Nitrous oxide uptake during spontaneous and controlled ventilation.

The rate of uptake of nitrous oxide was studied in 40 orthopaedic patients anaesthetised with either enflurane or isoflurane in nitrous oxide and with either spontaneous or controlled ventilation. A variant of the Douglas bag method was used in combination with low fresh gas flows to a circle system. There were no significant differences in nitrous oxide uptake between the groups and the uptake rates followed 'the square root of time concept', with an overall best fit curve of 1080.t-0.505 ml.70 kg-1 x min-1. During spontaneous ventilation, the nitrous oxide uptake rate was similar or even higher than the corresponding rate during controlled ventilation, in spite of lower minute volumes.