The presence of atelectasis during the perioperative period is likely a multifactorial process. Possible etiologies include the following:
- Compression: The properties of the thorax and lung change leading to decreased compliance and decreases FRC (decreased by 1 L supine + 0.4 L during general anesthesia). As the low lung volumes approach residual volume, small airways (less than 1 mm with no cartilage) collapse during expiration. Loss of diaphragmatic motion is crucial to the development of atelectasis as abdominal pressure displaces the dorsal portion of the diaphragm cephalad leading to increased pleural pressure in the lower lung portions. When pleural pressure exceeds transpulmonary pressure, atelectasis is favored.
- Absorption of alveolar gas: In areas with low V/Q ratios, more gas is absorbed from capillaries than enters the alveolus. This is commonly observed in patients with very low mixed venous saturation or in patients who are administered highly soluble gas mixtures (high FiO2, nitrous). At FiO2 of 0.3, atelectasis takes hours to develop secondary to absorption; however, this is reduced to approximately 8 minutes with an FiO2 of 1.0. Also, small airways may become completely occluded causing distal gas to be trapped and absorbed leading to airway collapse. It should be noted that the presence of even a small amount of nitrogen in inhaled gas significantly decreases the amount of absorption atelectasis. Akca et al found that the incidence and severity of atelectasis, lung volumes, and alveolar gas exchange were comparable in patients given 30% and 80% perioperative oxygen. The authors concluded that administration of 80% oxygen in the perioperative period does not worsen lung function if it is administered with a small amount of nitrogen.
- Alteration of surfactant: Once atelectasis forms, production of surfactant decreases, which leads to more airway collapse and instability if airways reopen. Decreased surfactant leads to increased surface tension and a decrease in FRC. Surfactant has a long half-life and is replaced in 14 hours so this mechanism is thought to be more important in prolonged mechanical ventilation.
O Akça, A Podolsky, E Eisenhuber, O Panzer, H Hetz, K Lampl, F X Lackner, K Wittmann, F Grabenwoeger, A Kurz, A M Schultz, C Negishi, D I Sessler Comparable postoperative pulmonary atelectasis in patients given 30% or 80% oxygen during and 2 hours after colon resection. Anesthesiology: 1999, 91(4);991-8