[Influence of inhaled nitrous oxide on the induction doses of propofol and thiopental assessed by auditory evoked potentials].

OBJECTIVES

To assess the influence of 50% nitrous oxide (N2O) on the induction doses of propofol and thiopental by means of the auditory evoked potentials index (AAI). To evaluate the time to induction of anesthesia.

PATIENTS AND METHOD

This was a prospective controlled trial in ASA 1-2 patients randomized to 4 groups for administration of propofol in 100% oxygen (group P1), propofol in a mixture of 50% N2O and 50% oxygen (group P2), thiopental in 100% oxygen (group T1) or thiopental in a 50% N2O and 50% oxygen mixture. An AAI monitor was used. Continuous intravenous infusions of 1% propofol or 2.5% thiopental were administered at a rate of 120 mL h(-1) until an AAI of less than 40 (clinical hypnosis) and then an AAI of less than 20 (surgical hypnosis) were obtained. Vital signs, induction doses required to reach an AAI of 40 and of 20, and induction times were recorded.

RESULTS

One hundred sixty patients with a body mass index of less than 30 kg/m2 were enrolled. The biological parameters and vital signs were similar in the 4 groups of 40 patients each. The induction doses of thiopental and propofol and the induction times were significantly reduced in the groups in which the mixture of 50% N2O and 50% oxygen was used in comparison with the groups breathing 100% oxygen, in terms of both AAI target values of 20 and 40 (P<.01). There were no complications in any patient.

CONCLUSIONS

The doses of both thiopental and propofol are reduced and induction times are shorter when the drugs are administered with a mixture of 50% N2O and 50% oxygen. Monitoring the AAI facilitates the titration of thiopental and propofol dosing during anesthetic induction.