Everything You Need to Know About Plethysmographs

A plethysmograph is a unique measurement tool and its primary function is detecting changes in the volume of organs and/or the whole body itself.

Nowadays, doctors most often use a computer-aided machine that shows more accurate and productive results. 

The name of the instrument comes from the Greek word "plethysmos," which roughly translates to "enlarging, increasing, bloated, and becoming full." 

Usually, the primary determining factor of the aforementioned volume changes is in the form of fluctuations in blood and/or air levels that a given organ or the whole body contains during a given period of time.


The main goal of this instrument is to perform plethysmography, a test that allows physicians to find the organ volume data and determine a diagnosis. Many sources have stated that the information gathered during this diagnostic procedure should be interpreted with care.

This text highlights some information about the history of the plethysmograph, the various types available on the market, and its overall medical value.


Some Historical Facts

Plethysmographs use Boyle’s Law (alternatively Boyle-Marriott law) as the primary measurement principle. So, the central concept that lies behind plethysmography has been well known for over 100 years. 

There have been a few technical difficulties in the past that didn't allow doctors to perform proper research in this area of medicine. For example, there weren’t any adequate measurement tools available for these purposes.


The first scientist who delivered a fully working animal plethysmograph was Paul Bert (1833-1886 )in the year 1868. Later, he described his invention in an article, which was published in 1915.

The version of this instrument that was invented for humans first appeared in 1955. The inventors were DuBois and Comroe, who used their new machine for measuring airway resistance in humans.


Following in the footsteps of these medical pioneers, humanity made a few more valuable improvements in the construction of plethysmographs. After many iterations, humanity ended up developing a device with a computer that operates using a set of programs that generally improve the main principles of older versions of this diagnostic apparatus.


What Can a Plethysmograph Measure?

Nowadays, these powerful tools have much more potential and use cases because they can be used for diagnostic purposes and measuring a vast number of parameters of humans. The body parts/organs that are examined most often are the following:


●      The entire human body

●      Lungs

●      Limbs

●      Genitals


During the examination of different organs, doctors usually use diagnostic apparatuses of different construction, shape, sizes, and other assets. The key function of most modern-day plethysmographs is the measurement of respiratory parameters.

The leading respiratory parameters that are measured include functional residual capacity, speed of exhalation/expiration, and total lung capacity.


When limbs are examined, the device is usually put on the arms or legs and blood circulation speed and capacity are measured. By using this method, doctors can determine many different conditions. An example is venous thrombosis disease.

Phallometry is another regular use case of a plethysmograph, which is usually performed to measure the blood flow inside the penis. It can help to determine the sexual problems present in the patient. This method was also used in the past during criminal research.


Types of Plethysmographs

According to the use cases and information mentioned so far, the main types of diagnostic apparatuses available for the various types of plethysmography are the following:


●      Penile

●      Pulmonary

●      Limb

●      Whole-body


There are various modern types of plethysmographs that perform several different standard measurements of parameters and volume analysis. Here are some important components of the device that allow the diagnostic tool to operate effectively:


●      Photoelectric: mostly uses infrared densitometry

●      Strain gauge: filled with gallium or mercury substances flexible tube

●      Impedance: works accordingly to the Ohm's law

●      Air: a cuff filled with air

●      Water: self-sealed chamber with water


Below there is more information about the most common types of these diagnostic devices. Understanding the principles of the device allows us to have an overall better understanding of the way the tool works and use cases/components associated with it.

The penile plethysmograph

This type of diagnostic device is most commonly used for measuring the blood flow in the patient's penis. There are a few variations of techniques used while performing a medical examination. Penile plethysmography allows us to measure certain aspects of the penis including:


●      Overall volume

●      Circumference


In order to perform both these tests, a physician only needs to change the cap on the diagnostic tool to get both measurements. There is no need for the physician to swap out the entire tool to acquire the second measurement

The pulmonary plethysmograph

This type of tool is the most commonly used in general medical practice. The three main types of diagnostic devices of such kind are usually defined as:


●      Closed-Type (Pressure);

●      Open-Type (Volume);

●      Pressure-Volume.


The main difference between them is whether they include a closed, opened, or combined-type chamber for the patient's breathing. Each type shows breathing parameters from different perspectives.

The limb plethysmograph

There is no specific differentiation among these types of diagnostic apparatuses. Dissociation may only occur in the main physical effect that makes the entire tool work (refer to the information above).

The whole-body plethysmograph

This device usually looks like a large closed telephone box with an improved glass or transparent plastic walls. Mostly, those tools can measure any parameter with a changing volume in the entire human body.

The main advantage of the whole-body plethysmograph is the significant ability to combine gathered data with artificial intelligence and computer programs for additional analysis. The second advantage is the lack of faulty results displayed to a medical professional after a test is run. And the third is a complex, highly informative test outcome.


Examples of Common Uses Cases

The plethysmograph can only be used as a tool for diagnostic purposes. There are a plethora of different pathologies that plethysmography can potentially discover. So let's review the most common.

Respiratory Issues

With the results presented by plethysmography, doctors can indirectly estimate the condition of a patient's respiratory system. The most frequent conditions they can diagnose are the following:


●      Bronchial obstruction due to the inflammatory process

●      Bronchial asthma

●      Chronic obstructive pulmonary disease (COPD)

●      Lung cancer

●      Bronchial cancer

●      Tuberculosis


They compare the usual indicators gathered from the plethysmography data for patients’ clinical groups and make conclusions according to the results.

Blood Vessels Pathology

Measuring the blood vessel's specific parameters, both venous and arterial (sometimes even arterioles, capillaries, and venules), can show a doctor a lot of information. It helps them diagnose many conditions including the following:


●      Vasculitis of any type

●      Venous hypotony

●      Varicose dilatation of the veins

●      Hidden arterial hypertension

●      Arterial hypotension

●      In-birth pathologies of the anatomy of blood vessels


Most commonly, they analyze the speed of blood flow, attempt to detect the regurgitation process, and compare vessel volumes of a patient with average indicators of control groups. That gives a doctor all the proper information to establish a diagnosis.

Sexual disorders

As mentioned before, while performing a test called a phallometry, doctors can discover serious issues associated with the blood flow in a patient's penis. This condition can be a sign of a various number of different sexual disorders, e. g.:


●      Aspermy

●      Erectile dysfunction (ED)

●      Hypoactive sexual desire disorder

●      Early ejaculation disorder

●      Among others


Issues with penis blood flow can potentially be a sign of a lack of testosterone, which may be caused by a wide variety of reasons. For example, this may be caused by varicocele, testicle failure, endocrine disorders, and others.


Market Leaders

Nowadays, we have a wide variety of different types of plethysmographs available on the market for any needs, at fair prices, and with exceptionally highly technical approaches. The greatest brands that sell these devices are:


●      SHILLER

●      Intellibio

●      Callegari

●      Geratherm

●      MGC Diagnostics

●      DSI


●      Bonther

●      Sensormedics

●      Carefusion

●      Resmed


Be patient and very attentive while buying this device, as there are lots of untruthful sellers nowadays on the Internet. You have always had to ask to show you all recommendations from the previous clients, certificates of quality, and certificates of authenticity before purchasing.


Final Thoughts

Generally, a plethysmograph is an excellent examination instrument that can provide doctors with valuable information. In combination with various other data about the patients, the results of the plethysmography can become the deciding factor when considering a diagnosis. 

There are different ways to use this medical device and interpret the results of the test to help with patient diagnosis and effectively, treatment.

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