The effect of anaesthesia and intermittent positive pressure ventilation with different frequencies on the anatomical and alveolar deadspace

Abstract

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. Physiological deadspace was increased by anaesthesia and IPPV, mainly as a consequence of increased rebreathing in the apparatus deadspace. There was no significant change in the anatomical deadspace. Thus, the expected reduction in deadspace brought about by endotracheal intubation was nullified by an increase in the anatomical deadspace distal to the carina. The VDanat/VT ratio remained constant on changing the respiratory frequency. A significant alveolar deadspace was measured during spontaneous breathing. 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.