Define energy expenditure and describe how chemical energy is changed to mechanical energy for muscle contractions, thermal energy for regulation of body temperature, and electrical energy for brain and nerve activity.

None specified.

None specified.

None specified.

Outcomes and other measures

• Defining respiratory quotient (RQ) and physiological range
• Indirect calorimetry (IDC) equipment types and how they operate
• How to complete a measure
• Troubleshooting and interpreting measures.

28 references 

• Number of articles identified not specified
• There were approximately 15 primary research articles dating from 1949 (Weir JB article) to article by Makk et al in 1990. The latter had 26 in study. Sample sizes ranged from eight to 61 individuals.
• Participants in the studies reflected critically ill patients. Two studies reported studies completed using children.
• Four narrative reviews were cited: Foster G, Knox LS, Depmsey DT, Mullen JL, 1987; Feurer ID, Fullen JL, 1986; Peronnet I, Massicotte D, 1991; and Branson JD, 1990). Two animal studies were cited.

REE, BEE and TEE

Basal energy expenditure (BEE) is the minimal heat production measured from 12 hours to 18 hours after ingestion of food and at complete rest. (Boothby WM, Sandiford I. Laboratory Manual of the Technique of Basal Metabolic Rate Determination. Philadelphia, PA: WB Saunders; 1920-2011.)

Resting energy expenditure (REE) accounts for 75% to 90% of total energy expenditure (TEE); the remainder is accounted for by thermogenesis of food, shivering and physical activity. (Elwyn DH, Kinney JM, Askanazi J. Energy expenditure in surgical patients, Surg Clin N Am, 1981; Feurer ID, Mullen JL. Bedside measurement of resting energy expenditure and respiratory quotient via indirect calorimetry, Nutr Clin Pract, 1986.)

INTERPRETING THE RESPIRATORY QUOTIENT

The RQ should be in physiologic range and consistent with the patient’s history and feeding.

If RQ:

• >1, then decease the total caloric load
• =1, then decrease carbohydrate and/or increase lipid
• >0.82, then increase total energy intake.

Physiological causes (i.e., oxidation) for RQ<0.71:

• Oxidation of ethanol and ketones
• Lipolysis
• Underfeeding
• Diabetes mellitus
• Ketoacidosis
• High rates of urinary glucose excretion.

Non-substrate utilization causes for RQ<0.71: Hypoventilation and technical difficulties associated with actual measurement.

Physiological causes (i.e., oxidation) for RQ>1. 0:

• Excess CO₂ production
• Hydrogen-ion buffering by bicarbonate-generating CO₂
• Lipogenesis
• Overfeeding.

Non-substrate utilization causes for RQ>1.0:

• Hyperventilation
• Metabolic alkalosis
• Postoperative period six hours to eight hours after general anesthesia
• Adaptation to ventilator settings
• Immediately after a meal, RQ is 1.0; during starvation it is 0.83; and in diabetes mellitus it is 0.71.

[Sources for this statement: Two animal models, one book; and primary studies of: Schultz Y, Ravussin E. Respiratory quotients lower than 0.70 in ketogenic diets. Am J Clin Nutr. 1980; 33: 131C; Frayn KN. Calculation of substrate oxidation rates in vivo from gaseous exchange. J Appl Physiol. 1983; 55: 628-634; Weissman C, Kemper M, Askanazi J, Hymen AL, Kinney JM. Resting metabolic rate of the critically ill patient: measured versus predicted. J Anesthesiol. 1986; 64: 673-679; Anderson CF, Loosbrock LM, Moxness KE. Nutrient intake in critically ill patients: too many or too few calories? Mayo Clin Proc, 1986.]

CLOSED CIRCUIT SYSTEMS

The major advantage of the closed system is that inspired minute ventilation can be measured rather than calculated using the Haldane equation. In ventilated patients with high forces inspiratory oxygen (FIO₂), the Haldane equation becomes less accurate as the denominator (1-FIO₂) approaches zero. Therefore, there is no FIO₂ upper limit with a closed system. O₂ analyzers are not required.

Breathing resistance increased; and inspiratory time is prolonged and may increase work of breathing by 10%. [Branson RD, Hurst JM, Davis K, Bower R. Comparison of open-circuit and closed-circuit methods for measuring oxygen consumption. (abstract). American
Society of Parenteral and Enteral Nutrition, Miami FL, 1989; Browning JA, Linberg SE, Turney SZ, Chodoff P. The effects of fluctuating FIO₂ on metabolic measuremtns in mechanically ventilated patients. Crit Care Med. 1982 Feb;10 (2): 82-85.]

Closed-circuit systems are affected by changes in lung volume or leaks in the system. Because O₂ consumption is interpreted as a volume loss per unit time, any leak will be interpreted as a change in VO₂.

OPEN CIRCUIT SYSTEMS

These should not be used for patients receiving FIO₂ more than 60% because the Haldane transformation is used in the calculation.

Because oxygen analyzers are used, open-circuit systems are susceptible to fluctuations in FIO₂. Many systems provide a display of oxygen, CO₂ and flow to permit direct monitoring of leak detection, FIO₂ stability and system functioning. Breath-by-breath systems that measure for FIO₂ each breath minimize changes caused by fluctuating FIO₂ because of the frequent sampling.

OTHER CONSIDERATIONS

Patients with incompetent endotracheal tube cuffs, leaking chest tubes and bronchopleural fistulas should be measured because complete gas collection cannot be guaranteed.

Water vapor must be eliminated from the gas before it reaches the analyzers, which may be accomplished by heating or cooling the sample or using desiccants, water traps or special tubing.

PERFORMING INDIRECT CALORIMETRY

Test validity

The overall RQ physiologic range is 0.67 to 1.3 (Source: Branson RD. The measurement of energy expenditure: instrumentation, practical considerations, and clinical application. Respir Care. 1990; 35(7): 640-659.)

The physiologic range of VO₂ is 1.7mL per minute per kilogram to 3.4mL per minute per kilogram, and the range of VCO₂ is 1.4mL per minute per kilogram to 3.1mL per minute per kilogram. (Source: Bursztein S, Elwyn DH, Askanazi J, Kinney JM. Energy Metabolism, Indirect Calorimetry, and Nutrition. Baltimore, MD: Williams & Wilkens; 1989;64-65,219-220.)

The measurement should be clinically evaluated. For example, an RQ more than 1.0 in fasting patients or a minute ventilation that is less than five liters or more than 10 liters probably signifies an invalid test.

INTERPRETING IDC MEASUREMENTS

RQ should be in the physiologic range and consistent with the patient’s history and feeding. If RQ is more than 1.0, decrease total calorie intake and adjust carbohydrate-to-lipid ratio. If RQ is less than 0.812, increase total calorie intake.

RQ can be used as a clinical guide when weight gain is desirable.

• IDC can be a useful tool in the provision of nutrition support
• The measurement must be obtained using good technique to ensure an accurate test. The dietetics professional must understand the methodology to evaluate the validity of the test.

• Good overview on RQ definition, how it is established and interpreted
• Findings generalizable to hospitalized critically ill patients
• Weaknesses include a publication year of 1997 and most current cited literature was 1990.