Effect of transdermally administered fentanyl on the minimum alveolar concentration of isoflurane in cats

OBJECTIVE: To determine the effect of two doses of fentanyl, administered transdermally, on the minimum alveolar concentration (MAC) of isoflurane in cats. STUDY DESIGN: Prospective, randomized study. ANIMALS: Five healthy, spayed, female cats. METHODS: Each cat was studied thrice with at least 2 weeks between each study. In study 1, the baseline isoflurane MAC was determined in triplicate for each cat. In studies 2 and 3, isoflurane MAC was determined 24 hours after placement of either a 25 or 50 microg hour(-1) fentanyl patch. In each MAC study, cats were instrumented to allow collection of arterial blood and measurement of arterial blood pressure. Twenty-four hours prior to studies 2 and 3, a catheter was placed and secured in the jugular vein and either a 25 or 50 microg hour(-1) fentanyl patch was placed in random order on the left thorax. Blood samples for plasma fentanyl determination were collected prior to patch placement and at regular intervals up to 144 hours. After determination of MAC in studies 2 and 3, naloxone was administered as a bolus dose (0.1 mg kg(-1)) followed by an infusion (1 mg kg(-1) hour(-1)) and MAC redetermined. RESULTS: The baseline isoflurane MAC was 1.51 +/- 0.21% (mean +/- SD). Fentanyl (25 and 50 micro g hour(-1)) administered transdermally significantly reduced MAC to 1.25 +/- 0.26 and 1.22 +/- 0.16%, respectively. These MAC reductions were not significantly different from each other. Isoflurane MAC determined during administration of fentanyl 25 micro g hour(-1) and naloxone (1.44 +/- 0.16%) and fentanyl 50 micro g hour(-1) and naloxone (1.51 +/- 0.19%) was not significantly different from baseline MAC (1.51 +/- 0.21%). CONCLUSIONS AND CLINICAL RELEVANCE: Fentanyl patches are placed to provide long-lasting analgesia. In order to be effective postoperatively, fentanyl patches must be placed prior to surgery. Plasma fentanyl concentrations achieved intraoperatively decrease the need for potent inhalant anesthetics in cats.     

The effect of inhalant anesthetic and body temperature on peri-anesthetic serum concentrations of transdermally administered fentanyl in dogs

Objectives To determine whether moderate hypothermia during anesthesia significantly affects the serum concentration of transdermally delivered fentanyl and whether halothane or isoflurane affect these concentrations. Study Design Randomized cross‐over experimental trial. Animals Six mature, healthy Beagles (three males, three females) weighing 10.6 ± 0.43 kg. Methods A 50‐µg hour^?1 fentanyl patch was applied 36 hours prior to anesthesia. Anesthesia was induced at time 0 (t = 0). Each dog received four treatments: isoflurane + normothermia (ISO‐NORM), isoflurane + hypothermia (ISO‐HYPO), halothane + normothermia (HAL‐NORM), and halothane + hypothermia (HAL‐HYPO). Dogs were intubated and maintained at 1.5 times MAC. Animals in the hypothermia treatments were cooled to 35 °C during anesthesia. Serum fentanyl analysis was performed at ?36, ?24, ?12, 0, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 18, and 26 hours. Direct arterial blood pressures and arterial blood gases were monitored. Results The mean body temperatures (±SEM) during the anesthetic period for the four treatments were: ISO‐NORM = 37.7 ± 0.07 °C, ISO‐HYPO = 35.8 ± 0.1 °C, HAL‐NORM = 37.7 ± 0.06 °C, and HAL‐HYPO = 35.8 ± 0.13 °C. The mean (±SEM) serum fentanyl concentrations (SFC) for both hypothermia treatments were significantly lower than baseline concentrations at t = 1 hour and persisted for the duration of anesthesia for the ISO‐HYPO treatment but only from t = 1 to 2 hours for the HAL‐HYPO treatment. Serum fentanyl concentrations returned to baseline within one hour of the end of anesthesia, regardless of body temperature. There were no significant differences between treatments for systolic or diastolic blood pressure but mean blood pressures were higher during normothermia versus hypothermia during the last hour of anesthesia. Conclusions and clinical relevance Hypothermia during inhalation anesthesia produced a significant reduction in SFC using transdermal administration and was more protracted with isoflurane than halothane anesthesia. While significant reductions in SFC occurred, the SFC were still within the range believed to confer analgesia.     

Effect of transdermally administered fentanyl on minimum alveolar concentration of isoflurane in normothermic and hypothermic dogs

OBJECTIVE: To determine whether the minimum alveolar concentration (MAC) of isoflurane was altered by transdermal administration of fentanyl in normothermic and hypothermic dogs. DESIGN: Randomized complete block crossover design. ANIMALS: 6 mature healthy dogs. PROCEDURE: Dogs received each of 4 treatments in random order. Following induction of anesthesia, normothermia was maintained in dogs that were treated with a fentanyl patch (F-NORM) or sham patch (C-NORM), or hypothermia was maintained in dogs that were treated with a fentanyl patch (F-HYPO) or sham patch (C-HYPO). The appropriate patch was applied 24 hours prior to induction of anesthesia. Anesthesia was induced with isoflurane in oxygen; the dogs were intubated and mechanically ventilated. Target esophageal temperatures were maintained within 1 degrees C of baseline values (normothermia) or at 34.5 degrees C (94.1 degrees F; hypothermia) for 1 hour prior to starting MAC determinations. Supramaximal stimulation was achieved with an electrical stimulator attached to needle electrodes placed in the buccal mucosa of the lower jaw of the dog. RESULTS: Mean MAC +/- SEM of isoflurane during C-NORM, C-HYPO, F-NORM, and F-HYPO treatments were 1.20 +/- 0.17, 0.89 +/- 0.18, 0.76 +/- 0.10, and 0.81 +/- 0.17, respectively. The mean MAC during C-NORM was significantly higher than values for the other treatments. There was no significant difference in mean MAC among the C-HYPO, F-NORM, and F-HYPO treatments. CONCLUSIONS AND CLINICAL RELEVANCE: Data suggest that transdermal administration of fentanyl significantly reduces isoflurane requirements in normothermic dogs. The isoflurane MAC-sparing effects of transdermal fentanyl are not apparent in hypothermic dogs.     

Effects of diazepam and flumazenil on minimum alveolar concentrations for dogs anesthetized with isoflurane or a combination of isoflurane and fentanyl

OBJECTIVE: To determine the effect of a constant-rate infusion of fentanyl on minimum alveolar concentration (MAC) of isoflurane and to determine the interaction between fentanyl and a benzodiazepine agonist (diazepam) and antagonist (flumazenil) in isoflurane-anesthetized dogs. ANIMALS: 8 mixed-breed adult dogs. PROCEDURE: Dogs were anesthetized with isoflurane 3 times during a 6-week period. After a 30-minute equilibration period, each MAC determination was performed in triplicate, using standard techniques. Fentanyl was administered as a bolus (10 microg/kg of body weight, IV) that was followed by a constant infusion (0.3 microg/kg per min, IV) throughout the remainder of the experiment. After determining isoflurane-fentanyl MAC in triplicate, each dog received saline (0.9% NaCl) solution, diazepam, or flumazenil. After 30 minutes, MAC was determined again. RESULTS: Fentanyl significantly decreased isoflurane MAC (corrected to a barometric pressure of 760 mm Hg) from 1.80+/-0.21 to 0.85+/-0.14%, a reduction of 53%. Isoflurane-fentanyl-diazepam MAC (0.48+/-0.29%) was significantly less than isoflurane-fentanyl-saline MAC (0.79+/-0.21%). Percentage reduction in isoflurane MAC was significantly greater for fentanyl-diazepam (74%), compared with fentanyl-saline (54%) or fentanyl-flumazenil (61%). Mean fentanyl concentrations for the entire experiment were increased over time and were higher in the diazepam group than the saline or flumazenil groups. CONCLUSIONS AND CLINICAL RELEVANCE: Fentanyl markedly decreased isoflurane MAC in dogs. Diazepam, but not flumazenil, further decreased isoflurane-fentanyl MAC. Our results indicate that diazepam enhances, whereas flumazenil does not affect, opioid-induced CNS depression and, possibly, analgesia in dogs.     

Pharmacokinetics of vatinoxan in male neutered cats anesthetized with isoflurane

ObjectiveTo characterize the pharmacokinetics of vatinoxan in isoflurane-anesthetized cats.Study designProspective experimental study.AnimalsA group of six adult healthy male neutered cats.MethodsCats were anesthetized using isoflurane in oxygen. Venous catheters were placed to administer the drug and sample blood. Vatinoxan, 1 mg kg^–1, was administered intravenously over 5 minutes. Blood was sampled before and at various times during and up to 8 hours after vatinoxan administration. Plasma vatinoxan concentration was measured using liquid chromatography/tandem mass spectrometry. Compartment models were fitted to the time–concentration data using population methods and nonlinear mixed effect modeling.ResultsA three-compartment model best fitted the data. Typical value (% interindividual variability) for the three volumes (mL kg^–1), the metabolic clearance and two distribution clearances (mL minute^–1 kg^–1) were 34 (55), 151 (35), 306 (18), 2.3 (34), 42.6 (25) and 5.6 (0), respectively. Hypotension increased the second distribution clearance to 10.6.Conclusion and clinical relevanceThe pharmacokinetics of vatinoxan in anesthetized cats were characterized by a small volume of distribution and a low clearance. An intravenous bolus of 100 μg kg^–1 of vatinoxan followed by constant rate infusions of 55 μg kg^–1 minute^–1 for 20 minutes, then 22 μg kg^–1 minute^–1 for 60 minutes and finally 10 μg kg^–1 minute^–1 for the remainder of the infusion time is expected to maintain the plasma concentration within 90%–110% of the plasma vatinoxan concentration previously shown to attenuate the cardiovascular effects of dexmedetomidine (25 μg kg^–1) in conscious cats.     

Intravenous regional anesthesia in isoflurane anesthetized cats: lidocaine plasma concentrations and cardiovascular effects

Objective To determine if intravenous regional anesthesia (IVRA) can be used in cats without resulting in excessive plasma lidocaine concentrations or adverse cardiovascular effects. Study design Prospective, blinded crossover study. Animals Seven healthy male young adult cats weighing 3.96 ± 0.63 kg. Methods At 2.3% end‐tidal isoflurane concentration, lidocaine (L) 3 mg kg^?1 (1%) or saline (S) was injected in a distal cephalic venous catheter after application of two tourniquets to that forelimb which remained in place for 20 minutes. Heart and respiratory rates, arterial blood pressures and ECG were recorded every 5 minutes during tourniquet application and for 20 minutes following tourniquet removal. Lidocaine plasma concentrations were measured 5 minutes after injection and 0.5, 1, 2, 4, 8, 20 and 40 minutes after tourniquet removal. End tidal isoflurane concentrations were reduced to 1.5–2.0% to elicit a response to toe pinch (RTP) in the contralateral leg. The study was repeated similarly in the contralateral leg and RTP was graded for 40 minutes. Response was also tested in the leg previously injected, the differences between the two scores determined and those differences compared between the L and S treatments. The data were analyzed using anova for repeated measures comparing values to baseline. Significance was set at p < 0.005 using the Bonferroni method for multiple comparisons. Results There were no significant differences in physiologic parameters at either isoflurane concentration. Differences in RTP were significantly larger in the lidocaine treatment. The highest mean lidocaine concentrations were measured 0.5 minutes after tourniquet removal after both injections and were 2.79 ± 1.05 and 3.10 ± 1.11 µg mL^?1. The highest individual plasma concentration was 6.46 µg mL^?1. Conclusion No adverse hemodynamic effects were evident after IVRA lidocaine in any cat. The lidocaine dose studied inhibited a RTP until 20 minutes after tourniquet removal. Lidocaine concentrations varied and were measurable prior to tourniquet removal. Clinical relevance IVRA may be a suitable technique for cats undergoing surgery of the distal limbs.     

Hemodynamic effects of intravenous lidocaine in isoflurane anesthetized cats

Lidocaine dose‐dependently decreases the minimum alveolar concentration (MAC) of isoflurane in cats. The purpose of this study was to determine the hemodynamic effects of six lidocaine plasma concentrations in isoflurane anesthetized cats. Six cats were studied. After instrumentation, end‐tidal isoflurane concentration was set at 1.25 times the individual minimum alveolar concentration (MAC), which was determined in a previous study. Lidocaine was administered intravenously to target pseudo‐steady state plasma concentrations of 0, 3, 5, 7, 9, and 11 μg ml^–1, and isoflurane concentration was reduced to an equipotent concentration, determined in a previous study. Cardiovascular variables; blood gases; PCV; total protein and lactate concentrations; and lidocaine and monoethylglycinexylidide concentrations were measured at each lidocaine target concentration, before and during noxious stimulation. Derived variables were calculated. Data were analyzed using a repeated measures anova , followed by a Tukey test for pairwise comparisons where appropriate. One cat was excluded from analysis because the study was aborted at 7 μg ml^–1 due to severe cardiorespiratory depression. Heart rate, cardiac index, stroke index, right ventricular stroke work index, total protein concentration, mixed‐venous PO₂ and hemoglobin oxygen saturation, arterial and mixed‐venous bicarbonate concentrations, and oxygen delivery were significantly lower during lidocaine administration than when no lidocaine was administered. Mean arterial pressure, central venous pressure, pulmonary artery pressure, systemic and pulmonary vascular resistance indices, PCV, arterial and mixed‐venous hemoglobin concentrations, lactate concentration, arterial oxygen concentration, and oxygen extraction ratio were significantly higher during administration of lidocaine than when no lidocaine was administered. Most changes were significant at lidocaine target plasma concentrations of 7 μg ml^–1 and above. Noxious stimulation did not significantly affect most variables. Despite significantly decreasing in inhalant requirements, when combined with isoflurane, lidocaine produces greater cardiovascular depression than an equipotent dose of isoflurane alone. The use of lidocaine to reduce isoflurane requirements is not recommended in cats.     

The cardiovascular sparing effect of fentanyl and atropine, administered to enflurane anesthetized dogs.

Cardiovascular effects of high dose opioid together with low dose inhalant were compared with inhalant alone to determine whether opioid/inhalant techniques were less depressant on the cardiovascular system. The effects of positive pressure ventilation and increasing heart rate to a more physiological level were also studied. Cardiovascular measurements recorded during administration of enflurane at 1.3 minimum alveolar concentration (MAC; 2.89 +/- 0.02%) to spontaneously breathing dogs (time 1) and during controlled ventilation [arterial carbon dioxide tension at 40 +/- 3 mmHg (time 2)] were similar. At time 2, mixed venous oxygen tension and arterial and mixed venous carbon dioxide tensions were significantly decreased, while arterial and mixed venous pH were significantly increased compared to measurements at time 1. After administration of fentanyl to achieve plasma fentanyl concentration of 71.7 +/- 14.4 ng/mL and reduction of enflurane concentration to yield 1.3 MAC multiple (0.99 +/- 0.01%), heart rate significantly decreased, while mean arterial pressure, central venous pressure, stroke index, and systemic vascular resistance index increased compared to measurements taken at times 1 and 2. Pulmonary arterial occlusion pressure was significantly increased compared to measurements taken at time 2. After administration of atropine until heart rate was 93 +/- 5 beats/min (plasma fentanyl concentration 64.5 +/- 13.5 ng/mL) heart rate, mean arterial pressure, cardiac index, oxygen delivery index, and venous admixture increased significantly compared to values obtained at all other times.(ABSTRACT TRUNCATED AT 250 WORDS)     

Renal effects of carprofen administered to healthy dogs anesthetized with propofol and isoflurane

OBJECTIVE: To evaluate renal effects of carprofen in healthy dogs following general anesthesia. DESIGN: Randomized clinical trial. ANIMALS: 10 English hound dogs (6 females and 4 males). PROCEDURE: Dogs were randomly assigned to control (n = 5) or carprofen (5) groups. Anesthesia was induced with propofol (6 to 8 mg/kg [2.7 to 3.6 mg/lb] of body weight, i.v.) and maintained with isoflurane (end-tidal concentration, 2.0%). Each dog underwent two 60-minute anesthetic episodes with 1 week between episodes, and mean arterial blood pressure was maintained between 60 and 90 mm Hg during each episode. Dogs in the carprofen group received carprofen (2.2 mg/kg [1 mg/lb], p.o.) at 9:00 AM and 6:00 PM the day before and at 7:00 AM the day of the second anesthetic episode. Glomerular filtration rates (GFR) were determined during each anesthetic episode by use of renal scintigraphy. Serum creatinine and BUN concentrations and the urine gamma-glutamyltransferase-to-creatinine concentration (urine GGT:creatinine) ratio were determined daily for 2 days before and 5 days after general anesthesia. RESULTS: Significant differences were not detected in BUN and serum creatinine concentrations, urine GGT:creatinine ratio, and GFR either between or within treatment groups over time. CONCLUSIONS AND CLINICAL RELEVANCE: Carprofen did not significantly alter renal function in healthy dogs anesthetized with propofol and isoflurane. These results suggest that carprofen may be safe to use for preemptive perioperative analgesia, provided that normal cardiorespiratory function is maintained.     

Ketamine and the arrhythmogenic dose of epinephrine in cats anesthetized with halothane and isoflurane

Epinephrine-induced arrhythmias were studied in 4 cats (group A), using a 4 X 4 Latin square design. Each cat was anesthetized 4 times, 1 week apart, with halothane (1.5% end expired), isoflurane (2.0% end expired), and halothane or isoflurane preceded by ketamine administered IM (8.8 mg/kg). Lead II of the ECG and femoral artery pressure were recorded. Epinephrine was infused in progressively doubled rates (initial rate = 0.125 micrograms/kg/min) for a maximum of 2.5 minutes or until at least 4 ventricular premature depolarizations occurred within 15 s of each other. The arrhythmogenic dose of epinephrine (ADE; micrograms/kg) was calculated as the product of infusion rate and time to arrhythmia. The ADE (means +/- SD) during anesthesia with halothane alone and with ketamine-halothane anesthesia were 1.33 +/- 0.65 and 1.37 +/- 0.59 micrograms/kg, respectively; during anesthesia with isoflurane alone and ketamine-isoflurane anesthesia, the ADE were 9.34 +/- 1.29 and 16.16 +/- 3.63 micrograms/kg, respectively. The ADE was significantly greater (P less than 0.05) during isoflurane anesthesia and ketamine-isoflurane anesthesia than during halothane anesthesia. The percentages of change in systolic blood pressure (means +/- SD) at the ADE during halothane, ketamine-halothane, isoflurane, and ketamine-isoflurane were 31 +/- 34, 41 +/- 17, 127 +/- 27, and 148 +/- 57, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pharmacokinetics of buprenorphine and its metabolite norbuprenorphine in neutered male cats anesthetized with isoflurane

ObjectiveTo characterize the pharmacokinetics of buprenorphine and norbuprenorphine in isoflurane-anesthetized cats.Study designProspective experimental study.AnimalsA group of six healthy adult male neutered cats.MethodsCats were anesthetized with isoflurane in oxygen. Catheters were placed in a jugular vein for blood sampling and in a medial saphenous vein for buprenorphine and lactated Ringer’s solution administration. Buprenorphine hydrochloride (40 μg kg^–1 over 5 minutes) was administered intravenously. Blood samples were collected before buprenorphine administration and at various times up to 12 hours after administration. Plasma buprenorphine and norbuprenorphine concentrations were measured using liquid chromatography/tandem mass spectrometry. Compartment models were fitted to the time-concentration data using nonlinear mixed effect (population) modeling.ResultsA five-compartment model (three compartments for buprenorphine and two compartments for norbuprenorphine) best fitted the data. Typical value (% interindividual variability) for the three buprenorphine volumes of distribution, and the metabolic clearance to norbuprenorphine, the remaining metabolic clearance and the two distribution clearances were 157 (33), 759 (34) and 1432 (43) mL kg^–1, and 5.3 (33), 16.4 (11), 58.7 (27) and 6.0 (not estimated) mL minute^–1 kg^–1, respectively. Typical values (% interindividual variability) for the two norbuprenorphine volumes of distribution, and the norbuprenorphine metabolic and distribution clearances were 1437 (30) and 8428 (not estimated) mL kg^–1 and 48.4 (68) and 235.9 (not estimated) mL minute^–1 kg^–1, respectively.Conclusions and clinical relevanceThe pharmacokinetics of buprenorphine in isoflurane-anesthetized cats were characterized by a medium clearance.     

Pharmacokinetics of dexmedetomidine during administration of vatinoxan in male neutered cats anesthetized with isoflurane

Dexmedetomidine is an alpha-2 adrenoceptor agonist, and vatinoxan is an alpha-2 antagonist believed to poorly cross the blood–brain barrier in cats. Dexmedetomidine–vatinoxan combinations are of interest in anesthetized cats because the anesthetic sparing effect of dexmedetomidine may be preserved while vatinoxan attenuates the adverse cardiovascular effects of dexmedetomidine. The aim of this study was to characterize the pharmacokinetics of dexmedetomidine in cats during administration of isoflurane and vatinoxan. Six healthy adult male castrated cats were anesthetized with isoflurane in oxygen. Vatinoxan was administered using a target-controlled infusion system intended to maintain a plasma concentration of 4 µg/ml. Dexmedetomidine, 35 µg/kg was administered intravenously over 5 min. Plasma dexmedetomidine and vatinoxan concentrations were measured at selected time points ranging from prior to 8 hr after dexmedetomidine administration using liquid chromatography/tandem mass spectrometry. Compartment models were fitted to the time-concentration data using nonlinear mixed-effect modeling. A three-compartment model best fitted the data. Typical value (% interindividual variability) for the three-compartment volumes (ml/kg), the metabolic clearance and the two intercompartment distribution clearances (ml min⁻¹kg⁻¹) were 168 (259), 318 (35), 1,425 (18), 12.4 (31), 39.1 (18), and 29.6 (17), respectively. Mean ± standard deviation plasma vatinoxan concentration was 2.6 ± 0.6 µg/ml.     

Minimum alveolar concentration measures of central nervous system activation in cats anesthetized with isoflurane

OBJECTIVE: To compare the minimum alveolar concentration (MAC) of isoflurane required to prevent corticocerebral activation, autonomic responses, and purposeful movements after somatic or visceral stimulation in cats anesthetized with isoflurane. ANIMALS: 17 healthy spayed female cats. PROCEDURE: Bispectral index (BIS), autonomic parameters, and purposeful movements were monitored before and after somatic or visceral stimuli in cats anesthetized with isoflurane. End-tidal (ET) isoflurane concentration was varied to determine MAC values for cortical arousal (MAC(BIS)), autonomic responsiveness (MAC(BAR)), and purposeful movement (MAC). Bispectral index values > or = 60 were considered to represent corticocerebral activation. RESULTS: Minimum alveolar concentration for purposeful movement was significantly less than MAC(BIS) and MAC(BAR) for both somatic and visceral stimulation. Individual MAC values for somatic stimulation were not significantly different from respective MAC values for visceral stimulation. The percentage of cats that had a BIS response > or = 60 was inversely related to the end-tidal isoflurane concentration. CONCLUSIONS AND CLINICAL RELEVANCE: Corticocerebral arousal and subcortical autonomic reflexes occured at isoflurane anesthetic concentrations at which reflexive or purposeful movements were absent. These results suggested that isoflurane had a preferential effect on voluntary motor output at low end-tidal isoflurane concentrations, and that sensory pathways, subcortical sympathetic output, and cortical responsiveness are less susceptible to the anesthetic effects of isoflurane. Bispectral index values obtained after somatic or visceral stimulation were sensitive for the detection of early changes in cortical excitability.     

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.