Burst Suppression Ratio Research Articles

Prediction of Neurological Outcome Using Bispectral Index Monitoring in Patients with Severe Ischemic-Hypoxic Brain Injury Undergoing Emergency Surgery

Predicting outcome from ischemic-hypoxic brain injury can be difficult in patients rushed to the operating room for time-critical emergency surgery. The authors chose to evaluate the prognostic ability of bispectral index (BIS) in this setting. Twenty-five critically ill, unconscious patients with ischemic-hypoxic brain injury undergoing emergency surgery were prospectively studied. Clinical evaluation, laboratory investigations, BIS, and burst suppression ratio were recorded before and during surgery. Neurologic outcome of the patients was measured according to the Glasgow outcome scale at 30 days after injury, with poor neurologic outcome defined as severe disability or death. The incidence of poor neurologic outcome was 68%. Neither clinical judgment (P = 0.40) nor pupillary responses (P = 0.21) were predictive of neurologic outcome after surgery. An abnormal BIS trace was strongly associated with poor neurologic outcome, positive likelihood ratio 6.6 (95% CI 1.7-36.4; exact test P = 0.002). Some BIS values were significantly different when comparing patients with and without poor outcome: c-statistics for the average BIS and maximal electroencephalographic burst-suppression were 0.80 (95% CI 0.62-0.98; P = 0.017) and 0.84 (95% CI 0.68-0.99; P = 0.007), respectively. A normal BIS (P < 0.0005) but not clinical judgment (P = 0.16) could identify a group of patients more likely to survive with a good neurologic outcome. BIS, when compared with clinical judgment and routine laboratory tests, provides useful information that may identify patients with a good chance of recovery after ischemic-hypoxic brain injury requiring emergency surgery.

Intraoperative use of somatosensory-evoked potentials: is general anaesthesia comparable to awake condition?

Editor, We have read with great interest the article by Fielmuth and Uhlig [1] on the role of somatosensory-evoked potentials (SSEPs) in detecting cerebral ischaemia during carotid endarterectomy. The authors made an assessment of the validity of SSEP monitoring during carotid endarterectomy in awake patients under locoregional anaesthesia. They correctly reported the current substantial variety of SSEP parameters and thresholds related to these SSEP parameters used to detect critical deterioration in cerebral blood flow (CBF). Their study was designed to compare SSEP tracings with the objective clinical–neurological examination in order to evaluate the reliability of the different SSEP-generated criteria. They referred to Shapiro and Aidinis [2], who developed a model, which can be seen as the methodological basis for current neuromonitoring during carotid surgery. This model correlates neuronal integrity and electrical activity with CBF. Using this approach to interpret changes in SSEP tracing as a perfusion-dependent neuronal metabolic situation, different parameters and values have been established to detect a relevant reduction in CBF.

Endogenous Activation of adenosine A 1 receptors promotes post-ischemic electrocortical burst suppression

Following transient global cerebral ischemia (GCI), spontaneous electrocortical activity resumes from the isoelectric line through a sequence of “bursts” of activity alternating with periods of electrical “suppression,” commonly referred to as the post-ischemic burst suppression (BS) pattern. Several lines of evidence suggest that BS reflects an impairment of neocortical connectivity. Here we tested in vivo whether synaptic depression by adenosine A 1 receptor (A 1R) activation contributes to BS patterns following GCI. Male Wistar rats were subjected to 1, 5 or 10 min of GCI using a “four-vessel occlusion” model under chloral hydrate anesthesia. Quantification of BS recovery was carried out using BS ratio. During GCI full electrocortical suppression was attained (BS ratio reached 100%). During the following reperfusion the BS ratio returned to 0. The time course of the decay was exponential after 1 and 5-min GCI and bi-exponential after 10-min GCI. The BS recovery was progressively delayed with the duration of ischemia. Administration of the A 1R antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 1.25 mg/kg i.p.) accelerated the post-ischemic BS recovery for all GCI durations. Following the 10-min GCI the effect of DPCPX was only apparent on the initial fast decay of the BS ratio. These data suggest that endogenous adenosine release promotes BS patterns during reperfusion following transient cerebral ischemia. Furthermore, the endogenous A 1R activation may be the primary underlying cause of post-ischemic BS patterns following brief ischemic episodes. It is likely that synaptic depression by post-ischemic A 1R activation functionally disrupts the connectivity within the cortical networks to an extent that promotes BS patterns.

Comparative effects of halothane, isoflurane, sevoflurane and desflurane on the electroencephalogram of the rat

Inhalant anaesthetic agents are commonly used in studies investigating the electroencephalographic (EEG) effects of noxious stimuli in animals. Halothane causes less EEG depression than isoflurane, however, the EEG effects of halothane, isoflurane, sevoflurane and desflurane have not been compared in the same model. This study aimed to compare the EEG effects of these inhalational agents in the rat. Forty male Sprague-Dawley rats were assigned to four groups and anaesthetized with halothane, isoflurane, sevoflurane or desflurane. EEG was recorded from the left and right somatosensory cortices for 5 min at three different multiples of minimal alveolar concentration (MAC) (1.25, 1.5 and 1.75). Median, 95% spectral edge frequency and total power were derived and a single mean value for each was calculated for the first 60 s of each recording period. When the raw EEG contained burst suppression (BS), the BS ratio (BSR) over 60 s was calculated. No BS was found in EEG recorded from the halothane group at any concentration. BS was present at all concentrations with the other anaesthetic agents. BS was almost complete at all concentrations of isoflurane, whereas BSR increased significantly with increasing concentrations of sevoflurane and desflurane. No significant differences were found between the BSR due to the 1.75 MAC multiple of isoflurane, sevoflurane or desflurane. Halothane causes significantly less depression of cortical activity than the newer inhalant agents at equivalent multiples of MAC. These data support the hypothesis that halothane has a fundamentally different mechanism of action than the other inhalant agents.

Effect-site half-time for burst suppression is longer than for hypnosis during anaesthesia with sevoflurane

The relationship between measures of drug effect such as bispectral index (BIS) and end-tidal (ET) levels of anaesthetic agents is described by the 'effect site equilibrium half-time', t(1/2)(ke0). There are limited data available on sevoflurane t(1/2)(ke0) during routine anaesthesia and surgery. Preliminary observations suggested t(1/2)(ke0) for the degree of hypnosis as estimated by BIS is different from that for burst suppression of the electroencephalograph, occurring at 'deep' levels of anaesthesia. This study aimed to determine and compare t(1/2)(ke0) for these two 'effects'. Large changes in ET sevoflurane were produced in 13 subjects during surgery. ET sevoflurane, BIS, and burst suppression ratio (BSR) were recorded every 10 s. Data were divided into epochs with BIS>30 (BIS) or with BSR>10% (burst suppression). Using a non-parametric modelling technique, t(1/2)(ke0) was determined for each epoch. There were 36 'BIS' and 20 burst suppression zones. Mean (sd) t(1/2)(ke0) for BIS was 3.48 (1.12) min and for BSR 9.9 (6.4) min. In all subjects, t(1/2)(ke0) BIS<t(1/2)(ke0) BSR. These differences were statistically significant (P<0.01). The pooled mean effect-site sevoflurane concentration producing a BIS of 50 was 1.23 (sd 0.34) vol% and for a BSR of 50% was 3.3 (0.50) vol%. There were considerable intra- and inter-subject variabilities. The different values of t(1/2)(ke0) for these effects suggest different sites or mechanisms of action. These results also establish values of t(1/2)(ke0) which can be used to provide the real-time estimates of effect-site sevoflurane concentration in clinical practice.

Quantification of Burst Suppression and Bispectral Index With 2 Different Bolus Doses of Thiopentone Sodium

Metabolic suppression caused by barbiturates is a major mechanism responsible for their cerebral protective potential. Maximal cerebral metabolic suppression is believed to coincide with electroencephalographic burst suppression. However, many neurosurgical procedures associated with cerebral ischemic threat are still performed in the absence of electroencephalogram monitoring, especially in developing nations. The present study was designed to assess the degree of burst suppression with 2 different doses of thiopentone sodium administered on the background of isoflurane anesthesia intraoperatively. Forty-one patients without any intracranial pathology undergoing elective spinal surgery under a general anesthetic consisting of N2O (60%) in O2 (40%) and isoflurane to maintain a bispectral index (BIS) value of 45 were randomized to receive a thiopentone bolus of either 3 or 5 mg/kg. BIS, burst suppression ratio (BSR), systolic blood pressure, and heart rate were recorded before the bolus and every 15 seconds for first 2 minutes and every 30 seconds for another 8 minutes. During the 10-minute study period after the administration of thiopentone bolus, BIS values were significantly lower in the group that received thiopentone 5 mg/kg compared with the group that received thiopentone 3 mg/kg (P<0.02). BSR>25% was seen in 7 out of 21 patients in the 3 mg/kg group and 10 out of 20 patients in the 5 mg/kg group. There was a statistically insignificant prolongation of the duration of burst suppression with thiopentone 5 mg/kg [243 s (range 75 to 435 s)] compared with thiopentone 3 mg/kg [171 s (30 to 465 s)]. The number of patients who had a BSR >50% was higher among patients who received thiopentone 5 mg/kg as compared with those who received a dose of 3 mg/kg [9/20 vs. 3/21(P<0.02)]. We conclude that thiopentone in a bolus dose of 3 to 5 mg/kg produces only a short duration of incomplete burst suppression. Also, in this dose range, burst suppression does not occur consistently in all patients. The present data suggest that bolus doses of thiopentone in the range of 3 to 5 mg/kg may have very limited value in providing significant metabolic suppression required for intraoperative cerebral protection during temporary ischemic episodes.

Performance of the Cerebral State Index During Increasing Levels of Propofol Anesthesia: A Comparison with the Bispectral Index

The cerebral state monitor is a new device to measure depth of anesthesia. In this study we compared the cerebral state monitor with the bispectral index (BIS) monitor during propofol anesthesia. Fifteen healthy patients received a continuous infusion of propofol (300 mL/h). The cerebral state index (CSI) and the BIS values were recorded until burst suppression ratio > or =60%. Baseline variability, prediction probability, and agreement analysis between indices were evaluated. Clinical markers of loss of consciousness were also assessed. Mean awake BIS and CSI values were 95.6 and 91.6, respectively (P = 0.01). BIS and CSI prediction probability values (mean +/- sd) were estimated to be 0.87 +/- 0.08 and 0.86 +/- 0.08, respectively (NS). The CSI tended to stabilize at values of 60-40 when estimated propofol concentrations at the effect site increased from 5 to 8 mug/mL. The BIS stabilized at values of 40-20 when the propofol concentrations at the effect site increased from 7 to 10 mug/mL. The mean BIS-CSI difference was -7.4 with 95% limits of agreement of 22.2 and -36.9. The BIS and CSI correlation with the burst suppression ratio was -0.60 and -0.97, respectively (P < 0.01). Predicted BIS and CSI values for loss of eyelash reflex in 50% and 95% of the patients were different (P < 0.05). The overall performance of both monitors during propofol induction was similar. However, the different dynamic profiles of these monitors indicate that BIS may be a more useful index for evaluating intermediate anesthetic levels, whereas CSI may be better for evaluating deeper anesthetic levels.

Elevated Burst Suppression Ratio: The Possible Role of Hypoxemia

To the Editor: We report a case characterized by the simultaneous occurrence of increased suppression ratio number (SR) and hypoxemia without a concomitant decrease in bispectral index (BIS). A 59-yr-old man was scheduled for tracheal granuloma removal by laser, using a rigid bronchoscopy. The patient’s history included two lobectomies for epidermoid carcinoma. Anesthesia induction was induced and maintained with infusions of propofol and remifentanil. A bolus of succinylcholine was injected to facilitate introduction of the bronchoscope. The lungs were mechanically ventilated through the lumen of the bronchoscope using a high frequency jet ventilator. Six minutes after the succinylcholine bolus, his Spo2 decreased dramatically below 80% despite ventilation with pure oxygen, and we also observed an increase of the SR. BIS was in the range of 40–60 and signal quality index was more than 75%. His mean arterial blood pressure was 80 mm Hg. The bronchoscope was withdrawn and replaced by an endotracheal tube; the lungs were mechanically ventilated using pure oxygen. High airway pressure and auscultation revealed bronchospasm. Spo2 improved after inhalation of a beta2-agonist and we observed a decrease of the SR (Fig. 1). The endotracheal tube was removed. The patient’s postoperative neurological examination was normal.Figure 1.: Acute decrease of Spo2 during rigid bronchoscopy. BIS, bispectral index; SR, burst suppression ratio; A, high frequency jet-ventilation; B, tracheal intubation; C to D, inhalation of beta2 agonist; E, recovery.In general, burst suppression patterns appear at BIS levels below 40. The effect of an increasing concentration of a volatile anesthetic on the electroencephalogram is a progressive slowing until burst suppression patterns occur. Our observation of an increased SR appearing while BIS was at an adequate anesthetic level could have been a coincidence, but we cannot exclude a relationship between SR and poor brain tolerance of hypoxemia. This hypothesis is supported by animal studies demonstrating that hypoxemia induces isoelectric electroencephalograms, oxygenation allowing the restoration of a baseline electroencephalogram (1), and a reduction in cerebral tissue damage (2). Ngai Liu, MD Thierry Chazot, MD Department of Anesthesiology Hôpital Foch Université Paris Ile-de-France Ouest Suresnes, France Catherine Mutter, MD Departments of Anesthesiology and Intensive Care Hôpital Hautepierre Strasbourg, France Marc Fischler, MD Department of Anesthesiology Hôpital Foch Université, Paris Ile-de-France Ouest Suresnes, France [email protected]

Cerebral state monitoring in Beagle dogs sedated with medetomidine

ObjectiveTo provide experience of monitoring the level of hypnosis with the Cerebral State Monitor (CSM), a device extracting a single numerical variable between 0 and 100 from the electroencephalogram in dogs sedated with medetomidine during dental scale removal. Study designProspective observational study. Animals Nine female Beagle dogs weighing 13.3 ± 1.3 kg. MethodsCerebral state index (CSI) and burst suppression ratio (BSR) were recorded from sub-dermal needle electrodes in dogs sedated after subcutaneous injection of 114 ± 11 μg kg−1 medetomidine. Ten minutes after injection CSI monitoring began, and after 5 minutes, dental scale removal with an ultrasonic probe was started. After approximately 30 minutes, the effects of medetomidine were antagonized with atipamezole. ResultsThe CSI had a median value of 43 (range 40–56) in undisturbed sedated dogs. During dental scale removal, CSI increased to a median value of 99 (range 92–100). The BSR in undisturbed sedated dogs ranged from 2 to 15, but fell to zero during dental scale removal. ConclusionsStimulation during dental scale removal might be expected to reduce the level of sedation and hypnosis in dogs to which medetomidine had been administered. The concurrent increase in CSI and decrease in BSR suggested that a higher CSI was associated with arousal from sedation and a reduction in the depth of hypnosis. More studies are needed to validate CSI in order to better understand the functioning of this monitor. Clinical relevance The CSM shows promise for monitoring the degree of sedation and hypnosis during anaesthesia, and after adequate validation, could contribute to the refinement of anaesthetic techniques in animals.

An effective correlation dimension and burst suppression ratio of the EEG in rat. Correlation with sevoflurane induced anaesthetic depth

Anaesthesiologists need parameters that measure the depth of anaesthesia. In the context of this need, the present study investigated in rats how two variables from the electroencephalogram, the burst suppression ratio and effective correlation dimension correlated with a measure of anaesthetic depth as measured in the strength of a noxious withdrawal reflex. Eight rats were exposed to different inspiratory concentrations of sevoflurane, each rat in two separate experiments. In the first experiment, spontaneously breathing animals could move freely and no painful stimuli were applied. In the second experiment, in mechanically ventilated restrained anaesthetized rats, the withdrawal reflex was measured every 80 s. In both experiments the electroencephalogram was continuously recorded. The concentration in the effector compartment was estimated using a first order two compartment model. Correlation dimension was computed following the Grassberger/Procaccia/Takens approach with optimized parameter settings to achieve maximum sensitivity to anaesthetic drug effects and enable real-time computation. The Hill, equation was fitted to the data, describing the effect as a function of sevoflurane concentration. Good correlations of Depth of Anaesthesia with correlation dimension as well as burst suppression ratio were established in both types of experiments. Arousal by noxious stimuli decreased burst suppression ratio and increased correlation dimension. The effective sevoflurane concentration associated with 50% of the maximum effect (C50) was higher in experiment II (stimulation) than in experiment I (no stimulation): i.e. for correlation dimension 2.18% vs. 0.60% and for burst suppression ratio 3.07% vs. 1.73%. The slope factors were: gammaCD = 4.15 vs. gammaCD = 1.73 and gammaBSR = 5.2 vs. gammaBSR = 5.4. Correlation dimension and burst suppression ratio both correlated with the strength of the withdrawal reflex with correlation coefficients of 0.46 and 0.66 respectively (P < 0.001). Both correlation dimension and burst suppression ratio are related to anaesthetic depth and are affected by noxious stimuli. The relationship between anaesthetic depth and burst suppression ratio is confirmed and the potential of correlation dimension is demonstrated.

Changes in the effect of propofol in response to altered plasma protein binding during normothermic cardiopulmonary bypass

During normothermic cardiopulmonary bypass (CPB), the effect on propofol pharmacokinetics of changes in its binding to plasma proteins is consistent with the predictions of the well-stirred model of hepatic elimination for nonrestrictively cleared drug. However, whether changes in binding lead to clinically significant changes in the drug effect remains unclear. The purpose of this study was to assess changes in the drug effect of propofol in response to altered plasma binding using quantitative EEG measurements. Thirty patients undergoing cardiac surgery were assigned randomly to receive propofol infusions at 4 (Group P-4) or 6 (Group P-6) mg kg(-1) h(-1) during surgery. The concentration of propofol in blood samples, collected from the radial artery at predetermined intervals, was determined by HPLC. The unbound fraction of drug in plasma was estimated using equilibrium dialysis. Bispectral index (BIS) and burst suppression ratio (BSR) were measured at the time blood samples were collected. The total concentration of propofol in blood was unchanged during CPB relative to the pre-CPB value in both groups. However, the fraction of unbound propofol in blood increased by 2-fold during CPB. While BIS values were unchanged during CPB in Group P-4, there was a slight, but significant, decrease in Group P-6. In both groups, BSR significantly increased during CPB. BIS values showed a weak correlation with the concentration of unbound propofol (r(2)=0.19, P<0.001). BSR showed a moderate correlation with the concentration of unbound propofol (r(2)=0.56, P<0.001). The anaesthetic effect of propofol significantly increased during CPB without any alteration in the total drug concentration. The enhanced efficacy may be caused by a reduction in plasma binding of the drug.

Changes in the Bispectral Index During Intraabdominal Irrigation in Patients Anesthetized with Nitrous Oxide and Sevoflurane

Surgical stimulation typically results in an activation of electroencephalographic activity. In some instances, painful stimulation in the presence of inadequate anesthesia results in a suppression of the electroencephalogram. This phenomenon has been referred to as a "paradoxical arousal." In our daily practice, we have noted a marked decrease in the bispectral index (BIS) with large delta waves during abdominal surgery when the abdominal cavity was irrigated with normal saline. In the present study, we sought to evaluate changes in BIS during intraabdominal irrigation. Eighteen ASA physical status I-II patients scheduled for elective abdominal surgery were enrolled in the study and allocated randomly to the control group (group C) or the fentanyl group (group F). Anesthesia was induced with 3 mg/kg of thiopental and was maintained with sevoflurane and 50% nitrous oxide. BIS, 95% spectral edge frequency (SEF95), and burst-suppression ratio were recorded using a BIS monitor. Near the end of the procedure, but before irrigation of the abdominal cavity, 1.5 microg/kg fentanyl was given IV to group F. There was no significant change in BIS or SEF95 in group F patients during subsequent irrigation of the abdominal cavity. In contrast, BIS and SEF95 decreased significantly after start of irrigation in group C patients. These data show that the stimulation occurring during intraabdominal irrigation might cause a paradoxical arousal response, as evidenced by a decrease in processed electroencephalographic parameters. Pretreatment with fentanyl suppressed these changes. Anesthesiologists should be aware of this paradoxical arousal response to avoid an inappropriate decrease in the anesthetic concentration in such situations.

Does induced hypertension reduce cerebral ischaemia within the traumatized human brain?

Recent changes in published guidelines for the management of patients with severe head injury are based on data showing that aggressive maintenance of cerebral perfusion pressure (CPP) can worsen outcome due to extracranial complications of therapy. However, it remains unclear whether CPP augmentation could reduce cerebral ischaemia, a finding which might prompt the search for CPP augmentation protocols that avoid these extracranial complications. We studied 10 healthy volunteers and 20 patients within 6 days of closed head injury. All subjects underwent imaging of cerebral blood flow (CBF), blood volume (CBV), oxygen metabolism (CMRO2) and oxygen extraction fraction (OEF) using 15O PET. In addition, for patients, the EEG power ratio index (PRI), burst suppression ratio and somatosensory evoked potentials (SEP) were obtained and CPP was increased from 68 +/- 4 to 90 +/- 4 mmHg using an infusion of norepinephrine and measurements were repeated. Following elevation of CPP, CBF and CBV were increased and CMRO2 and OEF were reduced (P < 0.001 for all comparisons). Regions with a reduction in CMRO2 were associated with the greatest reduction in OEF (r2 = 0.3; P < 0.0001). Although CPP elevation produced a significant fall in the ischaemic brain volume (IBV) (from 15 +/- 16 to 5 +/- 4 ml; P < 0.01) and improved flow metabolism coupling, the IBV was small and clinically insignificant in the majority of these patients. However, the reduction in IBV was directly related to the baseline IBV (r2 = 0.97; P < 0.001) and patients with large baseline IBV showed substantial and clinically significant reductions. CPP augmentation increased the EEG PRI (5.0 +/- 1.5 versus 4.3 +/- 1.4, P < 0.01), implying an overall decrease in neural activity, but these changes did not correlate with the reduction in CMRO2 and there was no change in SEP cortical amplitude (N20-P27). These data provide support for recent changes in recommended CPP levels for head injury management across populations of patients with significant head injury. However, they do not provide guidance on whether the intervention may be more appropriate at earlier stages after injury, or in patients selected because of high baseline IBV. It also remains unclear whether CPP values below 65 mmHg can be safely used in this population. Clarification of the significance of a reduction in CMRO2 and neuronal electrical function will require further study.

QUANTITATIVE EEG TO EVALUATE DEPTH OF ANESTHESIA IN THE CAT

Introduction: Quantitative measures derived from the electroencephalogram (qEEG) have been used extensively in human medicine to assess the function of the brain under clinical manipulation (such as anesthesia and sedation), as well as in disease states (such as head trauma, coma and stroke). The goal of this study was to determine if qEEG measures could be used to predict the response of cats under isoflurane concentration to noxious stimuli.Methods: Twelve cats aged 1–2 years were included. Neurological and physical examination, blood chemistries and CBCs were normal in all cats. Anesthesia was induced with isoflurane (ISO) in O2, the cat was intubated and the femoral artery catheterized. The following physiologic data were collected continuously throughout the course of the experiment: temperature, blood pressure, respiratory rate, heart rate, and end‐tidal CO2 and ISO concentrations. An Aspect Medical Systems A‐1050 monitor was used to continuously record the raw electroencephalogram (EEG) and several qEEG measures including bispectral index (BIS), spectral edge frequency (SEF), burst suppression ratio (BSR), and total EEG power (POW). ISO concentration was maintained constant for 15 minutes, after which a tail clamp was applied for 1 minute. If the cat responded, the concentration was increased 10% and the procedure repeated. If the cat did not respond, the ISO concentration was decreased 10% and the procedure repeated. The minimum alveolar concentration (MAC) was the midpoint between a positive and negative response and, for each cat, this was determined three times.Results: BIS was significantly lower in cats at the lowest concentration at which there was no response (MAC−, 21.3±28.3) than at the highest concentration at which there was a response (MAC+, 63.4±22.1; p&lt;.01, Wilcoxon signed rank). However, in 3/12 cats (25%), BIS was higher at MAC− than at MAC+. Of the other qEEG measures, only BSR was significantly different between MAC− and MAC+ (50.4±34.4 vs. 6.1±19.4, p&lt;.01). Of the physiologic data collected, only respiratory rate was different between MAC− and MAC+ (29.4±10.0 vs. 45.3±17.9, p&lt;.05).Conclusions: These results suggest that qEEG measures may be useful for anesthesia monitoring in cats, but measures that are less species‐specific than the BIS may hold more promise. In addition, this work suggests that further studies utilizing qEEG measures for evaluation of sedation during ventilation as well as outcome prediction in animals with head trauma and critical illnesses with CNS complications are indicated.

Comparability of Narcotrend™ index and bispectral index during propofol anaesthesia

The dimensionless Narcotrend (NCT) index (MonitorTechnik, Germany, version 4.0), from 100 (awake) to 0, is a new index based on electroencephalogram pattern recognition. Transferring guidelines for titrating the Bispectral Index (BIS, Aspect Medical Systems, USA, version XP) to the NCT index depends on their comparability. We compared the relationship between BIS and NCT values during propofol anaesthesia. Eighteen adult patients about to have radical prostatectomy were investigated. An epidural catheter was placed in the lumbar space and electrodes for BIS and NCT were applied as recommended by the manufacturers. After i.v. fentanyl 0.1 mg, anaesthesia was induced with a propofol infusion. After intubation, patients received bupivacaine 0.5% 15 ml via the epidural catheter. Forty-five minutes after induction, the propofol concentration was increased to substantial burst suppression pattern and then decreased. This was done twice in each patient, and BIS and Narcotrend values were recorded at intervals of 5 s. The efficacy of NCT and BIS in predicting consciousness vs unconsciousness was evaluated using the prediction probability (P(K)). We collected 38 629 artefact-free data pairs of BIS and NCT values from the respective 5-s epochs. Because of artefacts, another 5008 epochs had been excluded from data analysis (3855 epochs for the NCT index alone, 245 epochs for the BIS alone and 908 epochs for both indices). Mean (sd) values in awake patients were 94 (6) for Narcotrend and 91 (8) for BIS. With loss of the eyelash reflex, both values were significantly reduced, to 72 (9) for NCT (P<0.001) and to 77 (11) for the BIS index (P<0.001). The PK value for loss of eyelash reflex was similar for BIS (0.95) and NCT (0.93). Decreasing BIS values coincided with decreasing NCT values. A sigmoid model [NCT index=52.8+26.8/(1+exp(-(BIS-78.3)/4.8))(0.4); r=0.52] described the correlation between BIS and NCT index in a BIS range between 100 and 50. For BIS values lower than 50, a second sigmoid model with a correlation of r=0.83 was applied [NCT index=6.6+45.3/(1+exp(-(BIS-29.8)/2.4)) (0.6) r=0.83]. The relationship between burst suppression ratio (BSR) and NCT index was best described by the following sigmoid model: NCT index=265/(1+exp((-BSR+108)/-49); r=0.73. We found a sufficient correlation between BIS and NCT index, but deviations from the line of identity in some ranges require attention. Therefore, a simple 1:1 transfer from BIS to NCT values is not adequate. Our results might serve as a blueprint for the rational translation of BIS into NCT values.

The relationship between bispectral index and electroencephalographic parameters during isoflurane anesthesia.

Bispectral index (BIS) integrates various electroencephalographic (EEG) parameters into a single variable. However, the exact algorithm used to synthesize the parameters to BIS values is not known. The relationship between BIS and EEG parameters was evaluated during nitrous oxide/isoflurane anesthesia. Twenty patients scheduled for elective ophthalmic surgery were enrolled in the study. After EEG recording with a BIS monitor (A-1050) was begun, general anesthesia was induced and maintained with 0.5%-2% isoflurane and 66% nitrous oxide. Using software we developed, we continuously recorded BIS, spectral edge frequency 95% (SEF95), and EEG parameters such as relative beta ratio (BetaRatio), relative synchrony of fast and slow wave (SynchFastSlow), and burst suppression ratio. BetaRatio was linearly correlated with BIS (r = 0.90; P < 0.01; n = 253) at BIS more than 60. At a BIS range of 30 to 80, SynchFastSlow (r = 0.60; P < 0.01; n = 3314) and SEF95 (r = 0.75; P < 0.01; n = 3339) were linearly correlated with BIS. The correlation between BIS and SEF95 was significantly better than the correlation between BIS and SynchFastSlow (P < 0.01). At BIS less than 30, the burst suppression ratio was inversely linearly correlated with BIS (r = 0.76; P < 0.01; n = 65). At BIS less than 80, burst-compensated SEF95 was linearly correlated with BIS (r = 0.78; P < 0.01; n = 3404). In the range of BIS from 60 to 100, BIS can be calculated from BetaRatio. At surgical levels of anesthesia, BIS and SynchFastSlow (a parameter derived from bispectral analysis) or burst-compensated SEF95 (derived from power spectral analysis) are well correlated. However, our results show that SynchFastSlow has no advantage over SEF95 in calculation of BIS. The relationship between bispectral index (BIS) and electroencephalographic parameters was evaluated during nitrous oxide/isoflurane anesthesia. At surgical levels of anesthesia, BIS and the relative synchrony of fast and slow wave (a parameter derived from bispectral analysis) or burst-compensated spectral edge frequency 95% (a parameter derived from power spectral analysis) are well correlated.

A Low Voltage EEG Signal May Give Low Bispectral Index Values

To the Editor: We read with great interest the study by Messner et al., which shows a relationship between EMG activity and BIS (1). We agree that artifacts due to EMG activity may give spurious BIS values. However, we do not think that the article clearly demonstrates that EMG activity is a substantial element of BIS analysis as written in the conclusion. Case reports have shown that the BIS may be misled by a low voltage EEG signal and give values as low as 40 (2). This EEG pattern occurs with an incidence of approximately 10% of the population (3). Moreover, neuromuscular blockade in conscious individuals is certainly a stressful experience, and the authors report “some level of discomfort and stress.” It is also well known that emotional tension may induce low voltage EEG activity (3,4). Thus it seems possible to us that this occurred during the experiment, explaining the low BIS values. This is especially true for the minimal BIS value of 9 recorded in one volunteer. For BIS values below 30, there is a linear correlation between the burst suppression ratio and the BIS (5). From the data of Bruhn et al. (5), a BIS value of 9 corresponds to a burst suppression ratio above 80%. Thus it is very likely that most of the EEG was interpreted as a flat EEG in this patient. Unfortunately, the lack of EEG trace in the article by Messner et al. does not allow the conclusion that this was true or not. Of course, we agree that EMG activity may cause artifactual BIS readings, but every monitor we use in anesthesia is prone to artifacts. In conclusion, we would like to emphasize the advice from a recent letter on “the importance of looking at the EEG when presenting univariate variables to describe it”(6). Before using any EEG monitor, the anesthesiologist has to learn the basis of EEG. N. Bruder, MD L. Velly, MD M. Rey, MD, PhD

Time‐frequency balanced spectral entropy as a measure of anesthetic drug effect in central nervous system during sevoflurane, propofol, and thiopental anesthesia

Time-frequency balanced spectral entropy of electroencephalogram (EEG) and frontal electromyogram (FEMG) is a novel measure of hypnosis during anesthesia. Two Entropy parameters are described: Response entropy (RE) is calculated from EEG and FEMG; and State Entropy (SE) is calculated mainly from EEG. This study was performed to validate their performance during transition from consciousness to unconsciousness under different anesthetic agents. Response entropy, SE [S/5 Entropy Module, M-ENTROPY (later in text: Entropy), Datex-Ohmeda Division, Instrumentarium Corp., Helsinki, Finland] and BIS (BIS XP, A-2000, Aspect Medical Systems, Newton, MA) data were collected from 70 patients; 30 anesthetized with propofol 2 mg kg-1, 20 with sevoflurane inhalation, and 20 with thiopental 5 mg kg-1. Loss and regaining of consciousness (LOC, ROC) was tested every 10 s, and sensitivity, specificity, and prediction probability (Pk) were calculated. Behavior of the indices was studied. Sensitivity, specificity, and Pk values for consciousness were high and similar for all indices. During regaining of consciousness after propofol bolus, RE, SE, and BIS values recovered by 81 +/- 22%, 75 +/- 26%, and 59 +/- 18% (mean +/- SD), respectively, from the minimum relative to their baseline. After thiopental bolus, RE, SE, and BIS values recovered by 86+/-21%, 88 +/- 13%, and 63 +/- 14%, respectively. The relative rise was higher in RE and SE compared with BIS (P < 0.01). During deep levels of hypnosis, RE and SE decreased monotonously as a function of burst suppression ratio, while BIS showed biphasic behavior. On average, RE indicated emergence from anesthesia 11 s earlier than SE, and 12.4 s earlier than BIS. All indices, RE, SE, and BIS, distinguished excellently between conscious and unconscious states during propofol, sevoflurane, and thiopental anesthesia. During burst suppression, Entropy parameters RE and SE, but not BIS, behave monotonously. During regaining of consciousness after a thiopental or propofol bolus, RE and SE values recovered significantly closer to their baseline values than did BIS. Response entropy indicates emergence from anesthesia earlier than SE or BIS.

Orexin a elicits arousal electroencephalography without sympathetic cardiovascular activation in isoflurane-anesthetized rats.

We studied the effects of intracerebroventricular injection of the novel neuropeptide orexin A on electroencephalogram (EEG) and autonomic nervous system activity in rats under isoflurane anesthesia. The administration of orexin A changed burst suppression patterns to arousal patterns on the EEG at 1.0 minimum alveolar anesthetic concentration (MAC) isoflurane and decreased the burst suppression ratio at 1.5 MAC isoflurane. However, orexin A did not influence the heart rate or mean arterial blood pressure at either isoflurane concentration. These findings demonstrated that orexin A elicited anesthetic arousal under isoflurane anesthesia in terms of EEG pattern without sympathetic cardiovascular activation in the rat. The novel neuropeptides orexins induce arousal associated with activation of the sympathetic nervous system in conscious rats. It is not known whether orexins affect the electroencephalogram (EEG), autonomic nerve activity, or both under anesthesia. Orexin A induced EEG arousal without sympathetic cardiovascular activation in the isoflurane-anesthetized rat. Orexin A might influence the depth of anesthesia.

Effects of propofol on cerebral oxygenation and metabolism after head injury

Flow-metabolism coupling is thought to be deranged after traumatic brain injury, while the effects of propofol on flow-metabolism coupling are controversial. We have used a step increase in target plasma propofol concentration in head injured patients to explore flow-metabolism coupling in these patients. Ten patients with a moderate to severe head injury received a step increase in propofol target controlled infusion of 2 microg x ml(-1). Cerebral tissue gas measurements were recorded using a multimodal sensor, and regional chemistry was assessed using microdialysis. Arterial-jugular venous oxygen differences (AVDO(2)) were measured and all patients had cortical function monitoring (EEG). The step increase in propofol led to a large increase in EEG burst-suppression ratio (0% (range 0-1.1) to 46.1% (range 0-61.7), P<0.05); however, this did not significantly change tissue gas levels, tissue chemistry, or AVDO(2). Flow-metabolism coupling remains intact during a step increase in propofol after traumatic brain injury. The EEG burst-suppression induced by propofol after traumatic brain injury does not appear to be a useful therapeutic tool in reducing the level of regional ischaemic burden.