British journal of anaesthesia
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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.
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Comparative Study
The effect of anaesthesia and intermittent positive pressure ventilation with different frequencies on the anatomical and alveolar deadspace.
Deadspace was measured in nine healthy subjects in the supine position, premedicated but awake and breathing spontaneously at a rate of 12 b.p.m. and subsequently under anaesthesia with artificial ventilation with frequencies of 12 and 24 b.p.m. The minute volume was kept at a relatively constant value. The physiological deadspace was calculated using the Bohr equation and the division into anatomical and alveolar deadspace was made with the aid of capnography. ⋯ This was unchanged by the induction of anaesthesia and the institution of artifical ventilation. On changing the frequency, the VDalv/VT ratio remained constant. It is concluded that both the anatomical and the alveolar deadspaces increasing with increasing tidal volume, but are unaffected by the breathing rate.