Mechanical Ventilation: Metabolic Measurement Using Indirect Calorimetry (Respiratory Therapy)

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    Mechanical Ventilation: Metabolic Measurement Using Indirect Calorimetry (Respiratory Therapy)


    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)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:

    • Accurately determine the patient’s RQ to allow nutritional regimens to be adjusted to the patient’s needs
    • Accurately determine the REE and the RQ to monitor the adequacy and appropriateness of current nutritional support and, in mechanically ventilated patients, to guide appropriate nutritional support
    • Assess the contribution of metabolism to ventilation
    • Determine the oxygen demand of breathing as a guide to the selection of ventilator mode, settings, and weaning strategies
    • Monitor the patient’s VO2 as a guide to targeting oxygen delivery

    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

    • Burns
    • Gas leaks from the patient or ventilator system that prevent the collection of expired gases
    • Multiple traumas
    • Multisystem organ failure
    • Neurologic trauma
    • Peritoneal and hemodialysis treatments that remove carbon dioxide across the dialysis coil membrane so it cannot be measured by the indirect calorimeter
    • Sepsis
    • Use of paralytic agents or sedation
    • Maintenance of equipment
    • Competence in the specific manufacturer’s calibration instructions

    During open-circuit measurement, factors that cause inaccurate measurement of the REE and the RQ include:2

    • Anesthetic gases or gases other than oxygen, carbon dioxide, and nitrogen in the ventilation system
    • Bias flow from flow-triggering systems or intermittent mandatory ventilation systems or specific ventilator characteristics that prevent the ventilator from separating inspired and expired gases
    • Connection of the indirect calorimeter to certain ventilators, which causes an adverse effect on the triggering mechanism
    • Fraction of inspired oxygen (FIO2) greater than 0.62
    • Inadequate duration of the measurement period
    • Inappropriate calibration
    • Increased expiratory resistance, pressure measurement, or maintenance of the ventilator
    • Instability of delivered FIO2 because of changes in source gas pressure and ventilator blender or mixing characteristics
    • Leaks within the calorimeter
    • Total circuit flow exceeding the internal gas flow of the indirect calorimeter that incorporates the dilutional principle
    • Water vapor causing sensor malfunction

    During closed-circuit measurement, factors that cause inaccurate measurement of the REE and the RQ include:2

    • Functional residual capacity (FRC) changes resulting in changes in spirometer volume that are unassociated with VO2
    • Increased compressible volume and resistance that results in difficulty triggering the ventilator and increased work of breathing
    • Increased compressible volume in the circuit that prevents adequate tidal volume delivery, resulting in alveolar hypoventilation and changes in VCO2/VO2
    • Leaks drawing gas into the system during spontaneous breathing measurements that add volume to the system and cause erroneously low VO2 readings
    • Short duration of the measurement period (a function of carbon dioxide absorber life and VCO2) that may not allow the REE state to be achieved

    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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    • Provide developmentally and culturally appropriate education based on the desire for knowledge, readiness to learn, and overall neurologic and psychosocial state.
    • Explain the procedure to the patient and family.
    • Explain to the patient and family that the test may last 30 minutes.1,4
    • Encourage questions and answer them as they arise.



    1. Perform hand hygiene before patient contact. Don appropriate personal protective equipment (PPE) based on the patient's need for isolation precautions or the risk of exposure to bodily fluids.
    2. Introduce yourself to the patient.
    3. Verify the correct patient using two identifiers.
    4. Assess the patient to determine if metabolic measurements are indicated.
      1. Patients with known nutritional deficits
      2. Patients who have failed attempts at liberation from mechanical ventilation
    5. Assess the patient’s hemodynamic status.
    6. Assess the patient’s need for increased ventilatory requirements.
      Rationale: Indirect calorimetry is useful to determine the effects of metabolism on ventilation and VO2 that may help guide ventilation or weaning strategies.
    7. Assess the stability level of the delivered FIO2.
      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
    8. Assess the patient for any recent period of sedation holiday, agitation, or hemodynamic or respiratory instability.
      Rationale: The patient needs to remain in a calm and restful state to obtain the most accurate indirect calorimetry metabolic measurements.


    1. Ensure that the patient rests quietly before testing.
      Rationale: Allowing rest before testing permits the patient’s temperature, level of awareness, and body metabolism to stabilize.4
    2. Ensure that the testing environment is quiet.
    3. Determine the design of the indirect calorimeter (open- or closed-circuit).
    4. Calibrate the indirect calorimeter on the day of measurement and more often if errors in measurement are suspected.
    5. Ensure that the calibration gas mixture is relevant to the concentration of gas to be measured clinically.
    6. Gather clean tubing, low-resistance bacteria filters, gas analyzers (oxygen and carbon dioxide), and a flow and volume measuring device.
    7. Ensure that the ventilator circuit and tubing to the calorimeter are leak free.
    8. If using an open-circuit design, make available a method of stabilizing FIO2.
    9. Turn on the indirect calorimeter before the start of testing, based on manufacturer’s instructions, to warm up the dilution pump.


    1. Perform hand hygiene and don gloves. Don additional PPE based on the patient’s need for isolation precautions or the risk of exposure to bodily fluids.
    2. Verify the correct patient using two identifiers.
    3. Explain the procedure and ensure that the patient agrees to treatment.
    4. Position the patient comfortably in bed, usually in a supine or semi-Fowler position.
    5. Connect the sampling line to the water trap container.
    6. Perform gas calibration if required.
    7. Enter the patient data into the device.
    8. Connect the patient to a stand-alone device (metabolic cart) or follow the manufacturer's instructions for the specific indirect calorimeter used to initiate measurement on a ventilator that contains an integrated calorimeter.
      1. Connect the mixing chamber inlet to the expiratory limb of the mechanical ventilator circuit.
      2. Place the inspiratory sampling line in the inspiratory limb of the mechanical ventilator circuit. Ensure it is placed after the humidifier.
      3. Press start or begin the test.
    9. Ensure that there are no system leaks.
    10. Measure for 5 to 30 minutes5 until the desired steady state or the average interval is achieved according to the device and the percent variability.
      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
    11. Repeat metabolic measurements as necessary for unstable patients.
    12. Retrieve the metabolic measurement report.
    13. Disconnect the patient from the indirect calorimeter.
    14. Discard supplies, remove PPE, and perform hand hygiene.
    15. Document the procedure in the patient’s record.


    1. Observe the patient before and during the measurement to determine if the patient is at a steady state.
    2. Evaluate the metabolic measurement results.
    3. Adjust the metabolic nutritional care to avoid under-or over-nutrition.
    4. Monitor the patient’s comfort level and movement during the test.
    5. Monitor the equipment’s functionality.
    6. Monitor the patient’s FIO2 stability.
    7. Monitor to determine whether the patient is responding to current nutritional therapy.
    8. Observe the patient for signs and symptoms of pain. If pain is suspected, report it to the authorized practitioner.


    • Patient obtains steady state.
    • Both REE and RQ are accurately assessed.
    • Both REE and RQ are accurately measured.
    • RQ rests in the normal physiologic range (0.67 to 1.3).2,4
    • RQ is consistent with the patient’s nutritional intake.
    • Variations of VO2 and VCO2 are less than or equal to 5% for a 5-minute data collection.2


    • Hypoxemia
    • Bradycardia
    • Patient discomfort
    • Decrease in the trigger sensitivity of the ventilator, resulting in increased patient work of breathing in closed-circuit calorimeters
    • Increased work of breathing or dynamic hyperinflation
    • Inaccurate measurement of REE and RQ
    • RQ outside the normal physiologic range (0.67 to 1.3),2 suggesting technical errors in measurement


    • Education
    • Patient’s vital signs
    • Ventilator settings
    • Current nutritional support
    • Test quality
    • FIO2 stability
    • Unexpected outcomes and related interventions


    • If older patients have difficulty hearing, speak clearly and loudly enough for them to hear the information given.


    1. Achamrah, N. and others. (2021). Indirect calorimetry: The 6 main issues. Clinical Nutrition, 40(1), 4-14. doi:10.1016/j.clnu.2020.06.024
    2. American Association for Respiratory Care (AARC). (2004). AARC clinical practice guideline: Metabolic measurement using indirect calorimetry during mechanical ventilation—2004 revision & update. Respiratory Care, 49(9), 1073-1079. (classic reference)* (Level VII)
    3. Duan, J.Y. and others. (2021). Energy delivery guided by indirect calorimetry in critically ill patients: A systematic review and meta-analysis. Critical Care, 25(1), 88. doi:10.1186/s13054-021-03508-6 (Level I)
    4. Mtaweh, H. and others. (2018). Indirect calorimetry: History, technology, and application. Frontiers in Pediatrics, 6, 257. doi:10.3389/fped.2018.00257
    5. Singer, P., Fishman, G. (2024). Chapter E21: Indirect calorimetry. In J.L. Vincent and others (Eds.), Textbook of critical care (8th ed., pp. 1370.e133). St. Louis: Elsevier.


    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.

    Elsevier Skills Levels of Evidence

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    • Level II - At least one well-designed randomized controlled trial
    • Level III - Well-designed controlled trials without randomization
    • Level IV - Well-designed case-controlled or cohort studies
    • Level V - Descriptive or qualitative studies
    • Level VI - Single descriptive or qualitative study
    • Level VII - Authority opinion or expert committee reports
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