Impact of Anesthesia on Lung Function: Atelectasis, Shunt, and V/Q Mismatch

Abstract

Anesthesia significantly impacts lung function, primarily through reductions in functional residual capacity (FRC), airway closure, and atelectasis formation. These changes disrupt ventilation-perfusion (V/Q) relationships, impair oxygenation, and contribute to postoperative pulmonary complications. The decrease in FRC is largely due to the loss of respiratory muscle tone, particularly the upward displacement of the diaphragm. Airway closure occurs when extraluminal pressure exceeds intraluminal pressure, predominantly affecting dependent lung regions and leading to V/Q mismatch. Atelectasis, present in approximately 90% of anesthetized patients, is caused by airway closure and gas resorption, and can persist for several days postoperatively. Strategies to prevent atelectasis include positive end-expiratory pressure (PEEP), recruitment maneuvers, and careful management of inspired oxygen concentrations. During anesthesia, ventilation is redistributed from dependent to nondependent lung regions, while perfusion increases progressively from ventral to dorsal regions. Hypoxic pulmonary vasoconstriction (HPV) reduces perfusion in atelectatic regions but is inhibited by inhalational anesthetics. A three-compartment lung model, including normal V/Q, low V/Q, and shunt regions, effectively explains oxygenation impairment during anesthesia. Techniques such as the multiple inert gas elimination technique (MIGET) and single-photon emission computed tomography (SPECT) provide detailed insights into V/Q relationships. Individualized approaches, including continuous positive airway pressure (CPAP) during induction and appropriate PEEP during and after anesthesia, can optimize lung function and minimize postoperative complications.