NITROUS-OXIDE ELIMINATION AND DIFFUSION HYPOXIA DURING NORMOVENTILATION AND HYPOVENTILATION

We studied the elimination rate of nitrous oxide in 36 patients underg oing orthopaedic surgery. They were allocated randomly to one of six g roups which differed in time of nitrous oxide exposure and mode of ven tilation. In order to simulate recovery conditions, nitrous oxide admi nistration was discontinued after 30, 60 or 120 min of exposure. Eithe r normoventilation or hypoventilation was used. The mean excretion rat e was 1 litre min-1 at 1 min, declining to 100 ml min-1 at 30 min, wit h relatively small effects of different modes of ventilation and times of exposure. In spite of an FI(O2) of 0.30, there were significant de creases in Sp(O2) during both normo- and hypoventilation. The smallest end-tidal oxygen concentrations were reached at 10-15 min in the grou ps with hypoventilation, after 1 or 2 h of nitrous oxide exposure.