Body Plethysmography

Angela Lorenzo, MS, RRT, RPFTRespiratory Care DivisionSchool 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 Presentation

• TV: tidal volume• TLC: total lung capacity• VC: vital capacity• SVC: slow vital capacity• FVC: forced vital

capacity• FRC: functional residual

volume• RV: residual volume

• ERV: expiratory reserve volume

• IRV: inspiratory reserve volume

• 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 measured– Functional residual volume– Total lung capacity

Tests Measuring RV

• Body Plethysmography• Nitrogen Washout• Inert Gas Testing• Chest X-Ray• CT Scan• MRI• 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 • Or• RV = mean FRC - largest ERV,• TLC = RV + largest VC.

Helium Dilution Technique

• Helium is inert, does not cross the A-C membrane,

Images from www.morgansci.com

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

Technique• 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 www.morgansci.com

Body Plethysmography

• Understanding the principle helps get a good test

• 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 constant

• P and V vary inversely

Body Plethysmography

• P1

• V1

• P2

• V2

images from www.morgansci.com

Thermal Equilibration is Essential

• Can’t short cut this phase of the test or results are inaccurate

Graphic Representation

Image from Medgraphics: Disease Management with Body Plethysmography

Image from ERS: http://dev.ersnet.org/uploads/Document/53/WEB_CHEMIN_2555_1194522257.pdf

Poor Timing Closing Shutter Valve

Bronchiectasis

Sir Humphry Davy

• 12/17/1778-5/29/1829• Penzance, Cornwal, England