Which method is used to measure Functional Residual Capacity (FRC)?

The method used to measure Functional Residual Capacity (FRC) is helium dilution. This technique involves the patient inhaling a known concentration of helium mixed with a carrier gas, typically oxygen or air. The distribution of helium in the lungs is then measured, which allows for the calculation of FRC based on the dilution of helium in the alveolar space.

Helium dilution is particularly useful because helium is insoluble in blood, ensuring that the measurements reflect the volume of gas remaining in the lungs after normal expiration. This provides an accurate assessment of the FRC, which is the volume of air in the lungs at the end of a normal, passive expiration. By knowing the initial concentration of helium and measuring how much it has diluted in the lungs, clinicians can determine the FRC effectively.

Other methods, such as cardiac echo assessment, peak expiratory flow measurement, and oximetry, do not specifically measure FRC. Cardiac echo is primarily used for imaging the heart and diagnosing cardiovascular conditions. Peak expiratory flow measurement assesses the maximum speed of expiration, helping to evaluate airway obstruction but not lung volumes directly. Oximetry measures oxygen saturation in the blood, which also does not provide information about lung volumes. Thus, helium dilution stands out as the