2Mechanisms of atelectasis in the perioperative period
Section snippets
Occurrence of atelectasis during anaesthesia
Atelectasis appears in about 90% of all patients who are anaesthetised1 (Fig. 1). It is seen both during spontaneous breathing and after muscle paralysis and irrespective of whether intravenous or inhalational anaesthetics are used.2 The atelectatic area on a computed tomography (CT) cut near the diaphragm is about 5–6% of the total lung area but can easily exceed 15–20%. It should also be remembered that the amount of tissue that is collapsed is even larger; the atelectatic area comprising
Mechanism of atelectasis formation
There appear to be at least three potential causes of atelectasis in an anaesthetised subject:
- 1.
Absorption atelectasis behind closed airways;
- 2.
Compression of lung tissue; and
- 3.
Loss of surfactant or surfactant function.
- 1.
Absorption atelectasis
In an adult subject, the resting lung volume (functional residual capacity, FRC) is reduced by 0.7–0.8 l by changing the body position from upright to supine, and there is a further decrease by 0.4–0.5 l with the induction of general anaesthesia7 (Fig. 3). As a
Oxygen and atelectasis
The time it takes for a lung unit to collapse has been the subject of theoretical studies. Dantzker et al.27 calculated the influence of inspired alveolar ventilation/perfusion ratio (VAI/Q) and inspired oxygen concentration on alveolar stability. They found a critical VAI/Q (when alveoli eventually collapse) approaching 0.001 during air breathing and that was much higher, about 0.07 while breathing 100% oxygen. They also calculated the minimum time to collapse for units with different VAI/Q
Funding source
Supported by the Swedish Research Council 5315 and Swedish Heart–Lung Fund.
Conflict of interest statement
None.
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