EEG predicts movement response to surgical stimuli during general anesthesia with combinations of isoflurane, 70% N2O,and fentanyl

Objective. Our objective was to evaluate the performance of the EEG as an indicator of anesthetic depth by measuring EEG prediction of movement response to surgical stimuli.Methods. While using 5 different combinations of isoflurane, 70% N₂O, and fentanyl, we measured the EEG of 246 patients during pelvic laparoscopy and observed their movement responses to opening stimuli (defined as skin incision, CO₂ needle insertion, or trocar insertion) and also to closing stimuli (defined as sutures during incision closure). The EEG was expressed asF95, the frequency in hertz below which resides 95% of the power in the EEG frequency spectrum. The relations betweenF95 and movement response were expressed as logistic regression curves.F95-response logistic regression curves, which are analogous to dose-response curves, were calculated for each of the 2 stimuli administered during each of the 5 anesthetic techniques. The prediction of patient responsiveness byF95 was tested using (beta), a measure of the slope of anF95-response logistic curve. The presence of shifts among theF95-response logistic curves was tested using the differences inF95 values between curves. Hypothesis tests used a level of significance ofP = 0.05.Main Results. The slopes of theF95-response logistic regression curves showed a statistically significant ability to predict movement response to stimuli for 9 of the 10 combinations of stimuli and anesthetic techniques. We did not calculate anF95-response logistic curve for the tenth combination because it contained burst suppression, which our EEG analysis method was not designed to process. TheF95-response logistic curves were shifted relative to each other, and the shifts were affected by the type of stimulus and the combination of anesthetic agents. Referenced to opening curves, the mean shift of the closing curves was ± 4.2 ± 0.3 Hz (mean ± SD). With increasing doses of fentanyl, the use of 70% N₂O, or both, the curves shifted to higher values ofF95; the range in shifts was 0.2 to 8.1 Hz. The slope values of theF95-response logistic curves and the shifts among the curves were similar to the values and shifts that might be expected from changes in anesthetic agent doses.Conclusions. The EEG, expressed asF95, predicted movement response to surgical stimuli during combinations of isoflurane, 70% N₂O, and fentanyl. TheF95-response curves shifted upward on the frequency scale for the less intense stimuli and for anesthetic techniques using 70% N₂O, fentanyl, or both.F95 prediction of movement response appeared to be related to anesthetic agent doses. OurF95-response curves may provide helpful guidelines for usingF95 to titrate the administration of anesthetic agents and for assessing the depth of general anesthesia.     

Electroencephalographic mapping during isoflurane anesthesia for treatment of mental depression

Seven patients undergoing isoflurane anesthesia were studied using electroencephalographic (EEG) mapping, a computer-assisted technique of EEG processing that permits the analysis of the spatial distribution of frequency components of the human EEG. After induction with thiopental, the patients were hyperventilated (carbon dioxide tension, 25 to 28 mm Hg) with 4% isoflurane in oxygen until a burst-suppression EEG appeared. Brain electrical activity was compared before anesthesia, at 0.5 to 0.8% and 1.8 to 2.1% isoflurane, during bursts, and after anesthesia. Starting from an alpha EEG with occipital predominance, a distinct alpha activation appeared over the whole cortex during 0.5 to 0.8% isoflurane. Decreased alpha amplitude with a maximum over the frontotemporal regions was observed during 1.8 to 2.1% isoflurane. Bursts consisted of high amplitude in all frequency bands. In the delta and the theta bands, voltage was very low before isoflurane administration, and increased in relation to depth of anesthesia. A minimum in delta and theta amplitudes occurred over both central regions. The dominant frequency, defined as the frequency component with the highest amplitude in the frequency spectrum, decreased from 8 to 4 and finally to 1 Hz, while its amplitude increased from 3 to 13 µV. As anesthesia deepened, the dominant frequency shifted from the occiput to the frontal cortex.Brain mapping transfers an immense amount of electroencephalographic (EEG) data into a comprehensive, earsily readable image of brain function during anesthesia for therapy of mental depression. To make use of this technique in the operating room, further investigation and development toward on-line mapping are necessary.     

Effects of hypocapnia on canine spinal,subcortical, and cortical somatosensory-evoked potentials during isoflurane anesthesia

Although hyperventilation with hypocapnia is frequently used in the management of neurosurgical patients in whom sensory-evoked potentials may be monitored, the effects of hypocapnia on evoked potentials have not been described with precision. In the present experiment, the effects of randomized arterial carbon dioxide tensions of 20, 25, 30, and 35 mm Hg on spinal, subcortical, and cortical somatosensory-evoked potentials (SEPs) were measured in dogs anesthetized with 1.40% isoflurane. Other variables known to affect the SEP (temperature, blood pressure, and arterial oxygen tension) were stable throughout the experiment. Hypocapnia caused reductions in the latencies of the early peaks of the spinal and subcortical SEPs. These differences were small, consisting of a 2% shortening of latency at 20 mm Hg carbon dioxide tension when compared with 35 mm Hg. No changes were detected in the later subcortical and cortical latencies. SEP amplitudes were also unchanged. These results in a controlled animal study corroborate the direction and magnitude of changes due to hypocapnia observed by other investigators in surgical patients. The magnitude of the changes indicates that SEP monitoring sensitivity is not compromised by clinically useful levels of induced hypocapnia during isoflurane anesthesia. Because hypocapnia may produce small SEP changes, baseline recordings should be acquired prior to initiation of hyperventilation. It is not warranted, however, to impute a severe deterioration of the SEP to hypocapnia alone, and causes must be sought elsewhere in a patient's status and management.Supported by the Department of Anesthesiology Research and Development Fund, University of Wisconsin. Isoflurane was provided by Anaquest, Inc., Liberty Corner, NJ.     

Effects of hypocapnia on canine spinal, subcortical, and cortical somatosensory-evoked potentials during isoflurane anesthesia

Although hyperventilation with hypocapnia is frequently used in the management of neurosurgical patients in whom sensory-evoked potentials may be monitored, the effects of hypocapnia on evoked potentials have not been described with precision. In the present experiment, the effects of randomized arterial carbon dioxide tensions of 20, 25, 30, and 35 mm Hg on spinal, subcortical, and cortical somatosensory-evoked potentials (SEPs) were measured in dogs anesthetized with 1.40% isoflurane. Other variables known to affect the SEP (temperature, blood pressure, and arterial oxygen tension) were stable throughout the experiment. Hypocapnia caused reductions in the latencies of the early peaks of the spinal and subcortical SEPs. These differences were small, consisting of a 2% shortening of latency at 20 mm Hg carbon dioxide tension when compared with 35 mm Hg. No changes were detected in the later subcortical and cortical latencies. SEP amplitudes were also unchanged. These results in a controlled animal study corroborate the direction and magnitude of changes due to hypocapnia observed by other investigators in surgical patients. The magnitude of the changes indicates that SEP monitoring sensitivity is not compromised by clinically useful levels of induced hypocapnia during isoflurane anesthesia. Because hypocapnia may produce small SEP changes, baseline recordings should be acquired prior to initiation of hyperventilation. It is not warranted, however, to impute a severe deterioration of the SEP to hypocapnia alone, and causes must be sought elsewhere in a patient's status and management.Supported by the Department of Anesthesiology Research and Development Fund, University of Wisconsin. Isoflurane was provided by Anaquest, Inc., Liberty Corner, NJ.     

瑞芬太尼复合静脉麻醉与异氟醚静吸复合麻醉的比较

目的:研究瑞芬太尼复合异丙酚全凭静脉麻醉与异氟醚静吸复合全身麻醉对患者气管插管及苏醒期血流动力学变化和拔管条件的影响.方法:60例择期全麻手术的患者随机分为瑞芬太尼+异丙酚静脉麻醉(R)组和异丙酚+异氟醚静吸复合麻醉(C)组,每组30例.观察麻醉诱导、气管插管及术后恢复过程的血压、心率.记录术毕停药后病人清醒时间、拔管时间,记录病人拔管后即刻、离开手术间及术后3和24 h疼痛评分(VRS).观察恶心、呕吐等副反应.结果:麻醉诱导后气管插管1～5 min内血流动力学变化幅度R组明显小于C组(P＜0.05);术后清醒时间及拔管时间R组早于C组(P＜0.05),并且恶心、呕吐发生率低;术后VRS评分R组高于C组.结论:瑞芬太尼复合异丙酚静脉麻醉在气管插管和麻醉苏醒期的血流动力学稳定,拔管条件优,并发症少,但应注意尽早实施术后镇痛.     

脑电双频指数、听觉诱发电位指数用于老年人七氟醚麻醉深度监测的意义

目的探讨七氟醚的不同呼气末浓度对老年患者脑电双频指数(B IS)、听觉诱发电位指数(AAI)及血流动力参数的影响。方法行气管内全身麻醉术的老年患者30例,ASAⅠ～Ⅱ级,在8%七氟醚、罗库溴铵,瑞芬太尼麻醉诱导插管后机械通气,调节挥发罐浓度,使七氟醚呼气末浓度依次稳定在0.6最低肺泡气有效浓度(MAC)、1.0MAC、1.3MAC,每种浓度均维持15 m in,记录七氟醚呼气末浓度稳定在各点15 m in后的BIS、AAI、平均动脉压(MAP)和心率(HR)值。结果七氟醚呼气末浓度为0.6MAC、1.0 MAC、1.3 MAC时的BIS、AAI、MAP、HR值与麻醉前比较明显降低(P<0.01),1.0 MAC、1.3 MAC时的BIS、AAI值较0.6MAC时明显降低(P<0.01),而1.0 MAC和1.3 MAC时的B IS、AAI值比较差异无统计学意义(P>0.05)。七氟醚呼气末浓度在0.6～1.3MAC范围内,BIS、AAI、MAP随七氟醚浓度增高而降低,并呈显著的等级相关;HR与七氟醚浓度无相关性。结论 BIS和AAI可作为老年人七氟醚麻醉深度监测的有效指标。     

Acceptable ranges for vital signs during general anesthesia

Objective. Define the ranges for normal vital signs during general anesthesia.Methods. We studied 50 patients undergoing general anesthesia. We asked residents to state desirable ranges for each patient's systolic and diastolic blood pressure (BP), heart rate (HR), SpO₂, andPetCO₂ during induction, intubation, maintenance, and emergence from anesthesia. We called these ranges the clinical operating range (COR) and observed the frequency, duration, and magnitude of transgressions of these CORs. We also recorded whether the transgressions were treated or tolerated, or whether the COR values were changed.Results. Upper COR values in the maintenance phase for systolic BP were 38%±20% above the preoperative values and 30%±20% above the values recorded just before induction of anesthesia. Lower COR values in the maintenance phase for systolic BP were 27%±9% below preoperative, and 31%±11% below pre-induction values. For HR, upper and lower COR values in the maintenance phase were 53%±44% above and 38%±17% below preinduction values, respectively. Transgressions of COR values for BP and HR were common, treatment frequent, and redefinition of COR values rare.Conclusion. Clinicians recognize ranges for vital signs during uneventful anesthesia. These CORs may differ from one stage of anesthesia to the next. Transgressions of these ranges are common. Not all transgressions are treated.Reprints are not available.     

Self-tuning adaptive control of induced hypotension in humans: A comparison of isoflurane and sodium nitroprusside

Induced hypotension is commonly used during surgery to decrease arterial pressure. Sodium nitroprusside and isoflurane are well-known hypotensive agents. The use of self-tuning adaptive control of induced hypotension was assessed with the use of sodium nitroprusside and isoflurane as hypotensive agents. Nineteen surgical patients were studied during closed-loop control of hypotension induced with sodium nitroprusside. This group of patients was compared with 10 similar patients in whom infusions of sodium nitroprusside were controlled manually by an anesthesiologist. Although the results of the two studies varied, no conclusion could be drawn regarding the superiority of cither manual or closed-loop control. When manual versus automatic control of isoflurane-induced hypotension was assessed in a similar fashion, the two methods of induction were found to be comparable.     

Effects of intravenous anesthetic agents on middle cerebral artery blood flow velocity during induction of general anesthesia

Objective. Our objective was to quantify the effects of intravenous anesthetics on values measured by or derived from transcranial Doppler sonography (TCD) during induction of general anesthesia.Methods. We recorded blood flow velocity in the middle cerebral artery (V-MCA) before, during, and after induction of general anesthesia in six groups of young patients without intracranial pathology (n=10 each) using TCD. Patients were randomized to receive either 2 mg/kg propofol, 1.5 mg/kg methohexital, 5 mg/kg thiopental, 0.3 mg/kg etomidate, 2 µg/kg fentanyl and 0.15 mg/kg midazolam, or 1.5 mg/kg ketamine and 0.15 mg/kg midazolam intravenously. At 2 min after injection, each patient was intubated and given isoflurane 0.8% and nitrous oxide 66% in oxygen. Ventilation was set to achieve an end-tidalPco ₂ of 40 mm Hg. V-MCA, arterial blood pressure, heart rate, hematocrit, andPco ₂ (venous samples) were measured before and 1, 3, 5, 10, and 30 min after induction of anesthesia.Results. The preinduction data were not different between groups. At 1 min after injection, propofol, thiopental, methohexital, and etomidate significantly decreased V-MCA. TCD values were only slightly affected following fentanyl/midazolam. Ketamine/midazolam induced a modest rise in V-MCA. After endotracheal intubation, V-MCA increased in all groups, and slowly declined thereafter.Conclusions. Under the circumstances of our study, values derived from TCD measurements responded differently to the agents used to induce general anesthesia in nonneurosurgical patients.     

眼科小儿不同全麻方式苏醒期躁动与清醒时间的比较

目的 比较眼科手术患儿丙泊酚-瑞芬太尼静脉复合（PR）、丙泊酚-七氟醚静吸复合（PS）和七氟醚吸入（S）三种全麻方式苏醒期躁动及意识清醒的情况。方法 选择90例ASAⅠ～Ⅱ级4～9岁患儿,其中视网膜玻璃体手术和额肌悬吊术各45例,随机等比分成PR组、PS组和S组（n＝30）。观察拔管后1～40 min七个时段三组患儿躁动（五级躁动评分≥4分者）的发生率及清醒时间。结果 拔管后10、15、20、30 min四个时段S组与PR组及PS组比较,躁动发生率显著增高（P〈0.05）,15 min发生率最高（P〈0.01）,PR组与PS组比较,各时段差异均无统计学意义（P〉0.05）;PR组与PS组及S组比较,清醒时间显著减少（P〈0.01）,PS组与S组比较差异无统计学意义（P〉0.05）。结论 丙泊酚-瑞芬太尼麻醉苏醒期躁动发生率较低而且清醒时间短,苏醒质量优于其他两种麻醉。     

Recognition of mixed anesthetic agents by mass spectrometer during anesthesia

Anesthetic agents are sometimes added to the wrong vaporizer on an anesthesia machine. As a result, the vaporizer may deliver a mixture of anesthetic agents at concentrations inappropriate for use on a patient. However, untoward clinical complications related to vaporizers can be prevented with a time-shared mass spectrometer. This device accurately and rapidly indicates the gases and gas concentrations present in a vaporizer.     

An anesthesia information system for monitoring and record keeping during surgical anesthesia

We have developed an anesthesia information system (AIS) that supports the anesthesiologist in monitoring and recording during a surgical operation. In development of the system, emphasis was placed on providing an anesthesiologist-computer interface that can be adapted to typical situations during anesthesia and to individual user behavior. One main feature of this interface is the integration of the input and output of information. The only device for interaction between the anesthesiologist and the AIS is a touch-sensitive, high-resolution color display screen. The anesthesiologist enters information by touching virtual function keys displayed on the screen. A data window displays all data generated over time, such as automatically recorded vital signs, including blood pressure, heart rate, and rectal and esophageal temperatures, and manually entered variables, such as administered drugs, and ventilator settings. The information gathered by the AIS is presented on the cathode ray tube in several pages. A main distributor page gives an overall view of the content of every work page. A one-page record of the anesthesia is automatically plotted on a multicolor digital plotter during the operation. An example of the use of the AIS is presented from a field test of the system during which it was evaluated in the operating room without interfering with the ongoing operation. Medical staff who used the AIS imitated the anesthesiologist’s recording and information search behavior but did not have responsibility for the conduct of the anesthetic.     

Wakeful response to command indicates memory potential during emergence from general anesthesia

Objective. An important aspect of assessing anesthetic depth is determining whether a patient will remember events during surgery. We looked for a clinical sign that would indicate a patient's potential for memory formation during emergence from anesthesia. A clinical sign indicating memory potential could be a useful endpoint for measuring the performance of anesthetic depth monitors and for titrating administration of anesthetic agents.Methods. We evaluated patients' responses to commands to open the eyes, squeeze the hand four times, and count 20 numbers. These responses were correlated with results on recall, cued recall, and multiple-choice memory tests.Main Results. Patients did not have evidence of memory formation until they sustained wakefulness sufficiently long to complete at least four hand squeezes or count four numbers. Of 28 patients, 13 (46%) with this sustained wakeful response had memory. Of 22 patients, 0 (0%) had evidence of memory formation when they demonstrated a brief wakeful response, defined as being responsive to command but unable to complete more than one hand squeeze or count, or an intermediate response, defined as two or three hand squeezes or counts.Conclusions. We conclude that a brief wakeful response to command indicates that a patient is unlikely to form memories, while a sustained wakeful response indicates that a patient may form memories. Thus, a patient's wakeful response to command could be a useful indicator of potential for memory.This work was supported by the Kaiser Foundation Research Institute. Elements of this work were presented at the Society for Technology in Anesthesia, January 1992, San Diego, CA.     

Brief wakeful response to command indicates wakefulness with suppression of memory formation during surgical anesthesia

Objective. In a previous study of patients emerging from anesthesia following surgery, we found that a brief wakeful response to command of an eye opening or single hand squeeze or count was not associated with memory formation, while the response of four hand squeezes or counts was associated with memory. We wanted to determine the anesthetic requirements for obtaining this brief wakeful response endpoint during surgery and to determine if memory occurred at this endpoint during surgical anesthesia.Methods. Six different combinations of isoflurane, 70% N₂O, and fentanyl were administered to 326 patients undergoing pelvic laparoscopy. After insertion of the trocar, anesthesia was reduced while patients were given verbal commands, and they were observed for movement responses to surgery and to command. Patients were classified as either not arousing, arousing with a movement response to surgery, or arousing with a wakeful response to command. For the patients who aroused, we calculated the percentage of arousal responses that were wakeful responses to command. The effect of fentanyl dosage upon the percentage of arousal responses that were wakeful responses to command was determined by using a Mann-Whitney test to compare a group of patients receiving fentanyl 2 µg/kg or less, with a group receiving fentanyl 4 µg/kg. In a subset of 39 patients, the potential for memory formation was evaluated by presenting a target sound to 29 patients during a period of either no arousal, movement response to surgery, or wakeful response to command; for a control group of 10 patients, no target sound was presented. All 39 patients were tested for memory of the target sound; the results from each group receiving a target sound were compared with the results of the control group, using a Mann-Whitney test.Main Results. A total of 68 patients aroused with either a movement response or a wakeful response to command. Wakeful responses occurred with only 1 of 39 patients (3%) receiving fentanyl 2 µg/kg or less; but, wakeful responses occurred with 17 of 29 patients (59%) receiving fentanyl 4 µg/kg. The difference between the groups was significant atp=0.01. None of the 68 patients had recall of intraoperative events or unpleasant dreams. None of these patients who were in the multiple-choice memory subset recalled the target sound. There were no statistically significant differences on the multiple-choice memory test between the groups presented with the target sound and the control group. Patient anecdotes suggested that some patients may have had memory of the target sound; but, memory was no more likely in patients with a brief wakeful response to command than in those who responded with a movement to surgical stimulation or those who did not have an arousal response.Conclusions. A brief wakeful response to a command of opening the eyes or squeezing the hand was not associated with increased memory formation during surgery. A brief wakeful response to command was found during surgery when patients received fentanyl 4 µg/kg; but it was rarely found at fentanyl dosages of 2 µg/kg or less.     

Electroencephalogram spectral edge frequency,lower esophageal contractility,and autonomic responsiveness during general anesthesia

Both the electroencephalogram (EEG) spectral edge frequency (SEF) and lower esophageal contractility (LEC) indices have been reported to be useful indicators of anesthetic depth. We designed a prospective study to evaluate the relationship between changes in these two variables and objective measurements of physiologic responsiveness to surgical stress (i.e., changes in hemódynamic variables and plasma levels of norepinephrine, epinephrine, total catecholamines, and vasopressin). Eighty-nine consenting adult males undergoing radical prostatectomy procedures under a standardized general anesthetic technique were studied according to a randomized, single-blinded protocol. General anesthesia was induced with 30 µg/kg intravenous (IV) alfentanil, 2.5 mg/kg IV thiopental, and 0.1 mg/kg IV vecuronium, and subsequently maintained with 0.5 µg/kg/min alfentanil, nitrous oxide (N₂O) 67% in oxygen, and 0.8 µg/kg/min vecuronium. Following retropubic dissection, 81 patients (92%) manifested acute hypertensive responses, with mean arterial pressure increasing from 90±14 to 122±14 mm Hg (mean ± SD). This acute hypertensive response was treated with one of three different treatment modalities (20 to 60 µg/kg IV alfentanil, 0.5 to 2.0% inspired isoflurane, or 0.05 to 0.15 mg/kg IV trimethaphan) to return the mean arterial pressure to within 10% of the preincisional (baseline) value within 5 to 10 minutes. Although the mean arterial pressure, heart rate, and plasma levels of catecholamines and vasopressin significantly increased following the surgical stimulus, and decreased after adjunctive therapy, the EEG-SEF and LEC index (LECI) values did not significantly change during these study intervals. Furthermore, using a logistic regression analysis, we observed that preincision EEG-SEF and LECI values could not predict whether patients would manifest a hypertensive response. Therefore, the EEG-SEF and LECI were unreliable indicators of anesthetic depth.This study was supported in part by a grant from the Ambulatory Anesthesia Research Foundation, Los Altos, CA. (Dr White is a member of the Board of Directors.)The authors would like to thank Dan Kuni (Baxter Healthcare) for his assistance in obtaining the equipment used to perform the study; Vinod Kothapa, MD, for his valuable assistance with the anesthetic management of the study patients; Alex K. Mills, MD, for his assistance with the EEG interpretation; and Steven A. Bai, PhD, for his assistance with the plasma alfentanil analyses.     

Craniofacial Electromyogram Activation Response: Another Indicator of Anesthetic Depth

Objective. After finding that craniofacial EMG preceding a stimulus was a poor predictor of movement response to that stimulus, we evaluated an alternative relation between EMG and movement: the difference in anesthetic depth between the endpoint of EMG responsiveness to a stimulus and endpoint of movement responsiveness to that stimulus. We expressed this relation as the increment of isoflurane between the two endpoints. Methods. We measured EMG over the frontalis muscle, over the corrugator muscle, and between the F_p2 and the mastoid process as patients emerged from general anesthesia during suture closing of the surgical incision. Anesthesia was decreased by controlled washout of isoflurane while maintaining 70% N₂O, and brain isoflurane concentrations (^Ciso_Brain) were calculated. We studied a control group of 10 patients who received only surgical stimulation, and 30 experimental patients who intermittently received test stimuli in addition to the surgical stimulation. Patients were observed for movement responses and EMG records were evaluated for EMG activation responses. We defined an EMG activation response to be a rapid voltage increase of at least 1.0 µV RMS above baseline, with a duration of at least 30 s, in at least one of the three EMG channels. Patient responses to stimuli were classified as either an EMG activation response without a move response (EMG⁺, a move response without an EMG activation response (MV⁺), both an EMG activation response and a move response (EMG⁺MV⁺), or no response. We defined the EMG⁺ endpoint to be the threshold between EMG⁺ response and nonresponse to a stimulus, and estimated^C iso_Brain at this endpoint. We similarly defined the move endpoint and estimated the move endpoint^C iso_Brain. We then calculated the increment of^C iso_Brain at the EMG⁺ endpoint relative to the move endpoint. Main results. For the 30 experimental patients, the initial response to a test stimulus was an EMG⁺ in 14 patients (47%), an EMG⁺MV⁺ in 12 patients (40%), and a MV⁺ in 1 patient (3%); no response occurred by the time surgery was completed in 3 patients (10%). No response occurred in 7 of the control patients (70%). Of the 14 patients with an initial EMG⁺ response to a test stimulus, 9 patients later had a move response. For these 9 patients, the increment of ^Ciso_Brain between the EMG⁺ endpoint and move endpoint was 0.11 ± 0.04 vol% (mean ± SD). Conclusions. Our results suggest that, given the circumstances of our study, an EMG activation response by a nonmoving patient indicates that the patient is at an anesthetic level close to that at which movement could occur. However, because the first EMG activation response may occur simultaneously with movement, the EMG activation response cannot be relied upon to always herald a move response before it occurs. Our results also suggest that EMG responsiveness to a test stimulus may be used to estimate the anesthetic depth of an individual patient.