Disclaimer: This article is intended solely for informational and educational purposes only. It does not constitute medical advice.
Administration of 100% oxygen is a standard part of preoxygenation before general anesthesia. By replacing nitrogen in the lungs with oxygen, preoxygenation increases the body’s oxygen reserve and gives clinicians more time to safely secure the airway before blood oxygen levels begin to fall. However, high concentrations of oxygen can also promote atelectasis, a condition in which small areas of the lung collapse and no longer fill normally with air. Although atelectasis is often temporary, it can reduce the efficiency of gas exchange and contribute to lower oxygen levels during and after surgery. The relationship between 100% oxygen and atelectasis therefore reflects an important clinical balance: high concentrations of oxygen provide a safety net during airway management, but they can also make parts of the lung more likely to collapse (1).
High oxygen concentrations can lead to lung collapse due to a process known as absorption atelectasis. Normally, air contains a large amount of nitrogen. Unlike oxygen, nitrogen is not rapidly absorbed into the bloodstream, so it remains inside the small air sacs of the lungs, called alveoli, and helps keep them open. When a patient breathes 100% oxygen, much of this nitrogen is washed out of the lungs. If airflow to a small region of the lung then becomes reduced or blocked, the oxygen in that area is gradually absorbed into the blood. With little nitrogen remaining to maintain the volume of the alveolus, the air sac can shrink and eventually collapse (1).
General anesthesia makes this process more likely to occur. Anesthetic medications reduce muscle tone, change the position of the diaphragm, and decrease the amount of air remaining in the lungs at the end of a normal breath. These changes are especially important in the lower, dependent portions of the lungs, where small airways are more likely to close. Studies using computed tomography have shown that areas of atelectasis can develop within minutes after the induction of anesthesia. The concentration of oxygen being given also matters. Edmark and colleagues found that patients who received 100% oxygen during induction developed more atelectasis than those who received lower oxygen concentrations (2). However, patients receiving 100% oxygen also experienced a longer period of time before their blood oxygen levels began to fall during apnea, demonstrating the practical benefit of preoxygenation.
High concentrations of oxygen during emergence from anesthesia can also increase the risk of atelectasis. In one study, patients who received a higher inspired oxygen concentration before removal of the breathing tube developed more postoperative atelectasis than patients who received a lower concentration (3). Additional research has shown that lung collapse tends to develop more quickly and extensively as the concentration of inspired oxygen increases.⁵ These findings suggest that the effects of oxygen concentration are not limited to induction; exposure throughout anesthesia may influence how well the lungs remain expanded.
Importantly, the association between 100% oxygen and atelectasis does not mean that pure oxygen should be routinely avoided. In situations where oxygen levels may fall quickly, such as difficult airway management, obesity, pregnancy, or critical illness, maximizing oxygen reserves before anesthesia can be essential. Instead, clinicians can reduce the risk of atelectasis by using techniques that help keep the lungs open. Positive end-expiratory pressure, or PEEP, maintains a small amount of pressure in the lungs at the end of each breath, while recruitment maneuvers can help reopen collapsed areas.³ Once the airway is secured and the patient is stable, the oxygen concentration can be reduced to a lower level that still provides adequate oxygenation. Overall, 100% oxygen remains an important safety tool during anesthesia, but its use should be balanced with strategies that protect the lungs and limit unnecessary alveolar collapse.
References
- Rothen HU, Sporre B, Engberg G, Wegenius G, Reber A, Hedenstierna G. Prevention of atelectasis during general anaesthesia. Lancet. 1995;345(8962):1387-1391. doi:10.1016/s0140-6736(95)92595-3
- Edmark L, Kostova-Aherdan K, Enlund M, Hedenstierna G. Optimal oxygen concentration during induction of general anesthesia. Anesthesiology. 2003;98(1):28-33. doi:10.1097/00000542-200301000-00008
- Benoît Z, Wicky S, Fischer JF, et al. The effect of increased FIO(2) before tracheal extubation on postoperative atelectasis. Anesth Analg. 2002;95(6):. doi:10.1097/00000539-200212000-00058
- Neumann P, Rothen HU, Berglund JE, Valtysson J, Magnusson A, Hedenstierna G. Positive end-expiratory pressure prevents atelectasis during general anaesthesia even in the presence of a high inspired oxygen concentration. Acta Anaesthesiol Scand. 1999;43(3):295-301. doi:10.1034/j.1399-6576.1999.430309.x