Whole-body air displacement plethysmography
The Luftverdrängungsplethysmographie (Air Displacement Plethysmography ADP; Greek Plethore = wealth, graphein = write ) is a technique to determine body composition, especially body fat, using density measurement ( densitometry ) on a volume determination ( plethysmography ). The technique is based on the same foundations as the hydrodensitometry (underwater weighing), but uses air instead of water displacement. The Luftverdrängungsplethysmographie has in comparison to other established reference methods for the determination of body composition some advantages such as the short duration of the study, the non- invasiveness and feasibility for many subjects (children, obese, elderly or disabled people).
History of air displacement plethysmography
The principle of plethysmography has been used for collection of the body volume and body composition of young children in the early 20th century. Until the 1960s, however, no reliable measurement methods were available. The systems required a strict constant of the ambient conditions. The technical difficulties to standardize the temperature and humidity of the air at the surface of the body limited the investigation of people. Due to the variety of technological difficulties, none of the early Luftverdrängungsplethysmographen was developed for everyday use. Also, the development approach later, technologically more advanced systems from the 1980s has not continued. The mid- 1990s came the first commercially available Luftverdrängungsplethysmograph for adults on the market in early 2000 followed a system for infants.
Principle of operation
Basics
Wherein the density of its body Luftverdrängungsplethysmographie is determined from the determination of the body volume and the mass of a subject. Of which in turn is on body composition, in particular the fat content, is closed.
When Luftverdrängungsplethysmographie the volume of a person indirectly by measuring the displaced volume of air within a closed chamber determined ( plethysmography ) is. The body of the person within the chamber displaces a volume of air that is equal to the volume of the body. In order to calculate the body volume is subtracted, the remaining chamber volume from the volume of the empty chamber when the subject is in the chamber. To measure the air in the test chamber physical gas laws apply.
Isotherm measurement
Boyle's law states that the pressure of a gas at constant temperature is inversely proportional to the volume. If a constant temperature can be guaranteed ( isothermal state change), the Boyle's law is applicable. Early Plethysmographiesysteme tried isothermal conditions in the test chamber to produce and therefore have not been practicable. This problem could only be solved with the development of systems that did not require isothermal test conditions more.
Adiabatic measurement
Since the late 1990s available systems dispense with the production of isothermal conditions and choose an approach in which between the measurement space and the environment no thermal energy is exchanged (adiabatic change of state) and determine on this basis, the volume of displaced air and thus the body volume of subjects with the help of the Poisson equation. The fact that (also) these conditions can not be fully established, with the help of corrections, which in turn account for the isothermal approach, balanced.
Validation
The Luftverdrängungsplethysmographie has been validated against the most important methods of body composition analysis:
- Hydrodensitometry (underwater weighing)
- Important reference methods are underwater weighing, dual X-ray absorptiometry measurement of the Total Body Water ( TBW ) by isotope dilution analysis, measurement of total body potassium ( Total Body Potassium ) and multi -compartment models
- Deuterium method ( in infants )