About the Body Plethysmograph

Refer to the figure below for illustration.

Any lung volume that can be voluntarily maintained can be measured by the body plethysmograph.

It is most commonly used to measure FRC .

Here's how it works:

Topic 2: Body Plethysmograph

Roxanne Rapan

Helpful URLs:

http://infonet.welch.jhu.edu/~omie/res_phys/Encyclopedia/BodyPleth/
BodyPleth.HTML			(includes quick time movie!)
http://morgansci.com/body.htm		(order your own body box!)
http://morgansci.com/bodyap.htm
http://morgansci.com/bodym.htm

Definition:
	The body plethysmograph is an enclosed chamber used to measure
lung parameters and volumes.

Relevance:
	Body plethysmography can be used to measure some of the more
elusive lung volumes: functional residual capacity (FRC), residual volume,
and total lung capacity, as well as airway resistance (and its reciprocal,
conductance). Boyle’s law states that at constant temperature, the product
of volume and pressure (P*V) of a gas is constant. Accordingly, in an
airtight cabin, pressure and volume changes can be used to calculate the
lung volumes. Additionally, if the volume is known, alveolar pressures can
be calculated.
	Inside a plethysmograph, a subject breathes normally. At the end
of an expiration, a shutter next to the mouth is closed, and the subject
makes breathing efforts. We know the starting volume and pressure of the
chamber, and can measure the change in pressure and volume. Rearranging
Boyle’s Law gives us the final volume:

Vfinal=Pinit*Vinit/Pfinal

Vfinal here is the final volume of the box, not the lung. However, the
difference between Vinit and Vfinal is equal to the increase in lung
volume during the inspiratory effort (we’ll call it Vinsp). Now we need to
find the initial volume of air in the lung. Using Boyle’s law again,

			VFRC*Pinit=(VFRC+Vinsp)*Pfinal

In other words (i.e. Dr. Zhang’s):

FRC=-Pinit * (-)dV/dP 

Sample Problem:
	A subject in a body plethysmograph makes breathing efforts against
the shutter. The final pressure in the box is measured to be 750 mmHg, and
the volume change to be 30 mL. What is the subject’s functional residual
capacity? Assume constant temperature.			(2280mL)
What could be said if the same patient, using the gas dilution method of
measuring FRC, had been measured to have an FRC=1780mL?
(patient probably has a lung disease resulting in an obstructed airway or
airways)

References:	West, John B. Respiratory Physiology. pg. 12-14
Respir. Care 1994:39(12):1184-1190

Sample Problems:

Answers:

For more information on this topic, please refer to West , page .

Also, check out the following links that may be helpful:

• And, as always, if you have questions, or if you can suggest a helpful link, please e-mail me.

No!
Please don't make me go to any of those pages!
I want to go somewhere completely different!

This page was written by Roxanne Rapan , a student in this course.

BME 403 Pages maintained by the T.A., Douglas Miles.