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Pharmacokinetics and clinical effects of a subanesthetic continuous rate infusion of ketamine in awake horses

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  • 1 Department of Large Animal Medicine and Surgery, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX 77843-4474.
  • | 2 Physiology and Pharmacology, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX 77843-4474.
  • | 3 Small Animal Medicine and Surgery, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX 77843-4474.
  • | 4 Texas Veterinary Medical Diagnostic Laboratory, Texas A&M University, College Station, TX 77843-4474.
  • | 5 Department of Large Animal Medicine and Surgery, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX 77843-4474.

Abstract

Objective—To determine the pharmacokinetics and clinical effects of a subanesthetic, continuous rate infusion of ketamine administered to healthy awake horses.

Animals—8 adult horses.

Procedures—Ketamine hydrochloride was administered to 2 horses, in a pilot study, at rates ranging from 0.4 to 1.6 mg/kg/h for 6 hours to determine an appropriate dose that did not cause adverse effects. Ketamine was then administered to 6 horses for a total of 12 hours (3 horses at 0.4 mg/kg/h for 6 hours followed by 0.8 mg/kg/h for 6 hours and 3 horses at 0.8 mg/kg/h for 6 hours followed by 0.4 mg/kg/h for 6 hours). Concentration of ketamine in plasma, heart rate, respiratory rate, blood pressure, physical activity, and analgesia were measured prior to, during, and following infusion. Analgesic testing was performed with a modified hoof tester applied at a measured force to the withers and radius.

Results—No signs of excitement and no significant changes in the measured physiologic variables during infusion rates of 0.4 and 0.8 mg of ketamine/kg/h were found. At 6 hours following infusions, heart rate and mean arterial pressure were decreased, compared with preinfusion measurements. An analgesic effect could not be demonstrated during or after infusion. Pharmacokinetic variables for 0.4 and 0.8 mg/kg/h infusions were not significantly different.

Conclusions and Clinical Relevance—Ketamine can be administered to awake horses at 0.4 or 0.8 mg/kg/h without adverse behavioral effects. The observed pharmacokinetic values are different than those reported for single-dose IV bolus administration of this drug.

Abstract

Objective—To determine the pharmacokinetics and clinical effects of a subanesthetic, continuous rate infusion of ketamine administered to healthy awake horses.

Animals—8 adult horses.

Procedures—Ketamine hydrochloride was administered to 2 horses, in a pilot study, at rates ranging from 0.4 to 1.6 mg/kg/h for 6 hours to determine an appropriate dose that did not cause adverse effects. Ketamine was then administered to 6 horses for a total of 12 hours (3 horses at 0.4 mg/kg/h for 6 hours followed by 0.8 mg/kg/h for 6 hours and 3 horses at 0.8 mg/kg/h for 6 hours followed by 0.4 mg/kg/h for 6 hours). Concentration of ketamine in plasma, heart rate, respiratory rate, blood pressure, physical activity, and analgesia were measured prior to, during, and following infusion. Analgesic testing was performed with a modified hoof tester applied at a measured force to the withers and radius.

Results—No signs of excitement and no significant changes in the measured physiologic variables during infusion rates of 0.4 and 0.8 mg of ketamine/kg/h were found. At 6 hours following infusions, heart rate and mean arterial pressure were decreased, compared with preinfusion measurements. An analgesic effect could not be demonstrated during or after infusion. Pharmacokinetic variables for 0.4 and 0.8 mg/kg/h infusions were not significantly different.

Conclusions and Clinical Relevance—Ketamine can be administered to awake horses at 0.4 or 0.8 mg/kg/h without adverse behavioral effects. The observed pharmacokinetic values are different than those reported for single-dose IV bolus administration of this drug.

Contributor Notes

Dr. Fielding's present address is Loomis Basin Veterinary Center, 3901 Sierra College Blvd, Loomis, CA 95650.

Dr. Brumbaugh's present address is PO Box 248, Wellborn, TX 77881.

Presented in abstract form at the Veterinary Emergency and Critical Care Symposium, San Diego, September 2004.

Supported in part by Fort Dodge Animal Health, Fort Dodge, Iowa, and by the Link Colic Research Program.

Address correspondence to Dr. Fielding.