Functional residual capacity: the human windbag

Like the windbag of a bagpipe, the functional residual capacity (FRC) is the lung volume that acts as a reservoir of air for physiologic use. This reserve volume is particularly important during the period of apnea that occurs during induction of general anesthesia. The balance of the inward elastic recoil of the lung and the outward chest wall forces determines the FRC. Inward recoil forces are dependent on the interaction between the fibrous skeleton of the lung tissue and the alveolar surface tension regulated by pulmonary surfactant. Positioning and the use of inhaled and intravenous anesthetics influence outward chest wall forces. Factors that affect the FRC may be altered by volume recruitment maneuvers such as administration of vital capacity breaths, the application of positive end-expiratory pressure, and/or maintenance of anesthesia with a fraction of inspired oxygen of less than 1.0. This course reviews the basic anatomy and physiology of the FRC during the perioperative period. Understanding the processes that contribute to intraoperative loss of lung volume and knowledge of interventions that can allay them are paramount to providing a reliable and safe general anesthetic.