Inspiratory to end-tidal oxygen difference during nitrous oxide anaesthesia.

In order to evaluate the value of the inspiratory to end-tidal oxygen concentration difference (Io2-E'o2) as a monitor during general anaesthesia, we studied 40 orthopaedic patients allocated randomly to four groups: anaesthesia with enflurane or isoflurane in nitrous oxide with either spontaneous or controlled ventilation. (Io2-E'o2) followed an asymptotically increasing curve because of decreasing uptake of nitrous oxide. At 1 h, (Io2-E'o2) approached the end-tidal carbon dioxide concentration (E'o2). During spontaneous ventilation, (Io2-E'o2) correlated best with E'co2. During controlled ventilation, there was a negative correlation between (Io2-E'o2) and nitrous oxide uptake rate. Changes in oxygen uptake rate were reflected in (Io2-E'o2), provided that the total ventilation volume was constant and the nitrous oxide uptake rate approached steady state conditions.