CPFT Practice Exam 2025 – Complete Test Prep Guide

Plethysmography is a technique used to measure the volume of gas in the lungs, specifically focusing on the total lung volume present within the thoracic cavity. This includes both the gas that is actively exchanged during breathing and the residual volume that remains in the lungs even after a maximal exhalation.

The correct response highlights that plethysmography assesses all the gas in the thorax, providing a comprehensive evaluation of lung volumes. It is particularly useful in pulmonary function testing because it can capture volumes that cannot be measured directly by other methods, such as simple spirometry that only assesses the volume of air that can be inhaled or exhaled during specific maneuvers.

Other choices present narrower views of what plethysmography measures. The option suggesting measurement of only the inhaled gas overlooks the complexity of lung volumes and risks omitting significant residual volumes. Similarly, measuring only the volumes present during tidal breathing fails to account for various lung capacities, including the total lung capacity and residual volume. Lastly, mentioning gases within the lungs at atmospheric pressure does not apply to the function of plethysmography, as the lung volumes measured are within the dynamic pressures of the respiratory cycle, not atmospheric conditions.