To review the techniques used in measurement of metabolic rate in critically ill patients using indirect calorimetry, data interpretation and clinical applications.

Not specified.

Not specified.

• Search procedures and study quality were not assessed
• Primarily critical illness populations were included and various types of indirect calorimeters discussed.

Outcome(s) and Other Measures

• Factors affecting accuracy of IC measures
• Information was not integrated; rather, it was generally summarized
• Qualitative analytic methods were used by giving clinical examples of how to determine an inaccurate IC measure.

121 articles included:

• Number of articles identified was not specified
• Of the 121 articles, 40 articles were evaluated for inclusion into various evidence analysis questions
• Sample size of studies ranged from a case study (N=1). The majority of studies reported characteristics of critical illness patients, with very limited healthy adults.

The Respiratory Quotient

• Ratio of CO₂ elimination to O₂ uptake reflects the net substrate utilization of the organism and is called respiratory quotient (RQ) when taken during rest
• Metabolism of each substrate results in different amounts of O₂ consumed and CO₂ produced:

RQ of Individual Substrates

• RQs of 0.67 to 1.25 have been observed in humans, lower values reported in ketosis with ketonuria and higher values in patients fed excess CHO and presumably converting it to fat.

(No reference given)

• In non-resting conditions, the ratio of VCO₂ to VO₂ is called respiratory exchange ratio (RER, R)
• It is a parameter used in exercise testing to mark the anaerobic threshold, the point at which VO₂ no longer rises but VCO₂does, and the RER rises toward and even above one
• During the period immediately following the start of hyperventilation, the RER rises as more CO₂ is eliminated; with hypoventilation, the RER falls as CO₂ is retained.

Metabolic Measurements during Mechanical Ventilation

• Elevated O₂ concentrations, humidification, elevated and fluctuating airway pressures, positive end-expiratory pressure (PEEP) and rapid respiratory rates can interfere with the accuracy of measurements
• Breath-by-breath methodology has the theoretical advantage of decreasing errors caused by fluctuating FiO₂, incomplete mixing of gases and presence of water vapor. Yet, O₂ analyzers often measure peak-not mean O₂ concentration, resulting in errors in VO₂.
• Airway pressure fluctuations and PEEP cause errors in gas concentration measurements; to improve accuracy newer instruments measure the sample line pressure at the analyzer and then correct the output of the analyzer
• Douglas bags collect expired gas at time intervals and contents are analyzed. Volume of gas, and concentrations of O₂ and CO₂ are used to calculate RQ. Douglas bags are subject to leaks, diffusion of gas (especially CO₂) through bags. Gas collections are made over a short time (usually two to six minutes).
• Metabolic monitor with mechanical ventilation:
  • System must be leak-free
  • FiO₂ must be stable, i.e., ventilators must deliver O₂ at stable concentrations; unstable delivery related to gas line pressures, unstable blending, intermittent mandatory ventilation, flow-by and pressure control ventilation
  • High levels of PEEP and inverse-ratio ventilation may increase absolute sample line pressures and their duration; this affects accuracy of gas concentration measurements;
  • Gas analyzers must be able to perform accurately and reproducibly in O₂-rich environments. O₂ analyzers should be accurate to the 100^th% over the clinical range of O₂ concentrations and be linear over this range (21% to 100%). Of note, the greater the O₂ concentration, the greater potential for error because the Fi0₂ to  FEO₂ difference narrows as the absolute concentration increases. The Haldane transformation loses accuracy as the O₂ concentration rises above 70%, so many instruments are unable to accurately measure at FiO₂S above 0.6 to 0.8.
  • Gas volume or flow must be accurate over a wide range because tidal volumes may vary from 50ml to 1.5L and flow rates from 1L per minute to 100L per minute. A feature of bias flow ensures that the volume measuring device is in the linear range when flow starts.
  • Water vapor content of the air must be removed
  • Signal alignment, the volume measurements and gas concentrations are aligned
  • Metabolic measurements are reported as STPD-std temperature (0°C), pressure (760mm) and dry
  • Machine calibration
• The RQ, with its narrow physiological range (0.7 to 1.25, with only a rare ketonuric patient dipping below 0.7), makes it a sensitive measure 
• Comparing a patient’s recent (three- to four-day) nutritional intake and clinical condition with the measured RQ provides a gross check of the validity of the RQ
  • Example 1: Patient receiving 100ml per hour of IV 5% dextrose for past five days (440kcal per day), should have a RQ between 0.75 and 0.82 because of semi-starved state resulting in net lipid oxidation (i.e., RQ=0.71)
    • A RQ of 1 should create suspicion that a technical problem or physiologic aberration has occurred. Possibilities are hyperventilation (i.e., increase CO₂), hidden ingestion of CHO or inaccurate gas analyzers.
  • Example 2: RQ of 0.83 in a 50kg patient receiving six days of enteral nutrition providing 1.75 times estimated RMR. Thus, net substrate oxidation should tend toward glucose oxidation (i.e., RQ=1).
    • If RQ=0.83, it is likely because of glucose intolerance. Technical causes can include malfunctioning O₂ and CO₂ analyzers, or leak of room air into expiratory gas sampling line.
• Errors in minute volume measurements can result in a correct RQ but inaccurate VO₂ and VCO₂ values
• VO₂: is about 3ml to 4ml per kg in normal adults subjects; lower values (2.5 to 3.5ml per kg can be seen in sedated, mechanically ventilated patients, starvation, pre-morbid state and hypothyroidism
• Higher VO₂ values are seen in hyperthyroidism, fever, burns, pancreatitis, head trauma, certain drugs, surgery and trauma.

As stated by the author in body of report:

• Measurements of VO₂ and VCO₂ can be used to calculate REE, which can be used to determine the caloric requirements and metabolic state of critically ill patients
• Measure the minute ventilation and the differences between the inspired and expired concentrations of oxygen and carbon dioxide
• Mechanical ventilation provides a challenging environment in which to make these measurements because of elevated oxygen concentrations, fluctuating airway pressures and humidity
• Careful attention must be paid to details to ensure accurate measurements under these conditions.

• Experienced researcher providing 121 references, human and animal, to review the techniques used in performing indirect calorimetry
• Weaknesses include publication year of 1995 so it doesn’t describe indirect calorimeters currently being sold and used (i.e., those with improved technology).