How does indirect calorimetry work

Respiration is performed through a mask connected to the Omnical. Air flow and respiratory gas samples are continuously obtained and accurately analysed by gas analysers within the Omnical.

Volume of oxygen consumed VO 2 and carbon dioxide produced VCO 2 are derived from these measurements. During metabolism, basic macronutrients in food carbohydrates, fats and proteins are fuel for energy.

As such, macronutrients are catabolised for energy in the presence of inspired oxygen O 2 and converted into CO 2 , water and heat, a metabolic process known as substrate oxidation Figure 1. Likewise, catabolism of different macronutrients yield different amounts of energy relative to oxygen consumed, termed the caloric equivalent for oxygen.

Therefore by determining the VO 2 and VCO 2 , information about the type and rate of substrate oxidation within the body can be obtained. TABLE 1: Comparison of oxygen consumed, carbon dioxide released, respiratory quotient, energy generation during oxidation of 3 main macronutrients. Multiple outcome parameters can be derived from indirect calorimetry measurements depending on user needs and applications.

These include nutritional parameters e. Under resting steady state conditions, RQ is used to represent net substrate carbohydrates, fat and protein oxidation by the body based on respiratory gas exchanges. Under most circumstances, RQ is between 0. RQ values obtained from clinical patients outside this range should trigger further questioning. A larger RQ value could reflect an increase in VCO 2 caused by excess exhalation of carbon dioxide as a result of underlying compensatory mechanisms such as metabolic acidosis or hyperventilation.

On the other hand during intense exercise, RER values can be larger than 1. Energy expenditure is often used to calculate energy requirements. Energy cost of physical activity is the most variable component of TEE, which accounts for energy consumed in muscular work during spontaneous and voluntary exercise and varies depending on individual physical activity levels.

TEF refers to the energy cost in relation to food consumption, i. With the use of Haldane transformation, REE can be provided through indirect calorimetry. Steady state is described as a patient under resting conditions.

Many factors can effect achievement of these conditions Table 1. They include but are not limited to patients undergoing physical stress such as fever and pain. Patients should be resting in a quiet room without excess environmental stimuli. Changes in the environment around the patient also affect these results [ 26 ]. Best practices for performing indirect calorimetry in healthy and non critically ill patients. The Academy of Nutrition and Dietetics utilizes a grading system for determining strength of evidence of studies and reports based on five factors.

These factors are quality, consistency, quantity, clinical impact, and generalizability. There are three areas that can pose challenges during the technical performance of indirect calorimetry. These are patient interface, elevated oxygen concentrations, and variability of mechanical ventilators. One interface for spontaneously breathing at rest subjects is a large canopy that encompasses the patients head and shoulders Figure 1.

This must be adapted to the subject and be free of potential leaks to prevent the loss of exhaled air. The canopy is designed to ensure collection of exhaled air close to the subjects mouth yet spacious enough for comfort and visibility. There is also an interface of a large form fitting mask that has open slots for adequate flow circulation. Another type of mask fitting is shown here Figure 2.

Each manufacturer identifies the ideal interface that will work well with their systems. Many authors have described in detail about the effects of elevated oxygen on the VO 2 measurement. An error effect comes into play when the FiO 2 is close to 1, and the denominator of the Haldane equation 1-FiO 2 approaches zero [ 28 ]. The impact of these variables on data results is noted in Table 2 [ 28 ].

Carefusion canopy. MedGraphics interface. Performances of indirect calorimetry on subjects who are mechanically ventilated have less potential for system leaks.

This closed system provides a simplified way to collect exhaled gas, and the measurement of inspired gas can be determined easily at the inspiratory limb of a ventilator circuit. The problems that arise in indirect calorimetry during mechanical ventilation are fluctuations in FiO 2 , ventilator mode, and flow requirements.

The variability in FiO 2 occurs due to increased flow demands, bias flow to provide better patient interface, and high minute ventilation. Leaks can occur during mechanical ventilation due to unsealed airways or incompetent tracheal cuffs, through chest tubes or bronchopleural fistulas.

Other technical aspects to consider during mechanical ventilation also include: calibration errors, recent changes in ventilator settings that may not reflect steady state, moisture in the system, patient-ventilator dyssynchrony, and acute hyperventilation or hypoventilation impact physiologic CO 2. Equipment variables and methodology of measuring gas concentrations can lead to inaccurate results as well. At the point of data accumulation, the issue becomes how to decrease the variance in collected data points.

The variance can be due to differences in tidal volume, respiratory rate, and other patient factors [ 8 ]. In order to eliminate these data variations, several techniques have been developed. They include breath averaging, time averaging, and digital filtering [ 8 ]. Breath averages collect data points over a predefined number of breaths. The average of these data points is used as the final result.

Similarly, time averaging is the collection of data points over pre-designated period of time. The data accrued from all of the breaths during the pre-designated period are averaged to give one value [ 8 ]. Digital filtering removes data points that are not within a range of the median data points. In this instance, setting the range in any one direction can drastically alter the results.

If the range is noted to be too low then many data points that are valid can be excluded from the analysis and thus providing distorted data values [ 8 ]. Recommendations exist to add correction factor estimates to decrease the error in indirect calorimetry as well. Patients can be defined as hypometabolic, hypermetabolic, and normal.

Correction factors can be added for dietary thermogenesis. This is the energy required for metabolism of food. Other common correction factors are activity factors and spontaneous ventilation [ 29 ]. Recent trials are not in complete agreement with this practice.

Some clinicians advocate for the removal of correction factors. They contend that correction factors lead to overfeeding. Over feeding has known deleterious outcomes in patients and this should be avoided [ 29 ]. This issue is important when dealing with patient in respiratory distress.

Over feeding will lead to an increase in RQ and subsequently an increase in minute ventilation. Patients in respiratory distress will need to increase their minute ventilation at a time of respiratory compromise. This scenario is often seen in patients on mechanical ventilation.

These patients will be excess nutrition that leads to them having an increase in their baseline minute ventilation. When the time comes for spontaneous breathing trials to assess their readiness for spontaneous ventilation, they fail these trials as a consequence of overfeeding.

Measurement of indirect calorimetry is best performed with the patient at rest with a goal of achieving steady state. Nutrition, whether parental or enteral feeding, need not be held. It is recommended that the subject be at rest with minimal distractions or disturbances. Most studies consider 20—30 min of data collection an accurate reflection of hour energy expenditure.

Some literature also supports an abbreviated time of 5 min of steady-state data. While indirect calorimetry remains the gold standard for caloric assessment, predictive equations provide an alternate method of determining nutritional requirements. Given the expense associated with indirect calorimetry and advanced training required to perform accurate metabolic studies, as well as limited availability of equipment, predictive equations are a cost-effective strategy for broadly assessing metabolic requirements.

The Harrison-Benedict equation HBE is the most established method dating back to , from studies conducted in healthy, young volunteers to assess resting energy expenditure REE. Thus, application of these formulas in the critical care setting, particularly in the elderly, should be with caution.

While utilization of these equations remains widespread, it has been noted that they may a results in a significant error in estimating REE. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. On Twitter, he is precordialthump. This site uses Akismet to reduce spam. Learn how your comment data is processed.

OVERVIEW Indirect calorimetry is a technique that measures inspired and expired gas flows, volumes and concentrations of O2 and CO2 allows measurement of oxygen consumption and carbon dioxide production non-invasive and accurate the equipment used is also known as a metabolic cart USES determine energy requirements and response to nutrition over time calculation of energy expenditure allows determination of nutritional requirements helps to determine the proportion of substrates used for energy production measures O2 consumption in shock typically used in difficult patient groups e.

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