Body Plethysmography- A Pulmonary Function Test- Sayali

Pulmonary function tests (PFTs) are breathing tests to find out how well you move air in and out of your lungs and how well oxygen enters your body. The most common PFT’s are spirometry, diffusion studies and body plethysmography.

Here, We will be discussing about Body plethysmography. It is a test to find out how much air is in your lungs after you take in a deep breath and how much air is left in your lungs after breathing out as much as you can.

Body plethysmography is particularly appropriate for patients who have air spaces within the lung that do not communicate with the bronchial tree. With COPD (Chronic obstructive pulmonary disease), the amount of air left in your lungs is often more than normal. Measuring the total amount of air your lungs can hold and the amount of air left in your lungs after you breathe out gives your doctor information about your COPD and helps guide them in your treatment. 

In body plethysmography, the patient sits inside an airtight box, inhales or exhales to a particular volume and then a shutter drops across his breathing tube. The subject makes respiratory efforts against the closed shutter, causing his chest volume to expand and decompressing the air in his lungs. The increase in his chest volume slightly reduces the box volume (the non-person volume of the box) and thus slightly increases the pressure in the box.

To compute the original volume of air in the lungs, we first compute the change in volume of the chest. Using Boyle's Law (P1V1=P2V2, at constant temperature), we set the initial pressure in the box times the initial volume of the box (both of which we know), equal to the pressure times volume of the box at the end of a chest expansion (of which we know only the pressure).

For more understanding, you can watch this video:


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