Effect of Inhaled Concentration of Nitrous Oxide on Awakening Time during Emergence after Propofol-N2O-O2 Anesthesia

Dae-Woo Kim, M.D., Ho-Yeong Kil, M.D.^*

Department of Anesthesiology, College of Medicine, The Catholic University of Korea; ^*Hallym University, Seoul, Korea

Background and Goal of Study: Nitrous oxide has unique analgesic effect and reduce the amount of hypnotics for preventing surgical stimuli during maintenance of anesthesia. We considered that high concentration of nitrous oxide has some effect on recovery from anesthesia. The aim of this study was to evaluate the effect of inhaled nitrous oxide concentration on recovery from propofol-N2O-O2 anesthesia.

Materials and Methods: With IRB approval and informed consent, sixty ASA class I or II patients scheduled for elective orthopedic surgery were randomly allocated to one of three groups according to concentration of inhaled nitrous oxide during emergence. They were Group 1: no N2O and only oxygen inhalation, Group 2: 33% N2O inhalation until eye open to verbal contact, and Group 3: same as group 2 except 50% N2O inhalation(n=20 in each group). With all routine monitoring, anesthesia was induced and maintained with propofol(C_T: 3-6 mcg/ml)- 67% N2O- 33% O2 and target concentration(C_T) of propofol was kept at 4 mcg/ml using Diprifusorä at least 30 minutes before stop the infusion of propofol. At the time of skin closure, propofol infusion was stopped and checked vital signs, predicted current/effect concentration of propofol, bispectral index, and elapsed time every minute until eye opening to verbal contact(awakening time).

Results: Vital signs, current/effect site concentration of propofol and bispectral index did not show any difference among groups. However, awakening time were significantly increased as the inhaled nitrous oxide concentration was higher(Table 1).

Table 1. Awakening Time

	Group 1	Group 2	Group 3
Time(min)	12.0+4.0	19.8+4.2=	26.2+7.5=H

⁼P<0.05 compared to group 1, ^HP<0.05 compared to group 2

Conclusions: Inhalation of nitrous oxide during emergence significantly delayed the awakening time after propofol-N2O-O2 anesthesia.