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Depth of Anesthesia Monitoring: A Survey of Attitudes and Usage Patterns Among Australian Anesthesiologists

Ben-Menachem, Erez MBChB, FCICM, FANZCA*†; Zalcberg, Dave MBBS*

doi: 10.1213/ANE.0000000000000344
Economics, Education, and Policy: Research Report

BACKGROUND: Utility of depth of anesthesia (DoA) monitors is contentious as evidence appears ambiguous regarding their clinical effectiveness and exact role. We conducted a survey of Australian anesthesiologists to determine their attitudes toward, and how and why they use, DoA monitors.

METHODS: A random sample of 963 anesthesiologists was invited to participate in an anonymous online survey.

RESULTS: The overall response rate was 30% (289 respondents). Twenty-nine percent (95% confidence interval, 24%–34%) of respondents thought DoA monitoring was indicated in all cases under relaxant general anesthesia. During total IV anesthesia with muscle relaxants, 74% of respondents (69%–79%) opined that DoA monitoring should be mandatory. DoA usage was never used by 5% of respondents (3%–8%), used in less than one-third of cases by 66% (61%–72%), and in more than one-third of cases by 29% (24%–35%). Belief in the usefulness of DoA monitoring for prevention of awareness was strongly associated with higher usage (P < 0.0001, Pearson correlation 0.32). Anesthesiologists were more influenced by higher DoA numbers than lower ones. In comparison with end-tidal anesthetic concentration monitoring, 30% (25%–35%) of respondents felt that DoA monitoring was more effective for prevention of intraoperative awareness. Thirty percent (25%–36%) of respondents reported having a previous case of awareness.

CONCLUSIONS: The relatively frequent use of DoA monitoring contrasts with patterns in the UK, suggesting greater acceptance by Australian anesthesiologists. “Awareness prevention” rather than “recovery enhancement” appears to be the primary driver in DoA monitoring use in Australia. Highly variable usage patterns of DoA monitoring in the context of the current body of evidence suggest the need for greater education on the appropriate use of these monitors.

Published ahead of print July 8, 2014.

From the *Department of Anesthesia, St. Vincent’s Hospital, Sydney, Australia; and School of Medicine Sydney, University of Notre Dame Australia, Sydney, Australia.

Published ahead of print July 8, 2014.

Accepted for publication May 5, 2014.

Funding: None.

The authors declare no conflicts of interest.

Reprints will not be available from the authors.

Address correspondence to Erez Ben-Menachem, MBChB, FCICM, FANZCA, Department of Anesthesia, St. Vincent’s Hospital, Sydney, Australia. Address e-mail to

Electroencephalographic (EEG) depth of anesthesia (DoA) monitors have become increasingly ubiquitous in modern anesthetic practice. Recent attempts have been made to better delineate the use of these monitors,1,2 but their place in clinical practice remains controversial and continues to evolve.3,4 The debate surrounding their role has even entered the realm of popular media, with statements such as, “Today, the BIS monitor has become the most controversial medical device in anesthesiology, if not all of surgery.”5

There are several proprietary monitors, with the bispectral index (BIS, Covidien, Dublin, Ireland) and Entropy (GE Healthcare, Helsinki, Finland) being the 2 most commonly used. These monitors process and analyze EEG waveform data, with or without electromyography, collected from scalp electrodes. Proprietary algorithms analyze these data, which reflect brain cortical activity and provide a dimensionless number on a scale of 0 to 100. This value is intended to reflect the DoA.

The emergence of these monitors has led to an abundance of quality research into their utility in current clinical practice. Primarily, these monitors were seen as aids in the prevention of intraoperative awareness; an infrequent but potentially devastating complication of general anesthesia (GA) with a consistently reported incidence of 0.1%–0.2%.6,7 Aptly named trials, such as the B-Aware8 and B-Unaware trials,9 have sought to elucidate the role of BIS in the prevention of awareness in high-risk patients. More recently Mashour et al.10 found that alerting based on BIS monitoring was not superior to alerting based on end-tidal anesthetic concentration (ETAC) values in preventing intraoperative awareness in unselected patients. The evidence collected to date suggests BIS is no better than ETAC monitoring, providing it is used in conjunction with an alerting algorithm.

Other investigators have explored an expanded role of DoA monitors beyond prevention of intraoperative awareness. BIS monitoring has been used to reduce anesthetic drug use and guide emergence with the aim of reducing recovery times and improving operating room efficiency. Meaningful metrics, such as time to discharge home and patient satisfaction, have not been improved.11,12 BIS-guided delivery of anesthesia and patient outcomes have also been investigated, with an association being found between cumulative time of “deep anesthesia” (BIS <45 or <40) and mortality,13 although results have been conflicting.14

Previous surveys have either focused on awareness or were conducted before the advent of widespread DoA monitoring accessibility when a maximum of only half of respondents had ever used such a monitor; consequently, their findings can no longer be assumed to reflect current practice.15,16 Little is known about how, and for what benefit, anesthesiologists use these monitors during routine clinical practice. Despite the costs associated with these monitors and ongoing intense research into their role, to our knowledge there is no current literature documenting how clinicians currently use DoA monitoring. With the intention of better defining these variables we surveyed Australian anesthesiologists to test the hypotheses that (1) the perceived effectiveness of these monitors promotes their use among Australian anesthesiologists and (2) Australian anesthesiologists use these monitors primarily to prevent intraoperative awareness.

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Prospective approval was obtained from the Human Research Ethics and Governance Committees at St. Vincent’s Hospital, Sydney (LNR/12/SVH/272). The requirement for individual consent was waived because survey completion was anonymous and voluntary such that completion of the survey implied consent.

A simple self-administered survey was developed using previously published guidelines for survey development in anesthesia.17,18 The survey contained 23 questions, of which 5 questions were dedicated to demographic data. Of the 18 remaining questions, 9 required responses on a Likert scale (strongly agree, agree, disagree, and strongly disagree) and a further 7 questions invited a “one of” individual response in closed question format. One allowed multiple responses from a list and one question required a ranking. The final question was an open response allowing free text. Demographic data differentiated between those practicing predominantly in the metropolitan hospitals and outer metropolitan hospitals and between public and private hospitals. The survey was developed in consultation with senior anesthesiology colleagues within our department. Questions were developed using the major research themes represented in the literature relating to awareness prevention, enhanced recovery, and reduction in complications. Some questions were chosen to reflect previous surveys to allow comparisons to be made. The survey was reviewed by an anonymous independent reviewer chosen by the Australia and New Zealand College of Anaesthetists (ANZCA). The survey was subsequently piloted with 10 consultant anesthesiologists at St. Vincent’s Hospital with feedback analysis. Feedback demonstrated face validity; several questions were reworded to improve clarity and Likert scales were simplified. Before final distribution of the survey, it was reassessed by the ANZCA trials group for survey quality and clinical relevance.

The Web-based survey was constructed using Web-host SurveyMonkey (Portland, OR). The survey and a cover letter with a brief explanation of the project and encouraging participation were e-mailed in late 2012 to 963 randomly selected Australian Fellows (board-certified) of ANZCA with the assistance and support of the ANZCA trials group. Residents were not surveyed because their practice is heavily influenced by supervising anesthesiologists. Australian membership of ANZCA included 3612 anesthesiologists in 2011–12, with the size of the sampled population representing approximately 27% of the total population. ANZCA is the sole training and supervisory body for anesthesia in Australia and represents the majority of anesthesiologists practicing in Australia. A second mailing went out 3 weeks after the initial e-mail. The sampled anesthesiologists and sample size were determined by ANZCA to minimize survey fatigue. The researchers remain blinded to the sampled anesthesiologists; therefore, nonresponder data cannot be collected. All responses were anonymous with no computer Internet protocol information collected.

Data analyses were conducted using SPSS for Windows (IBM Corp., Armonk, NY, version 21.0). Data are expressed as proportions (rounded to the nearest integer) and are reported with 95% confidence intervals calculated using the Wilson score interval. Differences among categorical variables were assessed for statistical significance using the χ2 test, and correlation among interval variables was tested using a 2-tailed Pearson correlation. Differences among scale means were tested for statistical significance using a 2-tailed t test. P values were reported, and a value of <0.05 was considered statistically significant.

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Of the 963 e-mailed survey requests, 289 surveys were completed, resulting in an overall response rate of 30%. Demographics of respondents, practice location, and DoA monitoring availability are presented in Tables 1 and 2. All demographic variables closely reflect the membership of ANZCA, including geographical distribution, gender, and years of experience.a

Table 1

Table 1

Table 2

Table 2

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The Perceived Effectiveness of These Monitors Among Australian Anesthesiologists

Attitudes Toward DoA Monitoring

The majority (>50%, P < 0.0001) of respondents considered DoA monitoring useful for the prevention of awareness with 66% (61%–72%), either agreeing or strongly agreeing with this statement, while 25% (20%–30%) did not know and only 9% (6%–13%) disagreed or strongly disagreed. Covariates, including experience, geographical or practice location, gender, hours of clinical work, and a previous case of awareness, were not statistically significant (all P > 0.14). Compared with ETAC, DoA monitoring was thought to be more effective by 30% of respondents (25%–35%), as effective by 36% (31%–42%), and less effective by 34% (29%–40%) at preventing awareness, and perception of the usefulness of DoA monitoring was the only covariate with statistical significance (P < 0.001, χ2 test). If DoA monitoring did not offer any medicolegal protection in a case of awareness, most would still use it (>50%, P < 0.0001). Whether or not there was an association between prolonged low DoA readings (<40) and adverse outcomes is presented in Table 2. Practitioners of >20 years compared with <5 years tended to strongly disagree or disagree (31% vs 10%) with the association between low DoA readings and adverse outcomes, while the opposite was true when agreeing/strongly agreeing (23% vs 55%, P = 0.011, χ2 test).

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Use of DoA Monitoring

In individual practice, DoA monitoring was used in less than one-third of cases by 66% of respondents, and 20% used it in more than one-third of cases (Table 2). The respondents with the highest usage correlated with those who agreed most strongly that DoA monitoring was useful for the prevention of awareness (P < 0.0001, Pearson correlation 0.32), and bivariate analysis showed proportional use of DoA monitoring to be heavily influenced by belief in its usefulness in the prevention of awareness (P < 0.0001). Usage was not influenced by years of experience or by whether the respondent reported having a case of awareness (all P > 0.40). The influence of the DoA monitor on titration of anesthetic during a muscle relaxant GA case, when the patient’s hemodynamics and delivered anesthetic are in the target range, is presented in Table 3, with elevated readings being highly influential. A majority of respondents (62%; 56%–67%) felt DoA monitoring enabled delivery of less anesthetic, while 19% (15%–24%) did not know and 19% (15%–24%) disagreed. The primary benefit of reduced anesthetic delivery was thought to be faster emergence (34%), followed by less postoperative morbidity (30%) and intraoperative hemodynamic stability (26%). Cost and greenhouse gas effects were not viewed as significant factors.

Table 3

Table 3

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Australian Anesthesiologists’ Use of DoA Monitors in the Prevention of Intraoperative Awareness

Indications for DoA Monitoring

Regardless of personal case mix, 5% (3%–8%) of respondents thought DoA monitoring was indicated in all cases under GA. Overall, 29% (24%–34%) thought it was indicated in all relaxant GA cases. Those respondents who thought DoA monitoring was useful in the prevention of awareness were more likely to think it was indicated in all relaxant GA cases (P = 0.004), while respondents practicing >20 years were significantly more likely to think it was indicated than those practicing <20 years (39% vs 23%, P = 0.01). A large proportion of respondents thought DoA monitoring was indicated for cardiac surgery (53%, 47%–58%). A significant majority thought it was indicated for GA cesarean delivery (59%; 53%–65%), major trauma (59%; 53%–65%), total IV anesthesia (TIVA) (78%; 72%–82%), and a previous episode of awareness (87%; 82%–90%). In free text comments, multiple respondents included the elderly and illicit-drug and opioid users as additional indications. In regard to TIVA with muscle relaxation, a majority of respondents (74%; 69%–79%) either agreed or strongly agreed with DoA monitoring being mandatory, and this was strongly correlated with the belief that such monitoring was useful for the prevention of awareness (P < 0.0001, Pearson correlation 0.36). However, during TIVA without muscle relaxation, only 40% (34%–46%) either agreed or strongly agreed with DoA monitoring being mandatory.

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Attitudes Toward Awareness

Thirty percent of respondents reported having a case of awareness in their practice. Those in practice >20 years were twice as likely to have had a case of awareness as those in practice <20 years (51% vs 24%, P < 0.001). When rating the seriousness of the problem of awareness (0–10 scale), the median (interquartile range) rating was 4 (2–7). Those reporting having had a case of awareness had a higher mean rating compared with those who had not had a case of awareness (4.99 vs 4.24, P < 0.029). Respondents in this survey rated awareness as more of a problem than respondents in a 2006 UK survey using the same question and rating scale (mean rating 4.46 vs 3.65, P < 0.0001).16

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Secondary Observations

When reading the output from the monitor, 37% of respondents found the number most useful, 14% found the trace most useful, and 48% found both equally important. Respondents practicing >20 years were more likely than those practicing <5 years to find the number useful (49% vs 20%), while the reverse was true for both the number and trace being equally important (39% vs 63%, P = 0.028).

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DoA monitoring is almost universally accessible (92%–99%) throughout anesthetizing locations in Australia, with high penetration in all locations apart from day surgery centers (61%), where presumably the majority of patients are at low risk for awareness and are undergoing minimally invasive, short-duration procedures.

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Perceived Effectiveness Driving High Utilization in Australia

A relatively high rate of respondents (29%) would use DoA monitoring for all relaxant anesthetic cases, and about one-third of respondents felt DoA monitoring was superior to ETAC monitoring, despite evidence that there is no benefit over ETAC monitoring with alerts.19 Furthermore, a majority believe DoA monitoring is useful for the prevention of awareness. Most anesthesiologists use DoA monitoring to some degree in their practice, with only a small minority never using it. The majority uses it in less than one-third of cases, but nearly 30% use it in more than one-third of cases. This is in clear contrast to the findings of the UK 5th National Audit Project baseline survey that found 25% of anesthesiologists using DoA monitoring in selected cases only with 1.8% using it routinely,20 suggesting higher use and acceptance by Australian anesthesiologists. Usage was strongly influenced by belief in the usefulness of DoA monitoring in the prevention of awareness, while no other covariates were found to significantly influence use.

In view of the lack of evidence of benefit for prevention of awareness over and above ETAC monitoring, possible explanations may include that the increasing presence and perceived low risk of using DoA monitoring are decreasing the threshold for its use. The ubiquitous presence of DoA devices (most are now integrated into modern anesthetic monitors) may now be encouraging increasing use. Alternatively, while use appears to be driven primarily by a desire to prevent awareness, it may increasingly be used as a guide for titration of delivered anesthetic either to improve recovery or reduce complications. Our survey results support this notion given that a majority of respondents (62%) felt that DoA monitoring enables delivery of less anesthetic, with major benefits thought to include faster emergence, less morbidity, and improved hemodynamic stability. Again, this practice is in contrast to some of the current evidence provided by large trials. The B-Unaware trial compared BIS with ETAC monitoring and found no reduction in the use of volatile anesthetics in the BIS-guided arm of the study,9 with similar findings in several large studies,8,21 and a recent review of the role of DoA monitoring as a means of awareness prevention or to reduce anesthetic usage did not support the use of BIS.22 Additionally, the BAG-RECALL study did not show BIS was superior to ETAC monitoring in the prevention of awareness.21

Interestingly, the survey results suggest a disconnect between the evidence base and clinical practice patterns. Behavior of clinicians does not always reflect their knowledge of the evidence, and Australian usage of DoA monitors appears to be another example of this. Similar findings have been found with regard to antagonization of nondepolarizing neuromuscular blocking drugs where practice reflected heuristic decision making and qualitative judgments.23 Another survey about “best practice” and actual management of perioperative peripheral IV catheters showed that what providers think is best practice is not what is actually being done.24 Illumination of the disparity between evidence and practice may encourage all clinicians to reflect on their own behaviors and guide research into how evidence filters into clinical practice.

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How Do Australian Anesthesiologists Use Candidate DoA Monitors?

Certain indications for EEG DoA monitoring were identified by a significant majority (>70%) of respondents, namely, TIVA or a previous case of awareness, while an approximate majority thought it was indicated for major trauma or GA cesarean delivery. Indications were typically viewed as being high risk for awareness. Furthermore, although a majority believe DoA monitoring can reduce anesthetic usage, when posed with an anesthetic scenario, respondents are twice as likely to respond to an elevated than a suppressed DoA reading. Despite the accumulating evidence of adverse outcomes associated with deep anesthesia prompting the ongoing BALANCED anesthesia trial,25 considerable confusion remains among clinical anesthesiologists, with one-third disagreeing with the association and another third not knowing about it. Overall, this suggests “awareness prevention” is the main driver of use rather than “recovery enhancement.”

The number of respondents with a case of awareness (30%) was similar to a previous UK survey conducted in 2006 (33%)16 and was more likely among those in practice for longer (>20 years), supporting the validity of our survey. However, in a 2003 Australian survey, 52% reported having experienced a patient with awareness.15 While this change may be artifactual, it may also be the result of an increased appreciation of awareness, greater vigilance among anesthesiologists, and greater use of DoA monitors. In the intervening period, considerable research has been conducted, and DoA monitors have become widely accessible.

Differences in attitudes were noted between more (>20 years) and less (<5 years) experienced anesthesiologists. Those practicing >20 years were less likely to agree with the association between low DoA monitor values and adverse outcomes and valued the monitor number above the number and EEG trace combined. This may have resulted from the greater passage of time between training and practice, less exposure to the latest research, and less familiarity with modern monitors that display EEG waveforms. Interestingly, there was no difference in the attitudes toward the effectiveness of DoA monitoring or the frequency of DoA usage when comparing those with more than 20 years’ experience and less than 5 years’ experience.

There are some limitations to our findings. The sample size was limited (289 respondents from 963 surveyed anesthesiologists), with a response rate of 30% (resulting in a sample of 8% of all anesthesiologists in Australia), and although this rate is a typical response rate for an online survey,17 it introduces the possibility of nonresponse bias, because it may select for respondents with strong attitudes toward this subject and limit generalizability. However, the responders’ representativeness of the population may be more important than response rate.26,27 All demographic variables closely reflect the membership of ANZCA, including geographical distribution, gender, and years of experience, and are similar to a previous survey conducted specifically on awareness in Australia, supporting the representativeness of our survey.15 Furthermore, the survey only reflects Australian practice and cannot be generalized to other regions given international variations.

With the increased availability of these monitors, the clinical utility needs to be carefully considered. It appears that education about these monitors may be lagging behind current evidence. Our study reveals 3 disconnects between the available evidence and practice patterns. First, DoA monitors are beneficial for preventing awareness when TIVA is used, yet usage is far from universal for this indication. Second, these monitors do not reduce awareness when ETAC monitoring with an alerting system is used, yet many practitioners believe otherwise. Last, DoA monitors have not been shown to decrease overall anesthetic use in contrast to the opinions of a sizeable proportion of respondents. Our survey documents wide variation in use and attitudes to DoA monitors. The results of this study challenge anesthesiologists, both on the individual level and as a collective, to better define the appropriate clinical role of these monitors.

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Name: Erez Ben-Menachem, MBChB, FCICM, FANZCA.

Contribution: This author helped in study design, conduct of the study, data analysis, and manuscript preparation.

Attestation: Erez Ben-Menachem approved the final manuscript, attests to the integrity of the original data and the analysis reported in this manuscript, and is the archival author.

Name: Dave Zalcberg, MBBS.

Contribution: This author helped in study design, conduct of the study, data collection, and manuscript preparation.

Attestation: Dave Zalcberg approved the final manuscript and attests to the integrity of the original data and the analysis reported in this manuscript.

This manuscript was handled by: Franklin Dexter, MD, PhD.

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The authors acknowledge and thank Dr. Kathy Petoumenos for advice on statistical analyses and the ANZCA Trials Group for supporting and facilitating this survey.

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a Medical Training Review Panel: sixteenth report. Available at: Accessed October 4, 2013.
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