Air displacement plethysmography is used in medicine as a noninvasive diagnostic tool to determine the volume or volume changes of a body or specific body parts. In a closed capsule or shot cavity that is sealed airtight by a cuff, changes in volume of the body or body parts cause changes in air pressure. Computer programs calculate the corresponding volume changes from the air pressure changes, compensating for any temperature changes in the air.
What is air displacement plethysmography?
The examination method provides quick and reliable information for distinguishing between asthma and chronic obstructive pulmonary disease. Pulmonary emphysema can also be diagnosed. In medicine, plethysmography refers to non-invasive measurement methods for measuring volume or changes in volume of the whole body or parts of the body. Measurement procedures are either named after the purpose of the measurement procedure, such as occlusion plethysmography for measuring and detecting venous occlusion and venous valve dysfunction, or they are named after the medium used for the measurement procedure. Air displacement plethysmography is therefore the name given to a method based on the displacement of air or the generation of pressure changes during volume changes as the measurement principle. Air displacement plethysmography is usually used as body plethysmography, in which the person being examined is in a hermetically sealed cabin and the volume changes of the entire body are included. Alternatively, air displacement plethysmography can be used, for example, to test venous function in partial areas of the body. In this case, air-tight cuffs serve as functional units that respond to volume changes in the veins with corresponding pressure changes. Bodyplethysmography, which is somewhat comparable to Archimedes’ principle of water displacement, in which the volume of a body is measured by immersion in water. The amount of water displaced is equal to the volume of the immersed body and the total mass (weight) can be determined by a balance. In this way, the specific mass per unit volume can also be calculated. Because air, unlike water, is a compressible medium, the air displaced from the body does not have to be measured directly in air displacement plethysmography; instead, this is done indirectly in the sealed cabin by measuring the pressure changes. The same principle of pressure changes analogous to volume changes of the examined body part is used in occlusion plethysmography on arms and legs. In principle, this is also air displacement plethysmography, in which the body part being examined is encased in an airtight cuff.
Function, effect, and goals
A frequently used application of air displacement plethysmography in the form of body plethysmography is the determination of the specific mass of the body and the body composition derived from it. In particular, it can be used to determine the percentage of fat on the body. Compared to the water displacement method, this examination method has the advantage that it is also suitable for persons with physical problems, because the test persons do not have to be completely immersed in water several times. The method requires little time and is largely automated. There are special (small) chambers for the examination of newborns. Another important area of examination for body plethysmography offers comprehensive pulmonary function testing. Air displacement plethysmography can be used to analyze specific lung and respiratory parameters that are not accessible to simple spirometry. In particular, breathing resistance can be measured, an important parameter in the diagnosis of respiratory diseases. Furthermore, the examination method provides rapid and reliable indications for distinguishing between asthma and chronic obstructive pulmonary disease. Pulmonary emphysema can also be diagnosed. Spirometric results can also be used as a supplement and differential diagnosis. For the so-called occlusion plethysmography for functional testing of veins and arteries in the extremities, air displacement plethysmography is available as one of several possible methods.It offers the advantage that actual volume changes are measured and therefore diagnostically more valuable and differentiated data are available than, for example, with plethysmography using strain gauges. In occlusive plethysmography by air displacement, individual extremities such as the lower leg or arms are usually examined separately. The necessary closed air space is created by an air-tight cuff. Volume changes due to a varying degree of filling of the veins or arteries have a proportional effect on pressure changes within the cuff and can be evaluated directly by a specialized computer program. The measurement of volume changes on the finger (finger plethysmography) serve to demonstrate the functionality of the arteries. The same method can be used to measure penile erectility (penile plethysmography).
Risks, side effects, and hazards
The main problem with air displacement plethysmography is that in a closed system, pressure changes are proportional to volume changes only if the gas (or air) in the closed system is kept isothermal, that is, at the same temperature (Boyle-Mariott’s gas law). Practically, this can hardly be done with reasonable effort. With the development of computer technology and complex algorithms, it has been possible to resort to adiabatic measurements since the late 1990s. In this case, there is no thermal exchange with the environment. However, temperature and pressure change simultaneously in the closed system when the volume of the body or body part being measured increases. In principle, the calculation of the adiabatic change of state no longer poses any problems today, but the adiabatic state cannot be represented without further ado, since, for example, body heat or heat radiation from outside can falsify the measurement results. On the one hand, the use of insulating materials can minimize the falsifying influences, and the development of complex algorithms can calculate out and compensate for the not entirely avoidable falsifications caused by the supply of thermal energy from outside into the closed system.
