Ventilatory effects of oxygen in the dog under thiopentone anaesthesia

The ventilatory effects of prolonged oxygen administration were examined in seven dogs during thiopentone anaesthesia. Ventilation, tidal volume (VT), ventilatory rate (f), minute ventilation (VE), inspiratory time (TI), expiratory time (TE), period (Ttot), TI/Ttot and mean inspiratory flow (VT/TI) were measured during the inhalation of room air, after 30 min of oxygen inhalation, and finally after a return to breathing room air. Arterial blood-gas tensions were measured before and after 5, 10, 20 and 30 min of oxygen administration and 15 min after return to breathing room air. Oxygen administration produced an immediate, significant and persistent decrease in ventilation, principally from a decrease in ventilatory rate and changes in ventilatory times. This was in contrast to what occurred in awake animals. Modifications in ventilatory mechanics or suppression of an hypoxic stimulus to ventilation were probably not involved. Anaesthesia may modify centrally mediated ventilatory responses to hyperoxia.