Atelectasis formation during anesthesia: Causes and measures to prevent it

Pulmonary gas exchange is regularly impaired during general anaesthesia wit h mechanical ventilation. This results in decreased oxygenation of blood. A major cause is collapse of lung tissue (atelectasis), which can be demonst rated by computed tomography but not by conventional chest x-ray. Collapsed lung tissue is present in 90% of all subjects, both during spontaneous bre athing and after muscle paralysis, and whether intravenous or inhalational anaesthetics are used. There is a correlation between the amount of atelect asis and pulmonary shunt. Shunt does not increase with age. In obese patien ts, larger atelectatic areas are present than in lean ones. Finally, patien ts with chronic obstructive lung disease may show less or even no atelectas is. There are different procedures that can be used in order to prevent ate lectasis or to reopen collapsed lung tissue. The application of positive en d-expiratory pressure (PEEP) has been tested in several studies. On the ave rage, arterial oxygenation does not improve markedly, and atelectasis may p ersist. Further, reopened lung units re-collapse rapidly after discontinuat ion of PEEP. Inflation of the lungs to an airway pressure of 40 cm H2O, mai ntained for 7-8 seconds (recruitment or "vital capacity" manoeuvre), re-exp ands all previously collapsed lung tissue. During induction of anaesthesia, the use of a gas mixture, that includes a poorly absorbed gas such as nitr ogen, may prevent the early formation of atelectasis. During ongoing anaest hesia, pulmonary collapse reappears slowly if a low fraction of oxygen in n itrogen is used for the ventilation of the lungs after a previous VC-manoeu vre. On the other hand, ventilation of the lungs with pure oxygen results i n a rapid reappearance of atelectasis. Thus, ventilation during anaesthesia should be done if possible with a moderate fraction of inspired oxygen (FI O2, e.g. 0.3-0.4). Alternatively, if the lungs are ventilated with a high i nspiratory fraction of oxygen, the use of PEEP may be considered. In summar y, atelectasis is present in most humans during anaesthesia and is a major cause of impaired oxygenation. Avoiding high fractions of oxygen in inspire d 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.