How is functional residual capacity (FRC) measured with plethysmography?

Functional Residual Capacity (FRC) is measured using plethysmography through a method where the patient pants against a closed shutter valve. This technique helps to determine the FRC by evaluating changes in lung volume as the chest and abdominal cavity expand and contract with each breath. When the patient pants against the closed valve, it creates pressure changes within the plethysmograph, allowing for accurate measurements of lung volumes, including the total volume of air present in the lungs after a normal expiration.

The panting maneuver increases intrathoracic pressure variations, which enables the detection of FRC through calculations that involve changes in pressure and volume. This method is particularly useful because it allows the technician to obtain precise measurements without the need for the patient to perform deep inhalation or expiration, thus providing an accurate reflection of lung volumes in accordance with functional conditions.

In contrast, using a closed circuit or measuring residual volume only does not provide the comprehensive assessment required to accurately determine FRC, while tidal breathing alone would not allow for the necessary volume and pressure measurements during the panting maneuver.