Metabolic measurements using indirect calorimeters should be performed by individuals who are trained and competent in indirect calorimetry.
Indirect calorimetry is the gold standard test to determine energy expenditure requirements through metabolic measurements in mechanically ventilated patients. An individualized nutrition therapy plan may be prescribed once the energy expenditure is determined. It is important to have optimal nutrition to promote recovery in critical illness, because both under- and over-nutrition may negatively affect patient outcomes.undefined#ref1">1 Indirect calorimetry may reduce short-term mortality in critically ill patients.3
An indirect calorimeter (Figure 1), in an open- or closed-circuit design, is used to measure metabolic measurements of oxygen consumption (VO2) and carbon dioxide production (VCO2) via expired gas analysis. The test involves measuring the amount of oxygen the patient inhales and the amount of carbon dioxide the patient exhales. The measurements of VO2 and VCO2 are then used to calculate the respiratory quotient (RQ), where RQ = (VCO2 ÷ VO2), and to predict the resting energy expenditure (REE) (kilocalories per day) using the Weir equation, where REE (kcal) = [VO2 L/min(3.941) + VCO2 L/min(1.11)] × 1.44.2 In other words, from these gas exchange data, the number of calories burned per minute is determined. Total test duration may last 30 minutes.4
Indirect calorimetry measures the resting metabolic rate, or the number of calories the body burns at rest. Indirect calorimetry for metabolic measurements is used to:
These factors can decrease the number of days the patient is on ventilatory support in the intensive care unit.
Factors affecting the accuracy of caloric requirement estimates include:2,4
During open-circuit measurement, factors that cause inaccurate measurement of the REE and the RQ include:2
During closed-circuit measurement, factors that cause inaccurate measurement of the REE and the RQ include:2
Metabolic measurements should be obtained through indirect calorimetry repeated regularly according to the patient’s clinical status and indications for performing the test.1 More frequent measurement may be necessary in patients with a rapidly changing clinical course as recognized by hemodynamic instability and spiking fevers.2 Patients in the immediate postoperative period and those being weaned from mechanical ventilation may also need more frequent measurement.
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Rationale: Indirect calorimetry is useful to determine the effects of metabolism on ventilation and VO2 that may help guide ventilation or weaning strategies.
Rationale: When there is instability in the level of the delivered FIO2, it may cause inaccurate measurement of the REE and the RQ during open-circuit measurement, and an FIO2 that is greater than 0.6 causes inaccurate VO2 measurements.2
Rationale: The patient needs to remain in a calm and restful state to obtain the most accurate indirect calorimetry metabolic measurements.
Rationale: Allowing rest before testing permits the patient’s temperature, level of awareness, and body metabolism to stabilize.4
Steady state is generally considered a period of less than 5 consecutive minutes in which variations of VO2 and VCO2 are less than are less than or equal to 5%.2
Compher, C. and others. (2022). Guidelines for the provision of nutrition support therapy in the adult critically ill patient: The American Society for Parenteral and Enteral Nutrition. JPEN: Journal of Parenteral and Enteral Nutrition, 46(1), 12-41. doi:10.1002/jpen.2267
Elke, G. and others. (2019). Clinical nutrition in critical care medicine-guideline of the German Society for Nutritional Medicine (DGEM). Clinical Nutrition ESPEN, 33, 220-275. doi:10.1016/j.clnesp.2019.05.002
Lambell, K.J. and others. (2020). Nutrition therapy in critical illness: A review of the literature for clinicians. Critical Care, 24(1), 35. doi:10.1186/s13054-020-2739-4
*In these skills, a “classic” reference is a widely cited, standard work of established excellence that significantly affects current practice and may also represent the foundational research for practice.
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