Measurement Of Carbon Dioxide Production Research Articles

Correlation between Carbon Dioxide Production and Mean Arterial Blood Pressure in Fluid Response in Mechanically Ventilated Patients

AbstractBackground: Volume expansion through fluid administra-tion is one of the simplest approaches for initial treatment of hemodynamic instability. In the Intensive Care Unit (ICU) a newly available technique called volumetric capnography (Vcap) allows measurement of carbon dioxide production (VCO2) on a regular basis at the bedside.Aim of Study: The aim of the study was to evaluate VCO2 as an endpoint predictor for fluid resuscitation in mechanically ventilated patients.Methods and Material: This prospective cohort study was carried out on 70 patients in Tanta University Hospitals at surgical ICU from August 2017 till August 2018. All patients included in the study were on controlled mechanical ventilation with tidal volume 6-8ml.kg–1 ideal body weight. Wide bore intravenous lines were inserted and patients were completely sedated and monitored by; pulse oximeter, non-invasive blood pressure monitoring, ECG all connected to monitor (infinium medical-OMNI lll-FL33773-USA). Volumetric capnogram, end tidal capnogram, detected by Y-Piece and sample line connected to (GE Healthcare Finland Oy, E-sCOVX-00- Finland). Bispectral Index (BIS) to assess the depth of sedation (should be between 40-60). Intravenous fluids were given after hypotension till MAP >65mmHg.Results: There was significant increase in MAP and VCO2 and significant decrease in HR after fluid resuscitation and there was no significance difference in BIS, SpO2. There was positive correlation between VCO2 and MAP with sensitivity of VCO2 for fluid responsiveness 92%.Conclusions: VCO2 and EtCO2 are good monitoring toolswhichare well correlated with hemodynamic changes, thus can be used as an indicator for fluid responsiveness and endpoint prediction ofresuscitation.

Soil Respiration in the Profiles of Forest Soils in Inland Dunes

Forest soil profiles of two dunes within the European belt of inland dunes were analysed in the laboratory. We carried out respirometric measurements of carbon dioxide production and oxygen consumption for every horizon of the studied soils while simultaneously quantifying the organic matter and humidity. Oxygen consumption and carbon dioxide excretion decreased exponentially with depth. The oxygen consumption decrease was less rapid than the decrease in carbon dioxide production. We found a statistical significant linear dependence between oxygen consumption and carbon dioxide excretion, and organic matter content and soil water capacity. Respiration processes in the profiles were divided into two strata; oxygen respiration dominated in the first and fermentation processes in the second. We estimated total respiration in the studied profiles for an area of 1 m2 down to around 1 m depth. We concluded that when assessing the soil’s role in carbon cycling in an ecosystem, it is necessary to consider both the respiratory and fermentation strata, as both produce large quantities of carbon dioxide. The main factor determining carbon dioxide production intensity is organic matter content; thus the distribution of organic matter in the soil profile determines carbon cycling intensity.

Accuracy of Gas Exchange Monitoring During Noninvasive Ventilation

Gas exchange monitoring by indirect calorimetry (IC) during noninvasive ventilation (NIV) is desirable but currently not available. Leaks around the mask preclude reliable measurements of carbon dioxide production (VCO2) and oxygen consumption (VO2) in this population. We aimed to examine the impact of system leaks and gas flows on the accuracy of gas exchange measurements during NIV using an in vitro metabolic simulation. We examined the agreement between VCO2 and VO2 measurements by IC (using a novel canopy device) and reference values generated during an in vitro metabolic simulation of NIV at room air. The flow rate of gas sampled by the IC device (VIC) was set relative to the output flow of the ventilator (VVENT) to obtain a range of sample factors (SF = VIC/VVENT). Linear regression was used to determine the effect of SF on the accuracy of the system. An acceptable agreement between measured and reference values was observed, with mean bias (limits of agreement) of -3.3% (-6.9% to 0.3%) and -10.6% (-14.9% to -6.4%) for VCO2 and VO2, respectively. An SF of 1.25 was associated with the highest accuracy of measurement. VO2 measurement accuracy deteriorated with system leak and at SF >1.25 and was linearly related to sample dilution by ambient air entrainment. A novel canopy device with titration of IC sample flow in relation to the ventilator flow allowed in vitro gas exchange measurements during simulated NIV with acceptable accuracy. This model needs to be tested in clinical settings.

A new microbial biosensor for organic water pollution based on measurement of carbon dioxide production

As aerobic respiration proceeds it consumes oxygen and produces carbon dioxide; and the relation between these two parameters, the respiratory quotient (RQ), is related to the type of substances being respired. Therefore, for a given water or wastewater origin, a more or less stable RQ is expected, making possible the estimation of BOD5 by means of CO2 production measurement. The microbial breathing intensity was continuously measured using a potentiometric CO2 electrode as transducer. We named this new biosensor BODstCO2. Proof of concept in this study was carried out with Saccharomyces cerevisiae or a BODSEED mixed microbial community as a biological recognition component. The effect of microbial load (0.5, 5, and 25mgcm−2 membrane, dry weight) over the apparent linear range (up to 670mgL−1 BOD5, 5mgcm−2 membrane), detection limit (ca. 1mgL−1 BOD5), stabilization time, reproducibility (typically better than 10%) and bio-membrane type (membrane or PVA hydrogel entrapment) were studied. When the Nernstian biosensor response was used for calibration, up to 20,000mgL−1 glucose standard was measured without sample dilution. BOD calibrations were accomplished using the two more commonly used standard artificial wastewaters, GGA and OECD solutions. The results showed that the potentiometric CO2 electrode was an useful transducer, allowing us to build, calibrate and characterize a BOD-like biosensor. Moreover, limitations present at oxygen amperometric electrode (customarily used as BOD biosensor-based transducer) such as oxygen low solubility and its reduction at the cathode were avoided.

Flowering and changes in respiration in Asiatic hybrid lilies as influenced by bulb vernalization

Asiatic hybrid lilies, Lilium × elegans Thunb., ‘Red Carpet’ and ‘Sunray’ were used to investigate the effect of bulb vernalization at 2.5 °C on plant growth, flowering, and CO 2 production (respiration), and to use the CO 2 production pattern to monitor the time of flower bud initiation and development. Lily shoot emergence and flowering were accelerated when bulbs received 2.5 °C bulb vernalization; however, flowering was delayed when bulbs were stored at 20 °C before treatment at 2.5 °C; this indicated that bulbs were de-vernalized. The maximum CO 2 level, and the minimum level, reached in 78 h in non-vernalized bulbs and in 110 h in 6 weeks of 2.5 °C (6 weeks/2.5 °C) treated bulbs, was increased as the 2.5 °C duration was increased; this indicated that CO 2 level can be an useful parameter to measure the cold stimulus (i) accumulated in bulbs following bulb vernalization. The respiration rate higher than the predicted values of the best-fit curves derived from the quadratic equations was designated as Blip A and this was correlated to the time of flower bud initiation and development. Shoot elongation may follow the rise in carbon dioxide levels after reaching the minimum level. It is proposed that increased carbon dioxide levels higher than the predicted levels (Blip A), was correlated to the time of flower bud initiation and development. Measurement of carbon dioxide production upon receipt of bulbs may be a useful technique to provide important information for optimum vernalization treatments for bulbs that have accumulated different levels of low temperature stimulus after bulb vernalization.

The Effects of Endotracheal Suctioning on the Accuracy of Oxygen Consumption and Carbon Dioxide Production Measurements and Pulmonary Mechanics Calculated by a Compact Metabolic Monitor

Open endotracheal suctioning (ETS), which is performed regularly in mechanically ventilated patients to remove obstructive secretions, can cause an immediate decrease in dynamic compliance and expired tidal volume and result in inadequate or inaccurate sidestream respiratory monitoring, necessitating prolonged periods of stabilization of connected metabolic monitors. We investigated the immediate effect of open ETS on the accuracy of oxygen consumption (VO2) and carbon dioxide production (VCO2) measurements and calculated lung mechanics, respiratory quotient, and resting energy expenditure in mechanically ventilated children without severe lung pathology, when using a compact modular metabolic monitor (E-COVX) continuously recording patient spirometry and gas exchange measurements. Open ETS was performed when clinically indicated in 11 children mechanically ventilated for sepsis or head injury. A total of 2800 pulmonary 1-min gas exchange measurements were recorded in 28 ETS instances for 50 consecutive minutes before and 50 min after the standardized procedure. Pulmonary mechanics and indirect calorimetry did not differ between pre- and postsuction sets of measurements. Pre- and postsuction VO2, VCO2, dynamic airway resistance, dynamic compliance, and expiratory minute ventilation remained stable from 5 to 55 min after tracheal suctioning and did not differ among different ventilatory modes. Average paired differences of sequential pre- and postsuction VO2, VCO2, respiratory quotient, and resting energy expenditure were -0.6%, -1%, -0.1%, and -0.3%. Ratio differences between the first and the second periods of measurements (1-25 vs 26-50 sets of 1-min measurements) did not differ in the two groups. Pulmonary mechanics and indirect calorimetry measurements are not influenced after uneventful open ETS in well-sedated patients. The E-COVX is able to reliably record spirometry and metabolic indices as early as 5 min after suctioning at different ventilator modes.

Development of a system for the on‐line measurement of carbon dioxide production in microbioreactors: Application to aerobic batch cultivations of Candida utilis

We developed and applied a conductometric method for the quantitative online measurement of the carbon dioxide (CO(2)) production during batch cultivations of Candida utilis on a 100-microL scale. The applied method for the CO(2) measurement consisted of absorption of the produced CO(2) from the exhaust gas of the microbioreactor in an alkali solution, of which the conductivity was measured on-line. The measured conductivity change of the alkali solution showed a linear relation with the total amount of CO(2) absorbed. After calibration of the CO(2) measurement system, it was connected to a well of a 96-well microtiter plate. The mixing in the well was achieved by a magnetic stirrer. Using online measurement of the CO(2) production during the cultivation, we show reproducible exponential batch growth of C. utilis on a 100-microL scale. The CO(2) production measurements obtained from the microcultivation were compared with the CO(2) production measurement in a 4-L bioreactor equipped with a conventional off-gas analyzer. The measurements showed that on-line measurement of the CO(2) production rate in microbioreactors can provide essential data for quantitative physiological studies and provide better understanding of microscale cultivations.

Rates of metabolism in diapausing and reproductively active tropical butterflies, Euploea core and Euploea sylvester (Lepidoptera: Nymphalidae)

Abstract. Although the ecology of diapause has been widely studied in the field, the underlying physiological responses occurring in tropical diapausing insects remain virtually unexplored. This is especially the case with rates of respiration in diapausing tropical insect species. The present study compares rates of metabolism, as assessed by measurement of carbon dioxide production, between two species of diapausing and reproductively active tropical butterflies, Euploea core (W.S Macleay) and Euploea sylvester (Fabricius), independent of temperature. Measurement of metabolism over a day‐time/night‐time regime confirms that these tropical butterflies display a diurnal rhythm in accordance with many other tropical and temperate insect species, regardless of developmental state. In addition, diapausing Euploea butterflies display rates of carbon dioxide production only 28% lower than those of reproductively active butterflies, and can terminate diapause within days of receiving the correct cues. Maintaining a similar metabolic rate throughout diapause, as well as a rapid termination of diapause, would enable these tropical butterflies to respond immediately to larval host plant resources, without the disadvantage of missing optimum conditions, allowing the species to maximize their reproductive potential.

Measurement of carbon dioxide production in very low birth weight babies

BACKGROUNDCO2production is most commonly measured by using indirect calorimetry to quantify elimination of CO2 in breath (Vco2). An alternative is to measure the rate at which CO2 appears in the...

Comparison of experimental methods for determination of the volumetric mass transfer coefficient in fermentation processes

Mass transfer in bioreactors has been examined. In the present work, dynamic methods are used for the determination of K L a values for water, model media and a fermentation broth (Candida utilis) in an airlift reactor. The conventional dynamic method is applied at the end of the microbial process in order to avoid an alteration in the metabolism of the microorganisms. New dynamic methods are used to determine K L a in an airlift reactor during the microbial growth of Candida utilis on glucose. One of the methods is based on the continuous measurement of carbon dioxide production while the other method is based on the relationship between the oxygen transfer and biomass growth rates. These methods of determining K L a does not interfere with the microorganisms action. A theoretical mass transfer model has been used for K L a estimation for the systems described above. Some differences between calculated and measured values are found for fermentation processes due to the model is developed for two-phase air-water systems. Nevertheless, the average deviation between the predicted values and those obtained from the relationship between oxygen transfer and biomass production rates are lower than 25% in any case.

Kinetics, Stereospecificity, and Expression of the Malolactic Enzyme

Mass spectrometric measurement of carbon dioxide production was used to study malolactic fermentation (MLF) in Lactobacillus collinoides isolated from cider. The kinetics and stereospecificity of the malolactic enzyme (MLE) were studied, and the stoichiometry of the reaction sequence was investigated. The optimum pH for activity of the MLE was 4.9. MLF was more rapid (in both intact cells and cell extracts) when L-malic acid was used than when D-malic acid or the racemic mixture was added. The enzyme was found to be constitutively present in L. collinoides. Addition of L-malic acid (37 mM) to the growth medium resulted in increased MLE activity; addition of the D isomer alone or the racemic mixture resulted in lower activities. Addition of the main sugars in apple juice (fructose, sucrose, and glucose) to the growth medium in the presence of malic acid repressed production of MLE to similar extents in all three cases; in the absence of malic acid, instead of inhibiting MLF, addition of sugars to the growth medium somewhat increased the residual MLE activity.

Embryonic Energetics in Eggs of Two Species of Australian Skink, Morethia boulengeri and Morethia adelaidensis

-We used eggs of two species of Australian skinks, Morethia boulengeri and M. adelaidensis, to: (1) estimate the amount of energy consumed during embryonic development, and (2) to assess the assumption that lipid provides most of the energy to fuel development. We estimated energy consumption in two ways, using standard closed-system respirometry and by estimating energy loss during development by comparing energy contained in fresh eggs and in hatchlings. Eggs of M. boulengeri are smaller (173 mg ? 4) than those of M. adelaidensis (217 mg ? 8), as are the hatchlings (36.4 mg ? 1.4, and 46.2 mg ? 1.7 dry mass, respectively). Incubation period is virtually identical (M. boulengeri: mean = 57.6, range 54-63; M. adelaidensis: mean = 57.7, range 55-60; P = 0.942). The estimate of energy consumption during development was similar using both methods for M. adelaidensis (46.8 0.8 ml 02 or 898 + 15 J from respirometry and 807 + 57 J from bomb calorimetry) and M. boulengeri (33.3 ? 0.8 ml or 640 16 J and 576 ? 36 J). Commensurate with their larger size, embryos of M. adelaidensis consumed more energy during development than M. boulengeri, but there was no significant difference in the dry mass-specific metabolic cost for M. boulengeri (18.2 kJ g-1 ?0.7 kJ) and M. adelaidensis (19.9 ? 0.8 kJ g-1, P = 0.1123). The respiratory exchange ratio was 0.76 ? 0.01 in both species, indicating that approximately equal quantities of energy are derived from protein and lipid during development At the time of oviposition, lecithal eggs of amniotes contain all the nutrient required to synthesize and fuel development of the embryo and thus provide ideal models for the study of energetics of embryonic development (Vleck and Hoyt, 1991). Data for oviparous crocodilians, turtles, squamates, and birds suggest differences in the ways that yolk is utilized during development among taxa (Thompson and Stewart, 1997), but scant data for some taxa prevent detailed comparisons that would enable interpretation of the basis for the differences. Among the most poorly characterized taxa are the lizards (Thompson and Russell, 1998), despite the large diversity of lizard species. The usual method of estimating the total en291 This content downloaded from 157.55.39.136 on Thu, 19 May 2016 06:21:49 UTC All use subject to http://about.jstor.org/terms M. B. THOMPSON AND K. J. RUSSELL ergetic cost of development is to make repeated measures of rates of oxygen consumption (Vo2) throughout incubation, calculate the total amount of oxygen consumed (V02(TOT)) during development, and convert it to an energy equivalent by making assumptions about the metabolic substrates (Vleck and Hoyt, 1991). Usually lipid, which has a respiratory quotient (RQ) of 0.71, is assumed to provide most of the energy to fuel embryonic development (Rahn et al., 1974). However, that assumption is tested rarely, even though the additional measurement of carbon dioxide production (Vco) provides an easy test. Additionally, the assumption that Vo,(TOT) is a robust measure of energy consumption during development can be tested by direct measurement of the energy content of fresh eggs and hatchlings. The difference between the energy content of fresh eggs and hatchlings should equal metabolic losses during development and be the same as the total cost of development estimated using respirometry. We selected two species of Australian skinks, Morethia boulengeri and M. adelaidensis, as models for description of energy consumption during embryonic development with the aim of quantifying differences in energetics of development within lizards. In addition to measuring V0,, we measured Vco2 to test the assumption that lipid is the predominant source of energy, and we made direct measures of the energy content of components of fresh eggs and hatchlings as an independent check on the results of our respirometry. MATERIALS AND METHODS Female M. boulengeri and M. adelaidensis were collected within 100 km of Adelaide in South Australia in November, 1993. All lizards were housed individually in plastic boxes 130 mm x 220 mm x 75 mm high in a room at the University of Adelaide. Litter was provided in each box (5-10 mm organic potting mix) and one end of the box was heated to approximately 34 C to provide a thermal gradient for 12 h per day. The room was maintained at 20-25 C with a natural day length. Dried leaves and pieces of bark were provide as shelter for the lizards. Drinking water and mealworms Tenebrio molitor were provided ad libitum. Every day, the whole box was sprayed with a fine mist of water until the potting mix was moist to prevent desiccation of

The measurement of carbon dioxide production in VLBW babies • 1534

The measurement of carbon dioxide production in VLBW babies • 1534

A method for estimating respiratory muscle efficiency using an automated metabolic cart

We measured respiratory muscle efficiency (RME) in twelve healthy human subjects by dividing the added energy cost of breathing against a threshold resistance load by the associated increase in caloric expenditure. Caloric expenditure was calculated using steady-state measurements of oxygen consumption and carbon dioxide production during loaded and unloaded breathing. Calculated RME ranged from 1.7% to 5.5% (mean 3.5%). The coefficient of variation in six subjects averaged 13%. We compared these calculations with a previously described oxygen consumption-based method that did not incorporate carbon dioxide production measurements. We found that changes in the respiratory quotient during resistive breathing could cause significant errors in oxygen consumption-based calculations of RME. Limits of agreement of 95% suggest that the oxygen-consumption-based calculations could potentially overestimate efficiency by as much as 5.0% or underestimate by up to 3.4%. We recommend that carbon dioxide production be measured when this technique for estimation of RME is used. This can be easily accomplished through the use of an automated metabolic cart.

Climacteric Fruit Produce a Diminished Respiratory Climacteric When Ripened on the Plant

Respiration (i.e., carbon dioxide production and oxygen consumption) increases as ripening is initiated in a group of harvested fruit called climacteric. This group includes many horticulturally important fruit crops, such as apples, avocados, bananas, melons, peaches, pears, and tomatoes. Other fruit, which includes cherries, citrus, and strawberries, do not exhibit an increase in respiration as they ripen and are called nonclimacteric. Measurements of carbon dioxide production by ripening apples, melons, and tomatoes revealed a well-defined climacteric, but only in harvested fruit. The respiratory climacteric was greatly diminished or absent from these fruit when they ripened while attached to the plant. Fixation of respired carbon dioxide through photosynthesis or into organic acids was insufficient to account for the diminished amount of carbon dioxide evolved from ripening attached climacteric fruit. Unlike the respiratory climacteric, an increase in ethylene production occurred in both attached and harvested climacteric fruit. Ethylene stimulates respiration in most plant tissues. The rapid rise in respiration as soon as attached ripening climacteric fruit were harvested or abscised suggests that an inhibitor of ethylene-stimulated respiration may be translocated from the plant and prevent the climacteric rise in respiration in attached ripening fruit.

Energy expenditure, body‐weight and foraging performance of Storm Petrels Hydrobates pelagicus breeding in artificial nesting chambers

Many avian species, such as Storm Petrels Hydrobates pelagicus, are intolerant of disturbance at the nest, which complicates the collection of data relating to metabolic rate and the use of body reserves during incubation. I describe the design of an artificial nest chamber, which is simple and inexpensive to construct and facilitates the collection of such data. Eighty‐one nest chambers situated in a large colony of breeding Storm Petrels had high occupancy rates (29/81 in each of 2 years), and the breeding success of birds nesting in boxes was similar to that of pairs nesting in natural crevices. Direct measurement of carbon dioxide production using standard respirometry techniques and estimations of metabolic rate based on the rates of mass loss during incubation indicated close agreement between the two methods of estimating energy consumption. Assuming the metabolic requirements during incubation are furnished entirely from stomach oil, 76% of the daily mass lost represented stomach oil catabolism. The duration of incubation shifts was unrelated to the body mass, and presumably to body reserves, of Storm Petrels on arrival at the nest. Shifts were usually terminated by the return of the foraging partner. The body mass of birds returning from foraging was relatively constant and was unrelated to the amount of time spent foraging at sea, indicating that the decision rule to return from foraging was the acquisition of a threshold level of body mass (about 31 g). There was a negative relationship between the duration of foraging trips and the body mass of Storm Petrels at departure from the nest and a positive relationship between trip duration and the net mass gain at sea. The use of nestboxes based on the design described here would have a wide variety of applications in facilitating data collection for many cavity‐or burrow‐nesting species which are sensitive to disturbance.

Variation in the respiratory quotient of birds and implications for indirect calorimetry using measurements of carbon dioxide production

Determination of animal power consumption by indirect calorimetry relies upon accurate estimation of the thermal equivalent of oxygen consumed or carbon dioxide produced. This estimate is typically based upon measurement or assumption of the respiratory quotient (RQ), the ratio of CO2 produced to O2 consumed. This ratio is used to indicate the mixture of lipids, carbohydrates and proteins in the metabolic substrate. In this analysis, we report the RQ for two bird species, Passer domesticus and Auriparus flaviceps, under several dietary and fasting regimes. RQ commonly differed substantially from those typically assumed in studies of energy metabolism and often included values below those explainable by current knowledge. Errors that could result from these unexpected RQ values can be large and could present the primary limit to the accuracy of power consumption estimates based upon measurement of carbon dioxide production.

Effect of fasting and feeding on measurement of carbon dioxide production using doubly labeled water.

The doubly labeled water method is a noninvasive technique for measurement of rates of CO2 production and total energy expenditure in free-living human subjects. The experimental protocol used in validation studies usually involves prolonged fasting before and after the isotope dose is given to start the study, although it is not clear whether this intrusive aspect of the method is necessary. We investigated this issue in four healthy adults [3 women and 1 man: age 29 +/- 2 (SD) yr, body mass index 22 +/- 2.7 kg.m2] with monitored constant physical activity who underwent two doubly labeled water studies that differed only in the duration of fasting before and after isotope dosing (either 6 h of fasting before and 5 h after dosing or 0.5 h before and 3 h afterward). No significant difference between the two measurements was found in the rate constants for isotope disappearance, the ratio of the dilution spaces of the isotopes, or CO2 production rate. These results indicate that prolonged fasting before and after isotope administration should not be necessary in doubly labeled water studies.

An indirect calorimetry system for ventilator dependent very low birthweight infants.

With neurodevelopmental outcome of very low birthweight (VLBW) infants being adversely affected by inadequate nutrition during the first few weeks of life, there is an urgent need for more specific nutritional data on the sick VLBW ventilator dependent infant. The development of a new mass spectrometry gas analysis indirect calorimetry system which is non-invasive and can operate over several hours or days is described. Technical evaluation of each of the components of the system indicates a total random error of less than 5%. Systematic error was determined using gas infusions which simulated carbon dioxide production and oxygen consumption. The relative error in the measurement of carbon dioxide production was less than or equal to 1.5% (coefficient of variation (CV) 6.0%)) with carbon dioxide infusion rates ranging from 3.86 to 13.98 ml/min. The relative error in oxygen 'consumption' was less than or equal to 4.3% (CV 2.8%) for infusions of oxygen at rates of 7.5 to 14.80 ml/min. With nitrogen infusions simulating oxygen consumptions of 2.0 and 5.5 ml/min the relative error in the calculated nitrogen infusion was 1.5% (CV 4.1%) and 1.4% (CV 5.7%) respectively. Clinical studies on 10 infants demonstrated a mean energy expenditure of 161.7 kJ/kg/day and a respiratory quotient in excess of 1.0. The energy expenditure of ventilated VLBW infants may be less than previously indicated and the energy mix and nitrogen content of parenteral nutrition regimens recommended for these infants may be inappropriate.

Incorporating Tracer‐Tracee Differences into Models to Improve Accuracy

The ideal tracer for metabolic studies is one that behaves exactly like the tracee. Compounds labeled with isotopes come the closest to this ideal because they are chemically identical to the tracee except for the substitution of a stable or radioisotope at one or more positions. Even this substitution, however, can introduce a difference in metabolism that may be quantitatively important with regard to the development of the mathematical model used to interpret the kinetic data. The doubly labeled water method for the measurement of carbon dioxide production and hence energy expenditure in free-living subjects is a good example of how differences between the metabolism of the tracers and the tracee can influence the accuracy of the carbon dioxide production rate determined from the kinetic data.