Bispectral index during asleep-awake craniotomies

Background: Asleep-awake craniotomy presents challenges for the anesthetist who has to provide adequate sedation and analgesia but also requires an awake and cooperative patient for neurological testing. In this setting, we hypothesized that Bispectral Index (BIS) monitoring might be helpful in shortening the patient's awakening and in predicting recovery of consciousness in order to initiate reliable intraoperative brain mapping. Methods: An observational prospective study was performed on 27 consecutive asleep-awake craniotomies, in which BIS was monitored and BIS data collected offline. Nine critical intraoperative time points were defined and analyzed [preinduction, start of surgery, termination of hypnotic drug, eye opening, obeying simple commands, laryngeal mask airway (LMA) removal, initiation of brain mapping, initiation of closure, and end of surgery]. Results: A shorter time to LMA removal was associated with a higher BIS at the termination of the hypnotic drug (P=0.016, Mann-Whitney U test). From the initiation of surgery to the time of LMA removal, BIS was significantly lower than the preinduction values, whereas at the initiation of brain mapping, BIS returned to the preinduction values (Friedman test P<0.0001, Dunns multiple comparisons test). Compared with LMA removal, BIS values >85 predicted the initiation of brain mapping with a sensitivity of 44% (95% confidence interval, 25.5%-64.7%) and a specificity of 74% (95% confidence interval, 53.7%-89%). Conclusions: During asleep-awake craniotomies, higher BIS values at the end of the asleep phase are associated with shorter time to LMA removal, suggesting that BIS monitoring may be beneficial in shortening recovery from anesthesia. During the awake phase, the return of BIS to the preinduction values appeared to indicate full recovery of consciousness, thereby allowing a reliable language testing.