Effect of a course of electroconvulsive therapy on interictal bispectral index values: a prospective study

Bispectral index (BIS) value measures the level of hypnosis (sedation) during anesthesia. It uses electroencephalographic signals to measure the level of sedation. Electroconvulsive therapy (ECT)-induced changes in electroencephalography last for several hours to days. How this affects BIS values is unknown.

In 30 patients with schizophrenia or mood disorders who were prescribed ECT (bitemporal, brief-pulse ECT), BIS values were obtained before the start of anesthesia on the first (baseline), fourth (48 hours after the third ECT), and seventh ECT (48 hours after the sixth ECT) sessions. Bispectral index values were recorded in resting state and during a short conversation a few minutes before the induction of anesthesia. The changes in BIS values through the course of ECT were analyzed using repeated-measures analysis of variance and the McNemar test.

Mean (SD) resting BIS value dropped from 91.6 (6.6) at baseline to 79.7 (20.2) after the sixth ECT (repeated-measures analysis of variance occasion effect: F = 6.78; df = 2,58; P = 0.02). The drop in corresponding values during conversation was not significant. The proportion of patients whose resting BIS values were in the sedated/anesthetized range increased from 26% at baseline to 60% after the sixth ECT session (McNemar test: P = 0.031).

Bispectral index values of awake individuals in resting state drop significantly through a course of bitemporal brief-pulse ECT. Anesthesiologists should be aware that a considerable proportion of patients who have received a course of ECT may have BIS values in the sedated/anesthetized range even when they are awake. The BIS may not provide accurate estimation of the depth of anesthesia during ECT after the initial ECT sessions.