SummaryThe effects of pre-oxygenation were studied by continuous expired oxygen analysis in twenty pregnant patients and ten nonpregnant controls. The Magill system and a demand valve breathing system were both studied with tidal and vital capacity breathing. Rapid pre-oxygenation Most of the published work on the effects of preoxygenation has concerned the nonpregnant general surgical patient.' , 2 We feel. however, that the pregnant patient deserves special attention for thrcc reasons. First, pre-oxygenation ha5 become a standard feature in the rapid sequence induction of general anaesthesia for Caesarean section. Second. in the acute obstetric emergency. failure to achieve maximum pre-oxygenation can have grave consequences. Third. term pregnancy is associated with changes in lung volumes and function. The mean tidal volume is increased by about 40% over values found in the nonpregnantT3 due probably to progc~tcrone.~ Upward displacement of the diaphragm by the gravid uterus causes a reduction in functional residual capacity (FRC) to 9.5-25'1i0 below the values obtained in nonpregnant s~b j e c t s .~.~,~ The reduction in FRC associated with rclative alveolar hypcrventilation should facilitate rapid pre-oxygenation prior to Caesarean section.Two breathing systcms were invcstigated. One was a system that incorporates both demand and non-rebrcathing valves similar to the standard F.ntonox apparatus. This permits a pcak inspiratory flow of 500 litres/minute without rebreathing of expired gas. As such. it should theoretically be the ideal systcm for preoxygenation. The Magill system was also investigated because it is probably the breathing system most widely used for pre-oxygenation in the United Kingdom.

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