INSPIRATORY TO END-TIDAL OXYGEN DIFFERENCE DURING NITROUS OXIDE ANAESTHESIA

In order to evaluate the value of the inspiratory to end-tidaloxygen concentration difference (I_O2–É_O2) as amonitor during general anaesthesia. we studied 40 orthopaedicpatients allocated randomly to four groups: anaesthesia withenflurane or isoflurane in nitrous oxide with either spontaneousor controlled ventilation. (I_O2–É_O2) followed anasymptotically increasing curve because of decreasing uptakeof nitrous oxide. At 1 h, (I_O2–É_O2) approachedthe end-tidal carbon dioxide concentration (É_CO2 Duringspontaneous ventilation, (I_O2–É_O2) correlated bestwith É_CO2. During controlled ventilation, there was anegative correlation between (I_O2–É_O2) and nitrousoxide uptake rate. Changes in oxygen uptake rate were reflectedin (I_O2–É_O2) provided that the total ventilationvolume was constant and the nitrous oxide uptake rate approachedsteady state conditions.