What is the most accurate way to measure Functional Residual Capacity (FRC)?

The most accurate method for measuring Functional Residual Capacity (FRC) is through plethysmography, particularly body plethysmography. This technique measures the volume of air in the lungs after a normal expiration and accounts for all lung compartments, including those that are not participating in gas exchange.

Body plethysmography utilizes a sealed chamber where changes in pressure and volume can be accurately measured as the patient breathes in and out. When a patient breathes against a closed shutter, the changes in lung volume due to the inhalation and exhalation can be calculated using Boyle's law. This allows for a precise determination of FRC, as it captures the total volume of air, including that which is trapped in the lungs at the end of normal expiration.

Other methods, such as gas dilution and spirometry, while useful for assessing lung function, do not provide an accurate measurement of FRC due to their limited ability to account for residual air in the lungs. Gas dilution methods like helium dilution can underestimate FRC because they may not measure areas where gas cannot reach, such as poorly ventilated or collapsed alveoli. Spirometry primarily measures volumes during forced maneuvers and does not adequately calculate FRC as it focuses on the tidal volumes without