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.     

Bispectral analysis of the electroencephalogram: a review of its development and use in anesthesia

OBJECTIVE: To evaluate the different methodologies used to monitor anesthetic depth and to review the principles and potential applications of bispectral index (BIS) monitoring in veterinary anesthesia. DATABASES USED: Medline (1966 to present); Pubmed (mid-1960s to present); personal files. CONCLUSIONS: Current anesthetic monitoring techniques rely on indirect measures of corticocerebral arousal after a noxious stimulus. Some anesthetics and pre-anesthetics suppress responses that are used to gauge adequate hypnosis and analgesia. The BIS is a direct measure of corticocerebral activation and is inversely related to the degree of hypnosis. Dynamic changes in the BIS after a noxious stimulus may signify early nociceptive activation of the cerebral cortex and may be a useful marker of inadequate analgesia. However, application of this technique during the use of various drug combinations (i.e. clinical anesthesia) will require further research and understanding.     

Isoflurane Potency in the Northern Leopard Frog <Emphasis Type="Italic">Rana pipiens</Emphasis> is Similar to That in Mammalian Species and is Unaffected by Decerebration

Amphibians are commonly used in biomedical research, including studies of mechanisms of anaesthetic action. There is, however, little published work describing the kinetics of inhaled anaesthetic agents or the potency of isoflurane in amphibians. Ten Northern leopard frogs were exposed to a constant isoflurane concentration of 1.0%, 1.2% or 1.5% atm for 4 h, and their response to a noxious stimulus was tested every 20 min. Each frog was anaesthetized with each concentration in random order and allowed at least 16 h to recover between anaesthetic exposures. Frogs were then pithed and the protocol was repeated. Frogs first displayed immobility during stimulus application at 80 min, and the proportion of animals becoming immobile steadily increased to reach a stable level at 4 h. The 50% effective dose for isoflurane in intact and pithed frogs did not differ, and was 1.15 and 1.25% atm, respectively. The potency of isoflurane in leopard frogs was similar to that reported in mammalian species. Cutaneous uptake of anaesthetic is effective given sufficient time, approximately 4 h in this study. Forebrain structures appear to be unimportant for the immobilizing action of isoflurane in the frog.     

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%.     

Determination of the Minimum Infusion Rate (MIR) of alfaxalone required to prevent purposeful movement of the extremities in response to a standardised noxious stimulus in goats

ObjectiveTo determine the minimum infusion rate (MIR) of alfaxalone required to prevent purposeful movement of the extremities in response to noxious stimulation.Study DesignProspective, experimental.AnimalsEight healthy goats; four does and four wethers.MethodsAnaesthesia was induced with alfaxalone 3 mg kg⁻¹ intravenously (IV). A continuous IV infusion of alfaxalone, initially at 0.2 mg kg⁻¹ minute⁻¹, was initiated. Following endotracheal intubation the goats breathed spontaneously via a circle breathing circuit delivering supplementary oxygen. The initial infusion rate was maintained for 30 minutes before testing for responses. The stimulus was clamping on the proximal (soft) part of one digit of the hoof with Vulsellum forceps for 60 seconds. In the absence or presence of purposeful movement of the extremities, the infusion rate was reduced or increased by 0.02 mg kg⁻¹ minute⁻¹ and held constant for 30 minutes before claw-clamping again. Alfaxalone MIR was calculated as the mean of the infusion rates that allowed and abolished movement. Cardio-respiratory parameters were measured. Recovery from general anaesthesia was timed and quality scored. Results are presented as median (range).ResultsThe MIR of alfaxalone was 0.16 (0.14–0.18) mg kg⁻¹ minute⁻¹ or 9.6 (8.4–10.8) mg kg⁻¹ hour⁻¹. Induction of and recovery from anaesthesia were excitement-free. Cardio-respiratory changes were minimal, although compared to baseline HR increased, and at 2 minutes post-induction, (prior to oxygen supplementation), PaO₂ decreased significantly from 84 (80–88) to 70 (51–72) mmHg [11.2 (10.7–11.7) to 9.3 (6.8–9.6) kPa]. Sporadic muscle twitches, unrelated to depth of anaesthesia, were observed during the period of general anaesthesia. Time (minutes) to sternal recumbency and standing were 4.0 (3.0–10.0) and 41.5 (25.0–57.0) respectively.Conclusions and Clinical RelevanceAlfaxalone can be used for total intravenous anaesthesia (TIVA) in goats and is associated with minimal adverse effects. Oxygen supplementation is advised, especially when working at higher altitudes.