Indirect calorimetry

Last updated April 24, 2024

Indirect calorimetry calculates heat that living organisms produce by measuring either their production of carbon dioxide and nitrogen waste (frequently ammonia in aquatic organisms, or urea in terrestrial ones), or from their consumption of oxygen. Indirect calorimetry estimates the type and rate of substrate utilization and energy metabolism in vivo starting from gas exchange measurements (oxygen consumption and carbon dioxide production during rest and steady-state exercise). This technique provides unique information, is noninvasive, and can be advantageously combined with other experimental methods to investigate numerous aspects of nutrient assimilation, thermogenesis, the energetics of physical exercise, and the pathogenesis of metabolic diseases.^[1]

Scientific background

Indirect calorimetry measures O₂ and nitrogen consumption and CO₂ production. On the assumption that all the oxygen is used to oxidize degradable fuels and all the CO₂ thereby evolved is recovered, it is possible to estimate the total amount of energy produced from the chemical energy of nutrients and converted into the chemical energy of ATP, with some loss of energy during the oxidation process.^[1] Respiratory indirect calorimetry (IC) is a noninvasive and highly accurate method of metabolic rate, which has an error of less than 1%.^[2] It has high reproducibility and has been considered a gold standard method.^[3] This method allows estimating BEE and REE as well as identification of energy substrates that are predominantly metabolized by the body at a specific moment. It is based on the indirect measurement of the heat produced by oxidation of macronutrients, which is estimated by monitoring O₂ consumption and CO₂ production for a certain period of time.^[4] The calorimeter has a gas collector that adapts to the subject and through a unidirectional valve minute by minute collects and quantifies the volume and concentration of O₂ inspired and CO₂ expired by the subject. After a volume is met, Resting Energy Expenditure is calculated by the Weir formula and results are displayed in software attached to the system.^[4] Another formula used is:^[5]

M=VO_{2}\left({\frac {RQ-0.7}{0.3}}e_{c}+{\frac {1-RQ}{0.3}}e_{f}\right)

where RQ is the respiratory quotient (ratio of volume CO₂ produced to volume of O₂ consumed), $e_{c}$ is 21.13 kilojoules (5.05 kcal), the heat released per litre of oxygen by the oxidation of carbohydrate, and $e_{f}$ is 19.62 kilojoules (4.69 kcal), the value for fat. This gives the same result as the Weir formula at RQ = 1 (burning only carbohydrates), and almost the same value at RQ = 0.7 (burning only fat).

History

Antoine Lavoisier noted in 1780 that heat production, in some cases, can be predicted from oxygen consumption^{[ citation needed ]}, using multiple regression. Indirect calorimetry, as we know it, was developed around 1900 as an application of thermodynamics to animal life.^[6] Although the development of indirect calorimetry dates back over 200 years, its greatest use has been in the last two decades with the development of total parenteral nutrition, interdisciplinary nutrition support teams, and the production of portable, reliable, relatively inexpensive calorimeters.^[7]

Collection methods

Four different gas collection and measurement techniques can be used to perform this test:

Douglas Bag: Expired respiratory gases are collected on an inflatable airtight bag.^[8] After completion of any test using Douglas Bags, gas collected must be analysed for volume and composition.
Canopy (dilution): The dilution technique is considered the gold standard technology for Resting Energy Expenditure measurement in clinical nutrition.^[3] The test lasts just few minutes and consists of making a patient lie down relaxed on a bed or on a comfortable couch, with the head under a transparent hood connected to a pump, which applies an adjustable ventilation through it. Exhaled gas dilutes with the fresh air ventilated under the hood and a sample of this mixture is conveyed to the analysers, through a capillary tube and analysed. Ambient and diluted fractions of O₂ and CO₂ are measured for a known ventilation rate, and O₂ consumption and CO₂ production are determined and converted into Resting Energy Expenditure.^[9]
Face mask (breath by breath): Indirect calorimetry tests are also often performed with a face mask, which is used to convey exhaled and inhaled gas through a turbine flowmeter able to measure the patient's breath by breath minute ventilation, at the same time a sample of gas is conveyed to the analyser and VO₂ and VCO₂ are measured and converted in energy expenditure.
Interface with a Ventilator (Intensive Care settings): In case the patient is mechanically ventilated, an indirect calorimeter can still measure breath by breath inhaled/exhaled O₂ and CO₂ if interfaced with the ventilator through the endotracheal tube.

Applications

Indirect calorimetry provides at least two pieces of information: a measure of energy expenditure or 24-hour caloric requirements as reflected by the Resting Energy Expenditure (REE) and a measure of substrate utilization as reflected in the Respiratory Quotient (RQ). Knowledge of the many factors that affect these values has led to a much broader range of applications. Studies of indirect calorimetry over the past 20 years have led to the characterization of the hypermetabolic stress response to injury and the design of nutritional regimens whose substrates are most efficiently assimilated in different disease processes and organ failure states. Indirect calorimetry has influenced everyday practices of medical and surgical care, such as the warming of burn unit and surgical suites and the weaning of patients from ventilators.^[7]

Related Research Articles

<span class="mw-page-title-main">Calorimeter</span> Instrument for measuring heat

A calorimeter is a device used for calorimetry, or the process of measuring the heat of chemical reactions or physical changes as well as heat capacity. Differential scanning calorimeters, isothermal micro calorimeters, titration calorimeters and accelerated rate calorimeters are among the most common types. A simple calorimeter just consists of a thermometer attached to a metal container full of water suspended above a combustion chamber. It is one of the measurement devices used in the study of thermodynamics, chemistry, and biochemistry.

A ventilator is a type of breathing apparatus, a class of medical technology that provides mechanical ventilation by moving breathable air into and out of the lungs, to deliver breaths to a patient who is physically unable to breathe, or breathing insufficiently. Ventilators may be computerized microprocessor-controlled machines, but patients can also be ventilated with a simple, hand-operated bag valve mask. Ventilators are chiefly used in intensive-care medicine, home care, and emergency medicine and in anesthesiology.

Dead space is the volume of air that is inhaled that does not take part in the gas exchange, because it either remains in the conducting airways or reaches alveoli that are not perfused or poorly perfused. It means that not all the air in each breath is available for the exchange of oxygen and carbon dioxide. Mammals breathe in and out of their lungs, wasting that part of the inhalation which remains in the conducting airways where no gas exchange can occur.

Mechanical ventilation or assisted ventilation is the medical term for using a machine called a ventilator to fully or partially provide artificial ventilation. Mechanical ventilation helps move air into and out of the lungs, with the main goal of helping the delivery of oxygen and removal of carbon dioxide. Mechanical ventilation is used for many reasons, including to protect the airway due to mechanical or neurologic cause, to ensure adequate oxygenation, or to remove excess carbon dioxide from the lungs. Various healthcare providers are involved with the use of mechanical ventilation and people who require ventilators are typically monitored in an intensive care unit.

An arterial blood gas (ABG) test, or arterial blood gas analysis (ABGA) measures the amounts of arterial gases, such as oxygen and carbon dioxide. An ABG test requires that a small volume of blood be drawn from the radial artery with a syringe and a thin needle, but sometimes the femoral artery in the groin or another site is used. The blood can also be drawn from an arterial catheter.

Exhalation is the flow of the breath out of an organism. In animals, it is the movement of air from the lungs out of the airways, to the external environment during breathing. This happens due to elastic properties of the lungs, as well as the internal intercostal muscles which lower the rib cage and decrease thoracic volume. As the thoracic diaphragm relaxes during exhalation it causes the tissue it has depressed to rise superiorly and put pressure on the lungs to expel the air. During forced exhalation, as when blowing out a candle, expiratory muscles including the abdominal muscles and internal intercostal muscles generate abdominal and thoracic pressure, which forces air out of the lungs.

In physiology, respiration is the movement of oxygen from the outside environment to the cells within tissues, and the removal of carbon dioxide in the opposite direction to the surrounding environment.

Hypercapnia (from the Greek hyper = "above" or "too much" and kapnos = "smoke"), also known as hypercarbia and CO₂ retention, is a condition of abnormally elevated carbon dioxide (CO₂) levels in the blood. Carbon dioxide is a gaseous product of the body's metabolism and is normally expelled through the lungs. Carbon dioxide may accumulate in any condition that causes hypoventilation, a reduction of alveolar ventilation (the clearance of air from the small sacs of the lung where gas exchange takes place) as well as resulting from inhalation of CO₂. Inability of the lungs to clear carbon dioxide, or inhalation of elevated levels of CO₂, leads to respiratory acidosis. Eventually the body compensates for the raised acidity by retaining alkali in the kidneys, a process known as "metabolic compensation".

Basal metabolic rate (BMR) is the rate of energy expenditure per unit time by endothermic animals at rest. It is reported in energy units per unit time ranging from watt (joule/second) to ml O₂/min or joule per hour per kg body mass J/(h·kg). Proper measurement requires a strict set of criteria to be met. These criteria include being in a physically and psychologically undisturbed state and being in a thermally neutral environment while in the post-absorptive state (i.e., not actively digesting food). In bradymetabolic animals, such as fish and reptiles, the equivalent term standard metabolic rate (SMR) applies. It follows the same criteria as BMR, but requires the documentation of the temperature at which the metabolic rate was measured. This makes BMR a variant of standard metabolic rate measurement that excludes the temperature data, a practice that has led to problems in defining "standard" rates of metabolism for many mammals.

<span class="mw-page-title-main">Capnography</span> Monitoring of the concentration of carbon dioxide in respiratory gases

Capnography is the monitoring of the concentration or partial pressure of carbon dioxide (CO^₂) in the respiratory gases. Its main development has been as a monitoring tool for use during anesthesia and intensive care. It is usually presented as a graph of CO^₂ (measured in kilopascals, "kPa" or millimeters of mercury, "mmHg") plotted against time, or, less commonly, but more usefully, expired volume (known as volumetric capnography). The plot may also show the inspired CO^₂, which is of interest when rebreathing systems are being used. When the measurement is taken at the end of a breath (exhaling), it is called "end tidal" CO^₂ (PETCO₂).

Resting metabolic rate (RMR) is whole-body mammal metabolism during a time period of strict and steady resting conditions that are defined by a combination of assumptions of physiological homeostasis and biological equilibrium. RMR differs from basal metabolic rate (BMR) because BMR measurements must meet total physiological equilibrium whereas RMR conditions of measurement can be altered and defined by the contextual limitations. Therefore, BMR is measured in the elusive "perfect" steady state, whereas RMR measurement is more accessible and thus, represents most, if not all measurements or estimates of daily energy expenditure.

The respiratory quotient is a dimensionless number used in calculations of basal metabolic rate (BMR) when estimated from carbon dioxide production. It is calculated from the ratio of carbon dioxide produced by the body to oxygen consumed by the body. Such measurements, like measurements of oxygen uptake, are forms of indirect calorimetry. It is measured using a respirometer. The respiratory quotient value indicates which macronutrients are being metabolized, as different energy pathways are used for fats, carbohydrates, and proteins. If metabolism consists solely of lipids, the respiratory quotient is approximately 0.7, for proteins it is approximately 0.8, and for carbohydrates it is 1.0. Most of the time, however, energy consumption is composed of both fats and carbohydrates. The approximate respiratory quotient of a mixed diet is 0.8. Some of the other factors that may affect the respiratory quotient are energy balance, circulating insulin, and insulin sensitivity.

Doubly labeled water is water in which both the hydrogen and the oxygen have been partly or completely replaced with an uncommon isotope of these elements for tracing purposes.

Respirometry is a general term that encompasses a number of techniques for obtaining estimates of the rates of metabolism of vertebrates, invertebrates, plants, tissues, cells, or microorganisms via an indirect measure of heat production (calorimetry).

A carbon dioxide sensor or CO₂ sensor is an instrument for the measurement of carbon dioxide gas. The most common principles for CO₂ sensors are infrared gas sensors (NDIR) and chemical gas sensors. Measuring carbon dioxide is important in monitoring indoor air quality, the function of the lungs in the form of a capnograph device, and many industrial processes.

The respiratory exchange ratio (RER) is the ratio between the metabolic production of carbon dioxide (CO₂) and the uptake of oxygen (O₂).

<span class="mw-page-title-main">Sucrose intolerance</span> Medical condition

Sucrose intolerance or genetic sucrase-isomaltase deficiency (GSID) is the condition in which sucrase-isomaltase, an enzyme needed for proper metabolism of sucrose (sugar) and starch, is not produced or the enzyme produced is either partially functional or non-functional in the small intestine. All GSID patients lack fully functional sucrase, while the isomaltase activity can vary from minimal functionality to almost normal activity. The presence of residual isomaltase activity may explain why some GSID patients are better able to tolerate starch in their diet than others with GSID.

A cone calorimeter is an instrument used to study the behavior of fire in small samples of condensed phase materials. It is widely used in the field of fire safety engineering and in oxygen consumption calorimetry.

Nitrogen washout is a test for measuring anatomic dead space in the lung during a respiratory cycle, as well as some parameters related to the closure of airways.

References

1 2 Ferrannini E."The theoretical bases of indirect calorimetry: a review." Metabolism. 1988 Mar;37(3):287-301.
↑ Marson F, et al. "Correlation between oxygen consumption calculated using Fick's method and measured with indirect calorimetry in critically ill patients." Arq Bras Cardiol. 2004 Jan;82(1):77-81, 72-6. Epub 2004 Feb 12.
1 2 Haugen HA, et al. "Indirect calorimetry: a practical guide for clinicians." Nutr Clin Pract. 2007 Aug;22(4):377-88.
1 2 Pinheiro Volp AC, et al. "Energy expenditure: components and evaluation methods." Nutr Hosp. 2011 May-Jun;26(3):430-40. doi: 10.1590/S0212-16112011000300002.
↑ A.R. Bain; et al. (Jun 2012). "Body heat storage during physical activity is lower with hot fluid ingestion under conditions that permit full evaporation Authors". Acta Physiologica . 206 (2): 98–108. doi:10.1111/j.1748-1716.2012.02452.x. PMID 22574769. S2CID 23682662. citing Nishi, Y. (1981). "Measurement of thermal balance in man". In K. Cena & J. Clark (ed.). Bioengineering, Thermal Physiology and Comfort . Elsevier. pp. 29–39.
↑ Atwater WO, et al. "Description of neo respiration calorimeter and experiments on the conservation of energy in the human body." US Department Agriculture, Off Exp Sta Bull 63, 1899
1 2 McClave SA, et al. "Use of indirect calorimetry in clinical nutrition." Nutr Clin Pract. 1992 Oct;7(5):207-21.
↑ Douglas, C. Gordon (18 March 1911). "A method for determining the total respiratory exchange in man". Proceedings of the Physiological Society. Retrieved 28 August 2016.^{[ dead link ]} (Douglas Bag)
↑ Academy of Nutrition and Dietetics "Measuring RMR with Indirect Calorimetry (IC)." Nutr Clin Pract. 2007 Aug;22(4):377-88.