The measurement of energy expenditure.

Proper nutrition support depends upon the clinician's ability to estimate the patient's energy expenditure. The accuracy of estimation is inversely proportional to the severity of the patient's illness. This fact has spurred academic and industry groups to pursue the measurement of energy expenditure. Harris and Benedict used indirect calorimetry to develop their now-famous equation nearly 100 years ago. The concept of indirect calorimetry is simple; if you know the concentration of inspired gases and expired gases, along with the flow, you can determine the amount of a gas consumed or produced. The complexity and expense of indirect calorimeters suggest that this simple concept is technically challenging. Because we desire to know the energy expenditure of the most critically ill patients, indirect calorimetry is further confounded by the presence of oxygen and mechanical ventilation. This paper will discuss the myriad of variables and obstacles that complicate the measurement of energy expenditure and will suggest methods to avoid or overcome them.