Nitrous oxide does not alter bispectral index: study with nitrous oxide as sole agent and as an adjunct to i.v. anaesthesia.
We have studied the effect of nitrous oxide on bispectral index (BIS), calculated from a bipolar encephalogram. Inhalation of 70% nitrous oxide resulted in loss of consciousness in all healthy volunteers (n = 10) but no change in BIS. Brief inhalation up to 1.2% sevoflurane also resulted in loss of consciousness in volunteers (n = 5), but with sevoflurane, BIS decreased. BIS and the haemodynamic effects of adding nitrous oxide were also measured during coronary artery bypass surgery in patients (n = 10) receiving midazolam and fentanyl infusions. Measurements were made after 0%, 33%, 66% and 0% nitrous oxide, just before skin incision and after sternotomy. Nitrous oxide caused no change in BIS. BIS may indicate a sufficient hypnotic depth to prevent awareness during surgery, but our study demonstrated that pharmacological unconsciousness-hypnosis can also be reached by mechanisms to which BIS is not sensitive. Thus BIS is a sufficient but not a necessary criterion for adequate depth of anaesthesia or prevention of awareness.
Barr
G
G
Department of Cardiothoracic Anaesthetics and Intensive Care, Karolinska Hospital, Stockholm, Sweden.
Jakobsson
J G
JG
Owall
A
A
Anderson
R E
RE
D000328:Adult; D000368:Aged; D000704:Analysis of Variance; D000768:Anesthesia, General; D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D018686:Anesthetics, Intravenous; D001364:Awareness / Q000187:drug effects*; D001026:Coronary Artery Bypass; D004334:Drug Administration Schedule; D004569:Electroencephalography / Q000187:drug effects*; D005283:Fentanyl; D006801:Humans; D008738:Methyl Ethers; D008874:Midazolam; D008875:Middle Aged; D008991:Monitoring, Physiologic; D009609:Nitrous Oxide / Q000494:pharmacology*; D012044:Regression Analysis; D000077149:Sevoflurane
D018685:Anesthetics, Inhalation; D018686:Anesthetics, Intravenous; D008738:Methyl Ethers; D000077149:Sevoflurane; D009609:Nitrous Oxide; D008874:Midazolam; D005283:Fentanyl