Technology, Computing, and Simulation: Case Report
The bispectral index (BIS) is a dimensionless variable between 0 and 100 that correlates with the degree of sedation (1,2). Values <60 correlate with a small probability of consciousness (3). In general, a BIS between 40 and 60 is recommended for adequate depth of anesthesia, and, at values higher than 80, patients regain consciousness (4). We report a case in which the patient could respond completely to verbal commands at a BIS value of 52 after propofol anesthesia.
A 91-year-old, 55-kg male with hypertension and complete right bundle branch block presented to the operating room for right hemilateral colorectomy as a result of cancer of the ascending colon. He had no history of use of psychological drugs or alcohol abuse. His surgical history was negative. Electrocardiogram, peripheral oxygen saturation, and arterial blood pressure were continuously measured. A BIS sensor was fixed at the frontal positions as recommended (Aspect A-1050; Aspect Medical Systems, Newton, MA). The BIS value before the induction of anesthesia was 98. An epidural catheter was inserted at T9-10. A 1% lidocaine test dose (2 mL) containing 10 μg of epinephrine was given. Anesthesia was induced by IV fentanyl 100 μg and propofol 65 mg with neuromuscular block induced with vecuronium 6 mg. After endotracheal intubation, anesthesia was maintained with continuous infusion of propofol at 2 mg · kg−1 · h−1 to control BIS values between 40 and 60 (5). No additional muscle relaxants were given thereafter. During the surgical procedure, 6 mL of 1% lidocaine was injected via the epidural catheter or IV fentanyl was given according to changes in arterial blood pressure and heart rate. Intraoperative usage of epidural lidocaine and IV fentanyl was 180 mg and 300 μg, respectively. The surgical procedure (3.5 h) was performed uneventfully, and then propofol infusion was discontinued. Bupivacaine (0.25%) was infused at 3 mL/h for postoperative analgesia. Spontaneous breathing occurred without reversal of neuromuscular blockade. His arterial blood pressure and heart rate increased gradually. Although we decided not to assess the response to verbal stimuli until the BIS value increased to greater than 80, the BIS values remained between 40 and 60 even 2 hours after propofol infusion was discontinued. Despite BIS values of 52 at 2 hours and 15 minutes after propofol infusion was discontinued, the patient responded to the first verbal command from an anesthesiology resident. At that time, a blood sample was taken from the right femoral artery and retained for high-performance liquid chromatography analysis (MC Medical, Inc., Osaka, Japan). We confirmed that the patient had awakened and he was then tracheally extubated and transferred to the postanesthetic care unit in stable condition. In the postanesthetic care unit, BIS values remained between 50 and 60 with complete response to verbal commands. After the electrode was removed, a new electrode was replaced to exclude the possibility of electrode impairment. However, the new electrode showed BIS values between 50 and 60 for the next 15 minutes.
During postanesthesia interviews 24 hours and 72 hours after surgery the patient reported that he could recall our voices in the operating room and feel his trachea being intubated but, fortunately, not his abdomen being operated on. He described some of our conversations after the surgery as the nurses and anesthesiologists conducted the postoperative evaluation. High-performance liquid chromatography analysis revealed that the arterial blood propofol and lidocaine concentrations taken at BIS 52 were 0.74 and 0.69 μg/mL, respectively.
In this case, the patient could completely respond to verbal commands at a BIS value of 52 and a blood propofol concentration of 0.74 μg/mL. Electromyographic activity as a source of interference for BIS monitoring (6) might have resulted in a false interpretation of the BIS value in this case. In general, however, electromyographic signals are considered to increase BIS values because of the overlap of electroencephalogram (0.5–30 Hz) and electromyographic (30–300 Hz) signals (5). Therefore, we do not believe that an artifact induced by electromyographic signals was associated with the low BIS value during emergence from propofol anesthesia.
The emergence from anesthesia at a BIS value of 52 might result from a feature of the electroencephalogram in geriatric patients. However, Kazama et al. (7) showed no effect of age on the propofol dose-BIS relationship. Renna et al. (8) however, demonstrated that some elderly patients with dementia (Alzheimer’s disease or vascular dementia) had low BIS values in the awake state. In addition, it was reported that senile dementia may be associated with lower BIS values during anesthesia (9), suggesting that senile dementia might alter the pharmacodynamic response to propofol. Because our patient had not been diagnosed with senile dementia and the preinduction BIS value was 98, it is possible that this patient was an outlier in the normal relationship between BIS and plasma propofol concentration. There have been some studies demonstrating that epidural anesthesia might change the pharmacodynamics of general anesthetics (sevoflurane, isoflurane, and propofol) (10–12) via inhibition of tonic afferent spinal signaling to the brain. Our patient did not complain of postoperative pain in the postanesthetic care unit, suggesting suppression of afferent spinal signaling to the brain. In this case, therefore, it is also likely that epidural analgesia might have modulated the pharmacodynamic changes to propofol.
Although a BIS value between 40 and 60 is recommended for adequate depth of anesthesia (4), Glass et al. (3) demonstrated that a BIS value of 51 corresponded to a 95% chance of the patient being unconscious; i.e., a 5% chance of consciousness. A large prospective study (13) showed that one patient among 1225 had an episode of intraoperative awareness in the BIS range of 55–59. The authors suggested that awareness can occur when BIS is at the upper limit of the recommended (40–60) range.
Prospective, randomized studies (14,15) demonstrated that anesthetic titration using BIS monitoring can result in significantly reduced average drug use and faster patient recovery from anesthesia. In using BIS monitoring for propofol titration, Glass et al. (3) recommended that anesthesiologists should maintain BIS values below 50 if awareness is to be avoided in most cases. Based on the clinical experience in this episode, we agree with this recommendation.
The BIS index can be useful in guiding dosing of anesthetic drugs. However, the present case indicates that BIS may not be predictive of depth of anesthesia in all patients.
1. Katoh T, Suzuki A, Ikeda K. Electroencephalographic derivatives as a tool predicting the depth of sedation and anesthesia induced by sevoflurane. Anesthesiology 1998;88:642–50.
2. Sebel PS, Lang E, Rampil, et al. A multicenter study of bispectral electroencephalogram analysis for monitoring anesthetic effect. Anesth Analg 1997;84:891–9.
3. Glass PS, Bloom M, Kearse L, et al. Bispectral analysis measures sedation and memory effects of propofol, midazolam, isoflurane, and alfentanil in healthy volunteers. Anesthesiology 1997;86:836–47.
4. BIS Clinical Reference Manual. Newton, Massachusetts: Aspect Medical Systems, Inc., 1998.
5. Johansen JW, Sebel PS. Development and clinical application of electroencephalographic bispectrum monitoring. Anesthesiology 2000;93:1336–44.
6. Bruhn J, Bouillon TW, Shafer SL. Electromyographic activity falsely elevates the bispectral index. Anesthesiology 2000;92:1485–7.
7. Kazama T, Ikeda K, Morita K, et al. Comparison of the effect-site k(e0)s of propofol for blood pressure and EEG bispectral index inelderly and younger patients. Anesthesiology 1999;90:1517–27.
8. Renna M, Handy J, Shah A. Low baseline bispectral index of the electroencephalogram in patients with dementia. Anesth Analg 2003;96:1380–5.
9. Renna M, Venturi R. Bispectral index and anaesthesia in the elderly. Minerva Anesthesiol 2000;66:398–402.
10. Hodgson PS, Liu SS. Epidural lidocaine decreases sevoflurane requirement for adequate depth of anesthesia as measured by the bispectral index monitor. Anesthesiology 2001;94:799–803.
11. Morley AP, Derrick J, Seed PT, et al. Isoflurane dosage for equivalent intraoperative electroencephalographic suppression in patients with and without epidural blockade. Anesth Analg 2002;95:1412–8.
12. Ge SJ, Zhuang XL, Wang YT, et al. Changes in the rapidly extracted auditory evoked potentials index and the bispectral index during sedation induced by propofol or midazolam under epidural block. Br J Anaesth 2002;89:260–4.
13. Myles PS, Leslie K, McNeil J, Chan MT. Bispectral index monitoring to prevent awareness during anaesthesia: the B-Aware randomized controlled trial. Lancet 2004;363:1757–63.
14. Gan TJ, Glass PS, Windsor A, et al and the BIS Utility Study Group. Bispectral index monitoring allows faster emergence and improved recovery from propofol, alfentanil, and nitrous oxide anesthesia. Anesthesiology 1997;87:808–15.
15. Struys M, Versichelen L, Byttebier G, et al. Clinical usefulness of the Bispectral Index for titrating propofol target effect-site concentration. Anaesthesia 1998;53:4–12.