Plethysmographs

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).

Use for lungs
It is most commonly used to measure the functional residual capacity (FRC) of the lungs -- that is, the volume in the lungs when the muscles of respiration are relaxed -- as well as the total lung capacity. A typical value for the FRC is 3 liters. 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 glottis is closed and 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 body compartment has increased.

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 in full versus 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.

Use for limbs
Some plethysmograph devices are attached to arms, legs or other extremities and used to determine circulatory capacity. Impedance plethysmography is a non-invasive method used to detect venous thrombosis in these areas of the body.

Use for genitals
Another common but more controversial type of plethysmograph is the penile plethysmograph. This device is used to measure changes in blood flow in the penis. Although some researchers use this device to assess sexual arousal and sexual orientation, the data are usually not admissible in court cases in the United States.

An approximate female equivalent to penile plethysmography is vaginal photoplethysmography.