Participants
After we obtained the permission of the ethics committee of the First Affiliated Hospital of Zhejiang University and informed written consent from all of the patients (Chairperson: Ming Zheng, date of registration: 2019/10/15, registration number: ChiCTR1900026590, principal investigator's names: Guozheng Wang, Delin Zhang, Fan Hang, Kangli Dong), based on resources available, we enrolled 40 patients (American Society of Anesthesiologists grade 1 or 2, 22 male and 18 female participants between 18–70 years old). All selected patients were scheduled for lower limb surgery and had no hearing impairment or neurological disorders. Pregnant women, people taking opioids or sedatives within 24 h, or people at risk for aspiration were excluded. All of the patients fasted for a minimum of 6 h before surgery.
As shown in Figure 1, based on resources available, forty patients were initially assessed in this study, but 8 patients withdrew before allocation to a study group. Of these patients, 3 met the exclusion criteria, and 5 declined to participate. Thirty-two patients were enrolled in the study. Five patients were excluded because of poor signal quality caused by extreme body movements during the operation. In total, data from 27 patients were analyzed.
Anesthetic technique and task
When the patient arrived in the operating room, an 18-G intravenous (IV) cannula was secured, and a standard electrocardiograph, non-invasive blood pressure, and pulse oximetry (SpO2) monitoring were acquired. Baseline heart rate (HR), mean blood pressure (MBP), and SpO2 readings were also obtained. The bispectral index (BIS) value was displayed using an Aspect EEG monitor (Model A-1000; Aspect Medical Systems). We then obtained the patient’s EOG signals by the Waveguard EEG Cap from ANT Neuro. The patient put on binaural headphones to hear the rhythmic auditory stimulation. We selected the combined lumbar and epidural anesthesia to induce anesthesia in the patient’s lower limbs, specifically using sevoflurane for the induction and maintenance of anesthesia in the laryngeal mask airway. After all of the medical equipment was set up, an 18-G intravenous (IV) cannula was secured, and Lactated Ringer’s solution was infused at a rate of 10 ml/kg/h to all of the patients. The patient was placed lying on the lateral recumbent position with the L3-4 gap as the puncture point. After successfully puncture, a dose of 2.5 mg/kg of Ropivacaine hydrochloride was injected into the subarachnoid space, and the epidural tube was fixed into place. For patients whose anesthesia level did not reach the eighth thoracic level 10 min after the drug administration, a total of 5 ml of 0.5% Ropivacaine was also injected from the epidural tube. Anesthesia was then induced using sevoflurane in 100% oxygen. The initial concentration of sevoflurane was set to 0.4% and was then increased by 0.1% every 1 min until loss of consciousness (After the Etsevo is greater than 2%, the anesthesiologist verifies whether the consciousness has disappeared through oral commands.) We subsequently increased the sevoflurane concentrations to 8% for at least 4 min until the LMA was inserted, and then decreased the concentration of sevoflurane to 2% for maintenance. Throughout this procedure, Etsevo and PECO2 were continuously monitored, and ventilation was supported manually if respiration was inadequate, and spontaneous breathing was maintained. The rhythmic auditory stimulation (calling the patient’s name) were circulated throughout the entire procedure, and patients were instructed to concentrate on listening to the rhythmic auditory stimulation.
Auditory stimulation and experimental design
Before the experiment, the neospeech synthesizer synthesized the auditory stimulation (16-bit, 16k sampling rate, Chinese Mandarin, 87–90 dB intensity). The rhythmic auditory stimulation was played by binaural headphones during the experiment and lasted two h. The rhythmic auditory stimulation was a periodic signal with 30 s, with the first 20 s as blank sounds, and calling the patient’s name every 2 s in the last 10 s (Figure 2A). We compared the EOG signal during the “blank sound” segment with the signal during the “name-calling” segment to study the effect of rhythmic auditory stimulation on EOG. According to the Etsevo during anesthesia induction, we divided the patient’s depth of sedation into wakefulness, light sedation, and deep sedation. The Etsevo in the three states was 0%–-0.5%, 0.7%–0.9%, and 1.2%–1.4%, respectively. In comparing the EOG of patients in different depth of sedation, we studied sevoflurane's effect on the EOG driven by the auditory stimulus sequences (Figure 2B).
Data Processing
We used MATLAB (MathWorks, Natick, MA) for data analysis. The EOG recordings were bandpass (0.2–20 Hz) filtered and separated by a 30s analysis window (the period length of the auditory stimulation). According to the rhythmic auditory stimulation structure, we then divided every 30 s EOG signal into a 20 s "blank sound" segment and a 10 s "name-calling" segment. Each segment was converted to the frequency domain using a discrete Fourier transform (DFT) without any additional smoothing window. We normalized each segment of the EOG spectrum at 0.5 Hz and multiplied it by 100. In the case of a patient in the same end-tidal concentration receiving multiple auditory stimuli, we used the multiple spectrum values' arithmetic mean as the patient's spectrum value at this end-tidal concentration.
Statistical analysis
The statistical significance of a spectral peak at the frequency of 0.5 Hz was tested by comparing the power at 0.5 Hz with the power of two neighboring frequencies using a bootstrap method by resampling 10,000 times. If the response was stronger at the 0.5 Hz frequency than the neighboring frequencies in A% of the resampled data, the significance level was (100A+ 1)/10001. Bootstrap was also used to estimate the standard error of mean (SE) across participants, and for this purpose, participants were resampled with replacement 100 times.
One-way analysis of variance (ANOVA) was used to examine the significance of differences of patients’ Peak value of EOG spectrum under the rhythmic auditory stimulation (PERA) or the BIS value in the three different Etsevo levels. We applied Bonferroni corrections to the post hoc analyses, and P values<0.05 were considered to be significant. Results are designated as mean (SD) or 95% confidence interval (95% CI).
The prediction probability (PK) was used to evaluate the PERA or BIS value's predictive ability for sedation depth. The mathematical basis of PK was proposed and explained by Smith et al. in 1996.[14] In the process of calculating PK, we used the BIS value or the PERA as the predicting variable and the actual depth of sedation (determined by the Etsevo) as the value of the variable to be predicted. The closer the value of PK is to 1, the better the prediction effect. The PK values were subsequently calculated for BIS and PERA. Two methods calculated each type of PK. value described above. First, PKsev indicates the probability of correctly predicting the ETsevo. Second, PKDS represents the probability of correctly predicting the depth of sedation.