Determination of functional residual capacity, residual volume, and total lung capacity

2021-05-17 03:15 PM

Volumetric measuring device with a volume filled by air mixed with helium gas. Before breathing from this device, each person was exhaled normally. At the end of expiration, the volume maintained in the lungs by FRC.

Functional residual capacity (FRC) is the volume of air that maintains the lungs during a normal expiration, which is important in lung function.

Since it is of particular value in certain lung diseases, it is often used to measure volume. Pulmonometers cannot be used to accurately measure FRC because air in the residual lung volume cannot be exhaled into the device, and this volume produces about FRC. To measure FRC, the volumetric device must use the indirect, Helium gas dilution method.

Volumetric measuring device with a volume filled by air mixed with helium gas. Before breathing from this device, each person was exhaled normally. At the end of expiration, the volume maintained in the lungs by FRC. At this point, the substance immediately begins to breathe from the device and the device gas mixes with the lung gas. As a result, the helium begins to be diluted by the FRC gas, and the volume of the FRC can be calculated from the helium dilution, using the formula:

         FRC = (Ci(He) / Cf(He)-1) x Vi (device)

FRC: functional residual capacity.

Ci(He) initial concentration of He in the device.

Cf(He) final concentration of He in the device.

Vi (device) initial volume of the device.

For each FRC determined, the residual volume (RV) can be determined by the expiratory residual volume (ERV) measured by a normal volumetric device, from the FRC. Total lung capacity (TLC) can be determined by adding inspiratory capacity (IC) and FRC.

RV= FRC – ERV

TLC= FRC + IC