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© 2001 International Anesthesia Research Society
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Abstract |
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 Top Abstract IntroductionMethodsResultsDiscussionReferences |
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We performed this study to determine whether instituting monitoring of bispectral index (BIS) throughout an entire operating room would affect end-tidal gas concentration (as a surrogate for anesthetic use) or speed of recovery after outpatient surgery. Primary caregivers (n = 69) were randomly assigned to a BIS or non-BIS Control group with cross-over at 1-mo intervals for 7 mo. Data were obtained in all outpatients except for those having head-and-neck surgery. Mean end-tidal gas concentration and total recovery duration were compared by unpaired t-test. Overall, 469 patients (80%) received propofol for induction and sevoflurane for maintenance. This homogeneous group was selected for statistical analysis. Mean end-tidal sevoflurane concentration was 13% less in the BIS group (BIS, 1.23%; Control, 1.41%; P < 0.0001); differences were most evident when anesthesia was administered by first-year trainees. Mean BIS values were 47 in the BIS-Monitored group. Total recovery was 19 min less with BIS monitoring in men (BIS group, 147 min; Controls, 166 min; P = 0.035), but not different in women. We conclude that routine application of BIS monitoring is associated with a modest reduction in end-tidal sevoflurane concentration. In men, this may correlate with a similar reduction (11%) in recovery duration.
IMPLICATIONS: Adoption of Bispectral index monitoring throughout an entire operating room was associated with use of lesser concentrations of sevoflurane to maintain anesthesia and reduced recovery duration in men undergoing general anesthesia for ambulatory surgery.
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Bispectral analysis of the electroencephalogram is a new method of monitoring depth of hypnosis during anesthesia. It is advocated as a means of preventing episodes of awareness during general anesthesia (1,2). It might also have more general utility in permitting improved accuracy of titration of anesthesia to meet individual patient requirements. The theoretical advantages are that less drug would be used and patients might recover and be discharged sooner. These concerns are particularly relevant in ambulatory surgery, where drowsiness is a cause of delayed discharge in up to 23% of patients after general anesthesia (3). A possible explanation for prolonged drowsiness is that the amount of anesthetic delivered is in excess of what is required for an individual patient.
Under closely controlled circumstances in which a uniform anesthetic protocol has been followed, anesthetic dose was reduced and various aspects of recovery expedited as the result of monitoring the Bispectral index (BIS) (4,5). However, there is a possibility that such advantages may not be observed if applied under conditions of routine clinical practice. In this study, we endeavored to determine how the adoption of BIS monitoring throughout an entire operating room would affect duration of recovery and anesthetic use under conditions of standard clinical practice.
The study was initiated approximately 3 mo after the installation of BIS monitors in all but one operating room throughout our institution. The null hypotheses to be tested were as follows: BIS monitoring has no effect on maintenance concentration of potent inhaled anesthetic (end-tidal gas concentration), and BIS monitoring has no effect on speed of recovery and discharge after outpatient anesthesia and surgery.
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Methods |
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The study was approved by the IRB at the University of Washington as a randomized, prospective comparison of outcome of patients anesthetized with or without monitoring BIS. BIS monitors (model A-1050 EEG Monitor; Aspect Medical Systems, Natick, MA) were permanently installed in 18 of 19 operating rooms. After an initial introductory period of approximately 3 mo to allow anesthesia providers (anesthesia attending staff, nurse anesthetists, and residents) to become familiar with the device, a study was instituted in which all anesthesia providers were assigned on a monthly basis to a BIS or Control group by use of a randomized cross-over design. The primary anesthesia providers included 18 certified registered nurse anesthetists (CRNAs) and 51 residents-in-training (19 in the clinical anesthesia training (CA)-1 yr, 16 in the CA-2 yr, and 17 in the CA-3 yr). They were supervised by 41 different anesthesia attending faculty members.
The study was performed over 7 mo. The patients studied included all patients undergoing general anesthesia for outpatient surgery not involving the head and neck. Patients having head-and-neck surgery were excluded in advance to ensure the absence of motion artifacts related to the conduct of surgery. Patients having regional or local anesthesia were included only if they also received a general anesthetic. Before beginning the study, anesthesia providers were given written and verbal education regarding the use and significance of the BIS. They were told that 1) a suggested target level for BIS is 50–60, 2) a BIS of <70 is adequate to prevent conscious recall, 3) movement may occur at that depth of anesthesia, and 4) patient safety should be their first priority. They were also advised that movement would be less likely if patients were analgesic (6). BIS electrodes were applied before surgery only to patients in the BIS group and removed before leaving the operating room. Anesthetics to be used were not specified. All anesthesia providers were requested to participate, and a notice attached to the face of each anesthesia machine reminded them of the study. At the end of 1 mo, all CRNAs (nonrotating) and residents who were not rotating to other geographic sites participated in a cross-over design. At the end of 2 mo, all participants were again randomly assigned to BIS or Control groups, followed by cross-over after 1 mo. All incoming residents rotating into our institution during the study received equivalent training and random group assignment, and they underwent cross-over if they remained in the institution for >1 mo. During the period of study, BIS monitoring was not available at the three other sites of our residency rotation. Faculty and CRNAs did not rotate to other centers. Use of the BIS/Control mode was used for all patients (inpatients and outpatients) during the course of the study. Patients were excluded from the study if anesthesia providers failed to adhere to the randomization scheme or if the patient’s status was changed from outpatient to inpatient or limited stay (overnight). The latter group was excluded because the duration of recovery caused by anesthesia is obscured by a variety of other factors in patients who are admitted. The majority of patients are admitted for reasons unrelated to the conduct of anesthesia.
There were no financial or other types of inducements for anesthesia providers to participate. Anesthesia providers were told they would receive periodic reports showing average recovery times and Phase 1 bypass rates for their patients compared anonymously with patients managed by their peers. The purpose was to stimulate general interest and enthusiasm for expediting patient recovery and discharge regardless of group assignment. It was also intended that feedback might offset any effects of bias by the anesthesia care team. Two reports were generated during the course of the study; the reports did not differentiate between BIS and Control patients but did compare one individual’s patients with those cared for by their peers.
Patient demographic characteristics and details of anesthetic delivery were summarized by anesthesia providers on separate data collection forms that accompanied the patient to the operating room. End-tidal anesthetic gas concentrations and BIS values were recorded manually by anesthesia providers at 15-min intervals. End-tidal gas concentration (sevoflurane) was used as an indicator of anesthetic dose for inhaled anesthetics. Mean end-tidal gas concentration was calculated for each patient as the average of all concentrations recorded at 15-min intervals, excluding the first and last 15 min of the case. Duration of anesthesia, surgery, and time to exit the operating room after completion of surgery were obtained from nursing records used in patient billing and verified with patient records.
The duration of recovery was determined separately for Phase 1 and Phase 2 recovery units, and the total recovery time until discharge from the hospital was computed as the sum of the two. Other intermediate milestones of recovery documented included the number of patients intubated on arrival in Phase 1 recovery unit, incidence of Phase 1 bypass, and the time to achieve an Aldrete score of 9–10 (7). Criteria for bypassing Phase 1 recovery included that the patient be awake, oriented, able to follow commands, and able to maintain an adequate airway without assistance with an oxygen saturation of >95% on room air (or within 3% of baseline). There was also an expectation that pain and nausea would be easily controlled. Criteria for transfer from Phase 1 to Phase 2 included all of the above, as well as absence of drainage from the surgical wound and pain and nausea rated as mild or absent by the patient.
When recovery duration exceeded 50 min in Phase 1 or 70 min in Phase 2, nurses were asked to indicate reasons for delays in discharge by using a preprinted list of causes that included medical, surgical, and system factors. Up to three factors were accepted; when more than three were cited, the three most important factors were recorded.
To ensure accuracy and completeness of data, a nurse who specialized in quality assurance issues in anesthesia was employed solely to oversee the function of the study. Accuracy of information provided on data collection forms by anesthesia caregivers and recovery room nurses was verified by comparison with information recorded on anesthesia and nursing records on a daily basis for each patient. Missing information was obtained by interview of the caregiver on the day after surgery. Adherence to the randomization scheme was reviewed daily and cases deleted if not consistent with group assignment. Additional research personnel were employed to input the data into a computer database for analysis.
Sample size was calculated on the basis of the results of a previous study, in which we observed a mean recovery time (±SD) of 164 ± 59 min in women and 151 ± 40 min in men undergoing general isoflurane anesthesia for outpatient surgery of 
90 min duration (3). By using the previous data to calculate standardized differences, a power analysis predicted a required sample size of 120 women or 55 men for 80% power to detect a 30-min difference, with an 
of 0.05, or 250 women and 130 men to detect a 20-min difference with an of 0.05 (8). The duration of study was set at 7 mo. This was predicted to yield approximately 250–300 patients of each sex. Descriptive data (mean, SD, SE) were computed by standard statistical methods. Comparisons between two groups were made by unpaired t-test. Frequencies for binary data were compared by 
2 tests with the Yates continuity correction. A nominal P value of 0.05 was considered significant throughout. Because we had previously observed differences between men and women in speed of recovery in relation to type of anesthesia and surgery, recovery data for men and women were analyzed separately (3). Although the primary outcome variables of the study were end-tidal gas concentration and total recovery time, secondary analyses were performed of various potentially explanatory variables for which the results might contribute to our understanding of the results of the primary analysis.
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Results |
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Overall, 585 patients were studied. The anesthetic drugs used were not stipulated by the study protocol. However, preliminary analysis revealed that 80% of anesthetics were performed using propofol for induction and sevoflurane for maintenance. The remainder was a heterogeneous group that received a wide variety of other combinations of drugs for induction and maintenance. We therefore chose to limit our analysis to the relatively homogeneous group of patients in the former category. The final analysis involved 469 patients: 236 men and 229 women, all of whom had received propofol for induction and sevoflurane for maintenance. The population characteristics and details of anesthesia are summarized in Table 1. In women, there was a higher incidence of intubation in the BIS group (56% vs 38%, P = 0.008). There were some notable differences between men and women in the types of surgery performed. Only women underwent abdominal surgery (pelvic laparoscopy). The dose of fentanyl was slightly larger on a microgram per kilogram basis in women than in men (women, 2.0 µg/kg versus men, 1.4 µg/kg; P = <0.0001), and it was somewhat larger in BIS versus Control men (BIS men, 1.6 µg/kg versus Control men, 1.3 µg/kg; P = 0.016).
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Mean end-tidal gas concentrations are shown in Table 2. Overall, the mean end-tidal concentration of sevoflurane was 13% less (P < 0.0001) in BIS-monitored patients. Stratification by subgroup of anesthesia provider (i.e., junior residents = CA-1 yr, nonjunior residents = CA-2 and CA-3 yr, and CRNAs) suggested that inexperienced junior residents were more likely to lessen anesthetic depth when using the BIS monitor. The end-tidal sevoflurane concentration in the BIS group was 24% less for junior residents, 5% less for nonjunior residents (not significant), and 6% less for CRNAs. There was a weak but significant correlation between mean end-tidal sevoflurane concentration and mean values for BIS (R2 = 0.09, P
0.0001).
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In Table 3, various aspects of recovery are summarized as a function of BIS group. Overall, male patients in the BIS group were discharged 19 min faster than those in the Control group (P = 0.035). There was no difference in the incidence of Phase 1 bypass between the two groups. There was a trend toward a lesser incidence of drowsiness as a cause of delayed discharge in the BIS group (6% vs 13%, BIS versus Control; P = 0.08). The relationship of group assignment (BIS versus Control) to total recovery time was similar if all patients were included in the analysis (i.e., if we included all men regardless of anesthetic technique, BIS recovery time was 148 min versus control time, 166 min; P = 0.044).
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In women, there was no significant difference in recovery duration between BIS and Control groups. Overall, nausea and vomiting were more often reported as a cause of delayed discharge in women (20%–28% of women as compared with 7%–12% of men). Exclusion of females whose recovery was delayed by nausea or vomiting resulted in a trend toward reduced recovery times in the BIS group relative to the Control group (i.e., mean Phase 1 recovery time was reduced from 60 and 54 min in BIS and Controls to 52 and 51 min; Phase 2 recovery time was reduced from 107 and 104 min in BIS and Controls to 98 and 102 min, respectively). Although women in the BIS group were more often intubated than those in the Control group (Table 1, P = 0.008), exclusion of intubated patients did not alter outcome (recovery time for BIS nonintubated women, 158 min versus Control nonintubated women, 144 min; P = 0.17).
In Table 4, we explored the relationship between BIS values, Phase 1 bypass, and recovery duration. Overall, the duration of recovery was significantly less in bypass patients than in nonbypass patients (57 min less in men, P = 0.005; 36 min less in women, P = 0.002). There was no significant difference in the duration of anesthesia between bypass and nonbypass patients (duration of anesthesia, 72 min in bypass men versus 74 min in nonbypass men, P = 0.85; duration of 69 min in bypass women versus 78 min in nonbypass women, P = 0.35). Within the BIS group, mean BIS values did not differ between bypass and nonbypass patients. There was however, a significant relationship between BIS values at the end of surgery and total duration of recovery (R2 = 0.61, P = 0.02) in the small subgroup of men (n = 10) who bypassed Phase 1 recovery, and there was a similar trend in women (R2 = 0.5, P = 0.17).
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Overall, there were no significant trends over the duration of the study in mean BIS values, mean end-tidal sevoflurane concentrations, or duration of recovery, suggesting there was no significant change in the management of patients over time within the institution (Fig. 1) and no "Hawthorn effect."
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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In this study, instituting BIS monitoring throughout an entire operating room was associated with a reduction in mean end-tidal gas concentration of a potent inhaled anesthetic (sevoflurane), suggesting that a similar reduction in anesthetic use occurred if we assume equivalent total gas flows in the two groups. There was a significant inverse correlation between mean end-tidal sevoflurane concentrations and mean BIS values.
The observation that anesthetic concentration is reduced in BIS-monitored patients is in agreement with a report by Song et al. (5) of a 30%–37% reduction in end-tidal concentrations of sevoflurane or desflurane in BIS-monitored women undergoing tubal ligation. Several factors may account for the lesser magnitude of difference (13%) observed in our study. The mean BIS in our study was 47, and the majority of patients were not paralyzed (91%). In contrast, all patients in the study of Song et al. were paralyzed, allowing greater flexibility in upward titration of anesthetic depth without risking patient movement. In that study (in which a BIS of 60 was mandated by protocol), the mean BIS was 60–62, and coughing and bucking occurred in 23% of patients, despite paralysis. In our study, a BIS of 50–60 was suggested, with a caution that patients might move at that depth and that patient safety should be the first priority.
The reduction in anesthetic concentration reported in our study is similar to the 13% reduction in propofol use reported by Gan et al. (4) in BIS-monitored patients undergoing a variety of procedures with a propofol, nitrous oxide, and alfentanil combination. In that study, a suggested target for BIS was 45–60, and the mean BIS achieved was 44.
The results of all three studies considered together suggest that maintaining BIS in the range of 44–47 may be associated with a modest (13%) reduction of anesthetic use, whereas a BIS of >60 may reduce use by as much as 30% but with an increased likelihood of movement, particularly in the unparalyzed patient. Johansen et al. (9) also observed a 22% reduction in use of desflurane and a 5% reduction for isoflurane in a mixed group of inpatients and outpatients if BIS values were maintained predominantly within a specified target range of 50–65. However, only about half of the patients were maintained within that target zone.
In our study, the difference in anesthetic concentration between BIS and controls was most evident in cases performed by junior residents. The differences were fewer with CRNAs and insignificant with residents beyond the first year of anesthesia training. Without BIS monitoring, there was a tendency for junior residents to maintain patients at a larger end-tidal sevoflurane concentration than other providers. However, with BIS monitoring, there was a reverse trend; junior residents tended to maintain their patients at smaller end-tidal sevoflurane concentrations compared with more experienced providers. This observation suggests that the BIS may be used most effectively by individuals new to the practice of anesthesia, because they have less clinical experience, fewer preconceived biases concerning adjustment of anesthetic depth, or both of these.
In our study, recovery was 19 min faster in men in the BIS-monitored group. Our end points of recovery are not directly comparable to those reported by Song et al. (5) or by Gan et al. (4) because of differences in patient populations, institutional practices, and anesthetic protocols. However, Song et al. similarly observed no difference in recovery duration in women monitored by BIS during sevoflurane anesthesia; men were not studied. Patients in the study of Gan et al. included patients of both sexes undergoing a variety of types of surgery. They also failed to detect differences in total recovery duration between BIS and Control groups, but they did not perform separate analyses for men and women. In our study, differences in speed of recovery were observed only in men. Johansen et al. (9), in a study that involved primarily inpatients with a mean anesthetic duration of 212–214 minutes, reported that time spent in postanesthesia recovery unit (Phase 1 recovery) was 10 minutes less in patients managed with BIS monitoring, if the patients were kept predominantly within the 50–65 target zone for BIS values.
It is noteworthy that discharge of women in our study was more often delayed by pain and by emetic symptoms than in men. This may in part reflect differences in surgical procedures (abdominal operations were performed only in women). The increased incidence of emetic symptoms may also be caused in part by women having received larger doses of opioid during surgery. Thus, the effects of pain, emetic symptoms, and other side effects may have overwhelmed any potential benefits of reduced anesthetic delivery in women. Our data suggest that most significant reductions in recovery duration in females might be expected to derive from improved methods of preventing pain and emesis. The effects of emetic symptoms as a rate-limiting step determining recovery duration in females have previously been described (10,11). Additionally, a variety of system factors (i.e., escort unavailable, discharge medications not ready, and Phase 2 recovery full) frequently delay discharge in both sexes (3,12). In this study, bypassing Phase 1 recovery was associated with a shorter recovery time (57 minutes less in men, 36 min less in women). Because patients were not randomly assigned to bypass Phase 1 recovery, it is unclear whether this difference is the result of bypass itself or of selection bias. In our study, BIS values did not predict bypassing Phase 1 recovery. BIS values were the same in bypass versus nonbypass patients in either sex whether comparing mean BIS values, BIS values 15 minutes before the end of surgery, or BIS values at termination of surgery. However, within a small group of male patients who did bypass Phase 1 recovery, we observed a significant correlation between BIS at the end of surgery and duration of recovery (R2 = 0.61, P = 0.02), and we observed a similar trend in women. Because the number of bypass patients was small, this analysis must be viewed as exploratory in nature. The data do suggest, however, that there might be a strong correlation between BIS at the end of surgery and total recovery duration if patients avoid delays inherently associated with being sent to a Phase 1 recovery unit. Therefore, future studies evaluating the role of BIS monitoring on recovery duration might best be conducted in Phase 1 bypass patients, with independent analyses for male and female patients.
This study may be criticized for the lack of blinding of anesthesia personnel. Anesthesia providers were obviously aware of which group they were assigned to, and they were free to administer anesthesia in whatever manner they chose. It is possible that there was bias that influenced the willingness to use the BIS effectively. Such biases could result in deliberate or unconscious attempts to prove or disprove the value of monitoring. Enthusiasm for the device varied, but all participants were cooperative in using the monitor and recording BIS values. To minimize or offset effects of bias, we attempted to stimulate general interest in achieving rapid recovery and discharge, regardless of BIS status, by providing individual feedback as to recovery times and rates of Phase 1 bypass. Nevertheless, there may have been biases that affected the ability to use the BIS to greatest advantage. However, there may also be such biases in most practice groups, and these are part of usual clinical practice conditions. Our study was designed to evaluate the effect of BIS monitoring under usual clinical practice conditions, which include the effects of practitioner bias.
In summary, we observed that BIS monitoring instituted throughout a large academic institution resulted in a 13% reduction in anesthetic use in outpatients of both sexes and an 11% reduction in duration of recovery in men. BIS monitoring did not alter the incidence of Phase 1 bypass, nor did BIS values predict the likelihood of Phase 1 bypass. We conclude that the BIS monitor is a useful adjunct for monitoring depth of anesthesia, anesthetic requirements, or both, particularly for trainees in a teaching institution. However, it has limited capacity to influence duration of recovery when used to monitor unparalyzed patients for short procedures with a relatively insoluble anesthetic such as sevoflurane, particularly when patients are recovered in the traditional Phase 1/Phase 2 paradigm.
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Acknowledgments |
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Supported, in part, by an unrestricted gift from Aspect Medical Corp, Natick, MA, and by departmental funding, Department of Anesthesiology, University of Washington.
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References |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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[Abstract/Free Full Text] - Gan TJ, Glass PS, Windsor A, et al. Bispectral index monitoring allows faster emergence and improved recovery from propofol, alfentanil and nitrous oxide anesthesia. Anesthesiology 1997; 87: 808–15.[Web of Science][Medline]
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- Sebel PS, Lang E, Rampil IJ, White PF. A multicenter study of bispectral electroencephalogram analysis for monitoring anesthetic effect. Anesth Analg 1997; 84: 891–9.[Abstract]
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