Multi-level approach to anaesthetic effects produced by sevoflurane or propofol in humans: 1. BIS and blink reflex †

Intraoperative Mapping of the Sensory Root of the Trigeminal Nerve in Patients with Pontocerebellar Angle Pathology

OBJECTIVE

The aim of the present study was to determine the position of the 3 sensory branches of the trigeminal nerve in the preganglionic tract using intraoperative neurophysiological mapping.

METHODS

We included consecutive adult patients who underwent neurosurgical treatment of cerebellopontine angle lesions. The trigeminal nerve was antidromically stimulated at 3 sites along its circumference with different stimulus intensities at a distance of ≤1 cm from the brainstem. The sensory nerve action potentials (SNAPs) were recorded from each main trigeminal branch (V1 [ophthalmic branch], V2 [maxillary branch], and V3 [mandibular branch]).

RESULTS

We analyzed 13 patients. The stimulation points at which we obtained the greatest number of congruous and exclusive SNAPs (SNAPs only on the stimulated branch) was the stimulation point for V3 (20.7%). The stimulation intensity at which we obtained the highest number of congruent and exclusive SNAPs with the stimulated branch was 0.5 mA.

CONCLUSIONS

Using our recording conditions, trigeminal stimulation is a reliable technique for mapping the V3 and V1 branches using an intensity not exceeding 0.5. However, reliable identification of the fibers of V2 is more difficult. Stimulation of the trigeminal nerve can be a reliable technique to identify the V3 and V1 branches if rhizotomy of these branches is necessary.

Tonic down-rolling and eccentric down-positioning of eyes under sevoflurane anesthesia without non-depolarizing muscle relaxant and its relationship with depth of anesthesia

Purpose

To analyze the relationship between eccentric downward eye movement/eccentric downward eye-positioning (EDEM/EDEP) encountered in patients undergoing ophthalmic surgeries and its return to a centralized position under general anesthesia (GA) with the depth of anesthesia (DOA).

Methods

Patients undergoing ophthalmic surgeries (6 months-12 years) under sevoflurane anesthesia without non-depolarizing muscle relaxant (NDMR) who witnessed a sudden tonic EDEM/EDEP were both retrospectively (R-group) and prospectively (P-group) enrolled (ambispective study). R-group included data-points after induction (AI) till the time surgery lasted while P-group compiled data both during induction (DI) and AI. DOA in terms of MAC (minimum alveolar concentration) at the time of EDEM/EDEP and centralization of eyeball and their timings were noted and compared for both AI and DI data-points. Also, vertical eccentric eye positions were scored and correlated with MAC.

Results

AI data included 22 (14R+8P) events and their mean MAC of EDEM/EDEP and centralization were 1.60 ± 0.25 and 1.18 ± 0.17 respectively (p = 0.000). DI data included 62 (P) cases and its mean MAC of EDEM/EDEP and centralization was 2.19 ± 0.43 and 1.39 ± 0.26 respectively (p = 0.000). Median (IQR) eye positions during down-positioning in 84 events was -3 (-3.9 to -2.5). It was preceded by an eccentric upward drift of eyes in 10/22 (6R+4P) AI cases. A strong negative correlation was seen between DOA and eccentric eye positions (r = -0.77, p = 0.000).

Conclusions

Tonic down-rolling of eyes is not uncommon in children seen without NDMR with higher depths of sevoflurane anesthesia compared to point of centralization and fluctuations in DOA should be avoided to circumvent inadvertent complications during ocular surgery.

Monitoring surgery around the cranial nerves

Intraoperative neurophysiologic monitoring (IONM) of cranial nerve (CN) function is an essential component in multimodality monitoring of surgical procedures where CNs are at risk for injury. In most cases, IONM consists of localizing and mapping CNs and their pathways, and monitoring of CN motor function during surgery. However, CN VIII, which has no motor function, and is at risk for injury in many surgical procedures, can be easily and accurately monitored using brainstem auditory evoked potentials. For motor CNs, the literature is clear that function can be safely and adequately performed using basic electromyographic (EMG) techniques, such as recording of continuous EMG activity and electrically evoked compound muscle actions potentials. Newer techniques, such as corticobulbar motor evoked potentials and reflex studies, show good potential for a greater degree of functional assessment but require further study to determine their clinical utility. EMG remains the basic clinical neurophysiologic technique with the greatest clinical research supporting its utility in IONM of motor CN function and should be used as part of a comprehensive multimodality IONM protocol. Understanding the physiologic basis of EMG and the changes associated with altered motor function will allow the practitioner to alter surgical course to prevent injury and improve patient safety.

Remimazolam - current knowledge on a new benzodiazepine intravenous anesthetic agent

Intravenous anesthetic agents such as midazolam, propofol, and ketamine are routinely used to provide anesthesia and sedation. They have been shown to effectively induce and maintain amnesia, sedation, and hypnosis in various patient groups and clinical settings. However, all anesthetic agents have the potential to cause unwanted side effects such as hemodynamic instability, respiratory depression, or slow recovery due to prolonged post-procedural sedation. Remimazolam, a recently approved benzodiazepine for general anesthesia and procedural sedation in Korea, has been successfully used for these purposes. To date, inconclusive knowledge has been obtained regarding the use of remimazolam in different patient populations and under various surgical conditions. With respect to the specific pharmacokinetic and pharmacodynamic characteristics of remimazolam, the use of remimazolam is expected to increase providing safe general anesthesia and sedation. This review aims to provide an overview of the basic and clinical pharmacology of remimazolam and to compare it with midazolam and propofol.

Monitoring cerebellopontine angle and skull base surgeries

Cerebellopontine angle (CPA) surgery represents a challenge for neurosurgeons due to the high risk of iatrogenic injury of vital neurological structures. Therefore, important efforts in improving the surgical techniques and intraoperative neurophysiology have been made in the last decades. We present a description and review of the available methodologies for intraoperative neuromonitoring and mapping during CPA surgeries. There are three main groups of techniques to assess the functional integrity of the nervous structures in danger during these surgical procedures: (1) Electrical identification or mapping of motor cranial nerves (CNs), which is essential in order to locate the nerve in their different parts during the tumor resection; (2) Monitoring, which provides real-time information about functional integrity of the nervous tissue; and (3) Brainstem reflexes including blink reflex, masseteric reflex, and laryngeal adductor reflex. All these methods facilitate the removal of lesions and contribute to notable improvement in functional outcome and permit on the investigation of their physiopathology in certain neurosurgically treated diseases. Such is the case of hemifacial spasm (HFS). We describe the methodology to evaluate the efficacy of microvascular decompression for HFS treatment at the end of this chapter.

Isoflurane not at the expense of postoperative nausea and vomiting in cardiac anesthesia - an observational study

OBJECTIVES

Inhalative anesthesia is of common use, but is generally known to potentiate postoperative nausea and vomiting (PONV). With an internal change of anesthesia regimen from total intravenous anesthesia (TIVA) to isoflurane (in terms of myocardial protection) in cardiac anesthesia a higher incidence of PONV was to be expected. Therefore, we evaluated the incidence of PONV after the simultaneous implementation of PONV prophylaxis.

METHODS

The incidence of PONV, prospectively assessed in 197 cardiac surgery patients (68 y ± 10.4, 66.5% male) having isoflurane plus dual PONV prophylaxis with dexamethasone and droperidol, was compared with previous data of 190 controls (67 y ± 9.6, 71% male) having TIVA without and with single or dual PONV prophylaxis (n = 64 dexamethasone and droperidol, n = 25 dexamethasone, n = 101 only TIVA), and the Apfel-scoring (0-4 depending on PONV-risk). DRKS00014275. Statistics: Chi²-test, p < .05 (Bonferroni).

RESULTS

The incidence of PONV under isoflurane with antiemetic prophylaxis was 20.8% (95% confidence interval (CI) 15.4; 27.4) compared to 30.5% (95%CI 24; 37.6) under TIVA (p = .029; dexamethasone and droperidol 23.4% (95%CI 13.8; 35.7); dexamethasone 32% (95%CI 14.9; 53.5); only TIVA 34.7% (95%CI 25.5; 44.8)), but was not lower in high-risk patients than predicted according to Apfel-scoring 4 (71.4 vs. 78%).

CONCLUSION

In cardiac anesthesia, the use of isoflurane is not at the expense of PONV when using a risk-independent two-drug-prophylaxis. It is even beneficial resulting surprisingly in a lower incidence of PONV than under TIVA unless with and without prophylaxis. Patients with the highest risk for PONV and receiving isoflurane should receive a third antiemetic prophylactic drug.

Performance of blink reflex in patients during anesthesia induction with propofol and remifentanil: prediction probabilities and multinomial logistic analysis

Background

The amount of propofol needed to induce loss of responsiveness varied widely among patients, and they usually required less than the initial dose recommended by the drug package inserts. Identifying precisely the moment of loss of responsiveness will determine the amount of propofol each patient needs. Currently, methods to decide the exact moment of loss of responsiveness are based on subjective analysis, and the monitors that use objective methods fail in precision. Based on previous studies, we believe that the blink reflex can be useful to characterize, more objectively, the transition from responsiveness to unresponsiveness. The purpose of this study is to investigate the relation between the electrically evoked blink reflex and the level of sedation/anesthesia measured with an adapted version of the Richmond Agitation–Sedation Scale, during the induction phase of general anesthesia with propofol and remifentanil. Adding the blink reflex to other variables may allow a more objective assessment of the exact moment of loss of responsiveness and a more personalized approach to anesthesia induction.

Results

The electromyographic-derived features proved to be good predictors to estimate the different levels of sedation/anesthesia. The results of the multinomial analysis showed a reasonable performance of the model, explaining almost 70% of the adapted Richmond Agitation–Sedation Scale variance. The overall predictive accuracy for the model was 73.6%, suggesting that it is useful to predict loss of responsiveness.

Conclusions

Our developed model was based on the information of the electromyographic-derived features from the blink reflex responses. It was able to predict the drug effect in patients undergoing general anesthesia, which can be helpful for the anesthesiologists to reduce the overwhelming variability observed between patients and avoid many cases of overdosing and associated risks. Despite this, future research is needed to account for variabilities in the clinical response of the patients and with the interactions between propofol and remifentanil. Nevertheless, a method that could allow for an automatic prediction/detection of loss of responsiveness is a step forward for personalized medicine.

Electromyographic assessment of blink reflex throughout the transition from responsiveness to unresponsiveness during induction with propofol and remifentanil

General anesthesia is a reversible drug-induced state of altered arousal characterized by loss of responsiveness due to brainstem inactivation. Precise identification of the moment in which responsiveness is lost during the induction of general anesthesia is extremely important to provide information regarding an individual's anesthetic requirements and help intraoperative drug titration. To characterize the transition from responsiveness to unresponsiveness more objectively, we studied neurophysiologic-derived parameters of electromyographic records of electrically evoked blink reflex as a means of identifying the precise moment of loss of responsiveness. Twenty-five patients received a slow infusion of propofol until loss of corneal reflex while successive blink reflexes were elicited and recorded every 6 s. The level of anesthesia was assessed using an adapted version of the Richmond Agitation-Sedation Scale. Different variables of the blink reflex components were calculated and compared to the adapted version of the Richmond Agitation-Sedation score and the estimated effect-site propofol concentration. Baselines of the blink reflex responses were similar to those in literature. After propofol infusion started, the most susceptible component of the blink reflex to propofol was R₂ (EC₅₀ = 1.358 (95% CI 1.321, 1.396) µg/mL) and the most resistant was R₁ (EC₅₀ = 3.025 (95% CI 2.960, 3.090) µg/mL). Most of the patients (24 out of 25) lost the R₁ component when they were still responsive to shaking and shouting and corneal reflex could be elicited clinically (time = 102.48 ± 33.00 s). Habituation was present in R₂ but not in R₁. The R₁ component of the blink reflex was found to have a strong correlation with the adapted version of the Richmond Agitation-Sedation Scale, with amplitude correlating better than areas (ρ = - 0.721 (0.123) versus ρ = - 0.688 (0.165)). We found a strong correlation between the R₁ component with the estimated propofol effect-site concentration, with amplitude correlating better than areas (ρ = - 0.838 (0.113) versus ρ = - 0.823 (0.153)) and between the clinical scale and the propofol concentration (ρ = 0.856 (0.060)). The area and amplitude of the R₁ component showed to be indicators of predicting different levels of anesthesia (P_k = 0.672 (0.183) versus P_k = 0.709 (0.134)) and these are connected to the propofol concentrations (P_k = 0.593 (0.10)). Our results suggest that electrically evoked blink reflex could be used during the induction of anesthesia as a surrogate of the Richmond Agitation-Sedation Scale to provide an objective endpoint as far as a - 4. At this point, at the moment of loss of R₁, the propofol infusion may be stopped, as overshooting increases slightly the effect-site concentration afterward and eventually reaching loss of responsiveness. If the desired target is not achieved, the infusion can then be resumed.

Intraoperative Neuromonitoring of Blink Reflex During Posterior Fossa Surgeries and its Correlation With Clinical Outcome

PURPOSE

Blink reflex (BR) under general anesthesia as an intraoperative neuromonitoring method was used to monitor facial nerves in few studies. This study aimed to test the utility of intraoperative BR during cerebellopontine angle and skull base surgeries, assess its prognostic value for facial nerve functions, and compare it with facial corticobulbar motor evoked potentials (CoMEPs).

METHODS

Blink reflex and facial CoMEPs were recorded from 40 patients undergoing skull base surgeries. Subdermal needles were placed in the supraorbital notch for stimulation and in the orbicularis oculi muscle for recording the BR. A double train of 20 to 40 V intensity with an intertrain interval of 40 to 60 milliseconds, an interstimulus interval of 2.5 milliseconds, and a stimulus duration of 0.5 milliseconds were applied. Facial nerve functions were assessed with the House-Brackmann grading system in the postoperative day 1 and third-month period and correlated with intraoperative BR and CoMEPs measurements.

RESULTS

Of 40 patients, BR was recordable on the affected side in 32 (80%) and contralateral side in 35 (87.5%) patients. According to our statistical results, BR had a slightly better sensitivity than facial CoMEPs in predicting impairment of facial nerve functions for both postoperative and third-month time points. Blink reflex showed better accuracy for predicting postoperative nerve functions, whereas CoMEPs correlated better in predicting third-month outcome.

CONCLUSIONS

We suggest that BR is a valuable intraoperative neuromonitoring method that can be used in addition to facial CoMEPs during skull base surgeries to assess real-time facial nerve integrity and predict prognosis.

Intraoperative monitoring of sensory part of the trigeminal nerve using blink reflex during microvascular decompression for trigeminal neuralgia

Intraoperative monitoring during cerebellopontine angle surgery is widely accepted. While techniques which monitor cranial motor nerves are commonly used, monitoring the sensory afferents has been challenging. Considering the reflex arc, blink reflex (BR) might be useful in monitoring the sensory part of the trigeminal nerve, the brainstem connections and the facial nerve. We describe the case of a patient who developed hemifacial hypoesthesia after microvascular decompression surgery for trigeminal neuralgia. Intraoperative BR showed a severe loss of R1 amplitude. BR might be a useful intraoperative technique to monitor the sensory part of the trigeminal nerve.

Impact of high concentrations of sevoflurane on laryngeal reflex responses

BACKGROUND

Exaggerated defensive upper airway reflexes, particularly laryngospasm, may cause hypoxemic damage, especially in children. General clinical experience suggests that laryngeal reflex responses are more common under light levels of anesthesia, and previous clinical studies have shown an inverse correlation between laryngeal responsiveness and depth of hypnosis. However, this seems to be less obvious in children anesthetized with sevoflurane. The aim of this study was to assess the impact of high concentrations of sevoflurane on laryngeal and respiratory reflex responses in spontaneously breathing children. Accordingly, we tested the hypothesis that laryngeal and respiratory reflex responses were completely suppressed in spontaneously breathing children when anesthetized with sevoflurane 4.7% (=MAC_{ED95Intubation} ) as compared with sevoflurane 2.5% (=1 MAC).

METHODS

In this prospective observational study, we tested the hypothesis that the incidence of laryngospasm evoked by laryngeal stimulation is diminished under high concentrations of sevoflurane. Following Ethics approval, trial registration, and informed consent, 40 children (3-7 years) scheduled for elective surgery participated in the trial. All children received sevoflurane 2.5% (1 MAC) and 4.7% (ED_95Intubation ) in random order with 5-min equilibration between the states. Under both conditions, distilled water was sprayed under bronchoscopic view onto the larynx. Potential laryngeal and respiratory reflex responses were assessed offline by a blinded reviewer.

RESULTS

Laryngospasm (episodes lasting >10 s) occurred in 12/38 (32%) of the patients anesthetized with sevoflurane 2.5%, vs 7/38 (18%) in those anesthetized with sevoflurane 4.7% (difference: OR 3.5; 95% CI [0.72-16.84], P = 0.18). All other reflex responses (coughing, expiration reflexes, and spasmodic panting) were infrequent and were similar among the examined concentrations.

CONCLUSION

Against our hypothesis, laryngospasm could still be observed in 18% of children under the higher concentration of sevoflurane (4.7%, ED_95Intubation ).

Influence of general anaesthesia on the brainstem

The exact role of the brainstem in the control of body functions is not yet well known and the same applies to the influence of general anaesthesia on brainstem functions. Nevertheless in all general anaesthesia the anaesthesiologist should be aware of the interaction of anaesthetic drugs and brainstem function in relation to whole body homeostasis. As a result of this interaction there will be changes in consciousness, protective reflexes, breathing pattern, heart rate, temperature or arterial blood pressure to name a few. Brainstem function can be explored using three different approaches: clinically, analyzing changes in brain electric activity or using neuroimaging techniques. With the aim of providing the clinician anaesthesiologist with a global view of the interaction between the anaesthetic state and homeostatic changes related to brainstem function, the present review article addresses the influence of anaesthetic drug effects on brainstem function through clinical exploration of cranial nerves and reflexes, analysis of electric signals such as electroencephalographic changes and what it is known about brainstem through the use of imaging techniques, more specifically functional magnetic resonance imaging.

Anaesthetic interventions for prevention of awareness during surgery

BACKGROUND

General anaesthesia is usually associated with unconsciousness. 'Awareness' is when patients have postoperative recall of events or experiences during surgery. 'Wakefulness' is when patients become conscious during surgery, but have no postoperative recollection of the period of consciousness.

OBJECTIVES

To evaluate the efficacy of two types of anaesthetic interventions in reducing clinically significant awareness:- anaesthetic drug regimens; and- intraoperative anaesthetic depth monitors.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL, ISSUE 4 2016); PubMed from 1950 to April 2016; MEDLINE from 1950 to April 2016; and Embase from 1980 to April 2016. We contacted experts to identify additional studies. We performed a handsearch of the citations in the review. We did not search trial registries.

SELECTION CRITERIA

We included randomized controlled trials (RCTs) of either anaesthetic regimens or anaesthetic depth monitors. We excluded volunteer studies, studies of patients prior to skin incision, intensive care unit studies, and studies that only randomized different word presentations for memory tests (not anaesthetic interventions).Anaesthetic drug regimens included studies of induction or maintenance, or both. Anaesthetic depth monitors included the Bispectral Index monitor, M-Entropy, Narcotrend monitor, cerebral function monitor, cerebral state monitor, patient state index, and lower oesophageal contractility monitor. The use of anaesthetic depth monitors allows the titration of anaesthetic drugs to maintain unconsciousness.

DATA COLLECTION AND ANALYSIS

At least two authors independently scanned abstracts, extracted data from the studies, and evaluated studies for risk of bias. We made attempts to contact all authors for additional clarification. We performed meta-analysis statistics in packages of the R language.

MAIN RESULTS

We included 160 studies with 54,109 enrolled participants; 53,713 participants started the studies and 50,034 completed the studies or data analysis (or both). We could not use 115 RCTs in meta-analytic comparisons because they had zero awareness events. We did not merge 27 of the remaining 45 studies because they had excessive clinical and methodological heterogeneity. We pooled the remaining 18 eligible RCTs in meta-analysis. There are 10 studies awaiting classification which we will process when we update the review.The meta-analyses included 18 trials with 36,034 participants. In the analysis of anaesthetic depth monitoring (either Bispectral Index or M-entropy) versus standard clinical and electronic monitoring, there were nine trials with 34,744 participants. The overall event rate was 0.5%. The effect favoured neither anaesthetic depth monitoring nor standard clinical and electronic monitoring, with little precision in the odds ratio (OR) estimate (OR 0.98, 95% confidence interval (CI) 0.59 to 1.62).In a five-study subset of Bispectral Index monitoring versus standard clinical and electronic monitoring, with 34,181 participants, 503 participants gave awareness reports to a blinded, expert panel who adjudicated or judged the outcome for each patient after reviewing the questionnaires: no awareness, possible awareness, or definite awareness. Experts judged 351 patient awareness reports to have no awareness, 87 to have possible awareness, and 65 to have definite awareness. The effect size favoured neither Bispectral Index monitoring nor standard clinical and electronic monitoring, with little precision in the OR estimate for the combination of definite and possible awareness (OR 0.96, 95% CI 0.35 to 2.65). The effect size favoured Bispectral Index monitoring for definite awareness, but with little precision in the OR estimate (OR 0.60, 95% CI 0.13 to 2.75).We performed three smaller meta-analyses of anaesthetic drugs. There were nine studies with 1290 participants. Wakefulness was reduced by ketamine and etomidate compared to thiopental. Wakefulness was more frequent than awareness. Benzodiazepines reduces awareness compared to thiopental, ketamine, and placebo., Also, higher doses of inhaled anaesthetics versus lower doses reduced the risk of awareness.We graded the quality of the evidence as low or very low in the 'Summary of findings' tables for the five comparisons.Most of the secondary outcomes in this review were not reported in the included RCTs.

AUTHORS' CONCLUSIONS

Anaesthetic depth monitors may have similar effects to standard clinical and electrical monitoring on the risk of awareness during surgery. In older studies comparing anaesthetics in a smaller portion of the patient sample, wakefulness occurred more frequently than awareness. Use of etomidate and ketamine lowered the risk of wakefulness compared to thiopental. Benzodiazepines compared to thiopental and ketamine, or higher doses of inhaled anaesthetics versus lower doses, reduced the risk of awareness.

Evaluation of the aepEX™ monitor of hypnotic depth in pediatric patients receiving propofol-remifentanil anesthesia

BACKGROUND

The aepEX Plus monitor (aepEX) utilizes a mid-latency auditory evoked potential-derived index of depth of hypnosis (DoH).

OBJECTIVE

This observational study evaluates the performance of the aepEX as a DoH monitor for pediatric patients receiving propofol-remifentanil anesthesia.

METHODS

aepEX and BIS values were recorded simultaneously during surgery in three groups of 25 children (aged 1-3, 3-6 and 6-16 years). Propofol was administered by target-controlled infusion. The University of Michigan Sedation Scale (UMSS) was used to clinically assess the DoH during emergence. Prediction probability (P(k)) and receiver operating characteristics (ROC) analyses were performed to assess the accuracy of both DoH monitors. Nonlinear regression analysis was used to describe the dose-response relationships for the aepEX, the BIS, and propofol plasma concentrations (Cp).

RESULTS

The P(k) for the aepEX and BIS was 0.36 and 0.21, respectively (P = 0.010). ROC analysis showed an area under the curve of 0.77 and 0.88 for the aepEX and BIS, respectively (P = 0.644). At half-maximal effect (EC(50)), C(p) of 3.13 μg·ml(-1) and 3.06 μg·ml(-1) were observed for the aepEX and BIS, respectively. The r(2) for the aepEX and BIS was 0.53 and 0.82, respectively.

CONCLUSION

The aepEX performs comparable to the BIS in differentiating between consciousness and unconsciousness, while performing inferior to the BIS in terms of distinguishing different levels of sedation and does not correlate well with the C(p) in children receiving propofol-remifentanil anesthesia.

Direct measurement of nitrous oxide kinetics

BACKGROUND

Using conscious subjects, measurement of the effects of low concentrations of anaesthetic agents can allow the dynamics of onset and offset of the agent to be measured and kinetic values estimated. However, the tests have to be rapid and preferably assess cerebral function.

METHODS

We used a short version of the digit symbol substitution test (DSST) that allowed frequent measurement of the impairment caused by nitrous oxide. We compared 10 min of onset and offset of breathing 5% and 30% nitrous oxide in 30% oxygen, compared with 30% oxygen only. End-tidal nitrous oxide concentrations were used to predict the concentration in a central compartment, according to a range of T(1/2) values chosen to be consistent with possible cerebral blood flow values.

RESULTS

We studied 19 volunteers and estimated a mean response. Only 30% nitrous oxide decreased the DSST. When DSST scores were related to the values in the predicted central compartment, the best dose-effect relationship was found when the T(1/2) was 37 s, consistent with a regional blood flow of about 120 ml 100 g(-1) min(-1).

CONCLUSIONS

The onset of nitrous oxide effect on DSST is rapid, consistent with the perfusion of metabolically active cerebral cortical tissues. The rate of onset is greater than previous measures based on a motor test which involved the function of subcortical structures in the central nervous system.

The EEG signal: a window on the cortical brain activity

The accurate assessment of the depth of anesthesia, allowing a more accurate adaptation of the doses of hypnotics, is an important end point for the anesthesiologist. It is a particularly crucial issue in pediatric anesthesia, in the context of the recent controversies about the potential neurological consequences of the main anesthetic drugs on the developing brain. The electroencephalogram signal reflects the electrical activity of the neurons in the cerebral cortex. It is thus the key to assessment of the level of hypnosis. Beyond visual analysis, several monitoring devices allow an automated treatment of the electroencephalographic (EEG) signal, combining time and frequency domain analysis. Each of these monitors focuses on a specific combination of characteristics of the signal and provides the clinician with useful information that remains, however, partial. For a comprehensive approach of the EEG-derived indices, the main features of the normal EEG, in adults and children, will be presented in the awake state and during sleep. Age-related modifications accompanying cerebral maturation during infancy and childhood will be detailed. Then, this review will provide an update on how anesthetic drugs, particularly hypnotics, influence the EEG signal, and how the main available monitors analyze these drug-induced modifications. The relationships between pain, memory, and the EEG will be discussed. Finally, this review will focus on some specific EEG features such as the electrical epileptoid activity observed under sevoflurane anesthesia. The EEG signal is the best window we have on cortical brain activity and provides a fair pharmacodynamic feedback of the effects of hypnotics. However, the cortex is only one of several targets of anesthesia. Hypnotics and opiates, have also subcortical primary targets, and the EEG performances in the evaluation or prediction of nociception are poor. Monitoring subcortical structures in combination with the EEG might in the future allow a better evaluation and a more precise adaptation of balanced anesthesia.

Minimal alveolar concentration of sevoflurane inhibiting the reflex pupillary dilatation after noxious stimulation in children and young adults

BACKGROUND

In children, sevoflurane is the most commonly used anaesthetic. Its excellent haemodynamic tolerance gives it a wide therapeutic index. This halogenated agent can abolish movement [minimal alveolar concentration (MAC)] or haemodynamic responses (MACBAR) to noxious stimulus in children as in adults. Reflex pupillary dilatation (RPD) has been demonstrated as a very sensitive measure of noxious stimulation. In order to investigate the effect of sevoflurane on the RPD, a subcortical reflex, we determined the MAC of sevoflurane inhibiting the RPD in 50% of the subjects in response to skin incision (MACpup) in pre- and post-pubertal subjects.

METHODS

We included 30 pre-pubertal children and 19 post-pubertal subjects. Patients received sevoflurane at preselected concentrations according to an 'up and down' design, and after a steady-state period, skin incision was performed. The RPD was considered as significant when the pupillary diameter increased by more than 100%. Heart rate (HR) and bispectral index (BIS) changes were analysed according to the pupillary response.

RESULTS

The MACpup of sevoflurane was 4.8% (95% confidence interval, 4.6-5.1%) in pre-pubertal children vs 3.4% (3.5-3.3%) in post-pubertal subjects (P<0.001). Inhibition of RPD was always associated with lack of significant HR response. In pre-pubertal children receiving high concentrations of sevoflurane, RPD in response to noxious stimulation was frequently associated with lack of HR response and low BIS values.

CONCLUSIONS

MACpup was higher than MAC and close to the MACBAR. Inhibition of RPD in pre-pubertal children required higher sevoflurane concentrations compared with post-pubertal subjects, suggesting that the relationship between the brain structure sensitivities may differ with brain maturation.

Comparison between cerebral state index and bispectral index as measures of electroencephalographic effects of sevoflurane using combined sigmoidal E(max) model

AIM

The cerebral state index (CSI) was recently introduced as an electroencephalographic monitor for measuring the depth of anesthesia. We compared the performance of CSI to the bispectral index (BIS) as electroencephalographic measures of sevoflurane effect using two combined sigmoidal E(max) models.

METHODS

Twenty adult patients scheduled for laparotomy were studied. After induction of general anesthesia, sevoflurane concentrations were progressively increased and then decreased over 70 min. An analysis of the BIS and CSI with the sevoflurane effect-site concentration was conducted using two combined sigmoidal E(max) models.

RESULTS

The BIS and CSI decreased over the initial concentration range of sevoflurane and then reached a plateau in most patients. A further increase in sevoflurane concentration produced a secondary plateau in the pharmacodynamic response. The CSI was more strongly correlated with effect-site sevoflurane concentration (R(2)=0.95±0.04) than the BIS was (R(2)=0.87±0.07) (P<0.05). The individual E(max) and C(eff50) (effect-site concentration associated with 50% decrease from baseline to plateau) values for the upper and lower plateaus were significantly greater for BIS (12.7±7.3, 1.6±0.4, and 4.2±0.5, respectively) than for CSI (3.4±2.2, 1.2±0.4, and 3.8±0.5, respectively) (P<0.05). The remaining pharmacodynamic parameters for the BIS and CSI were similar.

CONCLUSION

The overall performance of the BIS and CSI during sevoflurane anesthesia was similar despite major differences in their algorithms. However, the CSI was more consistent and more sensitive to changes in sevoflurane concentration, whereas the measured BIS seemed to respond faster. The newly developed combined E(max) model adequately described the clinical data, including the pharmacodynamic plateau.

The Development of a System to Guide Volatile Anaesthetic Administration

We have developed and deployed within our operating rooms a system which provides real-time estimates of effect-site levels of inhalational anaesthetic agents along with forward predictions of end-tidal and effect-site concentrations. The initial aim of this project was to provide users of inhalational agents with tools similar to those available in target-controlled infusion systems. This paper describes the development and implementation of the system and outlines evaluation and uses of the system.

The prototype was developed by combining a locally developed data logging and trend display system with a model of uptake developed as a teaching tool in 1982. This uptake model performs as well as contemporary models of propofol uptake and distribution. Following initial evaluation, the system has been deployed in over half our operating rooms and uses data gathered from the Datex/GE Anaesthesia Delivery Unit anaesthetic machines. We have conducted a number of studies of the system itself, explored aspects of the underlying models, and used the system to investigate effect-site guided anesthesia and as a tool for data collection in other studies.

The system has been well accepted locally and has been shown to facilitate faster changes in inhalational levels. We have also seen a significant decrease in fresh gas flow rates over recent years and attribute this in part to the predictive system, which simplifies the task of determining the appropriate combination of gas flow and vapour dial setting. The system also provides a useful platform for a range of research projects.

Awareness under general anesthesia

BACKGROUND

Awareness while under general anesthesia, and the later recall of what happened during surgery, can be experienced by patients as horrific events that leave lasting mental trauma behind. Patients may have both auditory and tactile perception, potentially accompanied by feelings of helplessness, inability to move, pain, and panic ranging to an acute fear of death. For some patients, the experience of awareness under anesthesia has no sequelae; for others, however, it can lead to the development of post-traumatic stress disorder, consisting of complex psychopathological phenomena such as anxiety, insomnia, nightmares, irritability, and depression possibly leading to suicidality.

METHODS

The literature on the subject was selectively reviewed.

RESULTS

In the absence of risk factors awareness phenomena occur in one to two per 1000 operations under general anesthesia (0.1% to 0.2%) and are thus classed as an occasionally occurring critical event. In children, the risk of such phenomena occurring is 8 to 10 times higher. These phenomena are due to an inadequate depth of anesthesia with incomplete unconsciousness. They can be promoted by a number of risk factors that are either patient-related (ASA class III or above, medication abuse), surgery-related (Caesarean section, emergency procedures, surgery at night), or anesthesia-related (anesthesia without benzodiazepines, use of muscle relaxants).

CONCLUSION

Strategies for avoiding awareness phenomena under anesthesia include the training of staff to know about the problem and, specifically, the use of benzodiazepines, the avoidance of muscle relaxants if possible, and shielding the patient from excessive noise. EEG monitoring is effective but provides no guarantee against awareness. If awareness under anesthesia occurs despite these measures, the patient must be given expert, interdisciplinary treatment as soon after the event as possible in order to minimize its potential sequelae.

Pharmacokinetic-pharmacodynamic modeling in anesthesia, intensive care and pain medicine

PURPOSE OF REVIEW

Studies from the anesthesiology literature published in the last 2 years were selected to illustrate the most important developments in the field of pharmacokinetic-pharmacodynamic modeling.

RECENT FINDINGS

The pharmacokinetic models focused on incorporating covariate, especially age for pediatric-geriatric use, and altered physiological states. The pharmacodynamic models studied the effect of rate of anesthetic administration, age, experimental conditions, and delay within the monitor on estimation of drug concentration in the biophase. Models for the surrogate measure of the components of general anesthesia, hypnosis (bispectral index scale, entropy), immobility (limb tetanic stimulus-induced withdrawal reflex) and antinociception (surgical stress index, skin conductance algesimeter) were developed and validated. Response surface models were used to study drug interactions for important end-points during surgery and also to optimize dosing of anesthetic agents to maximize the desired/undesired effect ratio. The models for target-controlled infusions were improved by incorporating more covariates, and the closed-loop system was refined by using adaptive controllers that individualize the pharmacokinetic/pharmacodynamic parameters to the particular patient by using Bayesian, Kalman filters, fuzzy logic or neural networks.

SUMMARY

Progress was made by improving population pharmacokinetic/pharmacodynamic models, developing new indexes to measure drug effect and using them in an adaptive delivery system to the individual patient.

Recovery profiles from dexmedetomidine as a general anesthetic adjuvant in patients undergoing lower abdominal surgery

BACKGROUND

Dexmedetomidine induces less change in hemodynamic values during the extubation period. This drug may be useful in anesthetic management requiring smooth emergence from anesthesia. We sought to determine the effects of co-administration of dexmedetomidine on the recovery profiles from sevoflurane and propofol, which usually provide safe and rapid recovery when administered alone.

METHODS

Sixty patients undergoing lower abdominal surgery were randomly divided into four groups according to the anesthetic to be administered; namely, sevoflurane (group S), propofol (group P), both sevoflurane and dexmedetomidine (group SD), or propofol and dexmedetomidine (group PD) as maintenance general anesthetics. After induction, anesthesia was maintained with sevoflurane (0.6%-1.5%) in group S, propofol (2-5 mg/kg/h) in group P, sevoflurane and dexmedetomidine (1 microg/kg over 10 min followed by 0.4 microg/kg/h until the end of surgery) in group SD, and propofol and dexmedetomidine in group PD with continuous epidural infusion. Bispectral Index values were maintained within 45 +/- 5 by changing the concentration of sevoflurane or the infusion rate of propofol in all groups. The time between the interruption of maintenance general anesthetics and eye opening was measured. Postoperative cognitive function was evaluated using the Short Orientation Memory Concentration Test.

RESULTS

The time to eye opening of groups S (8.5 +/- 2.5 min, mean +/- SD; n = 15) and SD (12.0 +/- 3.3 min) were comparable, whereas that of group PD (21.7 +/- 7.1 min) was longer than that of group P (11.0 +/- 4.4 min). The time to eye opening of group PD was significantly (P < 0.001) longer than those of the other three groups. The scores of Short Orientation Memory Concentration Test between groups S and P were similar and were not changed by co-administration of dexmedetomidine.

CONCLUSION

When co-administered with dexmedetomidine, sevoflurane produced a shorter time to eye opening than propofol. Postoperative cognitive function was not affected by dexmedetomidine administration. These results suggest dexmedetomidine may delay recovery when given as an adjuvant to propofol during total i.v. anesthesia.

Awareness with recall during general anaesthesia: a prospective observational evaluation of 4001 patients

BACKGROUND

We have prospectively evaluated the incidence and characteristics of awareness with recall (AWR) during general anaesthesia in a tertiary care hospital.

METHODS

This study involves a prospective observational investigation of AWR in patients undergoing general anaesthesia. Blinded structured interviews were conducted in the postanaesthesia care unit, on postoperative day 7 and day 30. Definition of AWR was 'when the patient stated or remembered that he or she had been awake at a time when consciousness was not intended'. Patient characteristics, perioperative, and drug-related factors were investigated. Patients were classified as not awake during surgery, AWR, AWR-possible, AWR-not evaluable. The perceived quality of the awareness episode, intraoperative dreaming, and sequelae were investigated. The anaesthetic records were reviewed to search for data that might explain the awareness episode.

RESULTS

The study included 4001 patients. Incidence of AWR was 1.0% (39/3921 patients). If high risk for AWR patients were excluded, the incidence was 0.8%. After the interview on the seventh day, six patients denied having been conscious during anaesthesia; hence, the incidence of AWR in elective surgery was 0.6%. Factors associated with AWR were: anaesthetic technique incidence of 1.1% TIVA-propofol vs 0.59% balanced anaesthesia vs 5.0% O2/N2O-based anaesthesia vs 0.9% other anaesthetic techniques (mainly propofol boluses for short procedures), P=0.008; age (AWR 42.3 yr old vs 50.6 yr old, P=0.041), absence of i.v. benzodiazepine premedication (P=0.001), Caesarean section (C-section) (P=0.019), and surgery performed at night (P=0.013). More than 50% of patients reported intraoperative dreaming in the early interview, mainly pleasant. Avoidable human factors were detected from the anaesthetic records of most patients. Subjective auditory perceptions prevailed, together with trying to move or communicate, and touch or pain perception.

CONCLUSIONS

A relatively high incidence of AWR and dreams during general anaesthesia was found. Techniques without halogenated drugs showed more patients. The use of benzodiazepine premedication was associated with a lower incidence of AWR. Age, C-section with general anaesthesia, and surgery performed at night are risk factors.

Simulated drug administration: an emerging tool for teaching clinical pharmacology during anesthesiology training

A thorough understanding of the dose-response relationship is required for optimizing the efficacy of anesthetics while minimizing adverse drug effects. Nowadays, except for the inhaled anesthetics (for which end-tidal concentrations can be measured online), most of the drugs used in clinical anesthesia are administered using standard dosing guidelines, without giving due consideration to their pharmacokinetics and dynamics in guiding their administration. Various studies have found that introducing pharmacokinetics and pharmacodynamics as part of the inputs in clinical anesthesiology could lead to better patient care. With this in mind, it is extremely important that clinicians understand and apply the principles of clinical pharmacology that determine the time course of a drug's disposition and effect. Clinical pharmacology is one of the most challenging topics to teach in anesthesiology. The development of simulators to illustrate the time course of a drug's disposition and effect provides online visualization of pharmacokinetic-pharmacodynamic information during the clinical use of anesthetics. The aim of this review is to discuss the importance of simulation as a clinical pharmacology teaching tool for trainees in anesthesiology.

Applying a physiological model to quantify the delay between changes in end-expired concentrations of sevoflurane and bispectral index

BACKGROUND

The delay between changes in end-expired sevoflurane concentrations and bispectral index (BIS) may be characterized by a 'rate constant' (ke0). A smaller ke0 reflects a longer delay. Values for ke0 vary substantially among studies. The question arises how ke0 depends on experimental conditions, including ventilation and apparatus.

METHODS

Increasing and decreasing sevoflurane concentrations were cyclically delivered to our validated model. First, we quantified theoretical ke0 values for distinct alveolar ventilations, estimating ke0 from sevoflurane tensions in alveolar space and grey matter. Secondly, we investigated the impact of experimental conditions. To predict BIS, the model was extended with a pharmacodynamic section, including ke0. Known values, matching theoretical values, were assigned to this ke0. These were recovered from end-expired concentrations and BIS. Possible determinants of error (difference between assigned and recovered ke0) were varied, that is fraction of dead space gas in end-expired gas (d), and time delays in measuring BIS (tBIS) and end-expired concentrations (tEE).

RESULTS

Theoretical ke0s were 0.7, 0.53, 0.35, and 0.2 min(-1) for an arterial Pco2 of 8, 6.67, 5.33 (normocapnia), and 4 kPa, respectively. For spontaneous ventilation, ke0 = 0.53 min(-1). Recovered ke0s depended on d and Deltat (= tBIS - tEE) and were smaller than assigned values (if Deltat > 0). Errors increased with increasing d and Deltat. For normocapnia, ke0 was between 0.32 and 0.23 min(-1) (d = 0.1; any Deltat = 0-60 s). For spontaneous ventilation, ke0 was between 0.51 and 0.40 min(-1) (d = 0-0.1; Deltat = 5-20 s).

CONCLUSIONS

Published ke0s (0.22-0.53 min(-1)), including our own for sevoflurane-depressed spontaneous ventilation (0.48 min(-1)), are in the ranges dictated by investigation-specific conditions.

Multi-level approach to anaesthetic effects produced by sevoflurane or propofol in humans: 2. BIS and tetanic stimulus-induced withdrawal reflex †

BACKGROUND

General anaesthesia could be assessed at two sites: cortical structures and the spinal cord. However, the practicalities of measurement at these two sites differ substantially.

METHODS

We simultaneously analysed effects of sevoflurane (Group S; n = 16) or propofol (Group P; n = 17) on bispectral index (BIS) and the tetanic stimulus-induced withdrawal reflex (TIWR). TIWR was quantified by the area under the curve of the electromyogram of the biceps femoris muscle after electrical stimulation of the sural nerve. After loss of consciousness, TIWR was evoked once per minute. The anaesthetic was increased until TIWR disappeared. After discontinuation of the anaesthetic and reappearance of TIWR, the amount of anaesthetic was increased again. Using a sigmoid E(max) model and a first-order rate constant k(e0), we characterized the dose-response relationships for BIS and TIWR.

RESULTS

Concentration-dependent depression of TIWR was reasonably well modelled for sevoflurane, but poorly for propofol. TIWR was completely suppressed by sevoflurane, but not propofol. Sevoflurane reduced TIWR to 5 mV ms (very weak movement) at 1.68 vol% end-expired concentration [approximately minimum alveolar concentration (MAC value)]. The k(e0)s for TIWR were smaller than those for BIS: 0.25 (0.16-0.39) vs 0.41 (0.33-0.51) min(-1) for Group S; 0.25 (0.22-0.30) vs 0.34 (0.29-0.40) min(-1) for Group P [geometric mean (95% CI)].

CONCLUSIONS

High concentrations of sevoflurane depress TIWR more than propofol. With propofol, we frequently observed a paradoxical behaviour of muscles of the lower leg. TIWR lags behind BIS, indicating different effect sites for two intended anaesthetic effects: unresponsiveness to noxious stimulation and unconsciousness.