- Click here for explanation of classification scheme.

• Identify the factors that affect the measurement of resting energy expenditure (REE) and respiratory quotient (RQ) from indirect calorimetry
• Identify the potential sources of error that may occur when performing indirect calorimetry
• Describe the conditions necessary to obtain optimal results when performing metabolic tests
• Describe the clinical effect that indirect calorimetry has had in the nutritional management of critically ill patients.

 

 None specified.

 

None specified.

No search procedures and sorting criteria mentioned.

• Why type of information was abstracted from articles?  Not specified.
• How was it combined?  Not specified.
• What analytic methods were used, if any?  Not analyzed.

83 included in publication.

• The number of articles identified in searching process not stated.
• Five of 83 (six percent) citations by primary author
• Nine narrative reviews; six textbook citations; three abstracts
• Sample size of studies ranged from n=three healthy adults (Damask MC, Askanazi J et al, 1983, to n=164 mechanically ventilated patients on ventilator support (Swinamer DL, Grace MG, Hamilton SM, 1990).
• Characteristics of the study participants included critically ill, clinically stable, organ failure, spinal cord injury, patients with burns, inflammatory bowel disease and obese patients.

 

Terminology Defined 

• Indirect calorimetry=Weir equation (Weir JB, 1949)
• Energy expenditure=(3.94XVO₂)+(1.11XVCO₂) such that 80% O₂ consumption accounts for energy expenditure and 20% due to CO₂ production (Weir JB, 1949)
• Steady state or metabolic equilibrium=A five-minute interval during which VO₂ and VCO₂ change by less than 10%, or the coefficient of variation for these two values is less than five percent (Feurer ID, Crosby LO, Mullen JL, 1984; Feurer ID, Mullen JL, 1986)
• Respiratory quotient (RQ)=Ratio of VCO₂/VO₂; An associated physiologic range is from 0.67 to 1.3; Hence, values outside this range are generated though error and beneficially used to validate IC measurement accuracy (Branson RD, 1990).

Indirect Calorimetry Assumptions (Owen, 1988; Elwyn DH, Kinney JM, Askanazi J, 1981)

• IC correlates best to REE (10%+basal energy expenditure)

• IC is zero percent to three percent less than total energy expenditure (TEE)

• REE=70% to 90% TEE; remainder related to diet-induced thermogenesis, physical activity

• Variation in REE from day to day.

What are the main results of the review?

REE

• REE accounts for 75% to 90% of the total energy expenditure, the remainder of which is accounted for by thermogenesis resulting from nutrient intake (diet-induced thermogenesis), environment (shivering/non-shivering thermogenesis), and physical activity (Foster, 1987 in 100 critically ill patients; Feurer and Mullen, 1986 narrative review; Elwyn DH et al, 1981)

• Exposure to a cool environment and hypothermia cause increase in energy expenditure through shivering and non-shivering thermogenesis

• Shivering thermogenesis is the immediate response to cold exposure and involves increased muscle work mediated by hypothalamic control. Although the heat produced is close to the body surface, is associated with a rapid rate of heat loss, and thus, is very inefficient.

(Elwyn DH, KinneyJM, Askanazi J, 1981; Feuer and Mullen, 1986 narrative review)

• Postoperative patients, hypothermic from long surgical procedures in cool operating room suites demonstrated changes in non-shivering and shivering thermogenesis. In shivering thermogenesis, oxygen consumption increased 90%; in non-shivering thermogenesis, REE increase was avoided.

(Rodriguez, 1983)

Respiratory Quotient
When RQ is used for nutritional assessment, two assumptions are made:

• The patient is in true steady-s tate condition
• All VCO₂ measured reflects substrate utilization.

Individual Substrate Oxidation (RQ)

• Glucose: 1.0

• Fat: 0.7

• Protein: 0.8

• By excluding protein, the non-protein RQ (npRQ) provides a range of substrate utilization from 0.70 (indicating 100% fat utilization and 0% carbohydrate utilization) to 1.0 (indicating 100% carbohydrate oxidation and 0% fat oxidation). A npRQ of 0.85 (i.e., mid-point) indicates 50% fat and 50% carbohydrate oxidation. (Source: Anderson CF, Loosbrock LM, Moxness KE. Nutrient intake in critically ill patients: too many or too few calories? Mayo Clin Proc. 1986; 61: 853-858).

• Alcohol or XX (unable to read word) metabolism may reduce npRQ below range to 0.67 and overfeeding with lipogenesis may increase the npRQ to 1.3. (Sources: Feurer ID, Mullen JL. Bedside measurement of resting energy expenditure and respiratory quotient via indirect calorimetry. Weissman C, Kemper M et al. Resting metabolic rate of the critically ill patients: measured versus predicted. Nutr Clin Pract. 1986).

• All potential sources of error should be scrutinized. A 10% error in VO2 and a 10% error in VCO₂ cause seven percent and three percent errors, respectively, in the calculation of energy expenditure (Source: Burzstein S, Elwyn DH, ASkanazi J. Energy metabolism and indirect calorimetry in critically ill and injured patients. Acute Care. 1988-1989: 14-15: 91-110).

Clinical Application

• npRQ may be predicted on the basis of percentage exogenous fat and carbohydrate being infused.

• A study showed measured npRQ approximated predicted RQ (pRQ) (mRQ=pRQ±0.05) in 77% of patients tested. In 13% of cases, there was significant deviation of mRQ from pRQ. (Source: Makk LJK, McClave SA, Creech PW et al. Clinical application of the metabolic cart to the delivery of total parenteral nutrition. Crit Care Med. 1990;18:1320-1327; n=26 patients).

Causes for Discrepancies

• Hyperventilation through increase in CO₂ from the work of breathing and artifactual increased in VO₂ from release of tissue stores (Source: Kinney JM, 1987 narrative review)

• Overfeeding leading to lipogenesis; exceptions occur in some patients were the npRQ may plateau at 0.90 to 0.93 despite increasing levels of carbohydrate infusion more than energy requirements. This is thought to be related to hormonal counter-regulatory mechanisms and defects in fat and carbohydrate metabolism (Source: Wolfe BM, Ruderman RL, Pollard A. Basic principles for surgical nutrition: Metabolic response to starvation, trauma, and sepsis. In Dietal VM, ed. Nutrition in clinical surgery. Baltimore, MD: Williams & Wilkins, 1985: 14-23.)

• Hypoventilation or underfeeding

• Diabetes mellitus

• Any time gluconeogenesis is present

• Sepsis after injury or surgery where glucose production and gluconeogenesis continue despite hyperglycemi and glucose infusion

• Hypothermia

• Errors in calibration or leaks in the system.

[All other un-related topics to this particular evidence analysis question in the narrative review were not completed]

• “Indirect calorimetry has proved to be a valuable research tool over the past two decades and has shaped a number of the clinical physiologic principles involved in nutrient substrate assimilation and the stress-induced metabolic response.
• Unfortunately the impact of indirect calorimetry in patient care is limited by the fluctuations in the clinical condition of critically ill patients and the difficulties in the delivery of prescribed nutritional regimens. The potential for improving the usage of indirect calorimetry at a particular institution lies in developing strict protocols for performing metabolic testing, paying meticulous attention to accuracy and avoiding error, and forming a multidisciplinary team to interpret and implement test results.”
  

University/Hospital:

University of Louisville School of Medicine, Veterans Affairs Medical Center

Strengths: Comprehensive review of literature citations

 

Weaknesses: Most recent study cited was in 1991; therefore, missing 12 years of more recent literature.