Body Plethysmography – Angela Lorenzo MS, RRT, RPFT

Body Plethysmography
Angela Lorenzo, MS, RRT, RPFT
Respiratory Care Division
School of Health Science
We knew this the whole time!
• Thank you to those who
pioneered our
Profession to make it a
“Best Job”
• Thank you to all in the
Profession today who
continue to make it a
“Best Job”
From AARC CPG: Static Lung Volumes
Abbreviations Used in this
TV: tidal volume
TLC: total lung capacity
VC: vital capacity
SVC: slow vital capacity
FVC: forced vital capacity
FRC: functional residual
• RV: residual volume
• ERV: expiratory reserve
• IRV: inspiratory reserve
• IC: inspiratory capacity
• P: pressure
• V: volume
• Pleth: plethysmography
• FRCpleth:
• FRCN2:
Measuring Lung Volumes
• The VC and all of its components can be
directly measured
– Tidal volume
– Inspiratory reserve volume
– Expiratory reserve volume
• The RV and all it comprises cannot be directly
– Functional residual volume
– Total lung capacity
Tests Measuring RV
Body Plethysmography
Nitrogen Washout
Inert Gas Testing
Chest X-Ray
CT Scan
All tests have limitations
1799: First Recorded Measure of RV
• Sprigg’s article on Sir Humphry Davy:
• “describes the measurement of his own lung
volumes, including the first recorded measurement
of the residual volume. He measured his own rates of
oxygen consumption and carbon dioxide production.
He is famous for his investigations into nitrous oxide,
but he also investigated the effects of breathing nitric
oxide and carbon monoxide. He made these
observations with a gasometer and analysis of his
expired air, and his work anticipates the invention of
blood gas analysis.” (from abstract)
• Anaesthesia. 2002 Apr;57(4):357-64
Gas Dilution Techniques
• Open Circuit:
– Nitrogen washout
• Closed Circuit:
– Inert gas testing
• Helium Dilution
Nitrogen Washout
A good test
Easy to perform, easy to instruct
Cost effective in terms of equipment
Often time consuming, especially in
obstructive patient and patients who arrive
with portable oxygen system in use
• Difficult to repeat as at least 15 minutes are
needed between efforts
Calculating FRC and RV Determined by
Gas Dilution
TLC = mean FRC + largest IC,
RV = TLC - largest VC
RV = mean FRC - largest ERV,
TLC = RV + largest VC.
Helium Dilution Technique
• Helium is inert, does not cross the A-C
Images from
Body Plethysmography
• Pre testing patient quotes:
– “I’ve got to get IN that box?”
– “You’re not going to close that door are you?”
• Post testing patient quotes:
– “All of a sudden I couldn’t breathe!”
– “The air just cut off!”
– “You said that would happen but….”
Instructing the Patient
• Good/effective instructions often take longer
than the test
• Let the patient know that
• Let the patient know it is normal to feel like
the air was cut off but that within seconds it
will open and be easy to breathe again
• Establish a stable resting FRC
• At end-exhalation close the shutter valve and
have the patient pant gently against the
closed airway
• Patient must also perform a SVC
images from
Body Plethysmography
• Understanding the principle helps get a good
• Boyle’s Law states that P1 x V1 = P2 x V2 at
constant temperature
• In other words, the product of P and V of a gas
is constant when temperature remains
• P and V vary inversely
Body Plethysmography
• P1
• V1
• P2
• V2
images from
Thermal Equilibration is Essential
• Can’t short cut this phase of the test or results
are inaccurate
Graphic Representation
Image from ERS:
Image from Medgraphics: Disease
Management with Body
Poor Timing Closing Shutter Valve
Sir Humphry Davy
• 12/17/1778-5/29/1829
• Penzance, Cornwal, England

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