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Plethysmography

Pulmonary plethysmographs are commonly used to measure the functional residual capacity (FRC) of the lungs - the volume in the lungs when the muscles of respiration are relaxed - and total lung capacity.

In a traditional plethysmograph, the test subject is placed inside a sealed chamber the size of a small telephone booth with a single mouthpiece. At the end of normal expiration, the mouthpiece is closed. The patient is then asked to make an inspiratory effort. As the patient tries to inhale (a maneuver which looks and feels like panting), the lungs expand, decreasing pressure within the lungs and increasing lung volume. This, in turn, increases the pressure within the box since it is a closed system and the volume of the box compartment has decreased to accommodate the new volume of the subject.

Boyle's Law is used to calculate the unknown volume within the lungs. First, the change in volume of the chest is computed. The initial pressure and volume of the box are set equal to the known pressure after expansion times the unknown new volume. Once the new volume is found, the new volume minus the original volume is the change in volume in the box and also the change in volume in the chest. With this information, Boyle's Law is used again to determine the original volume of gas: the initial volume (unknown) times the initial pressure is equal to the final volume times the final pressure.

The difference between full and empty lungs can be used to assess diseases and airway-passage restrictions. An obstructive disease will show increased FRC because some airways do not empty normally, while a restrictive disease will show decreased FRC. Body plethysmography is particularly appropriate for patients who have air spaces which do not communicate with the bronchial tree; in such patients, gas dilution would give an incorrectly low reading.

Newer lung plethysmograph devices have an option which does not require enclosure in a chamber.

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Babu, Sateesh
Babu, Sateesh
Chief, Vascular Surgery
Vascular Surgery
Laskowski, Igor
Laskowski, Igor
Chief, Endovascular Surgery
Vascular Surgery
Carroll, Francis X.
Thoracic Surgery
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Goyal, Arun
Vascular Surgery
Mateo, Romeo B.
Vascular Surgery
Shah, Pravin M.
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