Indirect calorimetry in the critically ill: theoretical aspects and practical problems

Abstract

Indirect calorimetry measures both oxygen consumption (VO2) and carbon dioxide production (VCO2) and allows for the estimation of energy expenditure (EE). In the critically ill patient the measurements obtained can be used to evaluate the patient’s metabolic state, to assess the response to nutritional support and to estimate net substrates oxidation [1–3]. Moreover, indirect calorimetry measurements can be used to calculate cardiac output [4–6], to assess the relationship between oxygen delivery and VO2 [7–9], and to evaluate the effects of different levels of ventilatory support on the work of breathing [10, 11]. Here, I shall deal with some theoretical aspects and practical problems of indirect calorimetry. Knowledge of both theoretical and technical aspects of indirect calorimetry is fundamental to obtain accurate and meaningful results and to interpret correctly the results obtained, particularly in the critically ill mechanically ventilated patient.