Clinical Nutrition xxx (2014) 1e5
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Clinical Nutrition journal homepage: http://www.elsevier.com/locate/clnu
Original article
Accuracy of a simplified equation for energy expenditure based on bedside volumetric carbon dioxide elimination measurement e A two-center study Nilesh M. Mehta a, b, *, Craig D. Smallwood a, b, Koen F.M. Joosten c, Jessie M. Hulst c, Robert C. Tasker a, b, Christopher P. Duggan a, b a b c
Boston Children’s Hospital, Boston, MA, United States Harvard Medical School, Boston, MA, United States Erasmus MC-Sophia Children’s Hospital, Rotterdam, Netherlands
a r t i c l e i n f o
s u m m a r y
Article history: Received 16 October 2013 Accepted 12 February 2014
Background & aims: Accurate assessment of resting energy expenditure (REE) and metabolic state is essential to optimize nutrient intake in critically ill patients. We aimed to examine the accuracy of a simplified equation for predicting REE using carbon dioxide elimination (VCO2) values. Methods: We conducted a two-center study of metabolic data from mechanically ventilated children less than 18 years of age. Mean respiratory quotient (RQ) from the derivation set (n ¼ 72 subjects) was used to modify the Weir equation to obtain a simplified equation based on VCO2 measurements alone. This equation was then applied to subjects at the second institution (validation dataset, n ¼ 94) to predict resting energy expenditure. Bland-Altman analysis was used to assess the agreement between measured REE values, and REE estimated by the new equation as well as the Schofield equation. We also examined the accuracy of the new equation in classifying patients according to their metabolic state. Results: Mean respiratory quotient (SD) of 0.89 0.09 in the derivation set was used to obtain a simplified equation, REE (kcal/day) ¼ 5.534*VCO2 (L/min)*1440. In relation to the measured REE in the validation set, the mean bias (limits of agreement) for the REE predicted by this equation was 0.65% (14.4e13.1%); and the overall diagnostic accuracy for classifying subjects as hypometabolic or hypermetabolic was 84%. Mean bias (limits) of agreement between measured and Schofield equation estimated REE was 0.1% (40.5e40.7%). Conclusions: A simplified metabolic equation using VCO2 values was superior to the standard equation in estimating REE, and provided a reasonably accurate metabolic classification in mechanically ventilated children. In the absence of indirect calorimetry, bedside VCO2 monitoring could provide valuable continuous metabolic information to guide optimal nutrient intake. Ó 2014 Elsevier Ltd and European Society for Clinical Nutrition and Metabolism. All rights reserved.
Keywords: Indirect calorimetry VCO2 Energy expenditure Metabolic assessment Respiratory quotient
1. Background and aims Indirect calorimetry (IC) allows accurate assessment of resting energy expenditure (REE) from measured oxygen consumption (VO2) and carbon dioxide elimination (VCO2).1 IC is recommended
Abbreviations: VCO2, volumetric carbon dioxide elimination; VO2, volumetric oxygen consumption; RQ, respiratory quotient; REE, resting energy expenditure. * Corresponding author. Critical Care Medicine, Department of Anesthesiology, Perioperative and Pain Medicine, MSICU Office, Bader 634, Children’s Hospital, 300 Longwood Avenue, Boston, MA 02115, United States. Tel.: þ1 6173557327; fax: þ1 6177300453. E-mail address:
[email protected] (N.M. Mehta).
in critically ill patients for accurate measurement of REE and to guide energy prescriptions.2,3 However, IC requires specialized equipment and trained personnel, factors that limit its availability. In multicenter studies, IC was reportedly used in only a minority of centers worldwide.4,5 Estimates of REE and metabolic state utilizing prediction equations do not agree with measured values.6,7 Inaccurate energy prescriptions can lead to unintended underfeeding or overfeeding in the critically ill patient.8e10 Suboptimal feeding can lead to adverse consequences such as malnutrition, longer length of stay, longer time to wean from ventilator and increased complications. In a recent multicenter study, we observed significantly higher mortality in mechanically ventilated children that received inadequate (less than 66% prescribed) energy intake during their
http://dx.doi.org/10.1016/j.clnu.2014.02.008 0261-5614/Ó 2014 Elsevier Ltd and European Society for Clinical Nutrition and Metabolism. All rights reserved.
Please cite this article in press as: Mehta NM, et al., Accuracy of a simplified equation for energy expenditure based on bedside volumetric carbon dioxide elimination measurement e A two-center study, Clinical Nutrition (2014), http://dx.doi.org/10.1016/j.clnu.2014.02.008
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N.M. Mehta et al. / Clinical Nutrition xxx (2014) 1e5
stay in the PICU.5 A simplified and more readily available method to assess energy expenditure in the pediatric intensive care unit (PICU) to facilitate optimal energy prescription is desirable. We aimed to examine the accuracy of REE values and metabolic classification of patients by a simplified metabolic equation that uses VCO2 measurement alone. We hypothesized that a VCO2based REE equation would provide a clinically reliable estimate of REE and would be more accurate compared to standard equations that are currently used to estimate REE in this cohort. Independent VCO2 measurement capability is now available in most PICUs as stand-alone monitors or devices integrated into the mechanical ventilator. If the VCO2-REE equation is indeed reliable, then the benefits of metabolic assessment could be extended to a larger population in whom IC is currently not available, and for continuous metabolic measurement during mechanical ventilation. 2. Methods 2.1. Subjects Children less than 18 years of age who were mechanically ventilated in the multidisciplinary tertiary PICU at 2 academic centers, and in whom IC was performed according to institutional guidelines, were included in the study. Subjects were either on enteral or parenteral nutrition. Continuous enteral feeds or parenteral nutrition was not held for the test. Steady state gas exchange measurements from consecutive IC tests at Boston Children’s Hospital (Boston, USA) obtained with the VmaxÒ Encore (Viasys Healthcare, Loma Linda, CA) were used to derive the simplified metabolic equation that included VCO2 values only. IC measurements from Sophia Children’s Hospital (Rotterdam, Netherlands) obtained with the Deltatrac IIÒ (Datex-Ohmeda, Finland) were used as to test the accuracy of REE obtained by the simplified equation in comparison to the estimated energy expenditure by the Schofield equation. IC tests with respiratory quotient (RQ) outside the physiologic range (>1.3 or
