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Don appropriate personal protective equipment (PPE) based on the patient’s signs and symptoms and indications for isolation precautions.
Because of the level of technical and patient assessment skills required, metabolic measurements using indirect calorimeters should be performed by individuals who are trained and competent in indirect calorimetry.
Indirect calorimetry is considered the gold standard technique for assessing resting energy requirements through metabolic measurements of the oxygen consumption (VO2) and carbon dioxide production (VCO2) in mechanically ventilated patients.
An indirect calorimeter (Figure 1), in an open- or closed-circuit design, is used to measure metabolic measurements of VO2 and VCO2 via expired gas analysis. The test involves measuring the amount of oxygen the patient inhales and the amount of carbon dioxide he or she 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.undefined#ref1">1 In other words, from these gas exchange data, the number of calories burned per minute is determined. Total test duration is approximately 30 minutes.2
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:1,2
During open-circuit measurement, factors that cause inaccurate measurement of the REE and the RQ include:1
During closed-circuit measurement, factors that cause inaccurate measurement of the REE and the RQ include:1
Metabolic measurements should be repeated according to the patient’s clinical status and indications for performing the test. More frequent measurement may be necessary in patients with a rapidly changing clinical course as recognized by hemodynamic instability and spiking fevers.1 Patients in the immediate postoperative period and those being weaned from mechanical ventilation may also need more frequent measurement.
Rationale: Indirect calorimetry is indicated to determine the cause of the increased ventilatory requirement.
Rationale: The stability level of the delivered F
2 may cause inaccurate measurement of the REE and the RQ during open-circuit measurement, and an F
2 that is greater than 0.6
1 causes inaccurate V
Rationale: The patient needs to remain in a calm and restful state during metabolic measurement to obtain the most accurate indirect calorimetry data.
Rationale: Allowing the patient to rest before testing permits his or her temperature, level of awareness, and body metabolism to stabilize.
American Society for Parenteral and Enteral Nutrition and Society of Critical Care Medicine. (ASPEN). (2016). Guidelines for the provision and assessment of nutrition support therapy in the adult critically ill patient: Society of Critical Care Medicine (SCCM) and American Society for Parenteral and Enteral Nutrition (ASPEN). Journal of Parenteral and Enteral Nutrition, 40(2), 159-211. doi:10.1177/0148607115621863
*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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