Changes in arterial oxygenation and physiological deadspace under anaesthesia.

The changes in lung function commonly associated with anaesthesia and spontaneous or controlled ventilation are a reduction in lung compliance, an increase in physiological deadspace and an increase in alveolar-arterial oxygen tension difference. In 1959 Mead and Collier showed that there was a progressive fall in lung compliance in anaesthetized dogs whether ventilation was spontaneous or controlled. They also showed that this trend could be reversed by hyperinflation of the lungs. Ferris and Pollard (1960) reported a similar fall in compliance in conscious individuals. In 1963 Bendixen, Hedley-Whyte and Laver published the results of their studies on anaesthetized patients who were ventilated without periodic deep breaths. They showed that these patients suffered a progressive fall in arterial oxygen tension and total thoracic compliance which could be reversed by hyperinflation of the lungs. Furthermore, in 1964, Askrog and his colleagues claimed that there was a progressive increase in alveolar deadspace in patients who were anaesthetized and ventilated with a Bird ventilator, the increase in deadspace appearing to be most marked in those patients who were ventilated with the largest tidal volumes.