How do helium dilution and body plethysmography measure Functional Residual Capacity (FRC)?

From the Guidelines

Helium dilution and body plethysmography are two distinct methods for measuring functional residual capacity (FRC), with body plethysmography being more accurate, especially in patients with airflow obstruction. Helium dilution measures FRC by having the patient breathe from a closed circuit containing a known concentration of helium gas, which becomes diluted as the patient breathes, allowing FRC to be calculated using the principle of conservation of mass 1. However, this method only measures communicating air spaces and may underestimate FRC in patients with obstructive lung diseases where air trapping occurs. On the other hand, body plethysmography uses Boyle's law to determine FRC by having the patient sit in an airtight chamber and perform breathing maneuvers against a closed shutter, measuring changes in box pressure and mouth pressure to calculate thoracic gas volume 1. This method measures all air in the chest, including trapped air, typically yielding higher FRC values than helium dilution in obstructive diseases. Some key considerations for body plethysmography include:

• Calculating the mean slope of each respiratory effort against the occlusion, not just the inspiratory limb 1
• Excluding periods of noise immediately after shutter closure and during the peaks and troughs of each respiratory tug 1
• Subtracting the volume above FRC at the time of occlusion 1
• Using the technique to measure functional residual capacity (FRCp or FRCpleth) and airway resistance (Raw), and calculating other key parameters such as specific airway resistance (sRaw) and specific conductance (sGaw = Gaw/FRC) 1.

From the Research

Measurement of FRC by Helium Dilution and Body Plethysmography

• Helium dilution technique measures FRC by diluting a known volume and concentration of helium in the lungs with the air already present in the lungs 2, 3, 4, 5
• Body plethysmography measures FRC by detecting changes in pressure and volume within a sealed box as a person breathes in and out 2, 3, 4, 5, 6

Comparison of Helium Dilution and Body Plethysmography

• In patients with obstructive lung disease, helium dilution tends to underestimate FRC, while body plethysmography tends to overestimate it 2, 3, 4
• In patients with restrictive lung disease, both methods give comparable results 2, 3
• The difference between the two methods increases with increasing lung volumes 3

Principles of Body Plethysmography

• Body plethysmography relies on the fact that generation of airflow needs generation of pressure, which causes a shift in volume 6
• The shift volume is measured via the variation of box pressure, and the relationship between shift volume and alveolar pressure is assessed in a shutter maneuver 6

Clinical Use of Body Plethysmography

• Body plethysmography allows for the assessment of functional residual capacity (FRC), specific airway resistance (sRaw), total lung capacity (TLC), and residual volume (RV) 6
• It provides significant additional information compared to spirometry and has potential for further development 6