Nitrous Oxide Elimination and Diffusion Hypoxia During Normo- and Hypoventilation

We studied the elimination rate of nitrous oxide in 36 patients undergoing orthopaedic surgery. They were allocated randomly to one of six groups which differed in time of nitrous oxide exposure and mode of ventilation. In order to simulate recovery conditions, nitrous oxide administration was discontinued after 30, 60 or 120 min of exposure. Either normoventilation or hypoventilation was used. The mean excretion rate was 1 litre min-1 at 1 min, declining to 100 ml min-1 at 30 min, with relatively small effects of different modes of ventilation and times of exposure. In spite of an FIO2 of 0.30, there were significant decreases in SpO2 during both normo- and hypoventilation. The smallest end-tidal oxygen concentrations were reached at 10-15 min in the groups with hypoventilation, after 1 or 2 h of nitrous oxide exposure.