kg/day or to base the provision of calories on REE measured by indirect calorimetry.48 The author uses the former as a starting point to initiate nutrition support and obtains indirect calorimetry weekly and as needed based on the patient’s clinical course. Regardless of the method used, agitated or ambulatory patients may require as much as 25% more calories to account for activity.