Changes in rapidly extracted auditory evoked potentials during tracheal intubation

Limitations in the rapid extraction of evoked potentials using parametric modeling

The rapid extraction of variations in evoked potentials (EPs) is of great clinical importance. Parametric modeling using autoregression with an exogenous input (ARX) and robust evoked potential estimator (REPE) are commonly used methods for extracting EPs over the conventional moving time average. However, a systematic study of the efficacy of these methods, using known synthetic EPs, has not been performed. Therefore, the current study evaluates the restrictions of these methods in the presence of known and systematic variations in EP component latency and signal-to-noise ratios (SNR). In the context of rapid extraction, variations of wave V of the auditory brainstem in response to stimulus intensity were considered. While the REPE methods were better able to recover the simulated model of the EP, morphology and the latency of the ARX-estimated EPs was a closer match to the actual EP than than that of the REPE-estimated EPs. We, therefore, concluded that ARX rapid extraction would perform better with regards to the rapid tracking of latency variations. By tracking simulated and empirically induced latency variations, we conclude that rapid EP extraction using ARX modeling is only capable of extracting latency variations of an EP in relatively high SNRs and, therefore, should be used with caution in low-noise environments. In particular, it is not a suitable method for the rapid extraction of early EP components such as the auditory brainstem potential.

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.

Efficacy of intracarotid propofol infusion and impact of cerebral blood flow alteration

BACKGROUND

Intracarotid propofol infusion was studied in canines as an experimental basis for application of propofol in the Wada test.

METHODS

First, efficacy and side-effects of propofol anaesthesia were studied in eight mongrel dogs that received intracarotid and i.v. propofol infusions for 30 min according to a cross-over design. Auditory evoked potentials were used to guide anaesthesia. Secondly, eight mongrel dogs received intracarotid propofol infusion during both normal and cerebral hyperperfusion states using nicardipine. Haemodynamics and clinical endpoints were compared between the two infusion conditions.

RESULTS

We required 33 (7.6) mg propofol intracarotically vs 113 (17) mg propofol i.v. to achieve an anaesthetic state. The mean arterial pressure (MAP) decreased about 15-27% from the baseline during i.v. infusion. However, no obvious decrease of MAP was observed after intracarotid infusion. Administration of nicardipine increased the blood flow in the internal carotid artery by 17%. Then, the propofol dosage for achieving the anaesthetic effect increased from 7.7 (0.9) mg in the normal control to 11.3 (0.8) mg in the nicardipine group. The onset time of anaesthetic effect was prolonged and the recovery time was shortened during intracarotid infusion during cerebral hyperperfusion.

CONCLUSIONS

Compared with i.v. propofol infusion, intracarotid infusion could reach and maintain the target anaesthetic depth with less dosage and without affecting MAP. In addition, increase of cerebral blood flow requires a higher propofol dose, prolongs onset, and shortens recovery time during intracarotid propofol anaesthesia, indicating that patients with a cerebral hyperperfusion state may need higher dose of anaesthetics during the Wada test.

Population pharmacokinetics of sevoflurane in conjunction with the AnaConDa: toward target-controlled infusion of volatiles into the breathing system

BACKGROUND

The Anesthetic Conserving Device (AnaConDa) uncouples delivery of a volatile anesthetic (VA) from fresh gas flow (FGF) using a continuous infusion of liquid volatile into a modified heat-moisture exchanger capable of adsorbing VA during expiration and releasing adsorbed VA during inspiration. It combines the simplicity and responsiveness of high FGF with low agent expenditures. We performed in vitro characterization of the device before developing a population pharmacokinetic model for sevoflurane administration with the AnaConDa, and retrospectively testing its performance (internal validation).

MATERIALS AND METHODS

Eighteen females and 20 males, aged 31-87, BMI 20-38, were included. The end-tidal concentrations were varied and recorded together with the VA infusion rates into the device, ventilation and demographic data. The concentration-time course of sevoflurane was described using linear differential equations, and the most suitable structural model and typical parameter values were identified. The individual pharmacokinetic parameters were obtained and tested for covariate relationships. Prediction errors were calculated.

RESULTS

In vitro studies assessed the contribution of the device to the pharmacokinetic model. In vivo, the sevoflurane concentration-time courses on the patient side of the AnaConDa were adequately described with a two-compartment model. The population median absolute prediction error was 27% (interquartile range 13-45%).

CONCLUSION

The predictive performance of the two-compartment model was similar to that of models accepted for TCI administration of intravenous anesthetics, supporting open-loop administration of sevoflurane with the AnaConDa. Further studies will focus on prospective testing and external validation of the model implemented in a target-controlled infusion device.

A comparison of auditory evoked potentials and spectral EEG in the ability to detect marked sevoflurane concentration alterations and clinical events

BACKGROUND

Level of consciousness monitors can distinguish between consciousness and unconsciousness during anaesthesia induction and awakening. However, this distinction is rarely a clinical problem. What we do need is a peroperative indicator signalling when the anaesthetic depth comes close to awakening, or when it is too deep. We investigated the ability of the Alaris fast extracted AEP (AAI) and the GE Healthcare Spectral Entropy algorithms State- and Response Entropy (SE/RE) to respond to marked changes in sevoflurane concentration during stable surgery and to clinical incidents.

METHODS

Both monitors were used simultaneously in 9 patients during sevoflurane- based anaesthesia, which at low concentrations was combined with remifentanil. Additionally, most patients had an epidural block. The response of each monitor to sevoflurane concentration alterations within 0.5-1.5 age-adjusted MAC was recorded, mainly during periods with no surgical stimulation, as was the response to stimulation during surgery and at anaesthesia induction and awakening. Off-line, the numbers of correctly detected events were calculated.

RESULTS

In total, 114 events were found. The response rate of all events (95% c.i.) was 20-37% and 40-57% for the AAI- and the Entropy-monitors, respectively, P<0.05 (Wilcoxon Matched Pair test).

CONCLUSIONS

The Spectral EEG monitor performed significantly better, with a larger number of events detected, compared with the AAI-monitor. However, at the best half the number of events was detected. An anaesthetic ceiling effect might to some part explain this finding. Notwithstanding, continuous anaesthetic depth monitoring may add information to low sensitive semi-continuous standard autonomic monitoring.

Clinical study on effect of electro-acupuncture combined with different anesthetics on auditory-evoked potential index

OBJECTIVE

To observe the effect of electro-acupuncture (EA) on auto regressive with exogenous input model (ARX-model) auditory evoked index (AAI) in patients anesthetized with different anesthetics.

METHODS

Forty-eight adult patients undergoing scheduled surgical operation were enrolled and divided into two groups (24 in each group) according to the anesthetics applied, Group A was anesthetized with propofol sedation and Group B with Isoflurane-epidural anesthesia. Group A was subdivided into three groups of low, middle and high concentration of target effect-site of 1.0 microg/ml, 1.5 microg/ml and 2.0 microg/ml through target controlled infusion (TCI) and Group B into 3 subgroups of minimum alveolar effective concentration of isoflurane (0.4 MAC, 0.6 MAC and 0.8 MAC for B1, B2 and B3 subgroups) respectively, with 8 patients in every subgroup. EA on acupoints of Hegu (LI4) and Neiguan (P6) was applied on all the patients during anesthesia, and the change of AAI at various time points was recorded.

RESULTS

In the three subgroups of Group A, levels of AAI were significantly elevated in the first few minutes after EA, and significantly lowered 20 min after EA in subgroup A2. While in the subgroups of Group B, except the elevating in Group B1 1-2 min after EA, levels of AAI remained unchanged at other time points.

CONCLUSION

Pain response could be reflected by AAI during EA. EA could enhance the sedative effect of propofol in middle concentration, but its effect on isoflurane epidural anesthesia is insignificant.

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.

Combined use of Bispectral Index™ and A-Line™ Autoregressive Index™ to assess anti-nociceptive component of balanced anaesthesia during lumbar arthrodesis

BACKGROUND

This study evaluated the A-Line Autoregressive Index (AAI) response to surgical stimulation during lumbar arthrodesis, as an estimate of the anti-nociceptive component of a Bispectral Index (BIS) guided anaesthesia combined with epidural analgesia.

METHODS

An epidural catheter was inserted in 23 patients allocated randomly to receive ropivacaine plus clonidine (Group R) or normal saline (Group S) epidurally. General anaesthesia was induced with propofol, cis-atracurium and a remifentanil infusion that was stopped 3 min after tracheal intubation, and maintained using sevoflurane to keep BIS at 50 (range 40-60). Mean arterial pressure, heart rate, end-tidal sevoflurane, BIS and AAI were analysed from 2 min before to 17 min after surgical incision.

RESULTS

While BIS was maintained at 50, AAI significantly increased from a 2 min averaged value of 12 (4) to 21 (7) in Group S within the first 5 min after surgical incision, but did not change in Group R. Maximum AAI values reached during the study period were significantly higher in Group S than in Group R [38 (12) and 27 (10), respectively]. Binary logistic regression analysis allowed the calculation of AAI threshold values above which the probability of predominant nociception over anti-nociception was higher than 95%. At 1 MAC sevoflurane concentration, a 2 min averaged AAI of 35 or an AAI peak value of 62 were associated with such a probability.

CONCLUSIONS

During a BIS-guided constant level of hypnosis, AAI response to the onset of surgical stimulation significantly differs according to the analgesic regimen. Further studies are needed to refine the estimation of sensitivity and specificity of this variable in assessing the balance between nociception and anti-nociception during general anaesthesia.

A Comparison of Bispectral Index and Rapidly Extracted Auditory Evoked Potentials Index Responses to Noxious Stimulation During Sevoflurane Anesthesia

In 21 patients given sevoflurane anesthesia, we simultaneously compared the abilities of Bispectral Index (BIS) and rapidly extracted auditory evoked potentials index (AAI) to display the effect of an increasing cerebral concentration of sevoflurane, with and without noxious stimulation. In addition to BIS/AAI, hemodynamic variables were monitored. After titrating sevoflurane to BIS = 50-55 during 15 min, the end-tidal concentration of sevoflurane (1.46% +/- 0.20%) was doubled followed by a noxious stimulus, laryngoscopy, applied at random time points within the following 15 min. After the end-tidal concentration of sevoflurane was doubled, a substantial reduction in BIS was observed, whereas only a slight reduction in AAI was seen (P < 0.0001). BIS/AAI responses to laryngoscopy were not attenuated with increasing wash-in of sevoflurane. After noxious stimulation, AAI exceeded the highest recommended value, 25, in 3 cases, whereas BIS did not exceed the recommended threshold, 60, in any of the patients. Response times for BIS and AAI were 44.5 +/- 26 and 47 +/- 31 s, respectively. These results suggest that, at a hypnotic level associated with surgical sevoflurane anesthesia, BIS better displays drug-related alterations in the level of hypnosis than AAI or hemodynamic variables but there is no difference between BIS and AAI in the time to response to a noxious stimulus.

Neuromuscular block with vecuronium reduces the rapidly extracted auditory evoked potentials index during steady state anesthesia

PURPOSE

During clinical monitoring, vecuronium appeared to reduce the rapidly extracted auditory evoked potentials index (A-line ARX index or AAI) to some extent. A prospective and randomized study was designed to analyze this phenomenon.

METHODS

Forty adult patients undergoing elective surgery were studied. After tracheal intubation, anesthesia was maintained with an end-tidal isoflurane concentration (F(ET)ISO) of 1.0% for 20 min, then a 10-mL dose of either vecuronium 0.05 mg*kg(-1), 0.1 mg*kg(-1), 0.2 mg*kg(-1) or saline was administered in a randomized, double-blind design. The AAI and bispectral index (BI(hx)) were monitored throughout the study and analyzed off-line.

RESULTS

BI(hx) was unaltered after the administration of saline or vecuronium. The mean of the averaged (per patient) AAI values recorded from two minutes to ten minutes after the administration of saline or vecuronium 0.05 mg*kg(-1) did not differ significantly from the corresponding mean recorded from 15 min to 20 min after F(ET)ISO maintained 1.0% (P = 0.678, 0.169), however after the administration of vecuronium 0.1 mg*kg(-1) or 0.2 mg*kg(-1), AAI was reduced from 18.3, 18.0 to 14.8, 13.4 (P = 0.016, 0.017).

CONCLUSIONS

Neuromuscular block with vecuronium reduces AAI in patients during steady state anesthesia without surgical stimuli, while BI(hx) is unaltered. The cut-off values of AAI for events should be determined according to the level of neuromuscular blockade when monitoring the depth of anesthesia/sedation.

A comparison of bispectral index and ARX-derived auditory evoked potential index in measuring the clinical interaction between ketamine and propofol anaesthesia

We evaluated the effects of a bolus (0.4 mg.kg-1) and continuous infusion (1 mg.kg-1.h-1) of ketamine on Bispectral Index (BIS) and A-Line(R) ARX Index (AAI) during propofol anaesthesia. We included 15 ASA I patients scheduled for general anaesthesia. Induction was performed by infusion of propofol at 100 ml.h-1 until loss of consciousness. Both BIS and AAI monitors responded appropriately at that time. The calculated effect site concentration of propofol at loss of consciousness was maintained by means of a computer controlled infusion system. A 'pseudo' steady-state effect site concentration was reached after 4 min. After 1 min of baseline measurements, ketamine was administered. BIS values increased from the 3rd to the 8th min after the administration of ketamine. The AAI showed no significant increase or decrease, but between-patient variability increased.

Performance of the rapidly extracted auditory evoked potentials index to detect the recovery and loss of wakefulness in anesthetized and paralyzed patients

BACKGROUND

The rapidly extracted auditory evoked potentials index (A-lineTM ARX Index or AAI) has been proposed as a method to measure the depth of anesthesia. A prospective study was designed to assess the performance of AAI to detect the recovery and loss of wakefulness in anesthetized and paralyzed patients.

METHODS

Fourteen adult patients undergoing elective surgery were anesthetized with propofol 1.5 mg kg-1, vecuronium 0.1 mg kg-1 and further propofol 1.0 mg kg-1. Wakefulness was measured by the ability of the patient to respond to command using the isolated forearm technique (IFT). After the patient responded, propofol was infused at 10 mg kg-1. h-1 until wakefulness (responsiveness) was lost. The AAI was recorded continuously throughout the study and analyzed off-line.

RESULTS

The AAI showed a significant difference between the values registered during, 30 s before and 30 s after the recovery, and also between 30 s before and 30 s after the loss of wakefulness. The prediction probability (Pk) values for AAI were 0.786 and 0.864 during the transitions from unresponsiveness to responsiveness and from responsiveness to unresponsiveness. The area under the receiver operating characteristic curve for the responsive and unresponsive values was 0.926 (SE 0.002, 95% CI 0.922-0.931), and the AAI values of approximately 5%, 50% and 95% predicted probability of wakefulness were 19, 29 and 39, respectively.

CONCLUSION

The AAI may be a good predictor of recovery and loss of wakefulness for anesthetized and paralyzed patients.

Changes in the rapidly extracted auditory evoked potentials index and the bispectral index during sedation induced by propofol or midazolam under epidural block

BACKGROUND

The bispectral index (BIS) and the rapidly extracted auditory evoked potentials index (A-line ARX Index or AAI) have been proposed as methods to measure the depth of sedation. A prospective study was designed to assess the performance of both these methods for measuring the depth of sedation induced by propofol or midazolam under epidural block.

METHODS

Thirty-two ASA I and II adult patients undergoing elective gynaecological surgery under low-thoracolumbar epidural block were studied. Eighteen patients received propofol (Group P: 20 mg bolus every 3 min) and 14 received midazolam (Group M: 0.5 mg bolus every 5 min) until an observer's assessment of alertness/sedation (OAA/S) scale score of 1 was achieved. AAI and BIS were monitored for different OAA/S scores.

RESULTS

AAI and BIS decreased and increased following the changes on the patients' OAA/S scores and correlated with sedation significantly. During the onset phase, the coefficients of Spearman's rank correlation for AAI and BIS were respectively 0.958 and 0.898 (P < 0.001) for Group P, and 0.973 and 0.945 (P < 0.001) for Group M. During the recovery phase in Group P, the coefficients were respectively 0.946 and 0.702 (P < 0.001). Linear regression analysis showed that both AAI and BIS were linearly related to the OAA/S scores. The coefficients of Spearman's rank correlation and linear regression for AAI were all greater than those for BIS (P < 0.05).

CONCLUSIONS

Both AAI and BIS correlated well with the depth of sedation induced by propofol or midazolam under epidural block. AAI may be more valuable when monitoring depth of sedation.

Midlatency auditory evoked potentials do not allow the prediction of recovery from general anesthesia with isoflurane

PURPOSE

To investigate midlatency auditory evoked potentials (MLAEP) waveforms during recovery from anesthesia. The hypothesis was that MLAEP are sensitive variables to discriminate between states of consciousness and unconsciousness during emergence from anesthesia.

METHODS

MLAEP were recorded in the awake state and during the wake-up phase from isoflurane anesthesia in 22 female patients undergoing ophthalmologic surgery. During emergence from anesthesia the changes in latency and amplitude of MLAEP components Na, Pa and Nb were compared with the awake level. The next day the patients were asked for explicit memory for the recovery period.

RESULTS

In 72% of the patients the MLAEP waveforms were completely suppressed during isoflurane anesthesia. When the patients responded and opened their eyes spontaneously 38 +/- 12 min after anesthesia, the latencies of Na (18.3 +/- 1.2 vs 17.6 +/- 1.3; P = 0.013) and Nb (47.4 vs 7.1 vs 44.7 +/- 7.8; P = 0.048) remained prolonged compared with awake values. In contrast, the amplitudes NaPa and PaNb had regained baseline level. Nine patients had explicit memory for the immediate recovery period. However, there was no difference for any MLAEP component between patients with and without memory at any time.

CONCLUSIONS

The persistent changes of MLAEP latency components Na and Nb indicated impaired auditory signal processing 38 min after isoflurane anesthesia. There was a marked intra- and inter-individual variability during reversal of the anesthetic induced MLAEP changes. This limits the prediction of recovery of consciousness in the individual patient during emergence from anesthesia.

Intraoperative awareness: detected by the structured Brice interview?

BACKGROUND

Awareness is a rare complication in general anesthesia, but its consequences are stressful. Efforts must be undertaken to prevent, diagnose, and, if occurring, treat it. The incidence of awareness is higher following anesthesia involving the use of muscle relaxants. As a part of a quality assurance program at our short-stay surgery all patients exposed to general anesthesia are routinely subjected to a Brice interview, which aims to evaluate our standard anesthetic technique regarding awareness.

METHODS

The Brice interview was used prospectively in 5216 patients given a propofol/opioid anesthetic for day-case or short-stay surgery. Neuromuscular blocks were used only for surgical needs, not routinely. All patients were interviewed on discharge from the recovery room. A second interview, according to Brice, was undertaken by telephone 3-7 days later in the case of a notable intraoperative event, or otherwise after postoperative patient complaints. All patients were also interviewed by telephone 1-2 days postoperatively.

RESULTS

None of the patient interviews indicated awareness. This was also the case in five non-relaxed patients who had an incident of light anesthesia with eye opening and gross motor response without forewarning. Neuromuscular blockade was used in 7% of patients.

DISCUSSION

We were unable to detect intraoperative awareness. The anesthetic regimen, including minimal use of muscle relaxants, might be beneficial for awareness prevention. Alternatively, the diagnostic power, the timing of the Brice interview, or the number of interviews performed may be questioned.

Comparison of auditory evoked potentials and the A-line ARX Index for monitoring the hypnotic level during sevoflurane and propofol induction

BACKGROUND

Extraction of the middle latency auditory evoked potentials (AEP) by an auto regressive model with exogenous input (ARX) enables extraction of the AEP within 1.7 s. In this way, the depth of hypnosis can be monitored at almost real-time. However, the identification and the interpretation of the appropriate signals of the AEP could be difficult to perform during the anesthesia procedure. This problem was addressed by defining an index which reflected the peak amplitudes and latencies of the AEP, developed to improve the clinical interpretation of the AEP. This index was defined as the A-line Arx Index (AAI).

METHODS

The AEP and AAI were compared with the Modified Observers Assessment of Alertness and Sedation Scale (MOAAS) in 24 patients scheduled for cardiac surgery, anesthetized with propofol or sevoflurane.

RESULTS

When comparing the AEP peak latencies and amplitudes and the AAI, measured at MOAAS level 5 and level 1, significant differences were achieved. (mean(SD) Nb latency: MOAAS 5 51.1 (7.3) ms vs. MOAAS 1: 68.6 (8.1) ms; AAI: MOAAS 5 74.9 (13.3) vs. MOAAS 1 20.7 (4.7)). Among the recorded parameters, the AAI was the best predictor of the awake/anesthetized states.

CONCLUSION

We conclude that both the AAI values and the AEP peak latencies and amplitudes correlated well with the MOAAS levels 5 (awake) and 1 (anesthetized).