SNAP index and Bispectral index during different states of propofol/remifentanil anaesthesia

SNAP II index: an alternative to the COMFORT scale in assessing the level of sedation in mechanically ventilated pediatric patients

Sedation monitoring is essential in pediatric patients on ventilatory support to achieve comfort and safety. The COMFORT scale was designed and validated to assess the level of sedation in intubated pediatric patients. However, it remains unreliable in pharmacologically paralyzed patients. The SNAP II index is calculated using an algorithm that incorporates high-frequency (80-420 Hz) electroencephalogram (EEG) components, known to be useful in discriminating between awake and unconscious states, unlike other measurements that only include low-frequency EEG segments such as the bispectral index score. Previous studies suggested that the SNAP II index is a reliable and sensitive indicator of the level of consciousness in adult patients. Despite its potential, no data are currently available in the pediatric critically ill population on ventilatory support. This is the first pilot study assessing the potential application of the SNAP II index in critically ill pediatric patients by comparing it to the commonly used COMFORT scale.

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.

Use of cerebral state index to predict long-term unconsciousness in patients after elective craniotomy with delay recovery

Background

The major difficulty in postoperative care in patients after craniotomy is to distinguish the intracranial deficits from the residual effect of general anesthesia. In present study, we used cerebral state index (CSI) monitoring in patients after craniotomy with delayed recovery, and evaluated the prediction probability of CSI for long-term postoperative unconsciousness.

Methods

We enrolled 57 consecutive adult patients admitted to neurosurgical intensive care unit (NICU) after elective craniotomy with delayed recovery. CSI was continuously monitored for 6 hours after admission. Patient's level of consciousness was followed up for 24 hours. According to whether obeyed verbal command, patients were divided into awaken group and non-awaken group. CSI values were compared between the two groups. Prediction probability (P_K) was calculated to determine the probability of CSI in predicting unconsciousness 24 hours after operation.

Results

In awaken group (n = 51), CSI increased significantly after the 2nd NICU admitted hour (P < 0.05). At each time point, CSI values in awaken group were significantly higher than those in non-awaken group (n = 6) (P < 0.05). The values of P_K (SE) for CSI in the first 6 admitted hours ranged from 0.94 (0.06) to 0.99 (0.02).

Conclusions

In patients after craniotomy with delayed recovery, CSI monitoring in early postoperative hours had high prediction probability for long-term unconsciousness. CSI monitoring may be a reliable objective method to predict level of consciousness after elective craniotomy.

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.

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.

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.