Indirect calorimetry how does it work

TEF refers to the energy cost in relation to food consumption, i. Key determinants of the energy cost of the components are illustrated in Figure3. BMR can be derived accurately using indirect calorimetry by measuring VO 2 and VCO 2 under standard conditions being awake in the supine position, ten to 12 hours after a meal, following eight hours of physical rest and no strenuous exercise in the preceding day, and being in a state of mental relaxation and an ambient environmental temperature that does not evoke shivering or sweating , on the assumption that the volume of oxygen consumed is used to catabolise macronutrients and all the carbon dioxide produced is recovered.

Most indirect calorimetry systems use the abbreviated Weir equation to calculate EE, expressed as:. Furthermore, these equations are developed based on energy expenditure of healthy individuals. Indirect calorimetry is a reliable and accurate tool for studies of energy expenditure. Quantifying energy expenditure has multiple applications and is used most often to determine metabolic rate, assess physical fitness and nutritional needs and the efficacy of treatment or prevention programs.

Technologically advanced metabolic carts such as the Omnical, provide a user-friendly, portable and affordable option to support research and clinical applications. Contact us to find out more about indirect calorimetry. Save my name, email, and website in this browser for the next time I comment. Understanding Indirect Calorimetry 4 October In Energy expenditure , Geen categorie. By Pamela Leow.

New methods for calculating metabolic rate with special reference to protein metabolism. J Physiol. Matarese LE. Indirect calorimetry: technical aspects. The measurement of energy expenditure.

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Impact of withholding early parenteral nutrition completing enteral nutrition in pediatric critically ill patients PEPaNIC trial : study protocol for a randomized controlled trial. Leave a Reply Cancel reply. We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits.

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You also have the option to opt-out of these cookies. But opting out of some of these cookies may have an effect on your browsing experience. The products of this reaction are carbon dioxide and water, with heat as a byproduct. Extracting chemical energy from cellular fuels is accomplished by completely oxidizing the substrate to carbon dioxide and water [ 9 ].

The ratio of CO 2 production to oxygen consumption is referred to as the respiratory quotient RQ [ 9 ]. Under steady-state conditions, the blood and gas transport systems are keeping pace with tissue metabolism, thus RER can be used as index of metabolic events and assumed to be equivalent to RQ. The test was carried out when subjects are resting. During rest, the system is noted to be under steady state.

Measurement of protein metabolism was thought to be to clinically challenging through indirect calorimetry. In early s, assumptions were made that a fixed percentage of total calories arise from the metabolism of protein, and the contribution could be ignored without effecting the estimations of energy expenditure [ 10 ].

However, ignoring the contribution of protein metabolism does add a degree of error to the calculations. Per Turell and Alexander, a systemic error of 1. Varying the degree of protein intake does change the basal metabolic rate [ 12 ]. In , Weir demonstrated calculations that allowed for protein metabolism to be included.

The nitrogen backbone present in all proteins is metabolized through various pathways in the human body. Hence, measuring urea excretion allows for calculation of dietary protein intake. Weir demonstrated that quantifying urinary nitrogen over a hour period could then be modeled in an equation to measure contribution of protein metabolism. Hence, the equation can be written as below:.

Normal RQ values range between 0. Specific device and corresponding instruments are used for indirect calorimetry. The several technologies that are available require precise calibration and measurements of volume and gas analysis.

The traditional device used for decades was the Deltatrac and subsequently the Deltatrac II [ 16 ]. It is the set gold standard for indirect calorimetry. Since then, other products have become available. It is a European company based out of Italy. It is also able to distinguish the different substrates being utilized.

It can measure these values on spontaneously and mechanically ventilated patients. This device also measures REE in the usual manner. These new products have been compared to the Deltatrac II, as the acknowledged gold standard. Validation trials have been performed to certify the reproducibility of these calorimeters [ 17 — 19 ].

Other trials have not shown similar results. The variance between the Deltatrac II and these other calorimeters has been higher than clinically acceptable [ 16 ]. The disagreement between these new calorimeters requires for further refinements for them to be used in clinical practice. These machines need to demonstrate fidelity to the set gold standard in multiple clinical settings. These settings include spontaneously breathing as well as mechanically ventilated patients.

The mechanically ventilated patients are the critically sick ones [ 16 , 20 ]. It is manufactured by the Deltatrac parent company [ 21 ].

MedGem is a metabolic meter that measures VO 2 alone. It makes an assumption on RQ and is for use in spontaneously ventilated patients [ 21 ]. These calorimeters can be used in spontaneously breathing or mechanically ventilated patients. The inspired gas can be room air or a supplemental oxygen mixture.

In a spontaneously breathing patient, an overlying canopy, face tent, or facemask can be used for gas collection. A mouthpiece with a nose clip can also be used.

In mechanically ventilated patients, the calorimeter can be attached to the ventilator system. The total expired gas volume is recorded for calculations [ 21 — 25 ]. Although indirect calorimeters are easy to operate and understand, there are several variables that can impact accuracy in measurements.

Gas analyzers must be responsive and capable of measuring minimal changes in oxygen enriched and room air environments. The ability to measure minute changes in gas concentrations as small as 0. The principle of Haldane transformation assumes that nitrogen N 2 is an insoluble gas and does not participate gas exchange.

As a consequence, it is constant in both inspired and expired volumes. Assuming that oxygen O 2 and carbon dioxide CO 2 are the only gases exchanged in the lungs the inspired volume can be calculated from the expired volume [ 21 ].