Background: The use of N2O during low-flow anesthesia (LFA) causes difficulty in predicting inspired gas mixtures and oxygen concentration due to accumulation. An alternative technique, which uses a mixture of oxygen and air and a morphine infusion to replace N2O, has been evaluated in children during LFA, and the clinical effects and composition of gases in the system are presented here.
Methods: Thirty-five children aged 2-13 years undergoing major urogenital surgery were recruited into the study. Following a standardized induction, anesthesia was maintained with 2% sevoflurane in combination with a morphine infusion. After induction with 4 l min(-1), flow rates were reduced to 2 l min(-1) and finally to 1 l min(-1) at 10 min (0.5 l oxygen + 0.5 l air). The delivered oxygen concentration, oxygen saturation and the inspired and expired O2 sevoflurane and CO2 concentrations were recorded from the start of induction to the end of anesthesia.
Results: The duration of LFA was 132+/-89 min. The concentration of oxygen delivered by the flow meters during this period was 55-60%. Although the changes in inspired and expired oxygen and sevoflurane and inspired CO2 related to the duration of LFA were statistically significant (P < 0.0001), they were not clinically relevant. All vital parameters were stable. Four patients required supplemental morphine and nine presented emergence agitation.
Conclusion: We concluded that an N2O-free LFA technique with 0.5 l min(-1) of air and 0.5 l min(-1) of O2 supplemented by sevoflurane and a morphine infusion is safe and effective in children. The resulting high-inspired oxygen concentration prevents occurrence of hypoxic gas mixtures, and the use of supplemental morphine compensates for the absence of N2O and overcomes the possibility of light anesthesia arising from a decrease in the inspired sevoflurane concentration.