Journal of Clinical Monitoring and Computing

, Volume 24, Issue 4, pp 283–288 | Cite as

SNAP II versus BIS VISTA monitor comparison during general anesthesia

  • Candace Hrelec
  • Erika Puente
  • Sergio Bergese
  • Roger Dzwonczyk



Effectively monitoring the level of consciousness during general anesthesia is clinically beneficial to both the patient and the physician. An electroencephalogram (EEG)-based level-of-consciousness monitor can help minimize intraoperative awareness as well as the effects of over-sedation. In this study, we compared the SNAP II (Stryker Instruments, Kalamazoo, MI USA) and BIS VISTA (Aspect Medical Systems, Newton, MA USA) monitors’ primary metrics (SI and BIS, respectively) in terms of correlation, agreement and responsiveness to return to preoperative baseline in surgical cases involving general anesthesia.


With institutional approval and written informed consent, 33 patients received general anesthesia with isoflurane while undergoing abdominal surgery. We attached both the SNAP II and BIS VISTA electrodes to each patient. We collected data from each monitor simultaneously and continuously, beginning just prior to induction and ending after extubation. Each monitor’s level-of-consciousness index is a unit less metric that ranges from 0 to 100, with 100 indicating full consciousness. We performed a Bland–Altman and parameter difference analyses on the data. We calculated the time it took for each monitor to return to preoperative baseline level following cessation of anesthesia. We established an equivalence between the two indices over their entire range for our particular clinical scenario.


The indices were correlated (r = 0.736, P < 0.0001, N = 3,706 data point pairs). There was an overall difference between the two indices (median = 16.0, 25th/75th%ile  = 10.0/21.1) with BIS lower than SI. A 40-60 BIS range (the typical target range during general anesthesia) was approximately equivalent to a 54–74 SI range. In all 33 subjects, SI reached baseline before BIS at the end of the case (median = 3.3 min, 25th/75th%ile  = 1.6 min/8.2 min versus median = 8.9 min, 25th/75th  = 3.7 min/14.5 min, P = 0.0200), even though both metrics were equal at the beginning of the case.


Although the SI and BIS both can assess a patient’s level of consciousness and are correlated, they are not in agreement with each other numerically and therefore are not interchangeable. It is difficult to assess each monitor’s true responsiveness to acute changes in consciousness level from our study design. The differences between the metrics we observed in this study are most likely due to differences in signal processing methodologies, EEG frequencies employed and signal filtering utilized in the monitors.


BIS VISTA SNAP II EEG monitoring level of consciousness 


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. 1.
    Monk TG, Saini V, Weldon BC, Sigl JC. Anesthetic management and one-year mortality after noncardiac surgery. Anesth Analg. 2005;100:4–10.CrossRefPubMedGoogle Scholar
  2. 2.
    Sebel PS, Lang E, Rampil IJ, White PF, Cork R, Jopling M, Smith NT, Glass PS, Manberg P. A multicenter study of bispectral electroencephalogram analysis for monitoring anesthetic effect. Anesth Analg. 1997;84:891–9.CrossRefPubMedGoogle Scholar
  3. 3.
    Joint Commission on Accreditation of Healthcare Organizations. Preventing, and managing the impact of, anesthesia awareness. Sentinel Event Alert 2004; 32.Google Scholar
  4. 4.
    White PF, Tang J, Ma H. Is the patient state analyzer with the PSArray2 a cost-effective alternative to the bispectral index monitor during the perioperative period? Anesth Analg. 2004;99:1429–35.CrossRefPubMedGoogle Scholar
  5. 5.
    Rampil IJ. A primer for EEG signal processing in anesthesia. Anesthesiology. 1998;89:980–1002.CrossRefPubMedGoogle Scholar
  6. 6.
    Dressler O, Schneider G, Stockmanns G, Kochs EF. Awareness and the EEG power spectrum: analysis of frequencies. Br J Anaesth. 2004;93:806–9.CrossRefPubMedGoogle Scholar
  7. 7.
    Wong C, Fragen R, Fitzgerald P, McCarthy R. A comparison of the SNAP II and BIS XP indices during sevoflurane and nitrous oxide anesthesia at 1 and 1.5 MAC and at awakening. Br J Anaesth. 2006;97:181–6.CrossRefPubMedGoogle Scholar
  8. 8.
    Bruhn J, Boullion TW, Schafer SL. Electromyographic activity falsely elevates the bispectral index. Anesthesiology. 2000;92:1485–7.CrossRefPubMedGoogle Scholar
  9. 9.
    Wong CA, Fragen RJ, Fitzgerald PC, McCarthy RJ. The association between propofol-induced loss of consciousness and the SNAP index. Anesth Analg. 2005;100:141–8.CrossRefPubMedGoogle Scholar
  10. 10.
    Sing HC, Kautz MA, Thorne DR, Hall SW, Redmond DP, Johnson DE, Warren K, Bailey J, Russo MB. High-frequency EEG as a measure of cognitive function capacity: a preliminary report. Aviat Space Environ Med. 2005;76:114–35.Google Scholar
  11. 11.
    Draguhn A, Traub RD, Schmitz D, Jeffreys JG. Electrical coupling underlies high-frequency oscillations in the hippocampus in vitro. Nature. 1998;394:189–92.CrossRefPubMedGoogle Scholar
  12. 12.
    Vanhatalo S, Volpio J, Kaila K. Full-band EEG (fbEEG): a new standard for clinical electroencephalography. Clin EEG Neurosci. 2005;36:311–7.PubMedGoogle Scholar
  13. 13.
    Schmidt GN, Standl T, Lankenau G, Hellstern A, Hipp C, Bischoff P. SNAP-index and bispectral index during induction of anaesthesia with propofol and remifentanil. Anesthesiol Intensivmed Notfallmed Schmerzther. 2004;39:286–91.CrossRefGoogle Scholar
  14. 14.
    Schmidt GN, Bischoff P, Standl T, Lankenau G, Hellstern A, Hipp C, Schulte am Esch J. SNAP index and bispectral index during different states of propofol/remifentanil anaesthesia. Anaesthesia. 2005;60:228–34.CrossRefPubMedGoogle Scholar
  15. 15.
    Ruiz-Gimeno P, Soro M, Pérez-Solaz A, Carrau M, Belda FJ, Jover JL, Aguilar G. Comparison of the EEG-based SNAP index and the bispectral (BIS) index during sevoflurane-nitrous oxide anaesthesia. J Clin Monit Comput. 2005;19:383–9.CrossRefPubMedGoogle Scholar
  16. 16.
    Sigl JC, Chamoun NG. An introduction to bispectral analysis for the electroencephalogram. J Clin Monit. 1994;10:392–404.CrossRefPubMedGoogle Scholar
  17. 17.
    Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet. 1986;1:307–10.PubMedGoogle Scholar
  18. 18.
    Stanley WD. Realization and frequency response of discrete-time systems. In digital signal processing, vol. 5. 1st ed. Reston, VA: Reston Publishing Company, Inc.; 1975.Google Scholar

Copyright information

© Springer Science+Business Media, LLC 2010

Authors and Affiliations

  • Candace Hrelec
    • 1
  • Erika Puente
    • 1
  • Sergio Bergese
    • 1
  • Roger Dzwonczyk
    • 1
  1. 1.Department of AnesthesiologyThe Ohio State UniversityColumbusUSA

Personalised recommendations