Comparison of the EEG-based SNAP index and the Bispectral (BIS) index during sevoflurane-nitrous oxide anaesthesia

The SNAP index does not correlate with the State Behavioral Scale in intubated and sedated children

BACKGROUND

Ensuring appropriate levels of sedation for critically ill children is integral to pediatric critical care. Traditionally, clinicians have used subjective scoring tools to assess sedation levels. The SNAP II uses dual frequency processed electroencephalography to evaluate brain activity and may provide an objective assessment of sedation levels.

OBJECTIVE

This study attempts to find an objective method to monitor sedation in critically ill pediatric patients. We compared the SNAP II, a processed electroencephalography device, with the State Behavioral Scale (SBS), a subjective sedation scoring tool. We hypothesize that the SNAP II correlates with the SBS and has less observer bias.

METHODS

This was an IRB approved prospective, observational study. Patients receiving intravenous sedation while being mechanically ventilated were enrolled after informed consent. After the SNAP II monitoring electrodes were attached, blinded bedside nurses assessed sedation levels using the SBS. SNAP indices were collected and compared with SBS scores to determine correlation.

RESULTS

We compared 417 paired data points from 15 patients using Pearson's correlation and least squares means to determine correlation between the SBS and SNAP indices. No correlation was observed. Using covariance model patterning for repeated measures to adjust for covariates again showed no correlation.

CONCLUSION

The SNAP index does not correlate with SBS scores in our pediatric intensive care unit (PICU). Its use cannot be recommended to measure levels of sedation in our population. Future research should continue to explore objective ways of measuring sedation in critically ill children.

Comparison of SNAP™ II and BIS Vista indices during normothermic cardiopulmonary bypass under isoflurane anesthesia

OBJECTIVE

Processed EEG monitoring during cardiopulmonary bypass (CPB) may help determine loss of consciousness and depth of anesthesia. This study compared the SNAP(™) II and BIS Vista monitors in patients undergoing isoflurane anesthesia with normothermic CPB.

METHODS

40 subjects undergoing CPB with isoflurane anesthesia were enrolled. Subjects were premedicated with 1-2 mg midazolam approximately 5 min prior to acquisition of baseline index values and anesthesia induced with midazolam and fentanyl. Anesthesia was maintained with isoflurane, midazolam, and fentanyl and a cis-atracurium infusion. SNAP(™) II (version 1.2.9 algorithm 1.88) and BIS Vista (application version 3.00 platform version 2.03) indices were recorded at baseline, pre-induction, post-intubation, incision, start of CPB, every 15 min during CPB, end of CPB, and end of case. Agreement between methods was determined using Pearson correlation and the Bland-Altman method with repeated observa- tions.

RESULTS

Twenty-four male and 12 female subjects completed the analysis. The correlation between SNAP(™) II and BIS Vista index values was 0.61 (P < 0.005). A linear relationship between the difference in the indices and the average index values was observed following the induction of anesthesia. In awake subjects, the bias between the SNAP(™) II and BIS Vista was 5 (95% CI 3-7). The limits of agreement were 23 (95% CI 19-26) and -13 (95% CI -9--16). During anesthesia, the mean difference on a log scale was 0.11 (95% CI 0.09-0.12). The limits of agreement were 0.43 (95% CI 0.40-0.45) and -0.21 (95% CI -0.18--0.24). The antilog of the mean difference demonstrated that the SNAP(™) II value was 28% (95% CI 24-33%) higher than the BIS Vista value following induction of anesthesia.

CONCLUSIONS

The SNAP(™) II monitor demonstrates a consistently positive bias during cardiopulmonary bypass under isoflurane anesthesia compared with the BIS Vista.

SNAP II versus BIS VISTA monitor comparison during general anesthesia

INTRODUCTION

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.

METHODS

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.

RESULT

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.

DISCUSSION

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.

A comparison of SNAP II and bispectral index monitoring in patients undergoing sedation

Clinical signs and patients' verbal responses have traditionally been used to assess patients' comfort and the depth of sedation. Recently, level-of-consciousness monitors have been used to guide sedation. The SNAP II(c) is a single-lead electroencephalogram device that displays a SNAP(c) Index - a derived value based on both high and low frequency electroencephalogram signals. Much of the current clinical research on monitoring during sedation involves the bispectral index monitor. We compared simultaneous readings recorded by the SNAP II and bispectral index during sedation in 51 consecutive patients undergoing surgery. The anaesthesia team was blinded to the SNAP II and bispectal index values. Concurrent SNAP II and bispectral index readings displayed similarly-shaped trajectories during sedation, but further studies are needed to establish the routine clinical utility of both these monitors.

A comparison of the SNAP II™ and BIS XP™ indices during sevoflurane and nitrous oxide anaesthesia at 1 and 1.5 MAC and at awakening

BACKGROUND

Monitoring level of consciousness during anaesthesia, with the ability to predict the intentional or unintentional return to consciousness, is desirable. The purpose of this study was to compare two processed electroencephalographic depth of anaesthesia monitors (SNAP II and BIS XP) during sevoflurane and sevoflurane/nitrous oxide anaesthesia.

METHODS

In total, 42 subjects received an interscalene block, followed by general anaesthesia with sevoflurane or sevoflurane/nitrous oxide. The indices were recorded at baseline, at 1.5 and 1.0 minimum alveolar concentration (MAC) equivalents, and during emergence.

RESULTS

The SNAP and BIS indices decreased from baseline at 1.5 and 1.0 MAC equivalents, but there was no difference within groups between subjects who received nitrous oxide and those who did not. The SNAP index returned to baseline by 1 min before awakening and was higher than baseline at eye opening, but the BIS index remained below baseline at awakening. There was a bias of -1 (95% CI: -3 to 1) between the SNAP and BIS at baseline; this increased to 21 (95% CI: 19-23) during maintenance of anaesthesia and was 6 (95% CI: 4-8) at awakening.

CONCLUSIONS

The SNAP index tracks loss of consciousness and emergence from sevoflurane and sevoflurane/nitrous oxide anaesthesia. There is significant bias between the SNAP and BIS indices and therefore, the indices are not interchangeable. The SNAP index returns to baseline before awakening, whereas the BIS index remains below baseline at awakening, suggesting that the SNAP index may be more sensitive to unintentional awareness.