The effect of addition of nitrous oxide to a sevoflurane anesthetic on BIS, PSI, and entropy.
OBJECTIVE
N(2)O is a commonly used anesthetic that has amnestic and analgesic properties. Recently, devices that estimate depth of consciousness have been introduced in an attempt to better titrate anesthesia, however the effect of N(2)O on these monitors is unclear.


METHODS
General anesthesia was induced and titrated to maintain normal blood pressure and pulse in healthy adults. Data were collected in three 10 minute intervals (Sevo, Sevo + N(2)O, Sevo). In Phase A, sevoflurane concentration was held constant during the N(2)O trial in 60 subjects monitored with either BIS, PSI, or Entropy. In Phase B, sevoflurane concentration was reduced as N(2)O was added, maintaining a constant overall "MAC" in 20 subjects monitored concurrently with BIS and Entropy. Sample size for both phases was designed to detect a 10 unit change in measure of processed EEG with alpha = .05 and statistical power = .80.


RESULTS
In Phase A, supplementing sevoflurane with > 65% N(2)O increased MAC from 1.3 +/- 0.05 to 2.2 +/- 0.10, but did not significantly alter BIS nor PSI (p-value for differential MAC is < 0.05). Entropy, however, dropped significantly, with a change in state entropy (SE) from 31.1 +/- 7.3 to 18.9 +/- 3.7 and a corresponding rise when N(2)O was discontinued. In Phase B, supplementing sevoflurane with > 65% N(2)O with a concomitant reduction in sevoflurane resulted in an increase in both BIS (from 34 +/- 5 to 53.9 +/- 11.5) and SE (from 32 +/- 8.2 to 55.4 +/- 21.3).


CONCLUSIONS
Supplementing sevoflurane with > 65% N(2)O did not result in a significant change in either BIS or PSI when sevoflurane concentration was kept constant. Entropy, however, significantly decreased as anesthetic depth increased. When sevoflurane concentration was reduced during N(2)O administration, both BIS and Entropy rose despite maintenance of anesthetic depth, indicating a variable concentration effect between volatiles and N(2)O.
Soto
Roy G
RG
Department of Anesthesiology, University at Stony Brook, Stony Brook, NY 11794, USA. roysoto@yahoo.com
Smith
Robert A
RA
Zaccaria
Amy L
AL
Miguel
Rafael V
RV
D000328:Adult; D000465:Algorithms; D018685:Anesthetics, Inhalation / Q000008:administration & dosage; D001921:Brain / Q000187:drug effects* / Q000502:physiology; D003243:Consciousness / Q000187:drug effects* / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D004338:Drug Combinations; D004569:Electroencephalography / Q000187:drug effects*; D005260:Female; D006801:Humans; D008297:Male; D008738:Methyl Ethers / Q000008:administration & dosage*; D008875:Middle Aged; D009609:Nitrous Oxide / Q000008:administration & dosage*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D000077149:Sevoflurane
D018685:Anesthetics, Inhalation; D004338:Drug Combinations; D008738:Methyl Ethers; D000077149:Sevoflurane; D009609:Nitrous Oxide