A plethysmograph is a device that medicine uses to measure variations in volume. Depending on the type of plethysmograph, it can calculate the volume of blood vessels in the arms and legs, lungs or finger. In addition, this method is suitable for determining the volume of the finger (pulse) and the degree of erection in men. Particularly in pneumology, the plethysmograph helps to gain insights into the individual patient that are usually not measurable with other methods.
What is a plethysmograph?
The plethysmograph measures volume variations in different parts of the body, focusing on one body part at a time. Closure plethysmography, air displacement plethysmography, and body plethysmography appear to be the three most common forms of plethysmography. To determine the volume, the plethysmograph determines the filling quantity of another displaced substance. This is usually air or water. Common applications include studies of the lungs and body fat measurements.
Forms, types, and styles
Plethysmographs can be divided into different types. The classification depends on which criterion is used as a feature for differentiation. For example, a basic distinction can be made based on the medium whose displacement the plethysmograph detects. Theoretically, many different substances are conceivable that can fulfill this function – in fact, however, it is primarily a matter of deciding between water and air. Accordingly, water plethysmographs measure the extent of displacement based on the amount of water, and air plethysmographs determine the air displaced, for example, from a sealed chamber in which the patient sits during the examination. Another distinction can be made based on the functions or applications. Occlusive plethysmography deals with the blood flow in arteries and veins and, under favorable conditions, can determine whether there is a constriction or occlusion of the blood vessel. Physicians can perform this examination on a patient’s legs and/or arms. Air displacement plethysmography, on the other hand, is primarily concerned with determining a person’s body fat content. This is generally considered an important indicator of health. In contrast to simply determining weight, measuring body fat as accurately as possible allows better conclusions to be drawn regarding potential health risks resulting from obesity or other clinically relevant conditions. Some people also refer to the measurement method of air displacement plethysmography as densitometry (density measurement). A third form of volume measurement is body plethysmography. This is a method used primarily in pulmonary medicine (pneumology). It can pursue various goals and lead to a better understanding with regard to individual parameters such as breathing resistance, (total) lung volume or residual volume. The residual volume corresponds to the amount of air that remains in the two lungs after exhalation. Other forms of plethysmography are less common, such as penile plethysmography or phallometry, which can be used, for example, in the diagnosis of erectile dysfunction, or such as finger plethysmography, which can help provide insight into cardiovascular conditions.
Structure and mode of operation
In general, plethysmograph operation is based on a simple principle that has been known since ancient times. According to a historical anecdote, a Greek philosopher came up with the solution to a problem in the bath. He had been commissioned by the ruler to find out whether a crown was actually made of pure gold. As the philosopher lowered himself into his bath tub, he observed how his body displaced the water. He logically concluded that the displaced volume can be used to measure how large the volume of an object is. According to tradition, he then rushed out into the street, naked and beaming with joy, to shout “Eureka!” (“I’ve found it”) to celebrate his discovery. A plethysmograph works on the same basic principle. Instead of measuring displaced water, the device in most cases draws on displaced air volume. Of course, modern plethysmographs are not bath tubs, but are usually chambers in which the patient sits.However, water plethysmography in a contemporary form still represents a serious option alongside air plethysmography. In occlusion plethysmography, physicians can not only use one of the described approaches, but also alternatively resort to the determination of volume by means of strain gauges. As a rule, they are mercury strain gauges. They are also widely used in architecture or construction, for example, to detect material variations and changes.
Medical and health benefits
The benefits of plethysmographs can be seen in various medical fields. For example, body fat measurement allows near-accurate measurement of body composition. The procedure thus provides an alternative to other procedures, which can prove to be much more laborious. Pneumology also benefits greatly from knowledge gained from the plethysmograph. Physicians can use it to determine, among other things, lung and residual volume as well as respiratory resistance, about which it is otherwise difficult to obtain precise data. As a result, the plethysmograph can also play a role in the diagnosis and monitoring of lung diseases such as COPD. COPD (chronic obstructive pulmonary disease) can take several forms, all of which are characterized by progressively worsened airflow.
