- Click here for explanation of classification scheme.

Describe the clinical technique of bedside indirect calorimetry for the measurement of resting energy expenditure (REE) and respiratory quotient (RQ).

Definitions

• Steady state:
• Total daily energy expenditure: Includes basal metabolic rate (BMR), REE, dietary thermogenesis, and the caloric expenditure of voluntary physical activity.
• BMR: The minimal heat production of an organism, measured from twelve to eighteen hours after the ingestion of food and with the organism at complete muscular rest. (Boothby & Sandiford, 1920)
• Steady state or equilibration: A period of five or more consecutive 1-minute data points having a coefficient of variation for both oxygen consumption (VO₂) and carbon dioxide production (VCO₂) =5% (SD for each =5% of the respective mean) and an average resting minute ventilation that is appropriate for the patient’s size and clinical condition.”
• Respiratory quotient: Same as the project definition.

No article inclusion criteria given.

No article exclusion criteria given.

None reported

Outcome(s) and Other Measures

Summarized pertinent historical and current (e.g., up to 1985 articles).

No systematic search was completed; rather selection of articles based on authors' experiences.

• There were 33 references published dates from 1918 to 1984
  • Sample sizes represented by studies ranged from N=4 to N=239 (Harris Benedict
  • Chapters found within three books were cited as well as one letter to the editor.
• Study participants represented acute care, oncology and morbid obesity patients admitted into the hospital; the early, historical  study by Harris-Benedict represented a healthy population.
• The author cited most often was J. Askanazi (N=4), followed by I. Feurer (N=3).

Components of Total Daily Energy Expenditure

• "Total daily energy expenditure: Includes basal metabolic rate (BMR), REE, dietary thermogenesis and the caloric expenditure of voluntary physical activity
• BMR: The minimal heat production of an organism, measured from 12 hours to 18 hours after the ingestion of food and with the organism at complete muscular rest (Boothby & Sandiford, 1920); represents 90% of REE and during periods of sleep represents 80%.
• SDA: The digestion and metabolism of exogenous nutrients; is approximately 10% above postabsorptive REE over 24 hours
• Shivering/non-shivering thermogenesis: Normal response to cold includes shivering thermogenesis, which may result in a significant increase in heat production and rate of heat loss due to increase skin temperature. Non-shivering thermogenesis does not result in increased skin temperature or heat loss and is “zero" at the thermoneutral zone of body temperature."
• Resting energy expenditure represents the major part (75-100%) of total daily energy expenditure. 

Elwyn DH, Kinney JM, Askanazi J, 1981; Kinney JM, Long CL, Gump Fe et al, 1970

IC Analysis

All instruments consist of devices to measure the concentration of oxygen and carbon dioxide in a gas sample as well as the expire volume or low, temperature, barometric pressure and time. In addition, data acquisition and analysis procedures must provide for serial reports at regular intervals.

Measurement of gas exchange requires that mixed expired air is directed past gas analyzers. The concentrations of O₂ and CO₂ in the expired air are compared to the concentrations of O₂ and CO₂ in the inspired gas. The exact composition of inspired air must be known or measured and room air must not be inadvertently mixed with the expired gas. The volume (or flow rate) of gas expired over a precise time period must be accurately measured.

Key variables: Total expired volume of flow per unit time (e.g., minute ventilation), the fractional concentration of inspired oxygen (FIO₂) and carbon dioxide (FICO₂), the fractional concentration of mixed expired oxygen and carbon dioxide, barometric pressure, the temperature of mixed expired gas and time.

Clinical Technique

Sterilization of accessory equipment

All equipment coming into contact with the patient’s skin, secretions and expired air must be sterilized or disposable. Adapters, mouthpieces, noseclips and non-rebreathing valves should be washed and then sterilized (ethylene oxide) after each use. Masks are scrubbed with a disinfecting solution and all tubing used to direct expired air is replaced at regular intervals, depending on frequency and condition of use. The surfaces of the mixing chamber, canopy and instrument are wiped down regularly with a disinfecting solution.

Instrument preparation

The volume measurement system, temperature sensor and barometer are calibrated daily and gas analyzers are calibrated at the bedside before each measurement. Both 100% nitrogen and a calibration mixture (which approximates the composition of the patient’s mixed expired air) are used during the gas calibration procedures.

Measurement Conditions-REE

1. Patients have rested in the supine position (in bed or a recliner) for more than 30 minutes before the measurement to avoid the effects of previous voluntary activity on REE
2. Patients are without oral dietary intake or intermittent enteral or parenteral feedings for more than two hours before REE to avoid the transient thermic effect of intermittent nutrient intake.
3. Measurements are made in a quiet, thermo-neutral environment
4. The rate and composition of nutrients being infused on a continuous basis are noted
5. All sources of supplemental oxygen (e.g. nasal cannulas, masks or tracheostomy collars)  are turned off during routine room air measurements
6. Patients have no voluntary skeletal muscle activity (movement of the extremities) during the measurement
7. No leaks are present in the sampling system
8. All data used to derive the REE and RQ are taken from a period of equilibration or “steady state.”

Data Evaluation and Calculations

Our definition of equilibration (or "steady state") is a period of five or more consecutive one-minute data points having a coefficient of variation for both oxygen consumption (VO₂) and carbon dioxide production (VCO₂)=5% (SD for each equals 5% of the respective mean) and an average resting minute ventilation that is appropriate for the patient’s size and clinical condition. 

“In our experience if a patient does not equilibrate within the first 15 minutes of gas collection time, he or she is unlikely to do so.”

The abbreviated Weir formula same as project definition. The difference between REE derived via the abbreviated formula vs. complete equation is less than 2%.

Weir JB, 1949; Wilmore, DW 1977 (book citation).

Measurements of the whole body RQ reflect net substrate oxidation at the time of measurement (e.g., CHO oxidation net RQ=1.0; “mixed substrate” oxidation RQ=0.85; fat oxidation  RQ=0.70; lipogenesis  RQ =1.00 to 1.20; and non-steady state hyperventilation RQ≥1.00.

Measured, calculated and clinical data that are noted at the time of REE include:

• Anthropometric
  • Height
  • Weight, current, usual and ideal body weight
• Biochemical: none reported
• Clinical
  • Age
  • Primary diagnosis and relevant secondary diagnosis
  • Vital signs
  • Recent operative procedures
  • General assessment of patients' daily level of activity
• Indirect Calorimetry
  • Gas collection technique used
  • FIO₂ if pt is being mechanically ventilated
  • Number of hours since most recent intermittent nutrient intake
  • Composition and rate of any continuous feedings
  • Measured RE expressed as a percentage of Harris-Benedict standard
• Dietary (p.o.): IC taken in a fasting state.

Those patients who cannot tolerate 15 minutes without enriched oxygen cannot be tested.  Conditions that prohibit the complete collection of expired air such as the presence of chest tubes with documented air leaks or deflated endotracheal cannula cuffs also preclude measurement of REE.

• Mouthpiece and noseclip system: For short-term measurements (less than 15 minutes) on patients breathing room air, mouthpiece, noseclip and non-rebreathng valve system
• Mask: Used for short-term measurement if the patient cannot use the mouthpiece system, providing that extreme care is taken to ensure a complete seal at the mask cuff throughout the measurement. Under no circumstances should the patient hold the mask in place. Masks may not be used when nasoenteric tubing is present.
• Ventilator: The FIO₂ delivered to the patient must be stable and accurately measured.  Frequently, the internal blending system of the ventilator is not sufficiently consistent for the precision required during metabolic measurements and as FIO₂ increases, the magnitude of error associated with unstable or inaccurately measured FIO₂ also increases.

Browning JA, Linberg SE et al, 1982

• Canopy system: The rate of flow through the canopy is adjusted to maintain the FCO₂ in the canopy between 0.0065 and 0.0085 and the canopy air is directed to the mixing chamber.  Standard metabolic variables are then derived from the concentration differences between inspired and expired Ox and CO₂ and the measured flow rate.

Precautions should be taken to ensure that patients do not fall asleep and do not speak during the test as the former will result in a falsely low measurement and the latter will elevate the concentration of CO₂ in the canopy and preclude equilibration.

Troubleshooting Checklist

If a measurement does not reach equilibrium or data are physiologically unreasonable, the operator should:

1. Ascertain that the patient is rested and comfortable but not sleeping
2. Check for leaks at any point in the external sampling system between the patient and the collection chamber
3. Make sure that sources of supplemental oxygen have been turned off during room air measurements
4. Check for interruptions in the internal sampling pathway
5. Check gas calibration
6. Re-evaluate FIO₂ stability and sampling accuracy during ventilator measurements. 

 

As stated by the author in body of report:

“There exists significant individual variability in measured REE (percent standard) in a variety of malnourished or nutritionally at-risk hospitalized patient populations. When measured according to a strict protocol, REE eliminates the guesswork in estimating the combined effects of body composition, nutrition status, disease processes, trauma and clinical events on metabolic rate in patients requiring specialized nutrition/metabolic assessment, monitoring and support.

University/Hospital:

Hospital of University of Philadelphia

• The strength of this review is that the authors define the process that has been followed in the clinic setting over the past 20+ years. Machine calibration with span gas mixtures at various inspired temperatures (i.e., 21°C room air; 22 ° to 40° C ventilator and 20° canopy) are provided. Guidelines do not address healthy populations, although many techniques have possible extrapolation.
• With regards to nutrition parameters, use of “ideal body weight” is an outdated mode, especially considering an obese population. Use of HB, while historically a nutrition assessment standard, needs re-evaluation.