A man connected to a body plethysmograph for estimation of FRC inspires against a closed glottis. Which of the following statements is true?
**Core Concept**
The body plethysmograph is a device used to estimate functional residual capacity (FRC) in the lungs. When a person inspires against a closed glottis, the airway is occluded, preventing air from entering the lungs. This maneuver is known as the "occlusion technique" or "glottic occlusion method."
**Why the Correct Answer is Right**
When a person inspires against a closed glottis, the pressure inside the lungs increases due to the elastic recoil of the lungs and the inward movement of the chest wall. Since the airway is occluded, no air can enter the lungs, and the pressure increase is solely due to the elastic recoil of the lungs. The body plethysmograph measures the change in volume of the box surrounding the person, which is equal to the change in lung volume. By measuring the pressure increase and the change in volume, the FRC can be estimated.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the relationship between the occlusion technique and FRC measurement. The occlusion technique is used to estimate FRC, but it does not directly measure it.
**Option B:** This option is incorrect because it implies that the occlusion technique measures the residual volume (RV) rather than the FRC. While the occlusion technique can be used to estimate FRC, it is not a direct measure of RV.
**Option C:** This option is incorrect because it suggests that the occlusion technique is used to measure the total lung capacity (TLC). The occlusion technique is specifically used to estimate FRC, not TLC.
**Option D:** This option is incorrect because it implies that the occlusion technique is used to measure the inspiratory capacity (IC). While the occlusion technique can be used to estimate FRC, it is not a direct measure of IC.
**Clinical Pearl / High-Yield Fact**
The occlusion technique is a non-invasive method for estimating FRC, which is an important parameter in respiratory physiology. FRC is the volume of air left in the lungs after a normal expiration, and it is a critical factor in determining the respiratory status of patients.
**Correct Answer:** C. The occlusion technique measures the change in lung volume and pressure to estimate the functional residual capacity (FRC).