A wide variety of electroencephalographic (EEG)-based variables have been evaluated as indicators of the effect of anesthetic drugs on the central nervous system (1–9). However, many investigators have found that EEG-derived variables were drug specific and could not be monotonically related to either anesthetic effect or clinical response (6–8). The EEG bispectral index (BIS) monitor has a high probability of correctly predicting both the loss and recovery of consciousness (9–11) and has been reported to be more reliable for assessing the level of sedation and hypnosis during surgery than other available processed EEG algorithms (8). Nevertheless, the BIS has been reported to be susceptible to interference during use of the electrocautery (3).
The patient state index (PSI) is a quantitative EEG index for assessing the level of consciousness during sedation and general anesthesia (12). Drover et al. (4) reported that use of the PSI to titrate propofol administration significantly decreased drug use and improved the early recovery profile. A more recent study by Chen et al. (13) compared BIS and PSI values and suggested that the electrocautery unit was less likely to interfere with the PSI than the BIS readings during surgery. However, the study also revealed that application of the PSI electrode system was much more time consuming and less comfortable for the patients (13).
New disposable electrode monitoring strips have recently been introduced for both the patient state analyzer (PSA) (Physiometrix, Inc., North Billerica, MA) and the BIS™ monitor (Aspect Medical Systems, Natick, MA). We hypothesized that the changes in the PSA electrode system (PSArray2) would allow for a more rapid acquisition of the signal without reducing its accuracy, and that the BIS XP sensor would reduce the artifacts from electrocautery devices. In addition, this study was designed to compare the sensitivity and specificity of the new PSA and BIS electrode strips with respect to predicting loss of consciousness and emergence from a standardized general anesthetic technique. Finally, the effect of the modified electrode systems in assessing changes in the anesthetic concentrations and rejecting electrocautery interference were assessed during the intraoperative period.
After approval was obtained from the local IRB, 22 ASA physical status I–II patients scheduled for laparoscopic surgery under general anesthesia were enrolled in this study. No written informed consent was required by the IRB for this monitoring-only study. Patients with known neurological or psychiatric disorders or current use of anticonvulsant or other centrally active medications; those with clinically significant cardiovascular, respiratory, hepatic, renal, or metabolic disease; long-term drug or alcohol abuse; or a body weight >50% more than the ideal body weight were excluded from participating in this comparative study.
All patients received midazolam 2 mg IV for premedication. Both a BIS monitoring strip (XP sensor) and a PSA 4000 monitoring strip (PSArray2) were applied simultaneously to the forehead of all patients in the preoperative holding area (Fig. 1). On arrival in the operating room, routine clinical monitoring devices were placed, and baseline (awake) BIS and PSI values were recorded with the patient’s eyes closed for 1–2 min before the induction of anesthesia. Anesthesia was induced with propofol 2.0 mg/kg IV and fentanyl 1 μg/kg IV injected over 15–30 s. Cisatracurium 0.3 mg/kg IV was administered to facilitate tracheal intubation, followed by desflurane 4% (initial inspired concentration) in combination with nitrous oxide 60% in oxygen for the maintenance of anesthesia.
If the patient displayed autonomic signs consistent with inadequate anesthesia (e.g., increased heart rate, diaphoresis, or lacrimation), supplemental doses of propofol 20 mg IV were administered during the maintenance period. The inspired desflurane concentration was increased by 2% when the patient manifested a sustained (≥5 min) increase in mean arterial blood pressure (MAP) to ≥20% of the preincision baseline value. In response to clinical signs of excessive anesthetic effect (e.g., a decrease in MAP to ≥20% of the preincision value), the inspired concentration of desflurane was decreased by 2%. At the end of surgery, the inhaled anesthetics were discontinued, and residual neuromuscular blockade was reversed with glycopyrrolate 0.01 mg/kg IV and neostigmine 0.05 mg/kg IV.
The MAP, heart rate, BIS, and PSI values were measured at 1-min intervals during the induction and emergence periods, as well as immediately before and up to 5 min after a bolus of propofol or a change in the inspired concentration of desflurane. The end-tidal concentrations of desflurane, nitrous oxide, and oxygen saturation, as well as the BIS and PSI values, were recorded at 5-min intervals during the maintenance period. Two investigators were simultaneously involved in the conduct of the study. The staff anesthesiologists (RHW, AS, or RK) were responsible for administering the anesthetic drugs and for monitoring the depth of anesthesia by using standard clinical signs. Both the BIS and PSA monitor screens were positioned out of their line of sight, and the second investigator (JT) recorded data at specific intervals throughout the perioperative period. The incidence of electrocautery interference with the BIS and PSI values was determined by whether a displayed BIS or PSI value was present or absent each time the electrocautery unit was activated during the operation.
Data regarding the patient’s state of consciousness (e.g., ability to follow commands to open their eyes and squeeze the investigator’s hand; orientation to person, place, and time) were obtained at 15- to 30-s intervals from the start of the injection of the induction dose of propofol until loss of responsiveness to verbal commands and from discontinuing the inhaled anesthetics until the patient was awake (eye opening) and oriented (to person and place). Statistical analysis consisted of Student’s t-testing and repeated-measures of analysis of variance, with a post hoc test using Bonferroni’s correction for continuous variables. Categorical data were analyzed by the χ2 test. The relationship between BIS and PSI values during the induction and emergence periods was analyzed by using linear regression and the Bland-Altman technique to determine the correlation coefficients. Assessment of the nonlinear association between BIS and PSI values and the probability of unconsciousness was accomplished by using the logistic regression procedure, which estimated the probability of a binary yes/no response. The area under the receiver operating characteristic (ROC) curve for each index was determined by plotting the sensitivity (fraction of unresponsive participants who were correctly predicted to be unconscious) against 1 − specificity (fraction of responsive participants correctly identified as being awake), and it reflects the discriminating power of the indices. The area under the ROC curve summarizes the predictive power of the index to achieve a high specificity at any given sensitivity (14). An area >0.5 indicates that the measurement is predictive, and a measurement with 100% accuracy would have an area of 1.0. All tests were two sided, and a P value <0.05 was considered statistically significant. Data are presented as mean ± sd and percentages.
Ten male and 12 female patients with a mean age of 46 ± 11 yr (range, 24–66 yr) and mean weight of 69 ± 10 kg (range, 53–89 kg) were enrolled in this study. The mean duration of surgery was 85 ± 21 min (range, 47–128 min). The total doses of propofol and fentanyl were 155 ± 28 mg and 89 ± 25 μg, respectively. In addition, the average end-tidal concentration of desflurane was 3.3% ± 0.6%. Emergence times to eye opening and orientation were 4 ± 2 min and 5 ± 2 min, respectively. No patient reported intraoperative recall at the 24-h follow-up interview. The total time required to apply the electrode strip and to display the baseline PSI and BIS values was similar (66 ± 32 s versus 72 ± 41 s, respectively) for both monitoring systems (Table 1). The list price of the PSA monitor was less than that of the BIS monitor; however, the PSArray2 was more expensive than the BIS XP sensor (Table 1). The average selling prices of the two monitors and disposable sensors were similar.
The BIS and PSI values decreased progressively from preinduction values of 95 ± 5 and 97 ± 6 to preincision values of 52 ± 10 and 38 ± 17, respectively (Table 2). A similar degree of interpatient variability was observed in the PSI and BIS values. During the maintenance period, the PSI values tended to be lower than the BIS values. However, the pattern of the changes in the PSI and BIS values was similar after bolus doses of propofol and increases (or decreases) in the desflurane concentration (Table 3). Compared with the BIS monitor, the PSA monitor experienced significantly less interference (artifacts) during use of the electrosurgical unit (31% versus 73%, respectively) (Table 1). During the interval from discontinuing the maintenance anesthetics until the patients were oriented, the BIS and PSI values increased from 53 ± 10 and 47 ± 14 to 93 ± 8 and 86 ± 8, respectively (Table 4). Although the indices were comparable during the induction period, the PSI values were significantly lower than the BIS values during the emergence period. Nevertheless, the PSI correlated well with the BIS during both the induction (r = 0.85) and emergence (r = 0.74) periods (Fig. 2).
Logistic regression analysis demonstrated that the BIS and PSI were both significant predictors of unconsciousness (P < 0.01), with area under the ROC curve values of 0.97 ± 0.05 and 0.98 ± 0.05 for the BIS and PSI, respectively (Fig. 3). In addition, both the BIS and PSI values correlated with the end-tidal desflurane concentrations at eye opening (Fig. 4) and at tracheal extubation (Fig. 5). However, the correlation coefficients were consistently higher for the PSI than for the BIS.
A reliable monitor of anesthetic depth should display a good correlation between the measured index value and the patient’s physiologic responses during surgery, independent of the anesthetic that is being administered. In addition, its use would be associated with minimal interpatient variability (12). The induction of general anesthesia is usually accompanied by an increase in high-frequency EEG activity that spreads from the frontal region to more posterior regions of the brain, resulting in an increasing degree of sedation and eventually loss of consciousness (15). The pattern of changes in the PSI and BIS values was similar during the induction, maintenance, and emergence periods.
The BIS has been found to be a reliable monitor of the level of consciousness during sedation (10,11) and general anesthesia (3,9). The BIS possesses a good combination of sensitivity and specificity for assessing the level of consciousness even when different anesthetics are used (16). Most importantly, use of the BIS monitor has been found to improve the titration of both IV and inhaled anesthetics during general anesthesia (17–20). Analogous to the BIS, the PSI is a dimensionless number scaled from 100 to 0, with 100 representing an awake EEG and 0 representing complete electrical silence (12). Predictable changes in this quantitative EEG-based value have been associated with the loss and return of consciousness in volunteers anesthetized with propofol and sevoflurane (21). The enhanced sensitivity of the PSI is in part due to the use of a self-norming technique in the development of the algorithm (4,12). This method takes into account differences in individual background EEG activity, as well as interindividual patient variability in the brain’s response to different anesthetics (22).
This comparative study demonstrated that both of the new disposable sensor strips were able to provide reliable index values that could discriminate between the awake and anesthetized states. These cerebral monitoring devices displayed greater index values before anesthesia and upon recovery of consciousness compared with the index values during the maintenance period. The PSI values were consistently lower than the BIS values after supplemental boluses of propofol or changes in the inspired desflurane concentration, and this suggests that the PSI may be more sensitive to the effects of anesthetic drugs on the central nervous system. Furthermore, the failure of the PSI (versus BIS) to return to the preinduction baseline value after orientation to person and place suggests a potential difference between the two monitors with respect to their sensitivity to the residual (subhypnotic) effects of these particular anesthetics.
Analogous to earlier studies with the BIS monitor (17–20), these data suggest that the PSI could prove to be useful in optimizing the titration of IV and inhaled anesthetics to meet the needs of an individual patient, thereby facilitating a more rapid emergence from anesthesia. One study suggested that cerebral monitoring with either the BIS or auditory evoked potential devices can reduce the maintenance anesthetic and/or analgesic requirements, contributing to a shorter length of stay in the postanesthesia care unit and an improved quality of recovery (19). In outpatients undergoing ambulatory surgery, the anesthetic-sparing effect of EEG-based cerebral monitoring also contributed to a reduced time to discharge home (20). Compared with standard clinical practice, the cerebral-monitored groups were discharged home an average of 63 to 67 minutes earlier.
The ability of the PSA monitor with the PSArray2 to display PSI values during use of the electrocautery unit was superior to that of the BIS monitor with the XP sensor, confirming our earlier findings (13). In contrast to the original electrode system, application of the new PSArray2 electrode system required a similar amount of time to the BIS XP sensor (18 ± 8 seconds versus 12 ± 6 seconds). The area under the ROC curve data confirmed that both monitors possessed similar sensitivity and/or specificity with respect to changes in the level of consciousness. Moreover, both monitors demonstrated a good correlation during the induction of and emergence from general anesthesia. Finally, the costs of the monitors and disposable electrode units suggest that the PSA monitor is a viable alternative to the BIS monitor in the current health care environment.
This observational study can be criticized because only a small group of patients (n = 22) undergoing one type of surgical procedure was studied. In contrast to the extensive experience with the BIS monitor (23), there is a relatively limited database for the PSI algorithm (4,13). Further studies with the PSA monitor are clearly needed in larger populations of surgical patients undergoing different types of surgical procedures (e.g., cardiothoracic, obstetric, and trauma). It would also be important to determine whether use of the PSI for titrating volatile anesthetics during surgical procedures facilitates the early recovery process while avoiding intraoperative recall, analogous to the recent studies with the BIS and auditory evoked potential monitors (5,17–20). Questions still remain regarding the reliability of even the newest sensor systems (e.g., the BIS XP) in quantifying the level of consciousness. A recent report by Vuyk et al. (24) suggested that volunteers receiving a combination of propofol and midazolam were responsive with BIS values in the 40–50 range.
In conclusion, the changes in PSI values followed a pattern similar to that of the BIS values during the perioperative period. Furthermore, there was less interference with the PSI readings during electrocautery use, and application of the new PSI electrodes required a similar amount of time to the BIS electrode system. Analogous to the BIS, the PSI appeared to have a high degree of sensitivity and specificity in assessing consciousness during the induction of and emergence from general anesthesia, and was capable of detecting changes associated with the administration of anesthetic drugs during the maintenance period. Given the comparable costs, the PSA 4000 appears to be an acceptable alternative to the BIS for cerebral monitoring during general anesthesia.
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