Airway closure and intraoperative hypoxaemia: Twenty-five years later

Abstract

Purpose: The literature describing the pulmonary mechanisms of increased P(A)-PaO2 during general anaesthesia was examined to define the role of airway closure and sub-radiological atelectasis. Source: A Medline search was designed to include articles dealing with the stated purpose, which is thus selective rather than a meta-analysis. The MeSH consisted of the following words: Anesthesia: general/inhalational; Pulmonary gas exchange; Ventilation:perfusion ratio; Lung physiology; Lung volume measurements; Closing volume/capacity; Functional residual capacity; Atelectasis; Diaphragm. Also, Dr H. Rothen and Prof. G. Hedenstierna supplied raw data. Principal findings: Changes in shape and dimensions of the thorax and abdomen immediately after induction of anaesthesia result in marked alterations in the efficiency of oxygenation. Three pathways can be described: increased effects of airway closure, increased low ventilation: perfusion in dependent lung zones, and frank atelectasis. The magnitude of the alterations is determined by the patients' age and body habitus. Some of the changes may carry-over into the postoperative period. The data suggest that increasing tidal volume during anaesthesia will reduce the effects of airway closure and that vital capacity breaths will re-expand atelectatic areas. Conclusion: airway closure and atelectasis contribute equally to the increased ventilation: perfusion mismatching that occurs during general anaesthesia.