A comparison of bispectral index and entropy monitoring, in patients undergoing embolization of cerebral artery aneurysms after subarachnoid haemorrhage

Background. Processed EEG monitoring of anaesthetic depth couldbe useful in patients receiving general anaesthesia followingsubarachnoid haemorrhage. We conducted an observational studycomparing performance characteristics of bispectral index (BIS)and entropy monitoring systems in these patients. Methods. Thirty-one patients of the World Federation of Neurosurgeonsgrades 1 and 2, undergoing embolization of cerebral artery aneurysmsfollowing acute subarachnoid haemorrhage, were recruited tohave both BIS and entropy monitoring during general anaesthesia.BIS and entropy indices were matched to clinical indicatorsof anaesthetic depth. Anaesthetists were blinded to the anaestheticdepth monitoring indices. Analysis of data from monitoring devicesallowed calculation of prediction probability (P_K) constants,and receiver operating characteristic (ROC) analysis to be performed. Results. BIS and entropy [response entropy (RE), state entropy(SE)] performed well in their ability to show concordance withclinically observed anaesthetic depth. P_K values were generallyhigh (BIS 0.966–0.784, RE 0.934–0.663, SE 0.857–0.701)for both forms of monitoring. ROC curve analysis shows a highsensitivity and specificity for all monitoring indices whenused to detect the presence or absence of eyelash reflex. Areaunder curve for BIS, RE and SE to detect the absence or presenceof eyelash reflex was 0.932, 0.888 and 0.887, respectively.RE provides earlier warning of return of eyelash reflex thanBIS. Conclusion. BIS and entropy monitoring perform well in patientswho receive general anaesthesia after good grade subarachnoidhaemorrhage.     

Influence of volatile anaesthetics on hypercapnoeic ventilatory responses in mice with blunted respiratory drive

Background. Subanaesthetic concentrations of volatile anaestheticssignificantly affect the respiratory response to hypoxia andhypercapnoeia. Individuals with an inherited blunted respiratorydrive are more affected than normal individuals. To test thehypothesis that subjects with blunted hypercapnoeic respiratorydrive are diversely affected by different anaesthetics, we studiedthe effects of three volatile anaesthetics on the control ofbreathing in C3H/HeJ (C3) mice, characterized by a blunted hypercapnoeicrespiratory response. Methods. Using whole body plethysmography, we assessed respiratoryrate (RR) and pressure amplitude in 11 male C3 mice at rest,during anaesthesia with isoflurane, sevoflurane or desflurane,and during recovery. To test respiratory drive, mice were exposedto 8% carbon dioxide. Data were analysed by two-way-analysisof variance with post hoc tests and Bonferroni correction. Results. RR was unaffected during sevoflurane anaesthesia upto 1.0 MAC. Likewise, sevoflurane at 1.5 MAC affected RR lessthan either isoflurane (P=0.0014) or desflurane (P=0.0048).The increased RR to a carbon dioxide challenge was blocked byall three anaesthetics even at the lowest concentration, andremained depressed during recovery (P<0.0001). Tidal volumewas unaffected by all three anaesthetics. Conclusions. In C3 mice, spontaneous ventilation was less affectedduring sevoflurane compared with either isoflurane or desfluraneanaesthesia. However, the RR response to hypercapnoeia was abolishedat 0.5 MAC for all the anaesthetic agents and remained depressedeven at the end of recovery. Our data suggest that differentvolatile anaesthetics have varying effects on the control ofbreathing frequency but all block the respiratory response tocarbon dioxide. Therefore, a genetic predisposition to a bluntedcarbon dioxide response represents a susceptibility factor thatinteracts with hypercapnoeic hypoventilation during maintenanceof anaesthesia and in the emergence from anaesthesia, regardlessof the agent used. Br J Anaesth 2004; 92: 697–703     

Spinal anaesthesia indirectly depresses cortical activity associated with electrical stimulation of the reticular formation

Background. Neuraxial blockade reduces the requirements forsedation and general anaesthesia. We investigated whether lidocainespinal anaesthesia affected cortical activity as determinedby EEG desynchronization that occurs following electrical stimulationof the midbrain reticular formation (MRF). Methods. Six goats were anaesthetized with isoflurane, and cervicallaminectomy performed to permit spinal application of lidocaine.The EEG was recorded before, during and after focal electricalstimulation (0.1, 0.2, 0.3 and 0.4 mA) in the MRF while keepingthe isoflurane concentration constant. Results. During lidocaine spinal anaesthesia, the spectral edgefrequency (SEF) after MRF electrical stimulation (13.6 (SD 1.0)Hz, averaged across all stimulus currents) was less than theSEF during control and recovery periods (18.6 (3.6) Hz and 17.2(2.2) Hz, respectively; P<0.05). Bispectral index valueswere similarly affected: 69 (10) at control compared with 55(6) during the spinal block (P<0.05). Conclusions. These results suggest that lidocaine spinal anaesthesiablocks ascending somatosensory transmission to mildly depressthe excitability of reticulo–thalamo–cortical arousalmechanisms. Br J Anaesth 2003; 91: 233–8     

Recovery of memory after general anaesthesia: clinical findings and somatosensory evoked responses

Background. Mid-latency somatosensory evoked responses are usedto monitor the integrity of the sensory pathways intra-operatively.They can quantify the effects of anaesthetics on the centralnervous system. Mid-latency auditory evoked responses have beenrelated to cognition during anaesthesia, but there are no detailedstudies using median nerve somatosensory evoked responses (MnSSER). Methods. We studied 49 patients during recovery from generalanaesthesia (isoflurane/nitrous oxide or propofol) to assessimplicit and explicit memory function in relation to mid-latencyMnSSER. The MnSSER recordings were made before anaesthesia,during steady-state anaesthesia, and at the end of the recoveryperiod. The patients were interviewed 24 h later about theirmemory for the immediate wake up phase. Statistical analysiswas by multivariate analysis of variance. Results. Out of 49 patients, 23 recalled the recovery period,11 had implicit memory for an object shown to them during therecovery period, and 15 did not have any memory for the recoveryperiod. At RECOVERY the patients with recall had significantlyshorter MnSSER latencies N45 and P50 and inter-wave conductiontimes LatN35 – LatP45 than patients without memory (P<0.05). Conclusions. We conclude that MnSSER components warrant furtherinvestigation for studying the effects of anaesthetic drugson cognitive function. Br J Anaesth 2002; 88: 362–8     

Effect of tramadol on electroencephalographic and auditory-evoked response variables during light anaesthesia

Tramadol is a centrally acting opioid-like analgesic commonlyused for analgesia during surgery. It has been stated that theuse of tramadol increases the risk of awareness during anaesthesia.We studied 29 patients under steady state anaesthesia, ventilatedvia a laryngeal mask airway with 0.6 MAC isoflurane in50% nitrous oxide, and with no surgical stimulus. The electroencephalogram(EEG) and auditory-evoked response (AER) were recorded throughoutthe study period, as were pulse and arterial pressure. Patientswere given randomly a bolus of either saline (S), tramadol 100 mg(T1), or tramadol 200 mg (T2). Significant increases insystolic arterial pressure and decreases in heart rate wereseen in the tramadol groups compared to the saline group. Significant,dose-related activation in all EEG variables (median power frequency,spectral edge, Delta Power and Alpha/Delta ratio) but no significantchange in Pa or Nb amplitudes or latencies were noted. The EEGchanges were not at levels thought to be associated with awareness.This study indicates that tramadol, whilst causing EEG activation,has no effect on depth of anaesthesia as measured by the AER. Br J Anaesth 2000; 85: 705–7 ^* Corresponding author     

The Patient State Index as an indicator of the level of hypnosis under general anaesthesia

Background. This retrospective study describes the performanceof the Patient State Index (PSI), under standard clinical practiceconditions. The PSI is comprised of quantitative features ofthe EEG (QEEG) that display clear differences between hypnoticstates, but consistency across anaesthetic agents within thestate. Methods. The PSI was constructed from a systematic investigationof a database containing QEEG extracted from the analyses ofcontinuous 19 channel EEG recordings obtained in 176 surgicalpatients. Induction was accomplished with etomidate, propofol,or thiopental. Anaesthesia was maintained by isoflurane, desflurane,or sevoflurane, total i.v. anaesthesia using propofol, or nitrousoxide/narcotics. It was hypothesized that a multivariate algorithmbased on such measures of brain state, would vary significantlywith changes in hypnotic state. Results. Highly significant differences were found between meanPSI values obtained during the different anaesthetic statesselected for study. The relationship between level of awarenessand PSI value at different stages of anaesthetic delivery wasalso evaluated. Regression analysis for prediction of arousallevel using PSI was found to be highly significant for the combinationof all anaesthetics, and for the individual anaesthetics. Conclusions. The PSI, based upon derived features of brain electricalactivity in the anterior/posterior dimension, significantlyco-varies with changes in state under general anaesthesia andcan significantly predict the level of arousal in varying stagesof anaesthetic delivery. Br J Anaesth; 2004 92: 393–9     

Is prolongation of the QTc interval during isoflurane anaesthesia more prominent in women pretreated with anthracyclines for breast cancer?

Background. Inhalation anaesthetics and anthracycline chemotherapeuticdrugs may both prolong the QT interval of the electrocardiogram.We investigated whether isoflurane may induce or augment QTcprolongation in patients who had previously received cancerchemotherapy including anthracycline drugs. Methods. Forty women undergoing surgery for breast cancer wereincluded in the study. They were divided into two groups: (A)women previously treated with anthracyclines (n=20); and (B)women not treated with antineoplastic drugs (n=20). All patientsreceived a standardized balanced anaesthetic in which isoflurane0.5 vol% was used. The QT and corrected QT intervals were measuredbefore anaesthesia, after induction and tracheal intubation,after 1, 5, 15, 30, 60 and 90 min of anaesthesia, and duringrecovery. Results. In both groups we observed a tendency to QTc prolongation,but statistically significant differences among baseline valuesand values observed during isoflurane-containing anaesthesiawere seen only in group A. During anaesthesia, significant differencesin QTc values between the two groups were observed. Conclusion. In female patients pretreated with anthracyclinesfor breast cancer, the tendency to QTc prolongation during isoflurane-containinggeneral anaesthesia was more strongly expressed than in patientswithout previous chemotherapy. Br J Anaesth 2004; 92: 658–61     

Isocapnic hyperpnoea accelerates recovery from isoflurane anaesthesia

Background. Hyperventilation should speed up elimination ofvolatile anaesthetic agents from the body, but hyperventilationusually results in hypocapnia. We compared recovery from isofluraneanaesthesia in patients allowed to recover with assisted spontaneousventilation (control) and those treated with isocapnic hyperpnoea. Methods. Fourteen patients were studied after approximately1 h of anaesthesia with isoflurane. Control patients were allowedto recover in the routine way. Isocapnic hyperpnoea patientsreceived 2–3 times their intraoperative ventilation usinga system to maintain end tidal PCO₂ at 45–50 mm Hg. Wemeasured time to removal of the airway and rate of change ofbispectral index (BIS) during recovery. Results. With isocapnic hyperpnoea, the time to removal of theairway was markedly less (median and interquartile range valuesof 3.6 (2.7–3.7) vs 12.1 (6.8–17.2) min, P<0.001);mean (SD) BIS slopes during recovery were 11.8 (4.4) vs 4.3(2.7) min^–1 (P<0.01) for isocapnic hyperpnoea and controlgroups, respectively. Isocapnic hyperpnoea was easily appliedin the operating room. Conclusions. Isocapnic hyperpnoea at the end of surgery resultsin shorter and less variable time to removal of the airway afteranaesthesia with isoflurane and nitrous oxide. Br J Anaesth 2003; 91: 787–92     

The Bispectral Index in children: comparing isoflurane and halothane

Background. The Bispectral Index (BIS) has been calibrated forseveral general anaesthetic agents including isoflurane. Halothaneis still used in paediatric anaesthesia. Compared with othervolatile anaesthetics, halothane has a different receptor affinityand differing effects on the EEG. There are limited data evaluatingthe BIS with halothane. We set out to compare the BIS usinghalothane and isoflurane at a clinically relevant equipotentconcentration (1 MAC) and at a reproducible measure of anaestheticeffect (awakening). Methods. Forty children aged between 2 and 15 yr were enrolledin a masked randomized trial—20 in each group. Anaesthesiawas induced with sevoflurane or propofol. Either halothane orisoflurane were given to obtain an end-tidal concentration of1 MAC for 15 min. The BIS was then recorded. The BIS was alsorecorded at awakening. Values (mean (SD)) were compared witha t test. Results. At 1 MAC the BIS for halothane was significantly greaterthan isoflurane (56.5 (8.1) vs 35.9 (8.5), P<0.0001). Atawakening there was no significant difference (BIS halothane;81.1 (11.9), BIS isoflurane; 82.5 (16.4)). The difference inmeans at awakening was 1.4 (95% CI –8.2 to 11.1). Conclusions. At equipotent concentrations of halothane and isofluraneBIS valves were significantly greater with halothane. At awakeningthe BIS values were equivalent for each agent. This findingis consistent with the BIS being more affected by the agentused at higher concentrations of anaesthetic. The BIS must beinterpreted with caution when using halothane. Br J Anaesth 2004; 92: 14–17     

Effects of isoflurane and propofol on cortical somatosensory evoked potentials during comparable depth of anaesthesia as guided by bispectral index

Background. The aim of this study was to determine if propofolcaused less suppression of cortical somatosensory evoked potentials(SSEPs) during spine surgery compared with isoflurane duringcomparable depth of anaesthesia as guided by bispectral index(BIS) measurements. Methods. This was a randomized controlled trial of propofoland isoflurane involving 60 patients undergoing elective spinesurgery. BIS monitoring was used to guide a consistent and comparabledepth of anaesthesia, the index was maintained at between 40and 50 during anaesthesia. The cortical SSEP P40-N50 peak-to-peakamplitude and latency time to the P40 peak were measured beforeinduction of anaesthesia, after induction of anaesthesia, atthe start of skin incision, at the start of pedicle screw insertionand at the start of rod insertion, by a neurophysiologist blindedto drug allocation. Results. Both propofol and isoflurane decreased SSEP amplitudeand increased latency during the course of anaesthesia. Afterachieving a comparable depth of anaesthesia, the SSEP amplitudewas significantly lower with isoflurane, 1.5 (1.0) vs 2.4 (1.4)µV (P=0.005). Latency was significantly longer with isoflurane,39.5 (3.9) vs 37.3 (3.1) ms (P=0.024). Isoflurane was associatedwith greater variability of SSEP amplitude during the courseof anaesthesia and surgery, coefficient of variation 35.4 (18.0)vs 21.2 (10.2)% (P=0.008). Conclusions. Propofol anaesthesia caused less suppression ofthe cortical SSEP, with better preservation of SSEP amplitude,and less variability at an equivalent depth of anaesthesia.     

Uptake of isoflurane during prolonged clinical anaesthesia

Recent evidence has suggested that the rate of uptake of inhalationalanaesthetic is constant during maintenance of anaesthesia, contraryto the predictions of multi-compartment uptake models. We measuredisoflurane uptake using a totally closed anaesthetic systemduring up to 10 h of stable anaesthesia for maxillo-facial surgeryon 12 adult patients. Liquid isoflurane was injected into thesystem under computer control to produce an end tidal concentrationof 1.3 MAC of isoflurane. Bench tests demonstrated that theleakage from the system was less than 8 µl min^–1,confirming that the rate of injection of isoflurane into thesystem was a close upper bound on the patients’ uptake.Anaesthetic usage for a 70 kg patient was 0.44e^–0.51t+0.044e^–0.013t+0.058e^–0.00098t ml min^–1 ofliquid isoflurane, where t is duration of anaesthesia in minutes.There was a continuing reduction in anaesthetic requirementeven at the end of the period of study that was statisticallysignificant. These data do not support the notion that isofluraneuptake is constant during stable maintenance of anaesthesiabut is compatible with the conventional multi-compartment modelof anaesthetic uptake and distribution. Br J Anaesth 2001; 86: 645–9     

Does bispectral analysis of the electroencephalogram add anything but complexity?

Background. Analysis of the bispectrum of EEG waveforms is acomponent of the proprietary BIS index—a commonly usedcommercial monitor of depth of anaesthesia. Does the use ofthe bispectrum give more information about depth of anaesthesiathan the power spectrum? Methods. We collected and analysed EEG waveforms during inductionof general anaesthesia in 39 patients, comparing the changesin bispectral parameter (SynchFastSlow), with an analogous powerspectrum-based parameter (PowerFastSlow). Both compare the logarithmicratio of high frequency components (40–47 Hz) with thetotal (1–47 Hz). Because the changes in bispectrum areaffected by signal amplitude, we also calculated a third parameter(SFSbicoh) from the bicoherence, which is an amplitude-independentstatistic. Results. The SynchFastSlow and PowerFastSlow were correlated(r=0.84) and neither was superior in predicting the awake oranaesthetized state (area under receiver operating characteristiccurves = 0.85 vs 0.93). There was no change in the SFSbicohover the induction period, and it did not correlate with SynchFastSlow(r=0.07). Conclusions. We could not show that bispectral analysis gavemore information than power spectral-based analysis. Most ofthe changes in the bispectral values result from decreases inthe relative high frequency content of the EEG caused by anaesthesia. Br J Anaesth 2004; 92: 8–13     

Changes in cortical electrical activity during induction of anaesthesia with thiopental/fentanyl and tracheal intubation: a quantitative electroencephalographic analysis

Background. There are regional differences in the effects ofanaesthetics agents and perioperative stimuli on the EEG. Westudied the topography of the EEG during induction of anaesthesiaand intubation in patients receiving thiopental and fentanylto document regional electrical brain activity. Methods. EEG was recorded in 25 patients in the awake state,after pre-medication, during induction, at loss of consciousnessand after intubation. Eight bipolar recordings were made andthe relative power of the frequency bands delta, theta, alpha,and beta were used (after z-score transformation for age) tomeasure changes in regional EEG activity. Results. Noxious stimulation during tracheal intubation partiallyreversed the slowing of the EEG caused by anaesthesia. Duringinduction of anaesthesia alpha activity was most reduced intemporal and occipital regions. The most prominent EEG changesafter intubation were an increase in alpha and a decrease indelta power (P<0.001). The largest changes were in the frontaland temporal leads for alpha and in the frontal and centralleads for delta. Heart rate and arterial pressure remained constantduring intubation. Conclusions. Changes in alpha and delta power were identifiedas the most sensitive EEG measures of regional changes in electricalbrain activity during anaesthesia and noxious stimulation. Br J Anaesth 2004; 92: 33–8     

The effects of sevoflurane are similar to those of isoflurane on the neuromuscular block produced by vecuronium

We have examined the interactions of 1 MAC of isoflurane andsevoflurane (and 66% nitrous oxide in oxygen) with vecuronium,using the EMG response of the abductor digiti minimi to train-of-four(TOF) stimulation of the ulnar nerve. We constructed dose-responsecurves for vecuronium in 54 patients. The curves for both isofluraneand sevoflurane had a significant leftward shift compared withthat for fentanyl-nitrous oxide anaesthesia (P < 0.01). Whenthe amplitudes of the first response (T1) had recovered to 50%of control in another 32 patients, subsequently we comparedthe spontaneous recovery rate of the ratio of the fourth tothe first TOF response (T4:T1) at 3-min intervals during the15-min period, in the presence of two volatile anaestheticsor after discontinuation of administration of anaesthetic. Therate of recovery of T4:T1 was significantly greater when bothanaesthetics were discontinued. However, this rate was similarfor both anaesthetics, suggesting that the mechanism of actionof the two anaesthetics is similar. (Br. J. Anaesth. 1994; 72:465–467)     

Entropy of EEG during anaesthetic induction: a comparative study with propofol or nitrous oxide as sole agent

Background. The search continues for an anaesthetic monitorthat can define the level of anaesthesia in an individual patientirrespective of anaesthetic agent(s) used. Studies of availablemonitors based on bispectral analysis or evoked auditory potentialsshow the complexity of the problem. We assessed a new monitor,based on the entropy of the EEG, during induction of anaesthesiawith either propofol or nitrous oxide. Methods. In an open, randomized study (two groups; n=10) ofday surgical patients, we induced loss of response with incrementalboluses of propofol. The other group was given propofol 30 mgand then increasing concentrations of nitrous oxide until lossof response. We measured entropy with the M-Entropy Module S/5^TM(Datex-Ohmeda) using forehead electrodes and recorded responseentropy (RE; including frontal electromyogram) and state entropy(SE; only the cortical EEG). Values are median (range). Results. Baseline values were RE 98 (96–100), SE 89 (87–91)and RE 98 (96–99), SE 89 (87–91) for the propofoland nitrous oxide patients, respectively. During propofol induction,both entropy indices decreased with increasing sedation, withRE 40 (23–76) and SE 34 (17–70) at loss of response.Neither RE nor SE decreased during nitrous oxide inhalation,and at loss of response using nitrous oxide, RE and SE wereunchanged at 98 (96–100) and 88 (85–91) respectively. Conclusions. The entropy monitor of anaesthetic depth showsa successive decrease with propofol but loss of consciousnesswith nitrous oxide is not associated with change in entropyindices. Br J Anaesth 2004; 92: 167–70     

Nitrous oxide decreases shivering threshold in rabbits less than isoflurane

Background. Comparable minimum alveolar concentration (MAC)fractions of volatile anaesthetics produce similar thermoregulatoryimpairment. Nitrous oxide, however, decreases the vasoconstrictionthreshold less than sevoflurane or isoflurane. We tested thehypothesis that nitrous oxide also decreases shivering thresholdless than isoflurane alone or in combination. Methods. Twenty-four rabbits were assigned randomly to one ofthree 0.3 MAC anaesthetic regimens: (i) nitrous oxide 69%; (ii)nitrous oxide 35% and isoflurane 0.3%; or (iii) isoflurane 0.6%.Body temperature was lowered by perfusing 10°C water througha U-shaped thermode positioned in the colon. Shivering was evaluatedby inspection. Results. The rabbits anaesthetized with nitrous oxide aloneshivered at 37.0 (0.5)°C (P<0.01 vs other groups). Inthose given the nitrous oxide and isoflurane combination, theshivering threshold was 36.4 (0.5)°C and that in the isofluranegroup was 35.9 (0.4)°C. Conclusion. This study indicates that nitrous oxide reducesthe shivering threshold less than isoflurane. Br J Anaesth 2003; 90: 88–90     

Effect of sevoflurane/nitrous oxide versus propofol anaesthesia on somatosensory evoked potential monitoring of the spinal cord during surgery to correct scoliosis

Background. Use of intraoperative somatosensory evoked potential(SSEP) monitoring is helpful in spinal corrective surgery butmay be affected by anaesthetic drugs. An anaesthetic techniquethat has less effect on SSEP or allows faster recovery is anadvantage. We compared the effects on SSEP and the clinicalrecovery profiles of sevoflurane/nitrous oxide and propofolanaesthesia during surgery to correct scoliosis. Methods. Twenty adolescent patients were randomized into twogroups of 10. One group received sevoflurane–nitrous oxideanaesthesia and the other received propofol i.v. anaesthesia.An alfentanil infusion was used for analgesia in both groups. Results. Changes in anaesthetic concentration produced littleeffect on the latency of SSEP, but the effect on the variabilityof SSEP amplitude was significant (P<0.05). Sevoflurane produceda faster decrease in SSEP and a faster recovery than propofol(P<0.05). On emergence, patients who received sevofluranetended to have shorter recovery times to eye opening (mean 5.1vs 20.6 min, P=0.09) and toe movement (mean 7.9 vs 15.7 min,P=0.22). Those who had received sevoflurane were significantlymore lucid and cooperative in recovery. Conclusions. Sevoflurane produces a faster decrease and recoveryof SSEP amplitude as well as a better conscious state on emergencethan propofol. Br J Anaesth 2002; 88: 502–7     

Emergence and recovery in children after desflurane and isoflurane anaesthesia: effect of anaesthetic duration

Background. We hypothesized that increasing duration of inhalationanaesthesia is associated with slower emergence and recoveryin children, and that this effect would be less marked withdesflurane in comparison with isoflurane. Methods. Fifty-four infants and children assigned in groupsaccording to age and expected length of operation were prospectivelyrandomized to receive either isoflurane (I) or desflurane (D)for anaesthesia. After standard induction, the anaesthesia wasmaintained using an age-related 1.0 minimum alveolar concentration(MAC) equivalent for either agent in air and oxygen. Local analgesiawas used as appropriate. End-tidal volatile agent concentrationwas recorded until extubation. Clinical evaluation of recoverywas made by observers, blinded to group allocation. Results. For patients <4 yr of age, the median (95% CI) timesin minutes to first movement [5.27 (D), 9.22 (I)], eye opening[9.42(D), 13.3(I)] and extubation [7.18 (D), 12.5 (I)] weresignificantly shorter (P<0.05) for desflurane. In the group>4 yr of age, the median (95% CI) times in minutes to firstmovement [4.42 (D), 11.6 (I)], eye opening [8.55(D), 18.0(I)]and extubation [7.08 (D), 16.7 (I)] were significantly shorter(P<0.001) for desflurane. Times to leave recovery were notsignificantly different for the group <4 yr of age, but weresignificantly shorter for desflurane in the group >4 yr ofage (P<0.01). The isoflurane, but not desflurane, had a time-dependenteffect on arousal. There were no significant differences inincidence of airway irritation or emergence delirium betweenthe two agents. Conclusions. The rate of recovery in children after exposureto desflurane was faster than those patients receiving isoflurane;recovery from desflurane, but not isoflurane, was relativelyunaffected by the duration of anaesthesia.     

Relationship between awareness and middle latency auditory evoked responses during surgical anaesthesia

Background. Some studies support the view that meaningful auditoryinput can be processed by the brain during apparent surgicalanaesthesia. Consequently, patients may be able to remembersome information implicitly after anaesthesia as well througha ‘dream-like process’ (subconscious awareness).The aim of this study was to investigate the presence of subconsciousawareness during anaesthesia and to examine its relationshipto the mid-latency auditory evoked responses (MLAERs). Methods. We studied 40 patients, ASA I–II, undergoinglaparoscopic cholecystectomy. General anaesthesia was inducedwith thiopental 5 mg kg^–1, fentanyl 3 µg kg^–1,and vecuronium 0.08 mg kg^–1. For the maintenance of anaesthesia,patients were randomly assigned to one of four anaesthetic regimengroups: sevoflurane+air in oxygen 40%; sevoflurane+nitrous oxide60%; isoflurane+air in oxygen 40%; and isoflurane+nitrous oxide60%. MLAERs were recorded before anaesthesia, at 1 MAC of inhaledanaesthetic and then 30 min after awakening. An audiotape withone of four stories was played immediately after intraoperativeMLAER recording. Explicit and implicit memory was assessed 24h after awakening. Results. None of the patients had explicit recall. One of thepatients from the isoflurane–air group showed implicitmemory of listening to the audiotape. A dream-like process,in which they remembered implicitly the story played duringanaesthesia, occurred in one of the patients from the sevoflurane–nitrousoxide group. In the patients with subconscious awareness, MLAERswere similar to that of the awake state with a Pa latency increaseof less than 8.87. When there was a marked increase in Pa latencyduring anaesthesia, no subconscious awareness was observed.No statistically significant differences were found betweenPa latency before and after anaesthesia. Conclusions. MLAERs may help to predict subconscious cerebralprocessing of auditory inputs during anaesthesia. Br J Anaesth 2003; 90: 630–5     

Evoked EEG patterns during burst suppression with propofol

Background. During EEG suppression with isoflurane or sevofluraneanaesthesia, median nerve stimulation causes cortical responsesof two kinds: an N20 wave with a latency of 20 ms and an EEGburst with a latency of 200 ms. We tested the possibility thatmedian nerve stimulation during EEG suppression with propofolwould cause an EEG response that was consistent enough to beof use for neuromonitoring. Methods. Eight patients were anaesthetized with propofol toallow burst suppression. Electrical stimulation of the mediannerve was applied during general anaesthesia and the EEG wasmeasured. Results. The EEG response to a painful stimulus had four successivecomponents: (i) N20 and P22 potentials, reflecting activationof fast somatosensory pathways; (ii) a high-amplitude negativewave, possibly reflecting activation of the somatosensory cortexSII bilaterally; (iii) a burst (i.e. a negative slow wave withsuperimposed 10 Hz activity, probably reflecting an arousalmechanism); and (iv) a 13–15 Hz spindle, probably originatingfrom the thalamus, similar to sleep spindles. These could beseen separately and in different combinations. Bursts and spindlesduring burst suppression were also seen without stimulation.In deepening propofol anaesthesia, spindles were seen in thecontinuous EEG before burst suppression was achieved. In deepanaesthesia, spindles were seen when bursts had ceased, andpainful stimuli evoked sharp waves without subsequent bursts. Conclusion. In addition to SSEP (somatosensory evoked potentials),three different evoked responses are noted that could be usefulfor clinical monitoring. Br J Anaesth 2004; 92: 18–24