Air displacement plethysmography
The whole-body air displacement plethysmography ( air displacement plethysmography ADP; Greek . Plethore = Abundance, graphein = Write) is a technique, the body composition , in particular the body fat percentage to be determined by means of density measurement (densitometry) via a volume determination (plethysmography). The technique is based on the same principles as hydrodensitometry (underwater weighing , see also pycnometer ), but uses air instead of water displacement. Compared to other established reference methods for determining body composition, air displacement plethysmography has several advantages, for example the short duration of the examination, the non-invasiveness and the feasibility for many test subject groups (children, overweight, elderly or disabled persons).
History of air displacement plethysmography
The principle of plethysmography was already used at the beginning of the 20th century to record the body volume and body composition of small children. However, reliable measurement methods were not available until the 1960s. The systems required the environmental conditions to be kept strictly constant. The technical difficulties in standardizing the temperature and humidity of the air on the body surface limited the study of humans. Due to the multitude of technological difficulties, none of the early air displacement plethysmographs was further developed for everyday use. The development approach of later, more technologically advanced systems from the 1980s was also discontinued. The first commercially available air displacement plethysmograph for adults came on the market in the mid-1990s, and a system for small children followed in early 2000.
Working principle
Basics
In air displacement plethysmography, the density of the body is determined by determining the body volume and the mass of a test person. From this, in turn, conclusions are drawn about the body composition, in particular the fat percentage.
In air displacement plethysmography, the volume of a person is determined indirectly by measuring the volume of air displaced within a closed chamber (plethysmography). The person's body within the chamber displaces a volume of air that is equal to the body volume. To calculate the body volume, the remaining chamber volume is subtracted from the volume of the empty chamber when the test subject is in the chamber. The amount of air inside the chamber is calculated by changing the volume of the chamber slightly (e.g. by means of a sliding membrane) and then the amount of air in the chamber can be calculated using physical gas laws:
Isothermal measurement
The Boyle's law indicates that the pressure of a gas at a constant temperature is inversely proportional to the volume. If a constant temperature can be guaranteed ( isothermal change of state ), Boyle's law can be applied. Early plethysmography systems attempted to establish isothermal conditions in the test chamber and were therefore not practical. This problem could only be solved with the development of systems that no longer required isothermal test conditions.
Adiabatic measurement
Systems available since the end of the 1990s dispense with the creation of isothermal conditions and choose an approach in which no thermal energy is exchanged between the measuring room and the environment ( adiabatic change of state ) and on this basis determine the volume of the displaced air and thus the body volume of the Subjects using Poisson's equation . The fact that (also) these conditions cannot be fully established is compensated for with the help of corrections that in turn take the isothermal approach into account.
Validation
Air displacement plethysmography has been validated against the main body composition analysis methods:
- Hydrodensitometry (underwater cradles)
- The most important reference methods are underwater cradles, dual X-ray absorptiometry , measurement of total body water (TBW) using isotope dilution analysis , measurement of whole body potassium (total body potassium ) and multi-compartment models
- Deuterium method (in infants)
- Whole body magnetic resonance imaging.
literature
- Christine Becker: Formulation of population-specific algorithms to determine the body fat mass of children, adolescents and adults, based on field methods with air displacement plethysmography as a reference . (PDF; 1.9 MB) Diss., Kiel 2002
Web links
- Methodical focus . Different methods for determining body composition on the website of the Reference Center for Body Composition at Christian-Albrechts-Universität
Individual evidence
- ↑ a b c d e D. A. Fields et al .: Body-composition assessment via air-displacement plethysmography in adults and children: a review . In: Am J Clin Nutr , 75 (3), 2002, pp. 453-467.
- ↑ a b P. Dempster et al .: A new air displacement method for the determination of human body composition . In: Med Sci Sports Exerc , 27 (12), 1995, pp. 1692-1697, PMID 8614327 .
- ↑ A. Urlando et al .: A new air displacement plethysmograph for the measurement of body composition in infants . In: Pediatr Res , 53 (3), 2003, pp. 486-492, PMID 12595599 .
- ^ MA McCrory et al .: Evaluation of a new air displacement plethysmograph for measuring human body composition . In: Med Sci Sports Exerc , 27 (12), 1995, pp. 1686-1691, PMID 8614326 .
- ^ G. Ma et al .: Validation of a new pediatric air-displacement plethysmograph for assessing body composition in infants . In: Am J Clin Nutr , 79 (4), 2004, pp. 653-660, PMID 15051611 .
- ↑ Ute A. Ludwig, Florian Klausmann, Sandra Baumann, Matthias Honal, Jan-Bernd Hövener, Daniel König, Peter Deibert, Martin Büchert .: Whole-body MRI-based fat quantification: A comparison to air displacement plethysmography . In: Journal of Magnetic Resonance Imaging . tape 40 , no. 6 , December 1, 2014, ISSN 1522-2586 , p. 1437-1444 , doi : 10.1002 / jmri.24509 ( wiley.com [accessed January 17, 2017]).