The person breathes until the helium concentration reaches a new steady state at a lower level. CO 2 is absorbed by soda lime, and O 2 is added to the system to replace that consumed. If the initial and final concentrations of helium are known, it is easy to determine the FRC by calculating how much helium-free gas had to be added to the volume of the closed system to cause the observed drop in helium concentration. This ratio, when less than normal, is useful in indicating the severity of obstruction. The FVC maneuver is useful in assessing the mechanical properties of the lung.
Greater efforts give greater flow rates.
However, over approximately the lower two thirds of the VC, flow is limited by these mechanical properties in both healthy and diseased lungs. At modest pleural pressures, flow becomes limited over this volume range because further increases in pleural pressure fail to produce greater flows. The physiology and pathophysiology of expiratory flow limitation has been modeled in several ways. The most current and most likely mechanism for flow limitation is described by the wave speed theory. During a forced exhalation, alveolar pressure exceeds pleural pressure by an amount equal to the elastic recoil pressure of the lung.