Atelectasis formation during anesthesia: causes and measures to prevent it.

Pulmonary gas exchange is regularly impaired during general anaesthesia with mechanical ventilation. This results in decreased oxygenation of blood. A major cause is collapse of lung tissue (atelectasis), which can be demonstrated by computed tomography but not by conventional chest x-ray. Collapsed lung tissue is present in 90% of all subjects, both during spontaneous breathing and after muscle paralysis, and whether intravenous or inhalational anaesthetics are used. There is a correlation between the amount of atelectasis and pulmonary shunt. Shunt does not increase with age. In obese patients, larger atelectatic areas are present than in lean ones. Finally, patients with chronic obstructive lung disease may show less or even no atelectasis. There are different procedures that can be used in order to prevent atelectasis or to reopen collapsed lung tissue. The application of positive end-expiratory pressure (PEEP) has been tested in several studies. On the average, arterial oxygenation does not improve markedly, and atelectasis may persist. Further, reopened lung units re-collapse rapidly after discontinuation of PEEP. Inflation of the lungs to an airway pressure of 40 cm H2O, maintained for 7-8 seconds (recruitment or "vital capacity" manoeuvre), re-expands all previously collapsed lung tissue. During induction of anaesthesia, the use of a gas mixture, that includes a poorly absorbed gas such as nitrogen, may prevent the early formation of atelectasis. During ongoing anaesthesia, pulmonary collapse reappears slowly if a low fraction of oxygen in nitrogen is used for the ventilation of the lungs after a previous VC-manoeuvre. On the other hand, ventilation of the lungs with pure oxygen results in a rapid reappearance of atelectasis. Thus, ventilation during anaesthesia should be done if possible with a moderate fraction of inspired oxygen (FIO2, e.g. 0.3-0.4). Alternatively, if the lungs are ventilated with a high inspiratory fraction of oxygen, the use of PEEP may be considered. In summary, atelectasis is present in most humans during anaesthesia and is a major cause of impaired oxygenation. Avoiding high fractions of oxygen in inspired gas during induction and maintenance of anaesthesia may prevent formation of atelectasis. Finally, intermittent "vital capacity"-manoeuvres together with PEEP reduces the amount of atelectasis and pulmonary shunt.