title: Comparison of the effects of xylazine bolus versus medetomidine constant rate infusion on cardiopulmonary function and depth of anesthesia in horses anesthetized with isoflurane.
abstract: OBJECTIVE
  To compare the effects of xylazine bolus versus medetomidine constant rate infusion (MCRI) on cardiopulmonary function and depth of anesthesia in dorsally recumbent, spontaneously breathing, isoflurane-anesthetized horses.
  
  
  METHODS
  Prospective, randomized crossover study.
  
  
  METHODS
  10 healthy adult Standardbreds.
  
  
  METHODS
  Horses were premedicated with xylazine or medetomidine IV. Anesthesia was induced with diazepam and ketamine and maintained with isoflurane for 150 minutes. For the xylazine treatment, end-tidal isoflurane concentration was maintained at 1.7%, and xylazine (0.2 mg/kg [0.09 mg/lb], IV) was administered as a bolus at the end of anesthesia. For the MCRI treatment, end-tidal isoflurane concentration was maintained at 1.4%, and medetomidine (0.005 mg/kg/h [0.0023 mg/lb/h], IV) was infused throughout anesthesia. Physiologic data (ie, heart rate, respiratory rate, rectal temperature, bispectral index, and electromyographic values) were compared between treatments with xylazine bolus versus MCRI.
  
  
  RESULTS
  Heart rate was lower, but mean arterial blood pressure was higher from 20 to 40 minutes with MCRI treatment, compared with conventional treatment with xylazine. Respiratory rate and rectal temperature were greater with MCRI treatment. Bispectral index was lower with MCRI treatment from 80 to 150 minutes, and electromyographic values were lower with MCRI treatment from 30 to 150 minutes.
  
  
  CONCLUSIONS
  In isoflurane-anesthetized horses, premedication with medetomidine followed by administration of medetomidine as a constant rate infusion resulted in decreased heart rate, higher arterial blood pressure from 20 through 40 minutes after induction of anesthesia, and better preserved body temperature, compared with conventional treatment with xylazine. Greater depth of anesthesia and muscle relaxation were seen with MCRI treatment, despite the lower isoflurane concentration.
authors.affiliation: Departments of Companion Animals, Atlantic Veterinary College, Charlottetown, PE C1A 4P3, Canada. cate.creighton@adelaide.edu.au
authors.forename: Catherine M
authors.initials: CM
authors.lastname: Creighton
authors.forename: Kip A
authors.initials: KA
authors.lastname: Lemke
authors.forename: Leigh A
authors.initials: LA
authors.lastname: Lamont
authors.forename: Barbara S
authors.initials: BS
authors.lastname: Horney
authors.forename: Christopher B
authors.initials: CB
authors.lastname: Riley
mesh_terms: D058647:Adrenergic alpha-2 Receptor Agonists / Q000008:administration & dosage / Q000494:pharmacology; D000769:Anesthesia, Inhalation / Q000662:veterinary; D018685:Anesthetics, Inhalation / Q000008:administration & dosage / Q000494:pharmacology; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects*; D004334:Drug Administration Schedule; D005260:Female; D006339:Heart Rate / Q000187:drug effects*; D006736:Horses; D007530:Isoflurane / Q000008:administration & dosage / Q000494:pharmacology*; D008297:Male; D020926:Medetomidine / Q000008:administration & dosage / Q000494:pharmacology*; D014991:Xylazine / Q000008:administration & dosage / Q000494:pharmacology*
chemical_list: D058647:Adrenergic alpha-2 Receptor Agonists; D018685:Anesthetics, Inhalation; D014991:Xylazine; D007530:Isoflurane; D020926:Medetomidine