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What is a Plethysmograph?

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  • Written By: Alex Paul
  • Edited By: Jacob Harkins
  • Last Modified Date: 30 September 2016
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A plethysmograph is a piece of medical equipment that is used to measure the volume of different parts of the body. It is most commonly used to gauge the volume of a specific organ or limb, but can also measure the body as a whole. Generally, a body or organ will change volume due to a variation in the amount of blood or air in contains. There are several different types of plethysmographs, including ones for pulmonary, respiratory and penile measurements. Each is used to measure the volume of a different part of the body to help with issues such as air passage restrictions, poor blood circulation and problems with sexual arousal in men.

To measure the volume — and hence capacity — of the lungs, a pulmonary plethysmograph is used. This requires a sealed chamber and a mouthpiece in most cases although modern machines sometimes don’t need an enclosed area. The person undergoing the test then has to inhale before exhaling while the pressure inside the box is measured. Due to the fact that the box is closed from the outside world, any change in pressure of the box will be reflected in a change within the lungs. Using this information the volume of the lungs can then be calculated using formulas which relate pressure and volume.

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In some cases the absolute volume of the lungs may not be important but the difference between the volume of empty and full volume is. For this reason a pulmonary plethysmograph can measure the residual capacity of the lungs as well as the total capacity in order to compare the two. Diseases which cause obstruction can be investigated using this technique.

An air plethysmograph is used to estimate the volume of a particular area of the body. For example, a leg may be enclosed in a chamber that allows any changes in volume to be detected and measured. Water plethysmographs are similar but are generally more accurate than air versions. In many cases, however, air is used for convenience despite the fact that it doesn’t produce such reliable results.

Penile plethysmographs are also commonly used to detect changes in volume — and hence blood flow — to the penis. Although the same techniques cannot be used on females, there is a vaginal method called photoplethysmography that can be used to measure the flow of blood in the area. Some researchers have attempted to use results from these tastes to prove sexual orientation or different levels of arousal — penile plethysmography is not considered legal evidence, though.

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