Plethysmographs are medical devices that allow you to perform plethysmography, a term from the Greek plethysmo (increase) and graph (to write). A plethysmography is a functional study which takes into account the variation in volume or pressure of an organ and thus makes it possible to detect cases of occlusion, inflammation, etc.

Le Larousse Medical defines plethysmography as “an examination which makes it possible to record, under normal conditions and during various ailments, the variations in pressure prevailing in an organ or a segment of a limb, or its variations in volume”.

Plethysmographs allow this type of examination to be performed quickly, painlessly and non-invasively. They are used to record changes in pressure and volume:

• in blood vessels (in cardiology and angiology);
• in the lungs (in pulmonology);
• in the bladder or prostate (in urology).

Structure and functions

Pneumology plethysmography cabin

It is a transparent airtight cabin, equipped with a seat, generally swivel to facilitate access for people with reduced mobility, and a height-adjustable arm which can be managed by the manipulator from the outside. A microphone and loudspeakers make it possible to communicate and to give indications to the patient who is locked up there. The operating principle is as follows: “the fact of placing a subject in a space of reduced volume makes it possible to estimate the variations in his body volume from the variations in pressure in this space”.

The barometric plethysmograph is completely waterproof: variations in volume have direct repercussions on variations in pressure inside the chamber.

The flowmeter plethysmograph opens to the outside through a grid: variations in volume cause a gas leak which can be measured.

Plethysmographs in cardiology and angiology

• Mercury-gauge or impedance venous occlusion plethysmographs consist of a cuff installed at mid-thigh and electrodes placed on the calf connected to a monitor.
• Air plethysmographs consist of an air chamber connected to a pressure sensor, an amplifier and a recorder.

Respiratory Functional Exploration (FR)

The purpose of pulmonary plethysmography is to determine the total lung capacity and the resistance of the bronchi. He analyzes lung function and ventilatory mechanics. It allows to :

• diagnose emphysema;
• to follow the evolution of a pneumopathy;
• evaluate the effectiveness of a treatment;
• perform a preoperative assessment before thoracic surgery.

Functional venous exploration

The aim of a venous plethysmography is to assess in a limb the variations in blood volume linked to the venous and arterial circulation in various physiological or pathological situations. The results of the examinations make it possible to:

• assess venous function;
• assess peripheral venous pressure;
• study venous distensibility;
• diagnose deep vein thrombosis;
• assess post-thrombotic syndrome;
• provide therapeutic follow-up;
• assess chronic venous insufficiency (CVI);
• evaluate an arteriopathy obliterating of the lower limbs (PADI) (in this case, the examination will be completed by a Dopler).

The stages of operation

Pulmonary plethysmography

This examination is generally performed in the pulmonology department, without requiring hospitalization. It lasts 15 to 30 minutes. The patient is installed in the hermetic enclosure. He breathes exclusively through the mouth thanks to a mouthpiece (a nose clip prevents him from breathing through his nose). The manipulator gives him indications: breathe normally, deeply, pant, blow with all his strength, etc.

For some measurements, carbon monoxide or helium is sent to it through the mouthpiece. The fact of breathing causes variations in the volume of his thorax which influence the pressure in the hermetic chamber. Information about the pressure in the sealed chamber and mouthpiece helps assess the patient’s breathing capacity and the volume of air in their lungs.

Venous occlusion plethysmography

The patient is installed in a supine position. The cuff positioned at mid-thigh is inflated so as to block venous return. The calf volume increases until it reaches a plateau. When the cuff is deflated, the calf volume decreases according to the rate of venous emptying.

Air plethysmography

An inflated air chamber surrounds the patient’s leg. He is lying with his legs elevated and his venous system is emptied. When he stands up, the volume of his leg increases to a plateau corresponding to the functional venous volume. Then the subject does flexion extensions which will have the effect of ejecting the blood from the veins. At each step, measurements are made.

Precautions to take

Plethysmograms are non-invasive examinations without any risks. Nevertheless, it is necessary that the patient informs whether he is undergoing drug treatment, and, in the case of pulmonary plethysmography, he should not smoke beforehand. People with claustrophobia are unlikely to be able to perform the chamber examination. Desktop models of plethysmographs, without cab and without gas, are now available.

Plethysmographs are found in health centers, hospitals, medical offices, cardiologists, phlebologists, pulmonologists, pharmacies. The sale and use of plethysmographs are reserved for healthcare professionals (doctors, cardiologists, pulmonologists, etc.). Their handling requires an initiation. Plethysmographs are also used in animal research.

History

One of the first invented plethysmographs (late XNUMXth century) used water instead of air to assess changes in the volume of a limb. This instrument consisted of a sealed cylinder filled with water, in which the subject’s hand and forearm were enclosed. The cylinder was connected to a U-tube. The oscillations of the liquid level in the smaller branch of the U-tube were transmitted through a float to a lever equipped with a feather which traced the variations on a cylinder. turning.