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).
Organs studied
Lungs
Pulmonary plethysmographs are commonly used to measure the
functional residual capacity
Functional residual capacity (FRC) is the volume of air present in the lungs at the end of passive expiration. At FRC, the opposing elastic recoil forces of the lungs and chest wall are in equilibrium and there is no exertion by the diaphragm ...
(FRC) of the
lung
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...
s—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 his/her 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.
Methodological approach
Boyle's Law
Boyle's law, also referred to as the Boyle–Mariotte law, or Mariotte's law (especially in France), is an experimental gas law that describes the relationship between pressure and volume of a confined gas. Boyle's law has been stated as:
The ...
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:
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
Chronic obstructive pulmonary disease (COPD) is a type of progressive lung disease characterized by long-term respiratory symptoms and airflow limitation. The main symptoms include shortness of breath and a cough, which may or may not produce mu ...
, for instance, can easily be discovered because of the unique shape of the corresponding resistance loop.
Limbs
Some plethysmograph devices are attached to
arm
In human anatomy, the arm refers to the upper limb in common usage, although academically the term specifically means the upper arm between the glenohumeral joint (shoulder joint) and the elbow joint. The distal part of the upper limb between th ...
s,
leg
A leg is a weight-bearing and locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single, linear element ca ...
s 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
Impedance cardiography (ICG) is a non-invasive technology measuring total electrical conductivity of the thorax and its changes in time to process continuously a number of cardiodynamic parameters, such as stroke volume (SV), heart rate (HR), car ...
is a non-invasive method used to detect
venous thrombosis
Venous thrombosis is blockage of a vein caused by a thrombus (blood clot). A common form of venous thrombosis is deep vein thrombosis (DVT), when a blood clot forms in the deep veins. If a thrombus breaks off (embolizes) and flows to the lungs to ...
in these areas of the body.
Genitals
Another common type of plethysmograph is the
penile plethysmograph
Penile plethysmography (PPG) or phallometry is measurement of blood flow to the penis, typically used as a proxy for measurement of sexual arousal. The most commonly reported methods of conducting penile plethysmography involve the measurement of ...
. This device is used to measure changes in blood flow in the
penis
A penis (plural ''penises'' or ''penes'' () is the primary sexual organ that male animals use to inseminate females (or hermaphrodites) during copulation. Such organs occur in many animals, both vertebrate and invertebrate, but males do n ...
. Although some researchers use this device to assess
sexual arousal
Sexual arousal (also known as sexual excitement) describes the physiological and psychological responses in preparation for sexual intercourse or when exposed to sexual stimuli. A number of physiological responses occur in the body and mind as ...
and
sexual orientation
Sexual orientation is an enduring pattern of romantic or sexual attraction (or a combination of these) to persons of the opposite sex or gender, the same sex or gender, or to both sexes or more than one gender. These attractions are generall ...
, courts that have considered penile plethysmography generally rule that the technique is not sufficiently reliable for use in court.
[Myers JEB (2005). ''Myers on Evidence in Child, Domestic, and Elder Abuse Cases.'' Aspen Publishers Online ] An approximate female equivalent to penile plethysmography is
vaginal photoplethysmography
Vaginal photoplethysmography (VPG, VPP) is a technique using light to measure the amount of blood in the walls of the vagina. The device that is used is called a vaginal photometer.
Use
The device is used to try to obtain an objective measure o ...
, which optically measures blood flow in the vagina.
Use in preclinical research
Plethysmography is a widely used method in basic and
preclinical research
In drug development, preclinical development, also termed preclinical studies or nonclinical studies, is a stage of research that begins before clinical trials (testing in humans) and during which important feasibility, iterative testing and drug ...
to study respiration. Several techniques are used:
Respiratory parameters from conscious freely moving animals: whole-body plethysmography
Whole-body plethysmography is used to measure respiratory parameters in conscious unrestrained subjects, including quantification of
bronchoconstriction
Bronchoconstriction is the constriction of the airways in the lungs due to the tightening of surrounding smooth muscle, with consequent coughing, wheezing, and shortness of breath.
Causes
The condition has a number of causes, the most common bei ...
.
The standard plethysmograph sizes are for the study of mice, rats and guinea pigs. On request, larger plethysmographs can also be manufactured for other animals, such as rabbits, dogs, pigs, or primates.
The plethysmograph has two chambers, each fitted with a
pneumotachograph. The subject is placed in one of them (subject chamber) and the other remains empty (reference chamber).
The pressure change is measured by a differential pressure transducer with one port exposed to the subject chamber and the other to the reference chamber.
[Hong Wang, Venkatraman Siddharthan, Kyle K. Kesler, Jeffery O. Hall, Neil E. Motter, Justin G. Julander and John D. Morrey (2013). Fatal Neurological Respiratory Insufficiency Is Common Among Viral Encephalitide]
(free full text)
/ref>
Respiratory parameters from conscious restrained animals: double-chamber / head-out plethysmography
The double-chamber plethysmograph (dcp) measures respiratory parameters in a conscious restrained subject, including airway resistance
In respiratory physiology, airway resistance is the resistance of the respiratory tract to airflow during inhalation and exhalation. Airway resistance can be measured using plethysmography.
Definition
Analogously to Ohm's Law:
:R_ = \frac
Whe ...
and conductance. Different sizes of plethysmograph exist to study mice, rats or guinea pigs.
The head-out configuration is identical to the standard configuration described above except that there is no head chamber.
Of course the collar seal is still applied, so that the body chamber remains airtight. With only a thoracic signal, all parameters can be obtained except for specific airway resistance (SRaw) and specific airway conductance (Sgaw).
Resistance/compliance from sedated animals
In anesthetized plethysmography, lung resistance and dynamic compliance are measured directly because the subject is anesthetized.
Depending on the level of sedation, the subject may be spontaneously breathing (SB configuration) or under mechanical ventilation (MV configuration).
A flow signal and a pressure signal are required to calculate compliance and resistance.
Cerebral blood flow
Cerebral venous blood flow has been recently studied trying to establish a connection between Chronic cerebrospinal venous insufficiency
Chronic cerebrospinal venous insufficiency (CCSVI or CCVI) is a term invented by Italian researcher Paolo Zamboni in 2008 to describe compromised flow of blood in the veins draining the central nervous system. Zamboni hypothesized that it migh ...
and multiple sclerosis
Multiple (cerebral) sclerosis (MS), also known as encephalomyelitis disseminata or disseminated sclerosis, is the most common demyelinating disease, in which the insulating covers of nerve cells in the brain and spinal cord are damaged. This d ...
. The small study is not big enough to establish a conclusion, but some association has been shown.
See also
* Optoelectronic plethysmography
Optoelectronic plethysmography is a method to evaluate ventilation through an external measurement of the chest wall surface motion.
A number of small reflective markers are placed on the thoraco-abdominal surface by hypoallergenic adhesive tape. ...
* Photoplethysmograph
A photoplethysmogram (PPG) is an optically obtained plethysmogram that can be used to detect blood volume changes in the microvascular bed of tissue. A PPG is often obtained by using a pulse oximeter which illuminates the skin and measures cha ...
* Respiratory inductance plethysmography Respiratory inductance plethysmography (RIP) is a method of evaluating pulmonary ventilation by measuring the movement of the chest and abdominal wall.
Accurate measurement of pulmonary ventilation or breathing often requires the use of devices su ...
References
{{reflist
Further reading
* Glaab T, Taube C, Braun A, Mitzner W (2007) Invasive and noninvasive methods for studying pulmonary function in mice. ''Respiratory Research'' 8:63
free full text article
External links
Information on pulmonary function testing and physician reimbursement
Information on plethysmography used in preclinical research
Medical equipment
Measuring instruments
Respiratory therapy
Pulmonary function testing
Sexual health