Background: The rapidly extracted auditory evoked potentials index (A-lineTM ARX Index or AAI) has been proposed as a method to measure the depth of anaesthesia. In an attempt to give some guidance for clinical monitoring, prospective studies were designed: ⑴ to assess the performance of AAI for measuring the depth of sedation induced by propofol or midazolam under epidural block; ⑵ to assess the performance of AAI to detect the recovery and loss of wakefulness in anaesthetized and paralysed patients; ⑶ to test whether neuromuscular block with vecuronium can reduce AAI. Methods: ⑴ Thirty-two adult patients undergoing elective gynaecological surgery under low-thoracolumbar epidural block were studied to assess the performance of AAI for measuring the depth of sedation induced by propofol or midazolam. Eighteen patients received propofol (Group P: 20mg bolus every 3 min) and fourteen received midazolam (Group M: 0.5mg bolus every 5 min) until the observer’s assessment of alertness/sedation (OAA/S) scale score of 1 was achieved. AAI and the bispectal index (BIhx, Huaxiang Technology Co. Ltd, Heilongjiang, China) were monitored for different OAA/S scores. ⑵ Fourteen adult patients undergoing elective surgery were anaesthetized with propofol 1.5 mg·kg-1, vecuronium 0.1 mg·kg-1 and another propofol 1.0 mg·kg-1. Wakefulness was measured by the ability of the patient to respond to command using the isolated forearm technique (IFT). After the patient responded, propofol was infused at 10 mg·kg-1·h-1 until wakefulness was lost. AAI was recorded continuously throughout the study and analysed off-line. ⑶ Forty adult patients undergoing elective surgery were studied. After tracheal intubation, anaesthesia was maintained with an end-tidal isoflurane concentration (FETISO) of 1.0% for 20 min, then a 10 ml dose of either vecuronium 0.05 mg·kg-1, 0.1 mg·kg-1, 0.2 mg·kg-1 or saline was randomly administered. AAI and BIhx were monitored throughout the study and analysed off-line. Results: ⑴ AAI and BIhx decreased and increased following the changes on the patients’ OAA/S scores and correlated with sedation significantly. During the onset phase, the coefficients of Spearman’s rank correlation for AAI and BIhx were respectively 0.958 and 0.898 (P < 0.001) for Group P, and 0.973 and 0.945 (P < 0.001) for Group M. During the recovery phase in Group P, the coefficients were respectively 0.946 and 0.702 (P < 0.001). The coefficients of Spearman’s rank correlation and linear regression for AAI were all greater comparing with the corresponding ones for BIhx (P < 0.05). ⑵ AAI showed a significant difference between the values registered during, 30 s before and 30 s after the recovery, and also between 30 s before and 30 s after the loss of wakefulness. The prediction probability (Pk) values for AAI were 0.786 and 0.864 during the transitions from unresponsiveness to responsiveness and from responsiveness to unresponsiveness. The area under the receiver operating characteristic curve for the responsive and unresponsive values was 0.926 (SE 0.002, 95% CI 0.922-0.931), and the AAI values of approximately 5%, 50% and 95% predicted<WP=5>probability of wakefulness were 19, 29 and 39, respectively. ⑶ BIhx was unaltered after the administration of saline or vecuronium. The mean of the averaged (per patient) AAI values recorded from 2 min to 10 min after the administration of saline or vecuronium 0.05 mg.kg-1 did not differ significantly from the corresponding mean recorded from 15 min to 20 min after FETISO maintained 1.0% (P > 0.05). However, after the administration of vecuronium 0.1 mg.kg-1 or 0.2 mg.kg-1, the mean showed a significant reduction (P < 0.05). The mean percentages of decrease of AAI in the vecuronium 0.1 mg.kg-1 and 0.2 mg.kg-1 groups (16.3% and 21.6%) were also significantly greater than that in the saline group (0.3%) (P < 0.05). Conclusions: ⑴ Both AAI and BIhx correlate well with the depth of sedation induced by propofol or midazolam under epidural block. AAI may be more valuable to monitor the dept