Bispectral index,spectral edge frequency 95%, and median frequency recorded for various concentrations of isoflurane and sevoflurane in pigs

OBJECTIVE: To evaluate bispectral index (BIS), spectral edge frequency 95% (SEF), and median frequency (MED) in relation to a visual analogue scale (VAS) as indicators of anesthetic depth for various concentrations of sevoflurane and isoflurane in pigs. ANIMALS: 32 pigs. PROCEDURE: Pigs were randomly allocated to 8 groups (4 pigs/group). An electroencephalogram (EEG) was recorded in each conscious pig. Pigs were then anesthetized by use of sevoflurane (n = 16) or isoflurane (16). Agents were administered in oxygen at minimum alveolar concentrations (MACs) of 1, 1.25, 1.5, and 1.75 MAC in a randomized order. End-tidal sevoflurane and isoflurane concentrations were maintained for 30 minutes, after which an EEG was recorded for 5 minutes; BIS, SEF, and MED were then calculated. Anesthetic depth was evaluated by use of the VAS. Cardiovascular and EEG responses to nociceptive stimuli were evaluated for each anesthetic agent. RESULTS: BIS decreased significantly for the various concentrations of each anesthetic. At equivalent MACs, BIS values were significantly higher during sevoflurane-induced anesthesia than during isoflurane-induced anesthesia. Values of MED and SEF decreased significantly from basal values to 1 MAC of sevoflurane and isoflurane. For both agents, there was good correlation between VAS scores and BIS values and between VAS scores and SEF values. CONCLUSIONS AND CLINICAL RELEVANCE: BIS was useful for predicting changes in anesthetic depth at clinical dosages of inhalant anesthetics. Values of BIS, SEF, and MED were significantly higher during anesthesia induced by administration of sevoflurane than during anesthesia induced by administration of isoflurance at equivalent MACs.     

Evaluation of the bispectral index as an indicator of degree of central nervous system depression in isoflurane-anesthetized horses

OBJECTIVE: To determine whether the bispectral index (BIS) can be used as an indicator of degree of CNS depression in isoflurane-anesthetized horses. ANIMALS: 10 Standardbred and 6 Norwegian cold-blooded trotter stallions admitted for routine castration. PROCEDURE: A 2-channel referential electrode configuration was used to record EEG for calculation of BIS by the EEG monitor. The BIS was calculated before (awake) and after (sedated) administration of detomidine (0.01 mg/kg of body weight, IV) and butorphanol (0.01 mg/kg, IV). Anesthesia was induced with ketamine hydrochloride (2.5 mg/kg, IV) and diazepam (0.04 mg/kg, IV) and maintained with isoflurane delivered in oxygen. The BIS was calculated after 30 minutes of equilibration at an end-tidal isoflurane concentration of 1.4% (n = 8) or 1.9% (8) and recorded continuously during surgery. RESULTS: Bispectral index was significantly less in sedated and anesthetized horses, compared with awake horses. However, BIS was not significantly different between sedated and anesthetized horses. Mean BIS in horses anesthetized at 1.9% isoflurane was significantly greater, compared with horses anesthetized at an end-tidal concentration of 1.4%. Four horses in the 1.4% group moved during surgery, and BIS increased immediately prior to movement in 2 of these horses. CONCLUSIONS AND CLINICAL RELEVANCE: BIS is not a precise indicator of degree of CNS depression in isoflurane-anesthetized horses. Thus, determination of BIS may not be a useful technique for monitoring anesthetic depth in isoflurane-anesthetized horses.     

Use of the bispectral index as a monitor of anesthetic depth in cats anesthetized with isoflurane

OBJECTIVE: To determine whether the prestimulation bispectral index (BIS) value or relative change in BIS after noxious stimulation can be used to assess the depth of isoflurane anesthesia in cats. ANIMALS: 17 healthy female cats. PROCEDURE: Electroencephalogram (EEG) patterns and BIS values were examined in cats that received increasing end-tidal (ET) isoflurane concentrations. Subsequently, BIS values were determined before and after either a noxious somatic or visceral stimulus in cats that received ET isoflurane concentrations ranging from 1.8% to 2.4%. Electrical stimuli of the tail base and bladder distension to 50 cm of water were the somatic and visceral stimuli, respectively. RESULTS: he resting BIS at ET isoflurane concentrations from 1.4% to 1.9% steadily decreased concurrently with increasing degrees of EEG suppression. Prestimulation BIS values, however, were not related to 1.8% to 2.4% ET isoflurane concentrations and not useful for prediction of BIS values or hemodynamic and movement responses after a noxious stimulus. The poststimulation BIS value and the difference between mean BIS values before and after stimulation were inversely correlated with increasing ET isoflurane concentrations. Poststimulation BIS values > 60 were observed at ET isoflurane concentrations greater than those associated with a movement response after a stimulus. CONCLUSIONS AND CLINICAL RELEVANCE: The prestimulation BIS value has limited use in assessing anesthetic depth in cats during isoflurane anesthesia. The change in BIS values after a noxious somatic or visceral stimulus was a reliable measure of anesthetic depth and may be a useful measure of early arousal from the hypnotic state.     

Bispectral index and the clinically evaluated anaesthetic depth in dogs

Objective This study investigated whether the bispectral index (BIS monitor) corresponded with the clinical assessment of anaesthetic depth in dogs. Study design Prospective clinical study. Animals Sixty‐five dogs undergoing anaesthesia for surgery. Methods Dogs were assigned to one of three different anaesthetic techniques. A three point scale was devised to determine the clinical depth of anaesthesia (CDA); CDA 1 represented light, CDA 2 surgical and CDA 3 excessive depth of anaesthesia. BIS values were recorded and CDA assessed at specific times and points throughout surgery. Data were statistically analysed using mixed model regression. Results Clinical depth of anaesthesia was assessed as CDA 1 on 68, 2 on 748 and 3 on four occasions. The BIS recorded for CDA 1 differed significantly from that for CDA 2 (p < 0.001). However, individual BIS values measured at light and surgical levels of anaesthesia overlapped considerably. The sensitivities and specificities calculated for BIS to diagnose CDA 1 compared to CDA 2 in the three anaesthetic protocols were 28–86% and 55–85%. The accompanying positive predictive value was 0.08–0.29 and the negative predictive value was 0.95–0.97. End‐tidal isoflurane concentrations (anaesthetic techniques 1 and 3) and propofol infusion (technique 2) at CDA 1 was significantly lower than those at CDA 2 (p = 0.001). Conclusions Although BIS values overall distinguished between CDA scores, the calculated specificities, sensitivities and predictive values were low, and there were anomalous results in individual cases. Clinical relevance The use of the BIS as the sole method to determine anaesthetic depth in dogs is imprudent.     

Isoflurane Anaesthetic Depth in Goats Monitored Using the Bispectral Index of the Electroencephalogram

The bispectral index (BIS) of the electroencephalogram has recently been used to monitor the depth of anaesthesia in humans. The BIS is a dimensionless number that varies between 0 and 100. We hypothesized that the BIS could also be used to monitor depth of isoflurane anaesthesia in goats. Needle electrodes were placed over the frontal region of the scalp of goats and 5% isoflurane was administered via a mask. The BIS number was determined at clinically relevant end-points. The BIS number did not change when the animals became recumbent (95±5 to 94±7, n = 15), but decreased to 65±13 and 64±15 when the corneal reflex and withdrawal response to a noxious stimulus, respectively, were lost (p<0.001, n = 12). Direct laryngoscopy and intubation increased the BIS (56±7 to 83±11; p<0.05, n = 10), as did a noxious pinch to the dew-claw (57±9 to 76±9; p<0.05, n = 10). The spectral edge (frequency below which 95% of the total power resided) paralleled the change in BIS. We conclude that the depth of isoflurane anaesthesia in goats can be monitored using the BIS, although further work is needed to determine its sensitivity and specificity.     

Use of bispectral index to monitor depth of anesthesia in isoflurane-anesthetized dogs

OBJECTIVE: To evaluate the correlation between the bispectral index (BIS) and end-tidal isoflurane (ET(ISO)) concentration and compare the use of 3 BIS sensor positions in dogs. ANIMALS: 6 adult dogs. PROCEDURES: Mechanically ventilated dogs received pancuronium, and depth of anesthesia was altered by increasing ET(ISO) concentration from 1.5% to 2.3% and 3.0%. The BIS, suppression ratio (relative percentage of isoelectric electroencephalographic waveforms), and signal quality index (SQI) were recorded at each ET(ISO) concentration for each of 3 BIS sensor positions (frontal-occipital, bifrontal, and frontal-temporal positions). RESULTS: The BIS and ET(ISO) concentration were poorly correlated; regardless of sensor positioning, mean BIS values did not change significantly as ET(ISO) was increased. At 3% isoflurane, regardless of sensor positioning, there was an increase in suppression ratio coincident with BIS < 40 in some dogs, whereas paradoxic increases in BIS (> 60) were recorded in others. Furthermore, at 3.0% isoflurane, the SQI was significantly lower for the bifrontal sensor position (compared with values for the other positions), but low SQI values prevented recording of BIS values from the frontal-occipital sensor position in 2 dogs. Overall, BIS values derived from the 3 sensor positions did not differ. CONCLUSIONS AND CLINICAL RELEVANCE: In dogs, BIS values may not reflect changes in depth of isoflurane anesthesia in the absence of noxious stimulation. Of the 3 sensor positions, frontal-temporal positioning provided better correlation with changes in depth of anesthesia induced via changes in isoflurane concentrations. However, the sensor placements yielded similar results at SQI values > 50.     

Noxious stimulation during orthopaedic surgery results in EEG 'arousal' or 'paradoxical arousal' reaction in isoflurane-anaesthetised sheep

The effects of surgical stimuli on haemodynamic and electroencephalographic (EEG) variables were determined in 25 adult ewes undergoing an experimental orthopaedic procedure in isoflurane anaesthesia. Data were recorded after 15 min of constant end-tidal concentration of approximately 2.2% isoflurane (SS: steady state=baseline), during skin disinfection (DIS), incision (INC), drilling of the first hole through the tibia (DRI) and insertion of a threaded pin (PIN) for external fixation. Stimulation resulted in a significant increase in systolic and mean arterial pressure above SS at INC, DRI and PIN. Haemodynamic changes were accompanied by either significant increases or decreases in EEG median frequency (MF) and 80% spectral edge frequency (SEF80) above or below SS at all four stimulation time points suggesting 'arousal' or 'paradoxical arousal' reaction, respectively. We conclude, that either type of EEG activation pattern could be elicited dependent on stimulation intensity and level of anaesthetic depth.     

Comparison of electroencephalogram activity and auditory evoked responses during isoflurane and halothane anaesthesia in the rat

Objectives To compare the second differential index (SDI) calculated from the auditory evoked potential (AEP) and electroencephalogram (EEG) parameters: median frequency (MF), spectral edge frequency (SEF) and burst suppression rate (BSR) determined at four equivalent minimum alveolar concentrations (MAC) of isoflurane or halothane. Animals Twelve male Wistar rats weighing 418 g (SD ± 18.4 g). Methods Auditory evoked potentials and EEG responses were recorded in animals implanted with electrodes at established anaesthetic concentrations. Depth of anaesthesia was assessed using the strength of the pedal withdrawal reflex (PWR), and data were analysed using repeated measures anova and paired t‐tests. Results The SEF tended to decrease with increasing depth of halothane anaesthesia (F = 4.198, p = 0.05), but not with isoflurane. The MF and SDI were significantly higher during halothane than with isoflurane (F = 5.82, p = 0.036 and F = 5.263, p = 0.045, respectively) at equivalent depths of anaesthesia, and EEG burst suppression occurred at deeper planes of isoflurane but not halothane anaesthesia. Conclusions The study demonstrated that EEG and AEP characteristics recorded at MAC equivalent concentrations were suppressed to a greater degree by isoflurane than by halothane. These findings have strong implications for research projects where EEG recordings are collected, and also cast more general doubts upon the value of such parameters for evaluating depth of isoflurane anaesthesia in rats.     

Effects of morphine,lidocaine, ketamine,and morphine-lidocaine-ketamine drug combination on minimum alveolar concentration in dogs anesthetized with isoflurane

OBJECTIVE: To determine the effects of constant rate infusion of morphine, lidocaine, ketamine, and morphine-lidocaine-ketamine (MLK) combination on end-tidal isoflurane concentration (ET-Iso) and minimum alveolar concentration (MAC) in dogs anesthetized with isoflurane and monitor depth of anesthesia by use of the bispectral index (BIS). ANIMALS: 6 adult dogs. PROCEDURE: Each dog was anesthetized with isoflurane on 5 occasions, separated by a minimum of 7 to 10 days. Individual isoflurane MAC values were determined for each dog. Reduction in isoflurane MAC, induced by administration of morphine (3.3 microg/kg/min), lidocaine (50 microg/kg/min), ketamine (10 microg/kg/min), and MLK, was determined. Heart rate, mean arterial blood pressure, oxygen saturation as measured by pulse oximetry (Spo2), core body temperature, and BIS were monitored. RESULTS: Mean +/- SD isoflurane MAC was 1.38 +/- 0.08%. Morphine, lidocaine, ketamine, and MLK significantly lowered isoflurane MAC by 48, 29, 25, and 45%, respectively. The percentage reductions in isoflurane MAC for morphine and MLK were not significantly different but were significantly greater than for lidocaine and ketamine. The Spo2, mean arterial pressure, and core body temperature were not different among groups. Heart rate was significantly decreased at isoflurane MAC during infusion of morphine and MLK. The BIS was inversely related to the ET-Iso and was significantly increased at isoflurane MAC during infusions of morphine and ketamine, compared with isoflurane alone. CONCLUSIONS AND CLINICAL RELEVANCE: Low infusion doses of morphine, lidocaine, ketamine, and MLK decreased isoflurane MAC in dogs and were not associated with adverse hemodynamic effects. The BIS can be used to monitor depth of anesthesia.     

Effect of medetomidine administration on bispectral index measurements in dogs during anesthesia with isoflurane

OBJECTIVE: To determine the relationship between bispectral index (BIS) and minimum alveolar concentration (MAC) multiples of isoflurane after IM injection of medetomidine or saline (0.9% NaCl) solution in anesthetized dogs. ANIMALS: 6 dogs. PROCEDURE: Each dog was anesthetized 3 times with isoflurane. First, the MAC of isoflurane for each dog was determined by use of the tail clamp method. Second, anesthetized dogs were randomly assigned to receive an IM injection of medetomidine (8 microg x kg(-1)) or an equal volume of isotonic saline (0.9% NaCl) solution 30 minutes prior to beginning BIS measurements. Last, anesthetized dogs received the remaining treatment (medetomidine or isotonic saline solution). Dogs were anesthetized at each of 4 MAC multiples of isoflurane. Ventilation was controlled and atracurium (0.2 mg/kg followed by 6 microg/kg/min as a continuous infusion, IV) administered. After a 20-minute equilibration period at each MAC multiple of isoflurane, BIS data were collected for 5 minutes and median values of BIS calculated. RESULTS: BIS significantly decreased with increasing MAC multiples of isoflurane over the range of 0.8 to 2.0 MAC. Mean (+/- SD) MAC of isoflurane was 1.3 +/- 0.2%. During isoflurane-saline anesthesia, mean BIS measurements at 0.8, 1.0, 1.5, and 2.0 MAC were 65 +/- 8, 60 +/- 7 52 +/- 3, and 31 +/- 28, respectively. During isoflurane-medetomidine anesthesia, mean BIS measurements at 0.8, 1.0, 1.5, and 2.0 MAC were 77 +/- 4, 53 +/- 7, 31 +/- 24, and 9 +/- 20, respectively. CONCLUSIONS AND CLINICAL RELEVANCE: BIS monitoring in dogs anesthetized with isoflurane has a predictive value in regard to degree of CNS depression. During isoflurane anesthesia, our results support a MAC-reducing effect of medetomidine.     

Effect of inhalation of isoflurane at end-tidal concentrations greater than, equal to, and less than the minimum anesthetic concentration on bispectral index in chickens

OBJECTIVE: To determine the effect of inhalation of isoflurane at end-tidal concentrations greater than, equal to, and less than the minimum anesthetic concentration (MAC) on bispectral index (BIS) in chickens. Animals-10 chickens. PROCEDURES: For each chicken, the individual MAC of isoflurane was determined by use of the toe-pinch method. After a 1-week interval, chickens were anesthetized with isoflurane at concentrations 1.75, 1.50, 1.25, 1.00, and 0.75 times their individual MAC (administered from higher to lower concentrations). At each MAC multiple, a toe pinch was performed and BIS was assessed and correlated with heart rate, blood pressure, and an awareness score (derived by use of a visual analogue scale). RESULTS: Among the chickens, mean +/- SD MAC of isoflurane was 1.15 +/- 0.20%. Burst suppression was detected at every MAC multiple. The BIS and awareness score were correlated directly with each other and changed inversely with increasing isoflurane concentration. Median (range) BIS values during anesthesia at 1.75, 1.50, 1.25, 1.00, and 0.75 MAC of isoflurane were 25 (15 to 35), 35 (25 to 45), 35 (20 to 50), 40 (25 to 55), and 50 (35 to 65), respectively. Median BIS value at extubation was 70 +/- 9. Values of BIS correlated with blood pressure, but not with heart rate. Blood pressure changed with end-tidal isoflurane concentrations, whereas heart rate did not. CONCLUSIONS AND CLINICAL RELEVANCE: Assessment of BIS can be used to monitor the electrical activity of the brain and the degree of unconsciousness in chickens during isoflurane anesthesia.     

A comparison of the haemodynamic effects of isoflurane and halothane anaesthesia in horses

The purpose of this study was to compare the haemodynamic effects of equipotent isoflurane and halothane anaesthesia. Six adult horses were investigated on two separate occasions at least 4 weeks apart. On both occasions anaesthesia was induced by ketamine 2.2 mg/kg bwt given 5 min after i.v. administration 100 microg/kg bwt romifidine. Anaesthesia was maintained either by halothane or isoflurane (end-tidal concentrations 0.9-1.0% and 1.3-1.4%, respectively). Horses were ventilated by intermittent positive pressure to maintain PaCO2 between 40-50 mmHg. Haemodynamic variables were measured using catheter-mounted strain gauge transducers in the left and right ventricle, aorta, and right atrium. Cardiac output (CO), velocity time integral (VTI), maximal aortic blood flow velocity (Vmax) and acceleration (dv/dt(max)), left ventricular pre-ejection period (PEP) and ejection time (ET) were measured from aortic blood flow velocity waveforms obtained by transoesophageal Doppler echocardiography. Flow velocity waveforms were recorded from the femoral arteries and veins using low pulse repetition frequency Doppler ultrasound. Time-averaged mean velocity (TAV), velocity of component a (TaVa), velocity of component b (TaVb) and early diastolic deceleration slope (EDDS) were measured. Pulsatility index (PI) and volumetric flow were calculated. Microvascular blood flow was measured in the left and right semimembranosus muscles by laser Doppler flowmetry. Maximal rate of rise of LV pressure (LVdp/dt(max)), CO, Vmax, dv/dt(max), ET, VTI were significantly higher at all time points during isoflurane anaesthesia compared to halothane anaesthesia. Pre-ejection period and diastolic aortic blood pressure were significantly less throughout isoflurane anaesthesia compared to halothane. Isoflurane anaesthesia was associated with significantly lower systemic vascular resistance than halothane anaesthesia. Femoral arterial and venous blood flow were significantly higher and EDDS and PI were significantly lower during isoflurane anaesthesia compared to halothane anaesthesia. In addition during both halothane and isoflurane anaesthesia, femoral arterial flow was higher and EDDS and PI lower in the left (dependent) artery compared to the right (nondependent) artery. This study supports previous work demonstrating improved left ventricular systolic function during isoflurane compared to halothane anaesthesia. This improvement was still evident after premedication with a potent-long acting alpha2-adrenoreceptor agonist, romifidine, and induction of anaesthesia with ketamine. There was also evidence of increased hindlimb blood flow during isoflurane anaesthesia. However, there were differences observed in flow between the left and right hindlimb during maintenance of anaesthesia with each agent, suggesting that there were differences in regional perfusion in anaesthetised horses caused by factors unrelated to agents administered.     

Motor responses to stimulation during isoflurane anaesthesia in pigs

Objective To investigate motor responses to stimulation during the transition from ‘deep’ (burst suppression) to ‘light’ isoflurane anaesthesia in pigs. Study design Prospective, randomized observational study. Animals Five castrated male and five female Norwegian landrace pigs, weighing 19–29 kg. Materials and methods Anaesthesia was induced with isoflurane and the inspired concentration gradually increased until a burst suppression electroencephalogram (EEG) was recorded. End‐tidal isoflurane concentration ( F ) was then allowed to equilibrate for 30 minutes after which the eyelashes, cornea, nasal septum, anus, interdigital skin fold, periople, tail and claw were stimulated. The motor response to stimulation at each location was graded from 0 to 5. End‐tidal isoflurane concentration was decreased 0.3% and the areas re‐stimulated; this was repeated three times in each pig. A linear regression analysis using response as dependent and anaesthetic level as independent variable was performed for each stimulus in each pig. Using Student's t‐statistic a 95% confidence interval for the mean slope of each stimulus was constructed. Results No pig responded to eyelash brushing. The mean slopes for the other stimuli indicated increasing responses with decreasing F . Responses to periople pinching and tail and claw clamping showed significant increases. No stimuli consistently increased the magnitude of response in all pigs, and the appearance and absence of a response was inconsistent between pigs. Motor responses occurred in at least one pig during isoflurane burst suppression anaesthesia to all stimuli except eyelash brushing. Conclusions All the stimuli investigated may elicit movement responses during burst suppression anaesthesia with isoflurane except eyelash brushing. No consistent response pattern between pigs was observed with decreasing isoflurane concentration. Of the stimuli evaluated, clamping the tail or claw and pinching the periople appear the most reliable indicators of anaesthetic depth. Clinical relevance The absence or presence of single reflexes does not accurately reflect the degree of isoflurane‐induced cortical depression in individual pigs.     

Quantitative electroencephalography for measurement of central nervous system responses to diazepam and the benzodiazepine antagonist, flumazenil, in isoflurane-anaesthetized dogs

Quantitative EEG was assessed in six dogs anaesthetized with 1.8% end-tidal isoflurane concentration and following diazepam (0.2 mg/kg i.v.) administration. Ventilation was controlled to maintain normocapnia. Five dogs were subsequently given the benzodiazepine antagonist, flumazenil (0.04 mg/kg i.v.), and quantitative EEG was recorded. One dog received a saline injection following diazepam (as a control) and quantitative EEG was recorded for an additional 2.5 h. Heart rate, arterial blood pressure, esophageal temperature, arterial pH and blood gas tensions, end-tidal CO2 tension and end-tidal isoflurane concentration were monitored throughout the study. A 21 lead linked-ear montage was used for recording EEG. Quantitative EEG data were stored on an optical disc for analysis at a later date. Values for absolute power of EEG were determined for theta, delta, alpha, and beta frequencies. Cardiovascular parameters remained stable throughout the study. Diazepam administration was associated with decreased absolute power in all frequencies of EEG at all electrode sites. The duration of diazepam-induced decreased absolute power of EEG was at least 3 h in one dog. Administration of flumazenil antagonized diazepam-induced decreased absolute power of EEG in all frequencies at all electrode sites. We conclude that quantitative EEG provides a relatively non-invasive, objective measure of diazepam- and flumazenil-induced changes in cortical activity during isoflurane anaesthesia.     

Effect of morphine on the bispectral index during isoflurane anesthesia in dogs

ObjectiveTo assess the effect of morphine on the bispectral index (BIS) in dogs during isoflurane anesthesia maintained at a constant end–tidal concentration.Study designProspective, randomized, experimental trial.AnimalsEight adult Beagle dogs, weighing between 7.1 and 9.8 kg.MethodsAnesthesia was induced with isoflurane via a face mask. Dog's tracheas were intubated and anesthesia maintained with isoflurane at a constant end–tidal concentration (e′Iso) of 1.81% for a 30–minute equilibration period. Pulmonary ventilation was controlled to normocapnia. After equilibration, baseline values were recorded prior to intravenous administration of morphine sulfate (0.5 mg kg^?1) (MT) or an equal volume of saline (CT). Measurements for heart rate, systolic, diastolic and mean arterial pressure (SAP, DAP and MAP) were recorded at 10, 20, 30, 45, 60, 75, 90, 105 and 120 minutes after treatment. Bispectral index was recorded every 10 seconds for 3 minutes for each time measurement. Venous blood samples were collected at baseline, 10, 20, 30, 45, 60 and 120 minutes for determination of morphine serum concentrations. Anesthesia was discontinued after the last measurement and dogs were allowed to recover.ResultsBaseline BIS for MT and CT at 1.81%e′Iso were 63 ± 10 and 58 ± 9, respectively. Bispectral index in MT was 4–8% lower at 20, 75, 90 and 105 minutes compared with CT. There were no differences in BIS between baseline and any subsequent measurement within either MT or CT. Heart rate, SAP, MAP, and DAP decreased after morphine administration.Conclusion and clinical relevanceIntravenous administration of 0.5 mg kg^?1 morphine sulfate did not cause clinically significant changes in the BIS of unstimulated dogs during isoflurane anesthesia at an e′Iso of 1.81%.     

Effects of epidural administration of dexmedetomidine on the minimum alveolar concentration of isoflurane in dogs

Objective-To evaluate the effects of epidural administration of 3 doses of dexmedetomidine on isoflurane minimum alveolar concentration (MAC) and characterize changes in bispectral index (BIS) induced by nociceptive stimulation used for MAC determination in dogs. Animals-6 adult dogs. Procedures-Isoflurane-anesthetized dogs received physiologic saline (0.9% NaCl) solution (control treatment) or dexmedetomidine (1.5 [DEX1.5], 3.0 [DEX3], or 6.0 [DEX6] mug/kg) epidurally in a crossover study. Isoflurane MAC (determined by use of electrical nociceptive stimulation of the hind limb) was targeted to be accomplished at 2 and 4.5 hours. Changes in BIS attributable to nociceptive stimulation and cardiopulmonary data were recorded at each MAC determination. Results-With the control treatment, mean +/- SD MAC values did not change over time (1.57 +/- 0.23% and 1.55 +/- 0.25% at 2 and 4.5 hours, respectively). Compared with the control treatment, MAC was significantly lower at 2 hours (13% reduction) but not at 4.5 hours (7% reduction) in DEX1.5-treated dogs and significantly lower at 2 hours (29% reduction) and 4.5 hours (13% reduction) in DEX3-treated dogs. The DEX6 treatment yielded the greatest MAC reduction (31% and 22% at 2 and 4.5 hours, respectively). During all treatments, noxious stimulation increased BIS; but changes in BIS were correlated with increases in electromyographic activity. Conclusions and Clinical Relevance-In dogs, epidural administration of dexmedetomidine resulted in dose-dependent decreases in isoflurane MAC and that effect decreased over time. Changes in BIS during MAC determinations may not represent increased awareness because of the possible interference of electromyographic activity.     

Effects of different propofol infusion rates on EEG activity and AEP responses in rats

Parameters calculated from the auditory-evoked potential (AEP) recorded over the auditory cortex and from the electroencephalogram (EEG) recorded over the near vertex were compared in rats at three different infusion rates of propofol (62.5, 35 and 25 mg/kg/h). Depth of anaesthesia was assessed clinically using the strength of the pedal withdrawal reflex. Well-defined AEP responses were consistently obtained. As the propofol concentration was reduced, peak latencies decreased and peak to peak amplitudes increased. Amplitude and latency values were closely associated with the strength of the pedal withdrawal responses. Parameters calculated from the EEG showed no significant change as the propofol concentration was reduced. Periods of burst suppression became more frequent as the propofol infusion rate was increased. The study showed some of the difficulties that may be encountered when using EEG as a tool to assess depth of anaesthesia during propofol infusion. The AEP showed dose dependent changes in rats at different infusion rates of propofol. However, large variability between animals limits the use of this technique for monitoring depth of anaesthesia.     

Feline pyothorax - new insights into an old problem: part 1. Aetiopathogenesis and diagnostic investigation

Relationships between onset latency and peak-to-peak amplitude of magnetic motor evoked potentials (MMEP) after transcranial magnetic stimulation (TMS), together with the electroencephalographic parameters bispectral analysis index (BIS) and the autoregressive model with exogenous input (ARX)-derived auditory evoked potential index (AAI) were explored during different sedative and hypnotic drug combinations in six dogs. TMS was performed under sedation with acepromazine/methadone or medetomidine and after a single bolus injection of propofol or etomidate. Data for BIS and AAI were continuously collected during the periods of treatment with the hypnotic drugs. Changes in BIS and AAI during both periods were not statistically correlated with changes in onset latencies and peak-to-peak amplitudes of MMEP after TMS. Therefore, both electroencephalographic techniques are of limited use in titrating sedation and anaesthesia during TMS in the dog.