Is the ARX index a more sensitive indicator of anesthetic depth than the bispectral index during sevoflurane/nitrous oxide anesthesia?

Non-instrumental clinical monitoring does not guarantee an adequate course of general anesthesia. A prospective clinical study

BACKGROUND

Clinical monitoring is the most common method of adjusting the appropriate level of general anesthesia. However, episodes of intraoperative awareness (AWR) are still reported, suggesting that clinical observations may not be sufficient in some cases. The objective of this study was to compare the efficacy of clinical and instrumental neuromonitoring with auditory evoked potentials (AEP) in an intraoperative analysis of the proper level of general anesthesia.

METHODS

Patients scheduled for elective surgery were randomly divided into two groups. Subjects in the first group underwent intravenous, in the second group volatile anesthesia. The adequacy of anesthesia was analyzed using clinical parameters. All the participants were instrumentally monitored with the autoregressive AEP index (AAI). After the anesthesia, patients filled out a questionnaire on possible AWR.

RESULTS

Data of 208 patients (87 in the first, and 121 in the second group) were analyzed. Before surgery there were no changes in AAI values between groups (80 vs. 78, P=0.5192). The mean values of clinical parameters changed, but five minutes after the nociceptive stimuli. The mean values of AAI at analyzed time points were specific for general anesthesia. In patients under intravenous anesthesia, we found more episodes of too low (46/608 vs.15/847, P<0.000) anesthesia. One case of AWR was found in the TIVA group.

CONCLUSIONS

AAI index is good indicator of patients' level of consciousness during general anesthesia. Standard clinical monitoring provides appropriate level of the procedure. However, it is insufficient during TIVA and does not prevent episodes of AWR.

Composite-, plain-auditory evoked potentials index and bispectral index to measure the effects of sevoflurane

The composite auditory evoked potentials index (cAAI) uses both cortical electroencephalogram (EEG) and response to auditory stimuli, while the bispectral index (BIS) uses only the cortical EEG and auditory evoked potentials index (AAI) uses only response to auditory stimuli. We expected that the cAAI was more useful to monitor anesthetic effect of sevoflurane than the BIS and AAI. The present study compared the changes of cAAI, AAI, and BIS in different sevoflurane concentration. Forty-five adult patients were anesthetized with sevoflurane in 50 % nitrous oxide. AAI (AEP version 1.4), cAAI (AEP version 1.6), and BIS (A-2000) were compared (each 15 patients in AAI, cAAI, and BIS groups) before induction, just before and after intubation, at 10 min since sevoflurane was set to 1.0, 1.5 and 2.0 %, and after extubation. All three indices decreased significantly before intubation. The cAAI was significantly higher than the AAI at sevoflurane 1.0 and 1.5 %. The AAI and BIS were significantly lower at sevoflurane 2.0 % than those at sevoflurane 1.0 %, but the cAAI did not. The cAAI had the largest and AAI had the smallest inter-individual variation. In sevoflurane-nitrous oxide anesthesia, cAAI was inferior to AAI and BIS to discriminate different anesthetic effect. The cAAI had larger inter-individual variation than the AAI and BIS.

Recent advance in patient monitoring

Recent advance in technology has developed a lot of new aspects of clinical monitoring. We can monitor sedation levels during anesthesia using various electroencephalographic (EEG) indices, while it is still not useful for anesthesia depth monitoring. Some attempts are made to monitor the changes in sympathetic nerve activity as one of the indicators of stress, pain/analgesia, or anesthesia. To know the balance of sympathetic and parasympathetic activity, heart rate or blood pressure variability is investigated. For trend of cardiac output, low invasive monitors have been investigated. Improvement of ultrasound enables us to see cardiac structure and function continuously and clearer, increases success rate and decreases complication of central venous puncture and various kinds of nerve blocks. Without inserting an arterial catheter, trends of arterial oxygen tension or carbon dioxide tension can be monitored. Indirect visualization of the airway decreases difficult intubation and makes it easier to teach tracheal intubation. The changes in blood volume can be speculated non-invasively. Cerebral perfusion and metabolism are not ordinary monitored yet, but some studies show their usefulness in management of critically ill. This review introduces recent advances in various monitors used in anesthesia and critical care including some studies of the author, especially focused on EEG and cardiac output. However, the most important is that these new monitors are not almighty but should be used adequately in a limited situation where their meaning is confirmed.

The effects of auditory evoked potential click sounds on bispectral index and entropy

BACKGROUND

The click sounds of auditory evoked potentials (AEP) might have some effect on electroencephalogram indices and Bispectral Index (BIS) but many studies, unconcerned about this effect, have measured both indices simultaneously. In this study, I examined the effects, of AEP click sounds on the BIS, and also on the response entropy (RE) and state entropy (SE) of the entropy monitor.

METHODS

Forty patients aged 40-70 yr and scheduled for surgery of lower extremities under spinal anesthesia were anesthetized with 0.5% bupivacaine or tetracaine. Patients were sedated with midazolam 1 mg followed by propofol infusion started at 1 mg.kg(-1).h(-1). Propofol infusion was controlled to keep BIS or SE at 80, 60, or 40 for several minutes, and then click sounds (65 dB) of the AEP were given for 60 s. The changes in BIS, RE, and SE were observed continuously for 60 s after the click sounds had stopped.

RESULTS

BIS, SE, and RE significantly increased during the click sounds. The longest duration of increase was at BIS or SE 60.

CONCLUSION

AEP monitor click sounds transiently increased the simultaneously measured BIS, RE, and SE during different levels of sedation by propofol infusion during spinal anesthesia. Therefore, the effects of the click sounds should be considered when these monitors are used simultaneously in the same patient.

Level of sedation evaluation with Cerebral State Index and A-Line Arx in children undergoing diagnostic procedures

BACKGROUND

Monitoring of anesthesia depth is difficult clinically, particularly in children. The aim of this study was to assess the correlation existing between CSI (Cerebral State Index), or AAI (A-line ARX) and a clinical sedation scale such as UMSS (University of Michigan Sedation Scale), during deep sedation with propofol in children undergoing diagnostic procedures.

METHODS

Twenty ASA I and II children, scheduled to undergo deep sedation for magnetic resonance imaging (MRI) or Esophagogastroduodenoscopy (EGDS), were enrolled. The patients were randomly assigned to receive depth of anesthesia monitoring with CSI or AAI. The anesthetist administered repeated doses of propofol every 10 s to a UMSS score of 3-4. An attending anesthetist, not involved in drug administration, recorded time and doses of sedation medications, vital signs, UMSS score and CSI or AAI score. All the evaluations were recorded at awake state (baseline), every 10 s until an UMSS score of 3-4 and every 3 min until the children were awake.

RESULTS

We enrolled 13 males and seven females ranging in age from 8 months to 7 years. After induction of anesthesia CSI and AAI scores decreased and from the end of the procedure to emergence the two scores increased. The CSI data showed a strong correlation with the UMSS scores (r = -0.861; P < 0.0001); we found a similar correlation between the AAI data and the UMSS scores (r = -0.823; P < 0.0001).

CONCLUSIONS

Our study suggests that CSI and AAI may be two, real-time and objective tools to assess induction and emergence during propofol sedation in children undergoing EGDS and MRI.

Performance of AEP Monitor/2-derived composite index as an indicator for depth of sedation with midazolam and alfentanil during gastrointestinal endoscopy

BACKGROUND AND OBJECTIVE

The A-Line auditory evoked potential index (AAI) (AEP Monitor/2, Danmeter A/S, Odense, Denmark) is a newly developed composite parameter representing the degree of hypnosis. We conducted a prospective, observational study to explore the performance and validity of the AAI during conventional sedation for gastrointestinal (GI) endoscopy.

METHODS

Thirty adults of either sex, age <65, scheduled for combined oesophagogastroduodenoscopy (OGD) and colonoscopy under sedation with intravenous (i.v.) midazolam and alfentanil were enrolled. The sedative end-point was set at the Observer's Assessment of Alertness/Sedation (OAA/S) score less than 4. An AEP Monitor/2 was used in all patients. AAI, sedation scores, heart rate (HR), blood pressure (BP) and SPO2 were recorded every 2 min up to the end of the procedure. Receiver operator characteristic analysis was used to test validity and to select optimal sedation.

RESULTS

There was a significantly positive correlation between AAI and OAA/S scores (rho = 0.886; P < 0.001). The AAI also showed significant differences between subsequent levels of sedation scores (P < 0.001). AAI greater than 54 indicated fully awake or minimal sedation and values between 54 and 42 were suggestive of moderate sedation. Values between 42 and 34 were associated with moderate to deep sedation and readings below 34 were associated with deep sedation. The relative risk of SPO2 < 95% for OAA/S = 2 compared with 3 was 15.98 (95% confidence interval (CI): 3.94-64.81).

CONCLUSIONS

AAI is an effective tool for monitoring sedation during GI endoscopy induced by i.v. midazolam and alfentanil.

Auditory evoked potentials

This chapter will focus on the two auditory evoked potentials (AEP) most commonly used to assess the effects of general anesthetics on the brain, the auditory middle latency response (AMLR) and the 40 Hz auditory steady-state response (40 Hz-ASSR). We will review their physiological basis, the recording methodology, the effects of general anesthetics, their ability to track changes in level of consciousness and their clinical applications. Because of space constraints, this review will be limited to human studies.

Changes in the auditory evoked potentials index by induction doses of four different intravenous anesthetics

BACKGROUND

Many studies have investigated the electroencephalographic changes during the induction and maintenance of anesthesia. However, no comparative studies have been performed on the effects of intravenous anesthetics on the auditory evoked potentials index (AAI). The present study was performed to compare the changes in AAI caused by induction doses of thiopental, propofol, midazolam and ketamine.

METHODS

Eighty females, aged 30-70 years, referred for mastectomy, had anesthesia induced with thiopental 4 mg/kg, propofol 2 mg/kg, midazolam 0.1 mg/kg or ketamine 1 mg/kg (each 20 patients). The response to verbal command and the AAI were measured every minute for 5 min.

RESULTS

The AAI decreased to less than 40 within 1 min with thiopental and propofol. The AAI increased after 3 min with thiopental, but remained low with propofol. The AAI gradually decreased to less than 40 within 4 min with midazolam, but was higher than the AAI with propofol or thiopental. The AAI increased significantly with ketamine. The AAIs at the loss of verbal command were 19 +/- 7 with thiopental, 21 +/- 8 with propofol, 31 +/- 10 with midazolam and 92 +/- 2 with ketamine.

CONCLUSION

The AAI correlated with changes in hypnotic level, as measured by the response to verbal command, with induction doses of thiopental, propofol and midazolam, but not with ketamine. The AAI decreased to lower levels with propofol and thiopental than with midazolam at the induction of anesthesia.

Comparative effects of ketamine on Bispectral Index and spectral entropy of the electroencephalogram under sevoflurane anaesthesia

BACKGROUND

The Bispectral Index (BIS) and spectral entropy of the electroencephalogram can be used to assess the depth of hypnosis. Ketamine is known to increase BIS in anaesthetized patients and may confound that index as a guide to steer administration of hypnotics. We compared the effects of ketamine on BIS, response entropy (RE) and state entropy (SE) during surgery under sevoflurane anaesthesia.

METHODS

Twenty-two women undergoing gynaecological surgery were enrolled in this double-blind, randomized study. Anaesthesia was induced i.v. and maintained with sevoflurane. Under stable surgical and anaesthetic conditions, patients were assigned to receive either a bolus of ketamine 0.5 mg kg(-1) or the same volume of saline. Blood pressure, heart rate, BIS, RE and SE were measured every 2.5 min from 10 min before (baseline) until 15 min after ketamine or saline administration. The maximum relative increase in BIS, RE and SE compared with baseline was calculated for each patient. Values are mean (sd).

RESULTS

Baseline values were BIS 33 (4), RE 31 (5), SE 30 (5) for the ketamine patients and BIS 35 (3), RE 33 (5) and SE 32 (6) for the patients receiving saline. BIS, RE and SE increased significantly from 5 min (BIS) and 2.5 min (RE and SE) after ketamine administration, peaking at 46 (8) (BIS), 52 (12) (RE) and 50 (12) (SE) respectively. The maximum relative increase in RE [42.2 (10.4%)] and SE [41.6 (10.9)%] was higher than that of BIS [29.4 (10.4%)]. Blood pressure, heart rate and RE-SE gradient did not change in either group.

CONCLUSIONS

Ketamine administered under sevoflurane anaesthesia causes a significant increase in BIS, RE and SE without modification of the RE-SE gradient. This increase is paradoxical in that it is associated with a deepening level of hypnosis.