Pharmacodynamic interaction of nitrous oxide with sevoflurane, desflurane, isoflurane and enflurane in surgical patients: measurements by effects on EEG median power frequency.
OBJECTIVE
This study investigates the interaction of sevoflurane and nitrous oxide on EEG median power frequency of 2.5 Hz during surgery.


METHODS
Sevoflurane concentrations required for electroencephalographic median power frequency between 2 and 3 Hz were measured in 25 patients during gynaecological laparotomies. Nitrous oxide was randomly administered at 0, 20, 40, 60 and 75 vol%, subsequently two different concentrations in each patient. The data were analysed using isobolographic analysis together with previously published data on nitrous oxide-isoflurane, -enflurane, or -desflurane interaction.


RESULTS
The interaction is described by the equation: C volatile anaesthetic/C0 volatile anaesthetic + C N2O/C0 N2O=1 (C is the concentrations for a drug combination to achieve the desired effect; C0 is the concentration for single drug use). The parameters are C0 isoflurane=1.11 vol% (95% CI 1.03-1.19), C0 enflurane=1.64 (1.52-1.77), C0 desflurane=5.31 (4.92-5.73), C0 sevoflurane=2.12 (1.96-2.29), C0 N2O=174 (153-202). These parameters decrease by 6% (2.5-10) for every 10 years of patients' age > 40 years.


CONCLUSIONS
The interaction is compatible with additivity. The potency of nitrous oxide to substitute the volatile anaesthetics is less than anticipated from previously reported MAC values.
Röpcke
H
H
Department of Anaesthesiology and Intensive Care Medicine, University of Bonn, D-53123, Germany.
Wirz
S
S
Bouillon
T
T
Bruhn
J
J
Hoeft
A
A
D000328:Adult; D018685:Anesthetics, Inhalation / Q000009:adverse effects / Q000493:pharmacokinetics*; D000077335:Desflurane; D004347:Drug Interactions; D004569:Electroencephalography / Q000187:drug effects*; D004737:Enflurane / Q000009:adverse effects / Q000493:pharmacokinetics*; D005260:Female; D013509:Gynecologic Surgical Procedures; D006801:Humans; D007530:Isoflurane / Q000009:adverse effects / Q000031:analogs & derivatives / Q000493:pharmacokinetics*; D007813:Laparotomy; D008738:Methyl Ethers / Q000009:adverse effects / Q000493:pharmacokinetics*; D008875:Middle Aged; D008962:Models, Theoretical; D009609:Nitrous Oxide / Q000009:adverse effects / Q000493:pharmacokinetics*; D000077149:Sevoflurane
D018685:Anesthetics, Inhalation; D008738:Methyl Ethers; D000077149:Sevoflurane; D004737:Enflurane; D000077335:Desflurane; D007530:Isoflurane; D009609:Nitrous Oxide