Bispectral index as an indicator of seizure inducibility in electroconvulsive therapy under thiopental anesthesia.

UNLABELLED The prediction of seizure thresholds in electroconvulsive therapy (ECT) remains problematic. We examined the relationship between bispectral index (BIS) score and seizure duration in ECT performed under thiopental anesthesia in patients receiving their usual regimen of antidepressant medication for its potential as a predictor of seizure inducibility. One-hundred ECT treatments were evaluated in 16 adult patients diagnosed with depression. BIS scores were recorded at the preanesthetic and preictal periods and at recovery from ECT. Seizure duration was defined as the duration of the electroencephalogram seizure pattern. The relationships between preanesthetic and preictal BIS scores and seizure duration were evaluated. Effective seizure threshold was determined by receiver operator characteristic analysis, and the area under the curve (AUC) was calculated for seizure durations of more than 10 s, more than 20 s, and more than 30 s. The relationship between seizure duration and thiopental estimated effect-site and plasma concentrations was analyzed as well. Preictal BIS scores for seizures lasting longer than 30 s were significantly higher than those for seizures lasting <30 s. A preictal BIS score of 55 or more represents a strongly determinant condition for achieving seizures that last longer than 30 s (AUC, 0.937; receiver operator characteristic), as well as for briefer seizures that last more than 20 or 10 s (AUC: 0.938 and 0.959, respectively). There was no significant correlation between seizure duration and the estimated thiopental effect-site or plasma concentration. We conclude that during thiopental anesthesia, the minimum threshold for inducing seizures of any duration correlates with a preictal BIS score of 55. This threshold was independent of antidepressant regimen and thiopental dosage. We suggest that the preictal BIS score is useful in predicting the ictogenic threshold in ECT. IMPLICATIONS We found that the bispectral index (BIS) score serves as an indicator of seizure inducibility in electroconvulsive therapy (ECT) under thiopental anesthesia and that the relationship between BIS score and seizure duration was not linear, suggesting that the pharmacological mechanisms by which thiopental and propofol suppress ECT seizure activity are different.