Nutrition Support Blog: Calorie Needs of Critically Ill Patients
December 5, 2013
I have spent more of my life than I would care to admit estimating the calorie expenditure of patients. Plugging numbers into various prediction equations, wrestling with assorted stress factors, and calorie ranges and then agonizing over whether the result was really OK for the sick person in front of me.
I was fortunate to have access to numerous and frequent indirect calorimetry measurements for most of my ICU patients early in my nutrition support career. I quickly realized that the calorie expenditure of critically ill patients can vary day to day and the limitations of most prediction equations. Over time, I also recognized that precise calculations or pump settings are completely futile in critical care units because patients routinely receive incomplete and very variable amounts of nutrition due to essential procedures, tests, “intolerance”, etc.
Most critically, I learned that there is no adequate research to allow anyone to know with certainty what calorie provision (and at what time in the illness) will allow the best patient outcome. I am always surprised when professional organizations or guidelines suggest one method of estimating calorie needs over another considering that no one knows how many calories to give critically ill patients.
Research has validated that the calorie expenditure of critically ill patients can vary substantially from day to day1,2. The daily variability of calorie expenditure in the ICU means that almost all of the research comparing the accuracy of various prediction equations for calorie expenditure is completely inadequate, because the research used only a single indirect calorimetry measurement per patient, at some random point in the admission.
In view of this multi-level “tetrad of uncertainty” (1.Limitations of prediction equations, 2.No randomized outcome data, 3.Daily variability of calorie expenditure, 4.Patients don’t receive everything we order) we have chosen to use simple, time efficient calorie per kilogram estimations for calorie goals. Until we have more complete data, preventing gross overfeeding appears to be the best that we can do. We try to use the time we save doing calculations, or checking for math/calculator errors to make sure our patients receive what is ordered in a safe manner and educating our staff. Until there is a better understanding of patient outcomes from randomized studies, longer calculations are just a placebo for that mildly obsessive-compulsive desire in all of us for better accuracy (to what end we are not sure).
“…and though my lack of education hasn't hurt me none, I can read the writing on the wall…”
- Paul Simon, Kodachrome, 1973
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. Frankenfield DC, Ashcraft CM, Galvan DA. Longitudinal prediction of metabolic rate in critically ill patients. JPEN 2012;36(6):700-712.