[SNAP-index and bispectral index during induction of anaesthesia with propofol and remifentanil]

A Prospective, Multicenter, Single-Blind Study Assessing Indices of SNAP II Versus BIS VISTA on Surgical Patients Undergoing General Anesthesia

Background

Traditionally, anesthesiologists have relied on nonspecific subjective and objective physical signs to assess patients’ comfort level and depth of anesthesia. Commercial development of electrical monitors, which use low- and high-frequency electroencephalogram (EEG) signals, have been developed to enhance the assessment of patients’ level of consciousness. Multiple studies have shown that monitoring patients’ consciousness levels can help in reducing drug consumption, anesthesia-related adverse events, and recovery time. This clinical study will provide information by simultaneously comparing the performance of the SNAP II (a single-channel EEG device) and the bispectral index (BIS) VISTA (a dual-channel EEG device) by assessing their efficacy in monitoring different anesthetic states in patients undergoing general anesthesia.

Objective

The primary objective of this study is to establish the range of index values for the SNAP II corresponding to each anesthetic state (preinduction, loss of response, maintenance, first purposeful response, and extubation). The secondary objectives will assess the range of index values for BIS VISTA corresponding to each anesthetic state compared to published BIS VISTA range information, and estimate the area under the curve, sensitivity, and specificity for both devices.

Methods

This is a multicenter, prospective, double-arm, parallel assignment, single-blind study involving patients undergoing elective surgery that requires general anesthesia. The study will include 40 patients and will be conducted at the following sites: The Ohio State University Medical Center (Columbus, OH); Northwestern University Prentice Women's Hospital (Chicago, IL); and University of Miami Jackson Memorial Hospital (Miami, FL). The study will assess the predictive value of SNAP II versus BIS VISTA indices at various anesthetic states in patients undergoing general anesthesia (preinduction, loss of response, maintenance, first purposeful response, and extubation). The SNAP II and BIS VISTA electrode arrays will be placed on the patient’s forehead on opposite sides. The hemisphere location for both devices’ electrodes will be equally alternated among the patient population. The index values for both devices will be recorded and correlated with the scorings received by performing the Modified Observer’s Assessment of Alertness and Sedation and the American Society of Anesthesiologists Continuum of Depth of Sedation, at different stages of anesthesia.

Results

Enrollment for this study has been completed and statistical data analyses are currently underway.

Conclusions

The results of this trial will provide information that will simultaneously compare the performance of SNAP II and BIS VISTA devices, with regards to monitoring different anesthesia states among patients.

ClinicalTrial

Clinicaltrials.gov NCT00829803; https://clinicaltrials.gov/ct2/show/NCT00829803 (Archived by WebCite at http://www.webcitation.org/6nmyi8YKO)

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.

[Measurement of the depth of anaesthesia]

One of the most important mandates of the anaesthesiologist is to control the depth of anaesthesia. An unsolved problem is that a straight definition of the depth of anaesthesia does not exist. Concerning this it is rational to separate hypnosis from analgesia, from muscle relaxation and from block of cardiovascular reactions. Clinical surrogate parameters such as blood pressure and heart rate are not well-suited for a valid statement about the depth of hypnosis. To answer this question the brain has become the focus of interest as the target of anaesthesia. It is possible to visualize the brain's electrical activity from anelectroencephalogram (EEG). The validity of the spontaneous EEG as an anesthetic depth monitor is limited by the multiphasic activity, especially when anaesthesia is induced (excitation) and in deep anaesthesia (burst suppression). Recently, various commercial monitoring systems have been introduced to solve this problem. These monitoring systems use different interpretations of the EEG or auditory-evoked potentials (AEP). These derived and calculated variables have no pure physiological basis. For that reason a profound knowledge of the algorithms and a validation of the monitoring systems is an indispensable prerequisite prior to their routine clinical use. For the currently available monitoring systems various studies have been reported. At this time it is important to know that the actual available monitors can only value the sedation and not the other components of anaesthesia. For example, they cannot predict if a patient will react to a painful stimulus or not. In the future it would be desirable to develop parameters which allow an estimate of the other components of anaesthesia in addition to the presently available monitoring systems to estimate sedation and muscle relaxation. These could be sensoric-evoked potentials to estimate analgesia and AEPs for the detection of awareness.

Monitoring methods: SNAP

The SNAP electroencephalogram (EEG) monitor, described as an interesting, innovative EEG tool that samples raw EEG signals and uses its own unique algorithm, analyses both high- (80-420 Hz) and low- (0-20 Hz) frequency components of the signal. This is termed the SNAP index, and it ranges from 100 (arbitrarily representing the fully awake state) to 0 to provide functional data points for patient management. The SNAP is the first commercial EEG-monitoring tool to use Personal Digital Assistant computer technology. The first version of SNAP index was introduced in 2002, and so far there has been little experience with the SNAP device reported in the literature. Compared with other EEG devices, there is no evidence that SNAP is superior to others in generating more specific information about 'depth of sedation'. Moreover, the influence of high-frequency electromyographic activity on the SNAP needs to be clarified.

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

The BIS monitor (Aspect Medical Inc, Newton, USA) was the first electroencephalogram (EEG)-based monitor of the hypnotic effect reflected by a dimensionless figure ranging from 100 (awake state) to 0 (flat line EEG). Its widespread use makes it the most-studied and the best-known among same intended devices. Its algorithm processes low-frequency EEG oscillations in order to provide the Bispectral index. A BIS index ranging from 40 to 60 has been established as the proper for surgical performance. The BIS monitor permits a closer approach to the hypnotic component of anaesthesia beyond clinical signs and may reduce the probability of intraoperative awareness; therefore, it has become a recommended monitoring tool in routine practice. The SNAP monitor (Nicolet Biomedical, Madison WI, USA) is also intended for monitoring the hypnotic effect of anaesthetics, which is in turn displayed as an index ranging from 100 to 0, with 100 meaning a fully awake state and 0 meaning no brain activity. The algorithm of the SNAP monitor is featured by its additional processing of ultra-high EEG frequencies, which seem to be involved in the formation of consciousness. The use of these frequencies would theoretically improve responsiveness during increased brain activity. We studied its behaviour patterns and capability to monitor the hypnotic effect induced by sevoflurane-nitrous oxide by comparison with the BIS index. Seventy patients ASA I-III were induced with propofol, fentanyl and rocuronium, and maintained with sevoflurane-N(2)O. BIS and SNAP indices were simultaneously recorded before induction, after intubation, after incision, at the following 10, 30 and 50 minutes, awakening and extubation time points, together with heart rate and blood pressure. The Pearson correlation was R(2) = 0.68 (p < .05). The Bland and Altman test showed a bias of 14.3 for SNAP index values with respect to BIS index values. We concluded that the SNAP index correlates with variations in the hypnotic effect induced by sevoflurane-nitrous oxide anaesthesia when compared with the BIS index. In this context, a SNAP index ranging from 58 to 70 would be equivalent to the BIS index range 40 to 60 and, therefore, the accurate for surgical performance.