Energy expenditure in mechanical ventilation: is there an agreement between the Ireton-Jones equation and indirect calorimetry?

Objective: Assess the agreement between the energy expenditure measured by indirect calorimetry and that estimated by the Ireton-Jones formula of critically ill patients under assisted mechanical ventilation.

Methods: Participated in the study individuals able to interrupt ventilation support, admitted at the center of intensive care of the Hospital de Clínicas de Porto Alegre - RS, between August 2006 and January 2007. Energy expenditure was measured by indirect calorimetry using a specific monitor, as well as estimated by the Ireton-Jones formula. Values found were analyzed using the Student's t test and the Bland and Altman method and expressed in mean, ± standard deviation with a significance level of p<0.05.

Results: The study included forty patients with a mean age of 56±16 years and APACHE II score of 23±8. Energy expenditure measured by indirect calorimetry was of 1558±304kcal/24h, while that estimated by Ireton-Jones was of 1689±246kcal/24h. There was a significant statistical difference between means of energy expenditure measured and estimated of the same individual (p<0.004). The agreement thresholds between indirect calorimetry and the Ireton-Jones equation were of -680.51 to 417.81 kcal.

Conclusion: Energy expenditure estimated by the Ireton-Jones formula did not present good agreement with that measured by indirect calorimetry, however, considering aspects related to availability of the equipment, this equation may be useful in the nutritional planning for critically ill patients.