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October 17, 2009
2:00 PM - 4:00 PM
Room Area K
Feasibility of Awareness Decision Support Alerts Based on End-Tidal Anesthetic Concentrations or BIS
  *  Wolf H. Stapelfeldt, M.D., Scott D. Greenwald, Ph.D., Paul J. Manberg, Ph.D., Jacek B. Cywinski, M.D., Daniel I. Sessler, M.D.
Anesthesiology Institute, Cleveland Clinic Foundation, Cleveland, Ohio
Introduction : Recent studies have utilized automated notification systems to alert clinicians to instances of increased risk for intraoperative awareness. A recent trial compared outcomes based on using one of two alternate alarm triggers: End-tidal Anesthetic Gas (ETAG) concentration < 0.70 MAC-equivalents or BIS > 60 [1]. In that trial, reasonable compliance with the recommended alarm limits were reported to be less than 26% and 45%, respectively. The present investigation evaluated the clinical feasibility of each of these same two alarm strategies by calculating the frequency of notifications that would have occurred in a large, retrospective dataset of consecutive non-cardiac adult general anesthetics.

Methods : With IRB approval, BIS and end-tidal volatile anesthetic concentrations in MAC-equivalents (MAC) for a cohort of adult surgical patients receiving BIS monitored inhalational anesthesia were extracted from our electronic medical record-based registry (Perioperative Health Documentation System). MAC equivalents were derived from end-tidal concentrations of isoflurane, sevoflurane, and desflurane in the ratio of 1:2:6. The incidence and frequency of alarm notifications was modeled by generating a notification for every 5 contiguous minutes after incision of either condition being met. A Chi-square test was used to evaluate differences in the distributions of number of alarms. The Wilcoxon Signed Rank Test was used to evaluate differences in means between the mean number of alarms for each alarm condition.

Results : Data from 18,035 non-cardiac procedures were available for analysis. The mean (SD) of BIS, MAC and case duration for the patients were 44.9 (8.4) BIS units, 0.57 MAC equivalents (0.19), and 3.9 (2.0) hrs, respectively The table summarizes the distribution of alarm notifications that would have been triggered per case. The frequency of alarms would have been significantly higher using the ETAG condition compared to the BIS condition (11.5 ± 13.7 alarms per patient vs. 1.3 ± 3.4, respectively, P< 0.0001).[table1] Conclusions : The incidence of alarm notifications would have been significantly higher using the ETAG < 0.70 MAC condition than the BIS > 60 condition, suggesting that the ETAG alarm limit recommended previously would have been set too high to be clinically acceptable in our patient population. It is reasonable to assume that clinicians may disregard alarms if the false positive rate is too high, which may help to explain the lower compliance with ETAG-based notifications [1]. Further work is needed to demonstrate the effectiveness of an ETAG-based alarm strategy in routine clinical practice.

References :

[1] Avidan MS, Zhang L, Burnside BA, Finkel KJ, Searleman AC, Selvidge JA, Saager L, Turner MS, Rao S, Bottros M, Hantler C, Jacobsohn E, Evers AS. Anesthesia Awareness and the Bispectral Index. New England Journal of Medicine 2008; 358 (11): 1097-1108.

From Proceedings of the 2009 Annual Meeting of the American Society Anesthesiologists.

Some of the co-authors of this abstract are employees of Aspect Medical, Inc, which also provided financial support for the work.
Alarm frequency for BIS >60 and ETAG<0.7 MAC per patient
Condition (present for 5 min)0 Alarms*1 Alarm2 Alarms3 Alarms4+ AlarmsMAX # AlarmsMean (SD) # Alarms
BIS > 6012480 (69.2%)1064 (5.9%)882 (4.9%)1815 (10.1%)1794 (9.9%)1071.3 (3.4)
ETAG < 0.70 MAC1230 (6.8%)1200 (6.7%)1510 (8.4%)1652 (9.2%)12443 (69.0%)15911.5 (13.7)
*Distribution of alarms (0 vs 1+) is significantly different (p< 0.0001)