Plethysmograph

A plethysmograph is an instrument for measuring changes in volume within an organ or whole body (usually resulting from fluctuations in the amount of blood or air it contains). The word is derived from the Greek "plethysmos" (increasing, enlarging, becoming full), and "graphein" (to write).

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 (or "body box"), the test subject, or patient, 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.

With cabinless plethysmography, the patient is seated next to a desktop testing device and inserts the mouthpiece into their mouth. The patient takes a series of normal tidal breaths for approximately one minute. During this tidal breathing, a series of rapid interruptions occurs, with a shutter opening and closing, measuring pressure and volume. Lung volume measurements taken with cabinless plethysmography are considered equivalent to body plethysmography.

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 of the box times its volume is considered equal to the known pressure after expansion times the unknown new volume. Once the new volume is found, the original volume minus the new 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 in the chest: the initial volume (unknown) times the initial pressure is equal to the final volume times the final pressure. Starting from this principle, it can be shown that the functional residual capacity is a function of the changes in volume and pressures as follows:

${\displaystyle FRC=K_{V}{\frac {\Delta V_{box}}{\Delta P_{mouth}}},K_{V}\approx P_{Barometric}}$

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 helium dilution would give an incorrectly low reading.

Another important parameter, which can be calculated with a body plethysmograph is the airway resistance. During inhalation the chest expands, which increases the pressure within the box. While observing the so-called resistance loop (cabin pressure and flow), diseases can easily be recognized. If the resistance loop becomes planar, this shows a bad compliance of the lung. A COPD, for instance, can easily be discovered because of the unique shape of the corresponding resistance loop.