Inspiratory to end-tidal oxygen difference during nitrous oxide anaesthesia

Abstract

In order to evaluate the value of the inspiratory to end-tidal oxygen concentration difference (IO2-ÉO2) 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-ÉO2) followed an asymptotically increasing curve because of decreasing uptake of nitrous oxide. At 1 h, (IO2-ÉO2) approached the end-tidal carbon dioxide concentration (ÉCO2 During spontaneous ventilation, (IO2-ÉO2) correlated best with ÉCO2. During controlled ventilation, there was a negative correlation between (IO2-ÉO2) and nitrous oxide uptake rate. Changes in oxygen uptake rate were reflected in (IO2-ÉO2) provided that the total ventilation volume was constant and the nitrous oxide uptake rate approached steady state conditions. © 1992 British Journal of Anaesthesia.