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AMBULATORY ANESTHESIA

Can the Bispectral Index Be Used to Predict Seizure Time and Awakening After Electroconvulsive Therapy?

Paul F. White, PhD MD, FANZCA^*, Shivani Rawal, MD^*, Alejandro Recart, MD^*, Larry Thornton, MD, Mark Litle, MD, and Louis Stool, MD^*

Departments of *Anesthesiology and Pain Management and Psychiatry, University of Texas Southwestern Medical Center at Dallas

Address correspondence and reprint requests to Paul F. White, PhD, MD, FANZCA, Department of Anesthesiology and Pain Management, University of Southwestern Medical Center at Dallas, 5323 Harry Hines Blvd., F2.208, Dallas, TX 75390-9068. Address e-mail to paul.white{at}utsouthwestern.edu

	Abstract

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The electroencephalogram (EEG) bispectral index (BIS) measures the hypnotic component of the anesthetic state and correlates with emergence from general anesthesia. Therefore, we hypothesized that the BIS would be useful in predicting electroconvulsive therapy (ECT)-induced seizure times and awakening from methohexital anesthesia. Twenty-five consenting patients with major depressive disorders underwent 100 maintenance ECT treatments. All patients were premedicated with glycopyrrolate 0.2 mg IV, and anesthesia was induced with methohexital 1 mg/kg IV. The BIS was monitored continuously, and the values were recorded at specific end-points, including before anesthesia (baseline), after the induction of anesthesia (pre-ECT), at the end of ECT (peak), after ECT (suppression), and on awakening (eye opening). The pre-ECT BIS value correlated with the duration of both the motor (r = 0.3) and EEG (r = 0.4) seizure activity (P < 0.05). The peak post-ECT BIS value correlated with the duration of the EEG seizure activity (r = 0.5) (P < 0.05). A positive correlation was also found between the EEG seizure duration and the time to eye opening (r = 0.4) (P < 0.05). However, the BIS values on awakening from methohexital anesthesia varied from 29 to 97 and were <60 in 75% of the cases. We conclude that the BIS value before the ECT stimulus is applied could be useful in predicting the seizure time. However, the BIS values on awakening were highly variable, suggesting that it reflects both the residual depressant effects of methohexital and post-ictal depression.

IMPLICATIONS: The bispectral index (BIS) value immediately before the electroconvulsive therapy (ECT) stimulus correlates with the duration of the motor and electroencephalogram (EEG) seizure activity during methohexital anesthesia. In addition, the increase in the BIS value during the ECT-induced seizure was proportional to the duration of EEG seizure activity. However, the BIS value on awakening from anesthesia varied widely, from 29 to 97.

	Introduction

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The bispectral index (BIS) is an electroencephalogram (EEG)-derived multivariate scale that reflects the level of hypnosis in anesthetized patients (1). The BIS has been reported to correlate with both loss of consciousness and awakening from IV anesthesia (2–5). However, a recent report by Nishihara and Saito (6) questioned the reliability of the BIS monitor in predicting awakening after electroconvulsive therapy (ECT) with propofol anesthesia.

ECT is widely used to treat severe depression in patients who have not responded to standard pharmacotherapeutic drugs (7). Methohexital is the most commonly used IV anesthetic for ECT because it is less likely to interfere with ECT-induced seizure activity than propofol and thiopental (8). However, the relationship between the EEG-BIS values and clinical end-points in depressed patients undergoing ECT with methohexital anesthesia has not been studied.

We designed this study to test the hypothesis that BIS values immediately before ECT would be useful in predicting the duration of seizure activity and awakening after ECT with methohexital anesthesia. A secondary objective of this study was to assess the correlation between the changes in the BIS value during and after ECT and the durations of both motor and EEG seizure activity. Finally, we analyzed the relationship between the severity of the pre-ECT depressive symptoms and the baseline BIS value.

	Methods

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A total of 25 consenting patients, ranging in age from 30 to 81 yr and undergoing 100 maintenance ECT treatments for major depressive disorders, were enrolled in this prospective IRB-approved study. In addition to the standard monitoring devices (which included a noninvasive blood pressure cuff, five-lead electrocardiogram, pulse oximeter, and capnograph), the EEG signal was acquired by using a four-electrode disposable sensor (BIS^TM Sensor XP; Aspect Medical Systems, Inc., Newton, MA) applied on the patient’s forehead as recommended by the manufacturer. The impedance of each electrode was <5 k, and the EEG signal was recorded and analyzed with an A2000 BIS XP monitor. A lag time of 20 s was factored into the BIS values recorded at specific end-points because of the time required by the monitor to smooth the EEG signal; the BIS algorithm uses a rolling-averaging window to calculate the BIS value.

All patients were anesthetized with a standardized technique. After pretreatment with glycopyrrolate 0.2 mg IV, methohexital 1 mg/kg IV was administered as a bolus injection over 10 s to induce unconsciousness. On loss of responsiveness to verbal commands, a blood pressure cuff was inflated on the right lower leg to isolate the circulation to the foot before succinylcholine 0.75–1.25 mg/kg IV was administered. All patients received labetalol 10 mg IV 1 min after succinylcholine, and ventilation was assisted with a face mask and 100% oxygen to maintain an end-expiratory carbon dioxide level of 30–32 mm Hg during the ECT procedure. Patients were allowed to resume spontaneous ventilation on termination of the EEG seizure activity.

Before patients entered the study, their ECT-induced seizure threshold was determined by administering successive stimuli of increasing intensity at 30-s intervals until a generalized motor and EEG seizure was induced (9). All study patients were administered a suprathreshold electrical stimulus within 4 min after the induction of anesthesia via bifrontotemporal electrodes with a MECTA-SR1^TM ECT device (MECTA Corp., Portland, OR). The durations of seizure activity were calculated as the time intervals from the ECT stimulus until cessation of tonic-clonic motor activity in the isolated foot (motor seizure time) and until the EEG activity was suppressed (EEG seizure time). The times to eye opening and responsiveness to verbal commands were assessed at 1-min intervals after the patient resumed spontaneous ventilation.

The BIS values were recorded at 20 s after the following end-points: (1) preanesthetic baseline (before induction with methohexital), (2) before ECT (immediately before the ECT stimulus was administered), (3) post-ECT maximum (after the end of the motor seizure), (4) post-ECT minimum (after the end of the EEG seizure), and (5) awakening (eye opening in response to a verbal command).

Descriptive statistics were used to characterize demographic variables for the study patients, as well as the durations of motor and EEG seizure activity. Further statistical analysis was performed by using Spearman’s rank correlation test to determine the correlation between the seizure duration and BIS values at each of the end-points studied. Spearman’s correlation tests were also used to examine the relationship between the BIS values and awakening after the procedure. Data are presented as a mean ± SD, median (ranges), and correlation coefficients (r), with two-tailed P values <0.05 considered statistically significant.

	Results

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The changes in the BIS values during and after the ECT treatments followed a consistent pattern (Figure 1). The durations of both motor (38 ± 13 s) and EEG (54 ± 19 s) seizure activity were significantly correlated with the immediate pre-ECT BIS values (r = 0.3 and 0.4, respectively) (P < 0.05) (Table 1). In addition, the pre-ECT BIS values were significantly correlated with the peak post-ECT BIS values, the difference between the maximum (peak) and the minimum (post-ECT suppression) BIS values, and the time to eye opening (i.e., awakening) (r = 0.5, 0.4, and 0.5, respectively) (P < 0.05) (Table 1). The EEG seizure times correlated with the peak post-ECT BIS values (r = 0.5) and the maximum change in the BIS during the seizure (r = 0.4), as well as the time to eye opening (r = 0.5) (P < 0.05).

View larger version (7K): [in this window] [in a new window]   Figure 1. The averaged bispectral index (BIS) values at specific end-points before, during, and after the electroconvulsive therapy (ECT) treatments. Values are means ± SD (bars). All BIS values were significantly different from the preinduction (baseline) values (P < 0.05).

	Discussion

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The BIS value provides practitioners with information regarding the sedative-hypnotic effects of anesthetic drugs (2–4,10) and can predict the recovery of consciousness after IV anesthesia (5). In most clinical studies, the baseline BIS values are in the range of 90–100, and after the induction of anesthesia they decrease to 35–45. With return of consciousness, the BIS values generally exceed 60. However, after ECT treatments with methohexital anesthesia, we found that 75% of the BIS values were <60 at the time of eye opening.

A recent preliminary report suggested that patients with preexisting neurologic disorders have abnormal baseline BIS values (11). In contrast to patients with dementia, our study found no evidence that the patients’ level of depression before the ECT treatment affected their baseline BIS values. In fact, baseline BIS values exceeded 94 in 90% of the study cases despite the presence of moderate-to-severe depressive symptoms in the study patients. Furthermore, there was no correlation between the severity of the patients’ preexisting depressive symptoms (e.g., Hamilton depression score) and their baseline BIS values. These preliminary data suggest that cortical function may be less impaired in patients with depressive disorders than in those with cognitive disorders and dementias. Of importance, many of these patients were also receiving concurrent antidepressant medications.

The pattern of changes in BIS values during ECT procedures with methohexital does not follow the typical time course observed during IV anesthesia. The BIS values decreased from 97 ± 4 (baseline) to 39 ± 9 after the induction of anesthesia and then increased to a maximum value of 63 ± 15 at 60–80 s after the electrical stimulus. Because seizures can also occur in anesthetized (and comatose) patients, an increase in the BIS value during anesthesia (or sedation in the intensive care unit) may indicate an inadequate sedative-hypnotic state or the occurrence of seizure activity. After the EEG seizure ended (post-ECT suppression), the BIS value was reduced to 28 ± 11 (range, 26–56) as a result of post-ictal EEG depression.

In a case report, Tanabe et al. (12) reported a similar BIS pattern in a 28-yr-old patient with schizophrenia who was anesthetized with propofol. In a recent study by Gunawardane et al. (13), these investigators reported that post-ictal slow-wave EEG activity results in small BIS values that do not correlate with the patient’s level of consciousness after propofol anesthesia. In this study, the BIS values followed a highly variable recovery pattern from the post-ictal minimum value. Awakening occurred at BIS values ranging from 29 to 94, suggesting a variable contribution from both post-ictal EEG depression and residual methohexital-induced central nervous system depression.

The contribution of post-ictal EEG suppression suggests that even in ECT patients without preexisting EEG abnormalities, the BIS values during the emergence period may be difficult to interpret irrespective of whether methohexital or propofol is used for anesthesia. Although increased slower and wave activity can occur with increasing age (14), we did not find a significant correlation between the baseline BIS values and the patients’ ages (r = -0.2). However, an age-related increase in the duration of the EEG seizure activity was observed in this study. Of potential clinical importance, the postinduction BIS value (immediately before the electrical stimulus was applied) may prove to be useful in predicting the length of the ECT-induced seizure activity.

In conclusion, the pre-ECT stimulation BIS values correlated with the durations of both the motor and EEG seizure activity under methohexital anesthesia. However, the BIS values on awakening after ECT did not appear to correlate with the residual depressive effects of the anesthetic drug because of post-ictal EEG depression.

	Acknowledgments

 
The BIS monitoring equipment and supplies were provided by Aspect Medical Systems, Inc. (Newton, MA). Dr. White receives salary support from endowment funds provided by the Margaret Milam McDermott Distinguished Chair.

	References

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Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999