Pharmacodynamic interaction of nitrous oxide with sevoflurane, desflurane,isoflurane and enflurane in surgical patients: measurements by effects on EEG median power frequency

Background and objective This study investigates the interaction of sevoflu rane and nitrous oxide on EEG median power frequency of 2.5 Hz during surge ry. Methods Sevoflurane concentrations required for electroencephalographic med ian 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 p atient. The data were analysed using isobolographic analysis together with previously published data on nitrous oxide-isoflurane, -enflurane, or -desf lurane interaction. Results The interaction is described by the equation: C volatile anaestheti c/C0 volatile anaesthetic + C N2O/C0 N2O=1 (C is the concentrations for a d rug 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 s evoflurane=2.12 (1.96-2.29), C0 N2O =174(153-202). These parameters decreas e by 6% (2.5-10) for every 10 years of patients' age > 40 years. Conclusions The interaction is compatible with additivity. The potency of n itrous oxide to substitute the volatile anaesthetics is less than anticipat ed from previously reported MAC values.