Measurement of functional residual capacity during mechanical ventilation

Functional residual capacity (FRC) is defined, in classical physiology, as the volume of gas remaining in the lungs at the end of expiration. In other words, FRC is the volume at which the elastic recoil pressure of the chest wall equals that of the lung and, at FRC, the system is in equilibrium. If the mechanical properties of the system change, FRC will change as well: For example, if lung compliance decreases, elastic recoil pressure will increase, and FRC will decrease so that a new equilibrium with the elastic recoil pressure of the chest wall is reached. Moreover, if a fraction of the alveoli collapse or are flooded (as frequently occurs in the setting of acute lung injury [ALI]) this will also result in a decrease in FRC. On the other hand, if at end-expiration the airway pressure is kept above the atmospheric one by application of a positive-end expiratory pressure (PEEP), the system will reach a different equilibrium (i.e., FRC) at a higher lung volume, which is usually termed the end-expiratory lung volume (EELV, which corresponds to the FRC in the presence of PEEP, although in this chapter we will use the term FRC for FRC and EELV). The action of PEEP can, moreover, determine the re-opening of previously collapsed alveoli (recruitment). In this case, the increase in FRC will be greater than expected, because the system will shift to a different pressure-volume curve (Fig. 1). For this reason, FRC appears to be a very promising tool for monitoring lung recruitment.