Evaluation of the effect of alfentanil on the minimum alveolar concentration of halothane in horses

Peter J. Pascoe From the Department of Surgery, School of Veterinary Medicine, University of California, Davis, CA 95616-8745 (Pascoe, Steffey, Woliner), Department of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph, Ontario, N1G 2W1, Canada (Black, Claxton), and the Departments of Anesthesiology and Biomedical Engineering, Duke University Medical Center, Durham, NC 27710 (Jacobs).

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Eugene P. Steffey From the Department of Surgery, School of Veterinary Medicine, University of California, Davis, CA 95616-8745 (Pascoe, Steffey, Woliner), Department of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph, Ontario, N1G 2W1, Canada (Black, Claxton), and the Departments of Anesthesiology and Biomedical Engineering, Duke University Medical Center, Durham, NC 27710 (Jacobs).

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William D. Black From the Department of Surgery, School of Veterinary Medicine, University of California, Davis, CA 95616-8745 (Pascoe, Steffey, Woliner), Department of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph, Ontario, N1G 2W1, Canada (Black, Claxton), and the Departments of Anesthesiology and Biomedical Engineering, Duke University Medical Center, Durham, NC 27710 (Jacobs).

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Jean M. Claxton From the Department of Surgery, School of Veterinary Medicine, University of California, Davis, CA 95616-8745 (Pascoe, Steffey, Woliner), Department of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph, Ontario, N1G 2W1, Canada (Black, Claxton), and the Departments of Anesthesiology and Biomedical Engineering, Duke University Medical Center, Durham, NC 27710 (Jacobs).

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James R. Jacobs From the Department of Surgery, School of Veterinary Medicine, University of California, Davis, CA 95616-8745 (Pascoe, Steffey, Woliner), Department of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph, Ontario, N1G 2W1, Canada (Black, Claxton), and the Departments of Anesthesiology and Biomedical Engineering, Duke University Medical Center, Durham, NC 27710 (Jacobs).

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Michael J. Woliner From the Department of Surgery, School of Veterinary Medicine, University of California, Davis, CA 95616-8745 (Pascoe, Steffey, Woliner), Department of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph, Ontario, N1G 2W1, Canada (Black, Claxton), and the Departments of Anesthesiology and Biomedical Engineering, Duke University Medical Center, Durham, NC 27710 (Jacobs).

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Summary

The effect of 3 plasma concentrations of alfentanil on the minimum alveolar concentration (mac) of halothane in horses was evaluated. Five healthy geldings were anesthetized on 3 occasions, using halothane in oxygen administered through a mask. After induction of anesthesia, horses were instrumented for measurement of blood pressure, airway pressure, and end-tidal halothane concentrations. Blood samples, for measurement of pH and blood gas tensions, were taken from the facial artery. Positive pressure ventilation was begun, maintaining PaCO 2, at 49.1 ± 3.3 mm of Hg and airway pressure at 20 ± 2 cm of H2O. The mac was determined in triplicate, using a supramaximal electrical stimulus of the oral mucous membranes. Alfentanil infusion was then begun, using a computer-driven infusion pump to achieve and maintain 1 of 3 plasma concentrations of alfentanil. Starting at 30 minutes after the beginning of the infusion, mac was redetermined in duplicate. Mean ± sd measured plasma alfentanil concentration during the infusions were 94.8 ± 29.0, 170.7 ± 29.2 and 390.9 ± 107.4 ng/ml. Significant changes in mac were not observed for any concentration of alfentanil. Blood pressure was increased by infusion of alfentanil and was dose-related, but heart rate did not change. Pharmacokinetic variables of alfentanil were determined after its infusion and were not significantly different among the 3 doses.

Summary

The effect of 3 plasma concentrations of alfentanil on the minimum alveolar concentration (mac) of halothane in horses was evaluated. Five healthy geldings were anesthetized on 3 occasions, using halothane in oxygen administered through a mask. After induction of anesthesia, horses were instrumented for measurement of blood pressure, airway pressure, and end-tidal halothane concentrations. Blood samples, for measurement of pH and blood gas tensions, were taken from the facial artery. Positive pressure ventilation was begun, maintaining PaCO 2, at 49.1 ± 3.3 mm of Hg and airway pressure at 20 ± 2 cm of H2O. The mac was determined in triplicate, using a supramaximal electrical stimulus of the oral mucous membranes. Alfentanil infusion was then begun, using a computer-driven infusion pump to achieve and maintain 1 of 3 plasma concentrations of alfentanil. Starting at 30 minutes after the beginning of the infusion, mac was redetermined in duplicate. Mean ± sd measured plasma alfentanil concentration during the infusions were 94.8 ± 29.0, 170.7 ± 29.2 and 390.9 ± 107.4 ng/ml. Significant changes in mac were not observed for any concentration of alfentanil. Blood pressure was increased by infusion of alfentanil and was dose-related, but heart rate did not change. Pharmacokinetic variables of alfentanil were determined after its infusion and were not significantly different among the 3 doses.

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