The effect of extradural analgesia combined with light general anaesthesia and spontaneous ventilation on arterial blood-gases and physiological deadspace.

Arterial Po2 and Pco2 were measured at half-hourly intervals in eleven patients anaesthetized for lower limb vascular surgery with a combination of nitrous oxide 67% and halothane 0.3-0.5% in oxygen, and an extradural injection of bupivacaine. Values for the alveolar-arterial oxygen tension difference, physiological deadspace and deadspace-tidal volume ratio were calculated. Although both the alveolar-arterial oxygen tension difference and physiological deadspace were large, spontaneous ventilation was adequate to maintain both Po2 and Pco2 of arterial blood at acceptable values. Over the course of 2 hr following the extradural injection there were no significant changes in any measured value.