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).
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 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 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.
Some plethysmograph devices are attached to arms, legs or other extremities and used to determine circulatory capacity. In water plethysmography an extremity e.g. an arm is enclosed in a water-filled chamber where volume changes can be detected. Air plethysmography uses a similar principle but based on an air-filled long cuff, which is more convenient but less accurate. Another practical device is mercury-filled strain gauges used to continuously measure circumference of the extremity, e.g at mid calf. Impedance plethysmography is a non-invasive method used to detect venous thrombosis in these areas of the body.
Another common 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, which optically measures blood flow in the vagina.
Plethysmography is a widely used method in basic and preclinical research to study respiration. Several techniques are used:
This is the gold standard because it provides direct measurement of resistance (and its inverse compliance). The drawback is that it requires anesthesia and expertise in animal handling. The subject breathes spontaneously or is ventilated.
The subject is conscious but restrained. It is placed inside a plethysmograph with two chambers. A seal around the subject's neck forms two separate chambers. Measured parameters include specific airway resistance which is measured from the phase shift between thorax and nasal flow curves.
A variant on this method is head-out plethysmography, with one chamber, in which only thorax flow is measured.
The subject is conscious and can move about freely within the plethysmograph. Only one signal is measured. This signal represents the sum of two flows: nasal flow (due to movement of air into and out of the nose) and thoracic flow (due to rise and fall of chest cavity).
Measured parameters include enhanced pause (or penh), an indicator of bronchoconstriction.