Sevoflurane-induced isoelectric EEG and burst suppression: differential and antagonistic effect of added nitrous oxide

Determination of the minimal alveolar concentration of sevoflurane associated with isoelectric electroencephalogram in children: A prospective, randomized, double-blind study

OBJECTIVE

We investigated the minimal alveolar concentration (MAC) of sevoflurane associated with the occurrence of isoelectric electroencephalogram in 50% of children under steady-state conditions (MAC IE). The MAC IE was determined in 100% oxygen and with the addition of 50% nitrous oxide or after the injection of fentanyl.

METHODS

Seventy-two children (ASA I or II, age 3-8 years) undergoing elective surgery were allocated to one of three groups: in 100% oxygen (group O₂ ), in 50% oxygen and 50% nitrous oxide (group N₂ O), or in 100% oxygen with a bolus of 3 μg/kg fentanyl (group FENTANYL). The state of isoelectric EEG was considered as significant when a burst suppression ratio of 100% lasted for >1 minute. The MAC IE was determined by the Dixon's up-and-down method after a 15-minute period with a stable endtidal concentration of sevoflurane. The concentration of sevoflurane was determined by the electroencephalogram of the previous patient: increase of 0.2% if isoelectric EEG were absent or decrease of 0.2% if isoelectric EEG were present.

RESULTS

The MAC IE in group O₂ (MAC IE_O2 ) was 5.30 (5.12-5.48)% (median [95% CI]). The MAC IE in group N₂ O (MAC IE_N2O ) was 5.83 (5.67-5.99)%. The MAC IE in group FENTANYL (MAC IE_FENTANYL ) was 5.37 (5.21-5.53)%, which was close to MAC IE_O2 .

CONCLUSION

The MAC IE of sevoflurane calculated in 100% O₂ was 5.30% in children. Addition of 50% N₂ O modestly increased MAC IE of sevoflurane, while 3 μg/kg fentanyl had no effect on MAC IE of sevoflurane.

MAC-sparing effect of nitrous oxide in sevoflurane anesthetized sheep and its reversal with systemic atipamezole administration

INTRODUCTION

Nitrous oxide (N2O) is an anesthetic gas with antinociceptive properties and reduces the minimum alveolar concentration (MAC) for volatile anesthetic agents, potentially through mechanisms involving central alpha2-adrenoceptors. We hypothesized that 70% N2O in the inspired gas will significantly reduce the MAC of sevoflurane (MACSEVO) in sheep, and that this effect can be reversed by systemic atipamezole.

MATERIALS AND METHODS

Animals were initially anesthetized with SEVO in oxygen (O2) and exposed to an electrical current as supramaximal noxious stimulus in order to determine MACSEVO (in duplicates). Thereafter, 70% N2O was added to the inspired gas and the MAC re-determined in the presence of N2O (MACSN). A subgroup of sheep were anesthetized a second time with SEVO/N2O for re-determination of MACSN, after which atipamezole (0.2 mg kg-1, IV) was administered for MACSNA determinations. Sheep were anesthetized a third time, initially with only SEVO/O2 to re-determine MACSEVO, after which atipamezole (0.2 mg kg-1, IV) was administered for determination of MACSA.

RESULTS

MACSEVO was 2.7 (0.3)% [mean (standard deviation)]. Addition of N2O resulted in a 37% reduction of MACSEVO to MACSN of 1.7 (0.2)% (p <0.0001). Atipamezole reversed this effect, producing a MACSNA of 3.1 (0.7)%, which did not differ from MACSEVO (p = 0.12). MACSEVO did not differ from MACSA (p = 0.69). Cardiorespiratory variables were not different among experimental groups except a lower ETCO2 in animals exposed to SEVO/N2O.

CONCLUSIONS

N2O produces significant MACSEVO-reduction in sheep; this effect is completely reversed by IV atipamezole confirming the involvement of alpha2-adrenoreceptors in the MAC-sparing action of N2O.

Nitrous oxide-based vs. nitrous oxide-free general anaesthesia and accidental awareness in surgical patients: an abridged Cochrane systematic review

Accidental awareness during general anaesthesia can arise from a failure to deliver sufficient anaesthetic agent, or from a patient's resistance to an expected sufficient dose of such an agent. Awareness is 'explicit' if the patient is subsequently able to recall the event. We conducted a systematic review into the effect of nitrous oxide used as part of a general anaesthetic on the risk of accidental awareness in people over the age of five years undergoing general anaesthesia for surgery. We included 15 randomised controlled trials, 14 of which, representing a total of 3439 participants, were included in our primary analysis of the frequency of accidental awareness events. The awareness incidence rate was rare within these studies, and all were considered underpowered with respect to this outcome. The risk of bias across all studies was judged to be high, and 76% of studies failed adequately to conceal participant allocation. We considered the available evidence to be of very poor quality. There were a total of three accidental awareness events reported in two studies, one of which reported that the awareness was the result of a kink in a propofol intravenous line. There were insufficient data to conduct a meta- or sub-group analysis and there was insufficient evidence to draw outcome-related conclusions. We can, however, recommend that future studies focus on potentially high-risk groups such as obstetric or cardiac surgery patients, or those receiving neuromuscular blocking drugs or total intravenous anaesthesia.