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Abstract

Objective—To describe the effects of changes in circuit volume and oxygen inflow rate on inspired oxygen concentration for a large animal circle anesthetic system.

Study Population—A large animal circle anesthetic system, a 10 L/min flowmeter, and 20- and 40-L breathing bags.

Procedure—Circuit volume was determined by a carbon dioxide dilution technique. Oxygen flow rates of 3, 6, and 10 L/min were delivered to the circuit with the large breathing bag, and a flow rate of 6 L/min was used with the small bag. Gas samples were collected during a 20-minute period. The time constant (τ) and half-time (T1/2) were calculated and compared with measured values.

Results—Mean ± SEM volume of the breathing circuit with a 20- and 40-L breathing bag was 32.97 ± 0.91 L and 49.26 ± 0.58 L, respectively. The from measurements was 11.97, 6.10, and 3.60 minutes at oxygen flow rates of 3, 6, and 10 L/min, respectively, for the large breathing bag and 3.73 minutes at a flow rate of 6 L/min for the small breathing bag. The T1/2 was 8.29, 4.22, and 2.49 minutes at oxygen flow rates of 3, 6, and 10 L/min, respectively, for the large breathing bag and 2.58 minutes for the small breathing bag.

Conclusions and Clinical Relevance—This study emphasizes that there are delays in the rate of increase in the inspired oxygen concentration that accompany use of conventional large animal circle anesthetic systems and low rates of inflow for fresh oxygen. (Am J Vet Res 2005;66:1675–1678)

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in American Journal of Veterinary Research

Abstract

Objective—To characterize halothane and sevoflurane anesthesia in spontaneously breathing rats.

Animals—16 healthy male Sprague-Dawley rats.

Procedure—8 rats were anesthetized with halothane and 8 with sevoflurane. Minimum alveolar concentration (MAC) was determined. Variables were recorded at anesthetic concentrations of 0.8, 1.0, 1.25, and 1.5 times the MAC of halothane and 1.0, 1.25, 1.5, and 1.75 times the MAC of sevoflurane.

Results—Mean (± SEM) MAC for halothane was 1.02 ± 0.02% and for sevoflurane was 2.99 ± 0.19%. As sevoflurane dose increased from 1.0 to 1.75 MAC, mean arterial pressure (MAP) decreased from 103.1 ± 5.3 to 67.9 ± 4.6 mm Hg, and PaCO2 increased from 58.8 ± 3.1 to 92.2 ± 9.2 mm Hg. As halothane dose increased from 0.8 to 1.5 MAC, MAP decreased from 99 ± 6.2 to 69.8 ± 4.5 mm Hg, and PaCO2 increased from 59.1 ± 2.1 to 75.9 ± 5.2 mm Hg. Respiratory rate decreased in a dose-dependent fashion from 88.5 ± 4.5 to 58.5 ± 2.7 breaths/min during halothane anesthesia and from 42.3 ± 1.8 to 30.5 ± 4.5 breaths/min during sevoflurane anesthesia. Both groups of rats had an increase in eyelid and pupillary aperture with an increase in anesthetic dose.

Conclusions and Clinical Relevance—An increase in PaCO2 and a decrease in MAP are clinical indicators of an increasing halothane and sevoflurane dose in unstimulated spontaneously breathing rats. Increases in eyelid aperture and pupil diameter are reliable signs of increasing depth of halothane and sevoflurane anesthesia. Decreasing respiratory rate is a clinical indicator of an increasing dose of halothane. (Am J Vet Res 2003;64:470–474)

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in American Journal of Veterinary Research

Abstract

OBJECTIVE

To determine whether isoflurane-anesthetized cats with demonstrated resistance to the immobilizing effects of fentanyl would exhibit naltrexone-reversible sparing of the minimum alveolar concentration (MAC) of isoflurane when fentanyl was coadministered with the centrally acting catecholamine receptor antagonist acepromazine.

ANIMALS

5 healthy male purpose-bred cats.

PROCEDURES

Anesthesia was induced and maintained with isoflurane in oxygen. Baseline isoflurane MAC was measured by use of a standard tail clamp stimulus and bracketing study design. Afterward, fentanyl was administered IV to achieve a plasma concentration of 100 ng/mL by means of target-controlled infusion, and isoflurane MAC was remeasured. Next, acepromazine maleate (0.1 mg/kg) was administered IV, and isoflurane MAC was remeasured. Finally, isoflurane concentration was equilibrated at 70% of the baseline MAC. Movement of cats in response to tail clamping was tested before and after IV bolus administration of naltrexone. Physiologic responses were compared among treatment conditions.

RESULTS

Isoflurane MAC did not differ significantly between baseline and fentanyl infusion (mean ± SD, 1.944 ± 0.111% and 1.982 ± 0.126%, respectively). Acepromazine with fentanyl significantly decreased isoflurane MAC to 1.002 ± 0.056% of 1 atm pressure. When isoflurane was increased to 70% of the baseline MAC, no cats moved in response to tail clamping before naltrexone administration, but all cats moved after naltrexone administration.

CONCLUSIONS AND CLINICAL RELEVANCE

Acepromazine caused fentanyl to decrease the isoflurane MAC in cats that otherwise did not exhibit altered isoflurane requirements with fentanyl alone. Results suggested that opioid-mediated increases in brain catecholamine concentrations in cats counteract the opioid MAC-sparing effect.

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in American Journal of Veterinary Research

Abstract

Objective—To compare effects of 2 acetylcholinesterase inhibitors on recovery quality of horses anesthetized with isoflurane.

Animals—6 horses in phase 1, 7 horses in phase 2A, and 14 horses in phase 2B.

Procedures—The study comprised 3 phases (2 randomized, blinded crossover phases in horses undergoing orthopedic procedures and 1 prospective dose-determining phase). In phase 1, horses were anesthetized with isoflurane and received neostigmine or saline (0.9% NaCl) solution prior to anesthetic recovery. Phase 2A was a physostigmine dose-determining phase. In phase 2B, horses were anesthetized with isoflurane and received neostigmine or physostigmine prior to recovery. Objective recovery events were recorded and subjective visual analogue scale scores of recovery quality were assigned from video recordings.

Results—Recovery measures in phase 1 were not different between horses receiving neostigmine or saline solution. In phase 2A, 0.04 mg of physostigmine/kg was the highest cumulative dose that did not cause clinically relevant adverse behavioral or gastrointestinal effects. Horses receiving physostigmine had higher mean ± SD visual analogue scale recovery scores (70.8 ± 13.3 mm) than did horses receiving neostigmine (62.4 ± 12.8 mm) in phase 2B, with fewer attempts until sternal and standing recovery. Incidence of colic behavior did not differ among groups.

Conclusions and Clinical Relevance—Inhibition with physostigmine improved anesthetic recovery quality in horses anesthetized with isoflurane, compared with recovery quality for horses receiving neostigmine. Inhibition of central muscarinic receptors by inhalation anesthetics may underlie emergence delirium in horses recovering from anesthesia.

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in American Journal of Veterinary Research

Abstract

Objective—To evaluate effects of various doses of remifentanil on measures of analgesia in anesthetized cats.

Animals—6 healthy adult cats.

Procedures—Minimum alveolar concentration (MAC) for isoflurane and thermal threshold responses were evaluated in anesthetized cats. Remifentanil infusions of 0 (baseline), 0.0625, 0.125, 0.25, 0.5, 1, 2, 4, 8, and 16 μg/kg/min were administered; after a 45-minute equilibration period, isoflurane MAC and responses were determined. Isoflurane MAC was determined in anesthetized cats once for each remifentanil infusion rate by use of a standard tail clamp technique. Thermal threshold was measured in awake cats by use of a commercially available analgesiometric probe placed on the lateral portion of the thorax; remifentanil infusions were administered in randomized order to anesthetized cats, and thermal threshold determinations were made by an investigator who was unaware of the infusion rate.

Results—Mean ± SEM median effective concentration (EC50) for remifentanil and its active metabolite, GR90291, for the thermal threshold test was 1.00 ± 0.35 ng/mL and 307 ± 28 ng/mL of blood, respectively. Dysphoria was detected in all awake cats at the 2 highest remifentanil infusion rates. However, isoflurane MAC during remifentanil infusions was unchanged from baseline values, even at blood opioid concentrations approximately 75 times the analgesic EC50.

Conclusions and Clinical Relevance—Immobility and analgesia as reflected by thermal threshold testing were independent anesthetic end points in the cats. Results of MAC-sparing evaluations should not be used to infer analgesic potency without prior validation of an MAC-analgesia relationship for specific drugs and species.

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in American Journal of Veterinary Research

Abstract

Objective—To evaluate whether guaifenesin can prevent adverse anesthetic induction events caused by propofol and whether a guaifenesin-propofol induction combination has brief cardiovascular effects commensurate with rapid drug washout.

Animals—8 healthy adult horses.

Procedures—Guaifenesin was administered IV for 3 minutes followed by IV injection of a bolus of propofol (2 mg/kg). Additional propofol was administered if purposeful movement was detected. Anesthesia was maintained for 2 hours with isoflurane or sevoflurane at 1.2 times the minimum alveolar concentration with controlled normocapnic ventilation. Normotension was maintained via a dobutamine infusion. Plasma concentrations of propofol and guaifenesin were measured every 30 minutes.

Results—Mean ± SD guaifenesin and propofol doses inducing anesthesia in half of the horses were 73 ± 18 mg/kg and 2.2 ± 0.3 mg/kg, respectively. No adverse anesthetic induction events were observed. By 70 minutes, there was no significant temporal change in the dobutamine infusion rate required to maintain normotension for horses anesthetized with isoflurane or sevoflurane. Mean plasma guaifenesin concentrations were 122 ± 30μM, 101 ± 33μM, 93 ± 28μM, and 80 ± 24μM at 30, 60, 90, and 120 minutes after anesthetic induction, respectively. All plasma propofol concentrations were below the limit of quantitation.

Conclusions and Clinical Relevance—Guaifenesin prevented adverse anesthetic induction events caused by propofol. Guaifenesin (90 mg/kg) followed by propofol (3 mg/kg) should be sufficient to immobilize > 99% of calm healthy adult horses. Anesthetic drug washout was rapid, and there was no change in inotrope requirements after anesthesia for 70 minutes.

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in American Journal of Veterinary Research

Abstract

Objective—To characterize the pharmacokinetics of remifentanil in conscious cats and cats anesthetized with isoflurane.

Animals—6 cats.

Procedures—Remifentanil (1 μg/kg/min for 5 minutes) was administered IV in conscious cats or cats anesthetized with 1.63% isoflurane in oxygen in a randomized crossover design. Blood samples were obtained immediately prior to remifentanil administration and every minute for 10 minutes, every 2 minutes for 10 minutes, and every 5 minutes for 10 minutes after the beginning of the infusion. Blood was immediately transferred to tubes containing citric acid, flash frozen in liquid nitrogen, and stored at −80°C until analysis. Blood remifentanil concentration was determined by use of liquid chromatography–mass spectrometry. Remifentanil concentration-time data were fitted to compartment models.

Results—A 2-compartment model (with zero-order input because of study design) best described the disposition of remifentanil in awake and isoflurane-anesthetized cats. The apparent volume of distribution of the central compartment, the apparent volume of distribution at steady state, the clearance, and the terminal half-life (median [range]) were 1,596 (1,164 to 2,111) and 567 (278 to 641) mL/kg, 7,632 (2,284 to 76,039) and 1,651 (446 to 29,229) mL/kg, 766 (408 to 1,473) and 371 (197 to 472) mL/min/kg, and 17.4 (5.5 to 920.3) and 15.7 (3.8 to 410.3) minutes in conscious and anesthetized cats, respectively.

Conclusions and Clinical Relevance—The disposition of remifentanil in cats was characterized by a high clearance. Isoflurane anesthesia significantly decreased the volume of the central compartment, likely by decreasing blood flow to vessel-rich organs.

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in American Journal of Veterinary Research

Abstract

OBJECTIVE To evaluate pharmacokinetics, recovery times, and recovery quality in horses anesthetized with 1.2 times the minimum alveolar concentration of sevoflurane or desflurane.

ANIMALS 6 healthy adult horses.

PROCEDURES Anesthesia was maintained with sevoflurane or desflurane for 2 hours at 1.2 times the minimum alveolar concentration. Horses recovered without assistance. During recovery, end-tidal gas samples were collected until horses spontaneously moved. Anesthetic concentrations were measured by use of gas chromatography. After a 1-week washout period, horses were anesthetized with the other inhalation agent. Video recordings of anesthetic recovery were evaluated for recovery quality on the basis of a visual analogue scale by investigators who were unaware of the anesthetic administered. Anesthetic washout curves were fit to a 2-compartment kinetic model with multivariate nonlinear regression. Normally distributed interval data were analyzed by means of paired Student t tests; ordinal or nonnormally distributed data were analyzed by means of Wilcoxon signed rank tests.

RESULTS Horses recovered from both anesthetics without major injuries. Results for subjective recovery evaluations did not differ between anesthetics. Area under the elimination curve was significantly smaller and time to standing recovery was significantly less for desflurane than for sevoflurane, although distribution and elimination constants did not differ significantly between anesthetics.

CONCLUSIONS AND CLINICAL RELEVANCE Differences in area under elimination the curve between anesthetics indicated more rapid clearance for desflurane than for sevoflurane in horses, as predicted by anesthetic blood solubility differences in this species. More rapid elimination kinetics was associated with faster recovery times, but no association with improved subjective recovery quality was detected.

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in American Journal of Veterinary Research

Abstract

Objective—To test the hypothesis that postanesthetic sedation with romifidine would dose-dependently improve recovery quality of recovery from isoflurane anesthesia in horses more than postanesthetic sedation with xylazine.

Design—Prospective, randomized, blinded clinical trial.

Animals—101 healthy adult horses examined at the University of California-Davis Veterinary Medical Teaching Hospital from 2007 to 2009.

Procedures—Horses were sedated with xylazine, and anesthesia was induced with guaifenesin, diazepam, and ketamine via a standardized drug protocol. Anesthesia for surgical or diagnostic procedures was maintained with isoflurane in oxygen for 1 to 4 hours. At the end of anesthesia, horses were moved to a padded stall for recovery. Once the breathing circuit was disconnected and the patient was spontaneously breathing, either xylazine (100 or 200 μg/kg [45 or 91 μg/lb]) or romifidine (10 or 20 μg/kg [4.5 or 9.1 μg/lb]) was administered IV. Objective patient, surgical, and anesthesia data were recorded. Subjective visual analog scale (VAS) scores of recovery quality were assigned by a single individual who was unaware of the treatment received. A stepwise linear regression model was used to correlate patient and procedure factors with the VAS score.

Results—Painful procedures, longer anesthesia times, and the Arabian horse breed were associated with poorer VAS scores. Adjustment for these factors revealed an improved VAS recovery score associated with the use of a romifidine dose of 20 μg/kg.

Conclusions and Clinical Relevance—In healthy adult horses anesthetized with isoflurane for > 1 hour, the results of this study supported the use of 20 μg of romifidine/kg, IV, rather than lower romifidine doses or xylazine, for postanesthetic sedation to improve recovery quality.

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in Journal of the American Veterinary Medical Association

Abstract

Objective—To develop a method for surgical placement of a commercial microsensor intracranial pressure (ICP) transducer and to characterize normal ICP and cerebral perfusion pressures (CPP) in conscious adult horses.

Animals—6 healthy castrated male adult horses (1 Holsteiner, 1 Quarter Horse, and 4 Thoroughbreds).

Procedure—Anesthesia was induced and maintained by use of isoflurane as the sole agent. Catheters were inserted percutaneously into the jugular vein and carotid artery. A microsensor ICP transducer was inserted in the subarachnoid space by means of right parietal craniotomy. The burr hole was then sealed with bone wax, the surgical incision was sutured, and the transducer was secured in place. Measurements were collected 1 hour after horses were able to stand during recovery from anesthesia.

Results—Mean ± SD values for ICP and CPP were 2 ± 4 and 102 ± 26 mm Hg, respectively.

Conclusion and Clinical Relevance—This report describes a relatively facile technique for obtaining direct and accurate ICP measurements for adult horses. The ICP values obtained in this study are within reference ranges established for other species and provide a point of reference for the diagnosis of abnormal ICP in adult horses. (Am J Vet Res 2002;63:1252–1256)

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in American Journal of Veterinary Research