Nutrition Support Blog: Indirect Calorimetry as the "Gold Standard"?
July 22, 2011
I suspect that my pulse and blood pressure increase a bit every time that I hear or read that Indirect Calorimetry is the “Gold Standard” for determining calorie needs in critically ill patients. There are a lot of studies demonstrating that prediction equations are not consistently as accurate as we would like for predicting the energy expenditure of our critically ill patients. However, there are some serious flaws in the available research that should be considered before accepting these results at face value. The most important limitation is that most of the studies comparing indirect calorimetry with prediction equations used only a single indirect calorimetry study per patient, and these measurements were done at various times throughout their hospital stay. Some patients were measured in the early phase of illness, while others were stable or in the rehab phase when calorie needs were measured. This critical flaw is so important because when calorie expenditure was measured each day in the ICU, it was revealed that energy expenditure varies by an average of 30% each day (1,2).
Therefore, it is not clear that research comparing a prediction equation to one single indirect calorimetry measurement at some random point in the admission, is very useful. The day-to-day variability in energy expenditure in the ICU also means that a single indirect calorimetry does not appear any more accurate for predicting average energy expenditure of a patient than most of our prediction equations. Based on available data, it appears that indirect calorimetry would need to be done every single day in the ICU to be more “accurate” than most prediction equations.
There is also a more practical matter that needs to be considered. Even if a patient’s calorie expenditure is measured continuously, when you are dealing with enteral nutrition support, there are so many unavoidable interruptions in feeding, that patients do not receive the full amounts of what is ordered anyway!
While all of these facts are important, there is something even more crucial to consider before we label indirect calorimetry as “the gold standard.” The simple fact is that there is NO good evidence available demonstrating that an ICU patient’s outcome is optimized if we feed them what they are burning. Perhaps survival will be enhanced if we feed patients less, (or even more?) calories than they burn during the early phase of their admission. Until we have good evidence for when we should begin feeding in critically ill patients, and how much we should provide to have the best possible outcome it would seem very premature to label indirect calorimetry as the Gold Standard. Likewise it seems very premature to recommend any prediction equation as better than another because it compares well with a single indirect calorimetry study.
Indirect calorimetry may play a valuable role in clinical care for certain patients, especially those with altered body habitus. Patients with a very low BMI burn more calories/kg than the “average” patient and once these patients are in the rehab phase of their admission may need more than 40 kcals/kg to improve their nutrition status (3,4). However, some clinicians are reluctant to provide this many calories for fear of overfeeding, and indirect calorimetry can be useful to provide some objective data. However, it is important to remember the limitations of a single study, and consider additional needs that may be necessary to gain weight or meet goals and the amount of nutrition that actually enters the patient. It seems to me that clinical attention to detail for the individual patient may really be the Gold Standard.
1) Reid CL. Poor agreement between continuous measurements of energy expenditure and routinely used prediction equations in intensive care unit patients. Clin Nutr. 2007;26(5):649-657.
2) Vermeij CG, Feenstra BW, van Lanschot JJ, Bruining HA. Day-to-day variability of energy expenditure in critically ill surgical patients. Crit Care Med. 1989;17:623–626.
3) Ahmad A, Duerksen DR, Munroe S, Bistrian BR. An evaluation of resting energy expenditure in hospitalized, severely underweight patients. Nutrition. 1999;15(5):384-8.
4) Campbell CG, Zander E, Thorland W. Predicted vs measured energy expenditure in critically ill, underweight patients. Nutr Clin Pract. 2005;20(2):276-80.
"The emotion generated in scientific discussion increases proportionally with the softness of the data being discussed"
- Wolfe's Third Law
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