Indirect calorimetry (IC) is an accurate method of estimating a patient’s energy expenditure, particularly the complex critically ill patient who benefits most from an individualized regimen of nutritional support. This bedside technique measures variables related to gas exchange and replaces assumptions about physiologic stress. When indirect calorimetry data are augmented by an arterial blood gas analysis of carbon dioxide (PaCO<SB>2</SB>), the dead space to tidal volume ratio (V<SB>D</SB>/V<SB>T</SB>) can be determined for an individual patient. These data can be valuable to the healthcare team when checking reasons for weaning failure. A case study approach to a 69-year-old man with acute respiratory distress syndrome and biliary sepsis will demonstrate the utility of this measurement. Attention to precise nutritional support and optimal gas exchange can influence the outcome of critically ill mechanically ventilated patients. This discussion highlights the potential benefits of indirect calorimetry for critical care nurses.