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Reprinted from the September 2004 issue of RESPIRATORY CARE [Respir Care 2004;46(9):1073–1079]

AARC Clinical Practice Guideline

Metabolic measurements using indirect calorimetry During Mechanical Ventilation—2004 Revision & Update

Metabolic measurements using indirect calorimetry for determination of oxygen consumption (VO2), carbon dioxide production (VCO2), respiratory quotient (RQ), and resting energy expenditure (REE) as an aid to patient nutritional assessment and management; 1-5 assessment of weaning success and outcome; 6-8 assessment of the relationship between O2 delivery (DO2) and VO2;1,9,10 and assessment of the contribution of metabolism to ventilation.11,12 This guideline addresses metabolic measurement during mechanical ventilation.

Metabolic measurements use an indirect calorimeter to measure VO2 and VCO2 via expired gas analysis. The measurements of VO2 and VCO2 are used to calculate RQ (VCO2/VO2) and REE using the Weir equation:13

REE = [VO2 (3.941) + VCO2 (1.11)] 1440 min/day.

The measurement of REE in mechanically ventilated neonatal, pediatric, and adult patients has been shown to be more accurate than published formulas used to predict REE,14-38 to reduce the incidence of overfeeding and underfeeding,14-28 and to decrease costs associated with total parenteral nutrition (TPN).28 Measurement of REE and RQ has been shown to be helpful in designing nutritional regimens to reduce VCO2 in patients with chronic obstructive pulmonary disease (COPD) and patients requiring mechanical ventilation.15,39-44 Despite this evidence, studies demonstrating improved outcome, decreased time spent on the ventilator, or shorter ICU/hospital stay are lacking. The objectives of metabolic measurements by indirect calorimetry are

2.1 To accurately determine the REE of mechanically ventilated patients to guide appropriate nutritional support14-38
2.2 To accurately determine RQ to allow nutritional regimens to be tailored to patient needs14-44
2.3 To accurately determine REE and RQ to monitor the adequacy and appropriateness of current nutritional support14-44
2.4 To allow determination of substrate utilization when urinary nitrogen values are concomitantly measured45-47
2.5 To determine the O2 cost of breathing as a guide to the selection of ventilator mode, settings, and weaning strategies6-8
2.6 To monitor the VO2 as a guide to targeting adequate DO2 1
2.7 To assess the contribution of metabolism to ventilation11,12


3.1 Mechanically ventilated patients

3.1.1 In the hospital
3.1.2 In the extended care facility


Metabolic measurements may be indicated

4.1 In patients with known nutritional deficits or derangements.14-38 Multiple nutritional risk and stress factors that may considerably skew prediction by Harris-Benedict equation include

4.1.1 Neurologic trauma20-27,35
4.1.2 Paralysis27
4.1.3 COPD15,23,26,44
4.1.4 Acute pancreatitis18,36
4.1.5 Cancer with residual tumor burden16
4.1.6 Multiple trauma22,25,28,37,38
4.1.7 Amputations25
4.1.8 Patients in whom height and weight cannot be accurately obtained
4.1.9 Patients who fail to respond adequately to estimated nutritional needs21
4.1.10 Patients who require long-term acute care48
4.1.11 Severe sepsis18,34
4.1.12 Extremely obese patients49
4.1.13 Severely hypermetabolic or hypometabolic patients

4.2 When patients fail attempts at liberation from mechanical ventilation to measure the O2 cost of breathing and the components of ventilation6- 8,50
4.3 When the need exists to assess the VO2 in order to evaluate the hemodynamic support of mechanically ventilated patients1,9,10
4.4 To measure cardiac output by the Fick method51,52
4.5 To determine the cause(s) of increased ventilatory requirements11,12,53


When a specific indication is present, there are no contraindications to performing a metabolic measurement using indirect calorimetry unless shortterm disconnection of ventilatory support for connection of measurement lines results in hypoxemia, bradycardia, or other adverse effects.54,55


Performing metabolic measurements using an indirect calorimeter is a safe, noninvasive procedure with few hazards or complications. Under certain circumstances and with particular equipment the following hazards/complications may be seen.

6.1 Closed circuit calorimeters may cause a reduction in alveolar ventilation due to increased compressible volume of the breathing circuit. 5,56-58
6.2 Closed circuit calorimeters may decrease the trigger sensitivity of the ventilator and result in increased patient work of breathing.5,56-58
6.3 Short-term disconnection of the patient from the ventilator for connection of the indirect calorimetry apparatus may result in hypoxemia, bradycardia, and patient discomfort.54,55
6.4 Inappropriate calibration or system setup may result in erroneous results causing incorrect patient management.1,4,5
6.5 Isolation valves may increase circuit resistance and cause increased work of breathing and/or dynamic hyperinflation.
6.6 Inspiratory reservoirs may cause a reduction in alveolar ventilation due to increased compressible volume of the breathing circuit.59
6.7 Manipulation of the ventilator circuit may cause leaks that may lower alveolar ventilation.


Limitations of the procedure include

7.1 Accurate assessment of REE and RQ may not be possible60-63 because of patient condition or certain bedside procedures or activities.
7.2 Inaccurate measurement of REE and RQ may be caused by leaks of gas from the patient/ ventilator system preventing collection of expired gases including

7.2.1 Leaks in the ventilator circuit1,4,5
7.2.2 Leaks around tracheal tube cuffs or uncuffed tubes1,4,5
7.2.3 Leaks through chest tubes or bronchopleural fistula64

7.3 Inaccurate measurement of REE and RQ occurs during peritoneal and hemodialysis due to removal across the membrane of CO2 that is not measured by the indirect calorimeter1,4,5,17
7.4 Inaccurate measurement of REE and RQ during open circuit measurement may be caused by

7.4.1 Instability of delivered oxygen concentration (FIO2) within a breath or breath to breath due to changes in source gas pressure and ventilator blender/mixing characteristics65,66
7.4.2 FIO2 > 0.601,4,5,65,66
7.4.3 Inability to separate inspired and expired gases due to bias flow from flowtriggering systems, IMV systems, or specific ventilator characteristics1,4,5,67,68
7.4.4 The presence of anesthetic gases or gases other than O2, CO2, and nitrogen in the ventilation system66
7.4.5 The presence of water vapor resulting in sensor malfunction
7.4.6 Inappropriate calibration69
7.4.7 Connection of the indirect calorimeter to certain ventilators, with adverse effect on triggering mechanism, increased expiratory resistance, pressure measurement, or maintenance of the ventilator5
7.4.8 Total circuit flow exceeding internal gas flow of indirect calorimeter that incorporates the dilutional principle70
7.4.9 Internal leaks within the calorimeter71
7.4.10 Inadequate length of measurement72- 75

7.5 Inaccurate measurement of REE and RQ during closed circuit measurement may be caused by

7.5.1 Short duration of the measurement period (a function of CO2 absorber life and VCO2) that may not allow REE state to be achieved5,56-58
7.5.2 Changes in functional residual capacity (FRC) resulting in changes in spirometer volume unassociated with VO2 5,56-58
7.5.3 Leaks drawing gas into the system during spontaneous breathing measurements that adds volume to the system and cause erroneously low VO2 readings5,56-58
7.5.4 Increased compressible volume in the circuit that prevents adequate tidal volume delivery resulting in alveolar hypoventilation and changes in VCO2/VO2 5,56-58
7.5.5 Increased compressible volume and resistance that results in difficulty triggering the ventilator and increased work of breathing5,56-58


Metabolic measurements should be performed only on the order of a physician after review of indications (MMMV 4.0) and objectives.


9.1 Test quality can be evaluated by determining whether

9.1.1 RQ is consistent with the patient’s nutritional intake1-5
9.1.2 RQ rests in the normal physiologic range (0.67 to 1.3)1-5
9.1.3 Variability of the measurements for VO2 and VCO2 should be ≤ 5% for a 5- minute data collecton72-75
9.1.4 The measurement is of sufficient length to account for variability in VO2 and VCO2 if the conditions in 9.1.3 are not met72-75

9.2 Outcome may be assessed by comparing the measurement results with the patient’s condition and nutritional intake.
9.3 Outcome may be assessed by observation of the patient prior to and during the measurement to determine if the patient is at steady state.


10.1 Indirect calorimeter, open- or closed-circuit design

10.1.1 The calibration gas mixture should be relevant to the concentration of gas to be measured clinically.1-5
10.1.2 The indirect calorimeter should be calibrated on the day of measurement and more often if errors in measurement are suspected.1-5
10.1.3 When the measurement results are suspect and/or when repeated calibration attempts are marked by instability, the indirect calorimeter may be tested via an independent test method (burning ethanol or other substance with a known RQ or adding known flows of CO2 and nitrogen to simulate VO2 and VCO2).76-79 As a simple test, ventilation of a leak-free system should yield VO2 and VCO2 values of near 0. Routinely scheduled measurement of normal control subjects (volunteers) may be useful.

10.2 A method of stabilizing FIO2 during opencircuit
measurements should be available and
may include

10.2.1 An air-oxygen blender connected between the gas source and the ventilator inlets for high pressure gas65
10.2.2 An inspiratory mixing chamber between the ventilator main flow circuit and the humidifier (See MMMV 6.6)59
10.2.3 Ventilator changes, which may include mode, inspiratory flow rate, PEEP, or tidal volume to improve patient-ventilator synchrony53

10.3 An isolation valve, double-piloted exhalation valve, or other device to separate inspiratory and expiratory flow should be incorporated when using continuous flow in the ventilator circuit.67 (see MMMV 6.5)
10.4 Personnel: Due to the level of technical and patient assessment skills required, metabolic measurements using indirect calorimeters should be performed by individuals trained in and with the demonstrated and documented ability to

10.4.1 Calibrate, operate, and maintain an indirect calorimeter
10.4.2 Operate a mechanical ventilator, including knowledge of the air-oxygen blending system, the spontaneous breathing mechanisms, and the alarm and monitoring functions
10.4.3 Recognize metabolic measurement values within the normal physiologic range and evaluate the results in light of the patient’s current nutritional and clinical status
10.4.4 Assess patient hemodynamic and ventilatory status and make recommendations on appropriate corrective/therapeutic maneuvers to improve or reverse the patient’s clinical course. A relevant credential (eg, RRT, CRT, RN, or RPFT) is desirable.

10.5 A hood canopy system in combination with airway sampling may be employed to capture gas that leaks around an uncuffed endotracheal tube.80
10.6 If a stable FIO2 cannot be achieved, VCO2 may be used to estimate REE by assuming an RQ of 0.8381 and the largest expected error is an

10.6.1 Underestimation of 25% for RQ of 1.2
10.6.2 Overestimation of 19% for RQ of 0.67

10.7 A simultaneous measure of PaCO2 and VCO2 will allow calculation of pulmonary dead space and components of ventilation using the Bohr equation:82

VE = VCO2 x 0.863 PaCO2 x (1-VD/VT)


11.1 The following should be evaluated during the performance of a metabolic measurement to ascertain the validity of the results

11.1.1 Clinical observation of the resting state (See MMMV 9.3)
11.1.2 Patient comfort and movement during testing
11.1.3 Values in concert with the clinical situation
11.1.4 Equipment function
11.1.5 Results within the specifications listed in 9.1.3 or 9.1.4
11.1.6 FIO2 stability

11.2 Measurement data should include a statement of test quality and list the current nutritional support, ventilator settings, FIO2 stability, and vital signs.


12.1 Metabolic measurements should be repeated according to the clinical status of the patient and indications for performing the test. The literature suggests that more frequent measurement may be necessary in patients with a rapidly changing clinical course as recognized by

12.1.1 Hemodynamic instability60
12.1.2 Spiking fevers60

12.2 Patients in the immediate postoperative period and those being weaned from mechanical ventilation may also need more frequent measurement.60


Metabolic measurements using indirect calorimetry are relatively safe procedures, but a remote possibility of cross-contamination exists either via patient- patient or patient-caregiver interface. The following guidelines should be followed when a metabolic measurement is performed.

13.1 Standard Precautions should be exercised whenever there is potential for contamination with blood or other body fluids.83
13.2 Appropriate use of barriers and handwashing is recommended.83,84
13.3 Tubing used to direct expiratory gas from the ventilator to the indirect calorimeter should be disposed of or cleaned between patients.
13.4 Connections used in the inspiratory limb of the circuit proximal to the humidifier should be wiped clean between patients; equipment distal to the humidifier should be disposed of or subjected to high-level disinfection between patients.
13.5 Bacteria filters may be used to protect equipment in both the inspired and expired lines, but caution should be used that moisture does not increase filter resistance resulting in poor gas sampling flow or increased resistance to exhalation.

Revised by Charles D McArthur RRT RPFT, Immanuel St Joseph’s — Mayo Health System, Mankato, Minnesota, and approved by the 2003 CPG Steering Committee

Original Publication: Respir Care 1994;39(12):1170-1175.



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Reprinted from the September 2004 issue of RESPIRATORY CARE [Respir Care 2004;46(9):1073–1079]

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